What is the organization of the body?

Organization of Body Parts 
The structure of the body can be studied at different levels of organization. First, all substances, including body parts, are composed of chemicals made up of submicro­scopic particles called atoms. The atoms most frequently found in the body are carbon, hydrogen, oxygen, and ni­trogen. Atoms join together to form a molecule, which can join with other molecules to form macromolecules. The macromolecules in cells are called biomolecules in this text. For example, the molecules called amino acids join to­gether to form a biomolecule called protein. Muscles con­tain a significant amount of protein; therefore, meat is a rich source of this basic nutrient.
Proteins and also fats contribute to the makeup of the cell, the basic unit of all living things. Cells are the smallest living portion of any organism, and it is at the cellular level that health and disease are best understood. Within cells are organelles, tiny membranous structures that perform cell functions. For example, the or­ganelle called the nucleus is especially concerned with cell reproduction; another organelle called the mitochondrion supplies the cell with energy.
Cells are found in tissues, and tissues make up organs. A tissue is composed of similar types of cells and performs a specific function. An organ is composed of several types of tissues and performs a particular function within an organ system. For example, the stomach is an organ and is a part of the digestive system. It has a specific role in this system, in which the overall function is to supply the body 'dth the nutrients needed for growth and repair. The other systems of the body also have specific functions.
All of the body systems together make up the organism -in this case, the human being. Human beings are com­plex animals, but this complexity can be broken down and
studied at ever simpler levels. Each simpler level is organ­ized and constructed in a particular way.
The body has levels of organization that progress from atoms to molecules. macromolecules (biomolecules), cells, tissues. organs. organ systems. and finally, the organism.

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Anatomy and Physiology a complete defination

Anatomy and Physiology 
Anatomy is the study of the structure of body parts. For example, the stomach is a J-shaped, pouchlike organ. The stomach wall has thick folds, which disappear as the stomach expands to increase its capacity. Physiology is the study of the function of body parts. For example, the stomach temporarily stores food, secretes digestive juices, and passes on partially digested food to the small intestine.
Anatomy and physiology are closely connected in that the structure of an organ suits its function. For example, the stomach's pouchlike shape and ability to expand are suit­able to its function of storing food. In addition, the micro­scopic structure of the stomach wall is suitable to its secretion of digestive juices, as we shall see later in the text.
Anatomy is the study of the structure of body parts, and physiology is the study of the function of these parts. Structure is suited to the function of a part.

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Transportation in Plasma Membrane

Plasma Membrane Transport 
The plasma membrane allows only certain molecules to enter and exit the cytoplasm freely; therefore, the plasma membrane is said to be selectively permeable. Transport of molecules across the membrane can be active or passive. Active transport requires the use of ATP energy; passive trans­port does not. Passive transport mechanisms include diffu­sion, osmosis, and filtration.
Passive Diffusion
Diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentra­tion. For example, if a perfume bottle is opened in one cor­ner of a room, the perfume's scent will soon be apparent throughout the room because the perfume molecules have diffused from an area of high concentration (the corner) to areas oflower concentration. Another example of diffusion is putting a tablet of dye into water. The water eventually takes on the color of the dye as the tablet dissolves.
In the body, oxygen enters the blood from the alveoli (air sacs) ofthe lungs by diffusion. During kidney dialysis, waste molecules diffuse across a membrane from the area of higher concentration (the blood) to the area of lower concentration (the dialysate). Cells do not expend any energy when sub­stances can simply diffuse across the plasma membrane.
Some molecules that cannot cross the phospholipid bilayer of the plasma membrane diffuse through plasma membrane protein channels. Passive transport of this sort is a form of facilitated diffusion.
Osmosis 
Osmosis (oz-mo'sis) is the diffusion of water across a plasma membrane. It occurs whenever the concentrations of water on either side of a selectively permeable mem­brane are unequal. Normally, body fluids are isotonic (i"so-ton'ik) to cells, that is, the concentrations of substances (solutes) and water (solvent) on either side of the plasma membrane are equal and cells, therefore, maintain their usual size and shape. For this reason, most intravenous solutions are also isotonic to cells.
If red blood cells are placed in a hypotonic (hi "po­ton'ik) solution, which has a higher concentration of water (lower concentration of solute) than do the cells, water will enter the cells, and they will swell to bursting. Bursting of red blood cells is called hemolysis. On the other hand, if red blood cells are placed in a hypertonic (hi"per-ton'ik) solution, which has a lower concentration of water (higher concentration of solute) than do the cells, water will leave the cells, and they will shrink. The shrinking of red blood cells is called crenation.
Filtration 
Because capillary walls are only one cell thick, small mole­cules (water, small solutes) tend to passively diffuse across these walls, from areas of higher concentration to those of lower concentration. However, blood pressure aids matters by pushing water and dissolved solutes out of the capillary. This process is called filtration.
Filtration is easily observed in the laboratory when a so­lution is poured past filter paper into a flask. Large substances stay behind, but small molecules and water pass through.
Filtration of water and substances in the region of cap­illaries is largely responsible for the formation of tissue fluid, the fluid that surrounds the cells. Filtration is also at work in the kidneys when water and small molecules move from the blood to the inside of the kidney tubules.
Active Transport 
In active transport, substances accumulate either inside or outside the cell in the region of higher concentration. For ex­ample, iodine collects in the cells of the thyroid gland; sugar is completely absorbed from the gut by the cells lining the digestive tract; and sodium is sometimes almost completely withdrawn from uri ne by cells lining the kidney tubules.
Carrier proteins and an expenditure of energy are both needed to transport substances from an area of lower concentration to an area of higher concentration. A carrier is a plasma membrane protein that specializes in combining with and transporting substances across the plasma membrane. Because ATP energy is needed to cause a carrier to combine with the substance to be transported, cells primarily involved in active transport, such as kidney cells, have a large number of mitochondria near the mem­brane where active transport is occurring.
During active transport. vvhich requires plasma membrane carriers and ATP energy. substances move against a concentration gradient and accumulate in the area of higher concentration.
Endocytosis and Exocytosis
At times, substances are taken into cells by vesicle forma­tion. This is called endocytosis (en"do-si-to'sis). When the material taken in is quite large, the process is called phagocytosis (fag" o-si-to' sis) (cell eating). Phagocy­tosis is common to amoeboid-type cells, such as macro­phages. These white blood cells are called the body's scavengers because they engulf worn-out red blood cells and other types of debris. When cells take in material that is small enough to be dissolved or suspended in water, the process is called pinocytosis (pi" no-si- to' sis) (cell drinking).
Vesicles within the cytoplasm of the cell can fuse with the plasma membrane and release their contents to the outside of the cell. This is called exocytosis (ex" o-si-to'sis). Some cells of the nervous system release substances in­volved in the transfer of nerve impulses between adjacent cells via exocytosis.

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Composition and Functions of Nucliec Acids

Nucleic Acids 
Nucleic acids (nu-kla'ik as'idz) are huge biomolecules with very specific functions in cells. Genes, the hereditary factors that we receive from our parents and that control the characteristics of the cell and organism, are composed of a nucleic acid called DNA (deoxyribonucleic acid) (de­ok'sf-ri"bo-nu-kla"ik as'id). DNA is a molecule that stores coded information. Another important nucleic acid-RNA (ribonucleic acid)-works in conjunction with DNA to bring about protein synthesis in cells.
Both DNA and RNA are polymers of nucleotides joined together. Every nucleotide is a molecular complex of three types of unit molecules: a phosphate, a 5-carbon sugar, and a nitrogen base. The sugar in DNA is deoxyribose, while that in RNA is ribose, which accounts for the difference in their names. The nitrogen bases in DNA are adenine (A), guanine (G), thymine (T), and cytoine (C). The bases in RNA are the same, except that uracil (U) is substituted for thymine.
When nucleotides join together, they form a linear molecule called a strand, composed of a sugar-phosphate backbone, with the nitrogen bases projecting to one side. RNA is single-stranded, but DNA is double-
tranded. The two strands of DNA are twisted in the form of a double helix and are held together by hydrogen bonds between the bases. An unwound DNA helix re­embles a ladder: The steps of the ladder are the hydrogen­bonded nitrogen bases.
The sequence of nitrogen bases in DNA serves as a code for directing the sequence of bases in RNA and then the sequence of amino acids in a protein. In other words, the genes we inheri t determine the types of proteins we can produce in our cells.
Both DNA and RNA are polymers of nucleotides; only DNA is double-stranded. DNA makes up the genes. and along with RNA. controls protein synthesis.

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Proteins Structure and Funcitons

Functions and Structure of Proteins
Proteins Functions 
Proteins are huge biomolecules that sometimes have mainly a structural function. For example, in humans, ker­atin is a protein that makes up hair and nails, and collagen is a protein found in connective tissue, including cartilage, bone, and the fibrous connective tissue of ligaments and tendons. Muscles contain proteins that account for mus­cles' ability to contract.
Some proteins function as enzymes (en'z1mz), neces­sary contributors to the chemical workings of the cell and, therefore, of the body. Enzymes are organic catalysts that speed chemical reactions. The reaction occurs when the reactants are close to one another on the enzyme surface. Enzymes also catalyze reactions that break down reactants into their mo­lecular subunits. Enzymes work so quickly that a reaction that might normally take several hours or days without an enzyme takes only a fraction of a second when an enzyme is present.
Proteins Structure 
The unit molecules found in proteins are called amino acids (ah-me'no as'idz). The name amino acid refers to the fact that the molecule has two functional groups: an amino group and an acid group.
Amino acids differ from one another by their R groups, the remainder of the molecule. In amino acids, R groups vary from a single hydrogen atom (-H) to a complicated ring. Because about 20 different common amino acids are found in the proteins of living things, there are also about 20 different types of R groups.
A peptide bond (pep'tid bond) joins two amino acids together.
(sin'the-sis) the acid group of one amino acid reacts with the amino group of another amino acid, and water is given off. A dipeptide contains only two amino acids, but a polypeptide can contain hundreds of amino acids. Polypep­tides have three levels of structure. The pri­mary structure is the sequence of amino acids in that particular polypeptide. The secondary structure is often a helix, held in place by hydrogen bonding. The tertiary structure is the final three-dimensional shape of the polypeptide.
Some proteins have only one polypeptide chain, while others have more than one type of polypeptide chain, each with its own primary, secondary, and tertiary structures. These separate polypeptides are arranged to give some pro­teins a fourth level of structure, termed the quaternary structure. llemoglobin is a complex protein with a quaternary structure.
Proteins have both structural and enzymatic functions in the human body. Amino acids are the unit molecules for peptides and polvpentidea.

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Essential Functions and Structure of a Cell

Cell structure and functions
The cells of the body perform specific functions, and therefore, their structures vary greatly. Even so, because all cells have the same basic organization, we can begin the study of cell structure by examining a generalized cell. Knowledge of the generalized animal cell was obtained by using the light microscope and the electron microscope. The light microscope, which utilizes light to view the object, does not show much detail, but the electron microscope, which uses electrons to view the object, allows cell biologists to make out cell structure in great detail. The plasma membrane has also been examined using the electron microscope.
The Plasma Membrane 
Our cells are surrounded by an outer plasma membrane. The plasma membrane is the boundary between the inside of the cell, termed the cytoplasm, and the outside of the cell. Plasma membrane integrity is necessary to the life of the cell.
The plasma membrane is a phospholipid bilayer with attached or embedded proteins. The phospholipid mole­cule has a polar head and nonpolar tails. Be­cause the polar heads are charged, they are hydrophilic (water-loving) and face outward, where they are likely to encounter a watery environment. The nonpolar tails are hy­drophobic (water-fearing) and face inward, where there is no water. When phospholipids are placed in water, they natu­rally form a spherical bilayer because of the chemical prop­erties of the heads and the tails.
At body temperature, the phospholipid bilayer is a liq­uid; it has the consistency of olive oil, and the proteins are able to change their positions by moving laterally. The fluid-mosaic model, a working description of membrane structure, suggests that the protein molecules have a chang­ing pattern (form a mosaic) within the fluid phospholipid bilayer. Our plasma membranes also contain a substantial number of cholesterol molecules. These mole­cules lend stability to the phospholipid bilayer and prevent a drastic decrease in fluidity at low temperatures.
Short chains of sugars are attached to the outer surfaces of some protein and lipid molecules (called glycoproteins and glycolipids, respectively). These carbohydrate chains, specific to each cell, mark the cell as belonging to a partic­ular individual and account for such characteristics as blood type or why a patient's system sometimes rejects an organ transplant. Some glycoproteins have a special config­uration that allows them to act as a receptor for a chemical messenger like a hormone. Some plasma membrane pro­teins form channels through which certain substances can enter cells, while others are carriers involved in the passage of molecules through the membrane.
The Nucleus 
The nucleus (nu'kle-us) is enclosed by a nuclear envelope that is continuous with the endoplasmic reticulum (ER), another part of the cell. Pores, or openings, in this nuclear envelope allow the passage oflarge molecules from the nucleoplasm, the fluid portion of the nucleus, to the cytoplasm.
The nucleus is the control center that oversees the cell's metabolic functioning and ultimately determines the cell's characteristics. Within the nucleus are masses of threads called chromatin (kro'mah-tin), so named because they take up stains and become colored. Chromatin is indistinct in the nondividing cell, but it condenses to rodlike struc­tures called chromosomes (kro'mo-somz) just prior to cell division.
Chromosomes contain DNA, which makes up the genes. DNA is double-stranded; each strand carries a particular sequence of nitrogen bases. These serve as a genetic code that is passed on to a type of RNA called messenger RNA (mRNA) for the purpose of di­recting protein synthesis in the cell. Another type of RNA, called transfer RNA (tRNA) , is also made in the nucleus and functions in protein synthesis. Some of the synthe­sized proteins have a structural role, and some are enzymes involved in metabolism, which is all the chemical reac­tions that occur in the cell. In this way, DNA controls cell structure and function.
Occasionally, the sequence of bases in DNA changes, and this mistake, called a mutation, is copied by mRNA and may result in a faulty protein. The individual in which this occurs is said to have a genetic disease because the cells cannot function properly, and the result is a notice­able illness.
The nucleolus (nu-kle' o-lus) is a spherical body found in the nucleus. Here, another type of RNA, called ribosomal RNA (rRNA), is formed and contributes to the manufacture of small granules called ribosomes (ri'bo-somz). After their formation in the nucleolus, ribo­somes are transported from the nucleus to the cytoplasm, where they function in protein synthesis.
The nucleus contains chromatin, which condenses into chromosomes just prior to cell division. Genes, composed of DNA. are on the chromosomes, and they code for the production of proteins in the cytoplasm. The nucleolus is involved in ribosome formation.

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Complete Functions and Structure of a Cell

Cell structure and functions-Complete insight (golgi bodies and mitochondria)
Golgi Bodies
The Golgi Bodies (gol'je ba'dez) is named for Camillo Colgi, the Italian scientist who first dis­covered its presence in cells. It is composed of a stack of six or more saccules that look like flattened vacuoles (Iarge, membranous sacs). At the edges of the saccules are rounded vacuoles and vesicles.
The Golgi bodies are especially well developed in cells that secrete (export) a product-for example, in the pancreatic cells that make digestive enzymes or the bronchial cells that produce mucus. When the Colgi appa­ratus packages a product for export, the product is enclosed within a vesicle that moves toward the plasma membrane, where the vesicle discharges its contents.
Lysosomes
A lysosome (Ii'so-s6m) is a special type of vesicle formed by the Golgi apparatus. All lysosomes carry out intracellu­lar digestion and contain digestive enzymes. Following for­mation, the lysosome may fuse with an incoming vesicle that contains a substance to be digested. The products of digestion enter the cytoplasm, and only nondigested residue is retained.
Tay Sachs (ta silks) is a genetic disease in which a new­born has a faulty lysosomal digestive enzyme. The cells fill with nonfunctioning lysosomes, and the death of the child follows.
Lysosomes cany out autodigestion when they dispose of worn-out or damaged cell components, such as mitochon­dria. This is an essential part of the normal process of cyto­plasmic maintenance and turnover. Turnover refers to the cell's constant process of breaking down and remaking its parts.
Mitochondria
A mitochondIion (mi"to-kon'dre-on), a rather complex organelle with an outer membrane and convoluted inner membrane, produces ATP molecules. As discussed previ­ously, every cell needs a supply of ATP molecules to carry out general cell functions. Some cells have specialized functions. For example, muscle cells use ATP for muscle contraction, and nerve cells need ATP to conduct nerve impulses.
Mitochondria are often referred to as the powerhouses of the cell because, just as powerhouses burn fuel to pro­duce electricity, mitochondria burn glucose products to produce ATP molecules. The word burn is used advisedly because mitochondria use up oxygen and give off carbon dioxide and water. Several systems of the body function to make this possible. The digestive system digests food, and as a result, glucose enters the blood vessels and is taken to the cell. Within the celL glucose is first broken down to a molecule called pyruvate, and then pyruvate en­ters mitochondria. In the meantime, oxygen that has en­tered the lungs of the respiratory system is also transported to the cell, where it enters mitochondria. Following ATP formation, water and carbon dioxide exit the mitochondria and the cell. The lungs expel the carbon dioxide.
Since gas exchange is involved, mitochondria are said to carry on aerobic cellular respiration. One way to indi­cate the chemical transformation associated with aerobic cellular respiration is:
carbohydrate + oxygen ---->  Carbon dioxide + water + ATP energy
Centrioles and Related Organelles Centrioles
Centrioles (sen'tre-olz) are short cylinders that contain ine tubules called microtubules. Usually, two centrioles lie at right angles to one another near the nucleus. Before a cell divides, the centrioles duplicate, and the members of each pair are also at right angles to one another.
Centrioles are believed to give rise to basal bodies that direct the formation of cilia and flagella. Centrioles may be involved in the movement of material throughout the cells and in the organization of the spindle during cell division.
Cilia and Flagella
Cilia (sire-ah) and flagella (flah-jel'ah) are plasma mem­brane extensions that contain microtubules. They can ove either in an undulating fashion, like a whip, or stiffly, like an oar. Cells that have these organelles are capable of producing movement. For example, sperm cells, carrying genetic material to the egg, move by means of flagella. The cells that line the upper respiratory tract are cili­zied. The cilia sweep debris trapped within mucus back up the throat, which helps keep the lungs clean.
Centrioles lie near the nucleus and may be involved in the production of the spindle during cell division and in the formation of cilia and flagella.

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Cell Divisions - Nuclear division and reduction division

Cell Division 
The two types of cell division are termed mitosis and meio­sis. Mitosis occurs during growth and repair, and meiosis occurs during gametogenesis, the production of gametes­that is, the sperm and eggs.
Mitosis 
During ordinary cell division, called mitosis, a mother cell divides, producing two daughter cells. In humans, the mother cell has 46 chromosomes, and the two daughter cells that result also have 46 chromosomes.
A mother cell, has only two pairs of chromosomes. These chromo­somes are at first single, containing a single DNA helix. Be­fore cell division takes place, DNA replication occurs as the
chromosomes duplicate. A duplicated chromosome contains two identical parts (called chromatids). During mitosis, these portions separate, becoming daughter chromosomes. A daughter chromosome contains one DNA helix again.
Mitosis actually requires several stages, dur­ing which the nuclear envelope disappears and a spindle apparatus with spindle fibers forms. The chromosomes are attached to the spindle fibers by structures called cen­tromeres. Once the chromosomes have moved to the cen­ter of the mother cell the centro meres split, and the daughter chromosomes move toward the poles. Daughter nuclei form, and the cytoplasm divides by furrowing.
Mitosis is the type of cell division required for growth and tissue repair. The process of mitosis assures that each cell in the body has the same number and kinds of chro­mosomes and, th ere rore, the same genes (since the genes are on the chromosomes).
Ordinarily, a cell divides only about 50 times; with maturity, a cell stops dividing. However, cancer cells never mature, and instead, continue to divide indefinitely.
Mitosis ensures that each cell in the bdy in generically identical. At the time of division, a chromosome has two portions. When these separate, each daughter celll receives the same number of the mother cell.
Meiosis 
Meiosis is a special type of cell division that occurs only during the production of eggs and sperm. During meio­sis, which takes two rounds of cell division, the chromo­some number is reduced from 46 chromosomes to 23 chromosomes.
In females, meiosis occurs during oogenesis (o"o-jen'e-sis) (egg production) in the ovaries. In males, meiosis occurs during spermatogenesis (sper"mah-to­jen'e-sis) (sperm production) in the testes. Following sexual intercourse, a sperm fertilizes an egg, and a new indi­vidual begins development. Because a sperm carries 23 chromosomes and an egg carries 23 chromosomes, the new individual has 46 chromosomes, which is the normal number for human beings. In this way, both parents con­tribute 23 chromosomes to the new individual.
Meiosis is a special type of cell division that reduces the chromosome number. Meiosis occurs during oogenesis and spermatogenesis.

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5 Types of Membranes in Human body

Human Body Membranes 
Membranes line the internal spaces of organs and tubes that open to the outside, and they also line the body cavi­ties.
Mucous Membranes 
Mucous membranes line the interior walls of the organs and tubes that open to the outside of the body, such as those of the digestive, respiratory, urinary, and reproductive systems. These membranes consist of an epithelium overlying a layer of connective tissue. The epithelium contains goblet cells that secrete mucus.
The mucus secreted by mucous membranes ordinarily protects interior walls from invasion by bacteria and viruses; hence, more mucus is secreted when a person has a cold and has to blow his or her nose. In addition, mucus usually protects the walls of the stomach and small intes­tine from digestive juices, but this protection breaks down when a person develops an ulcer.
Serous Membranes 
Serous membranes line cavities, including the thoracic and abdominopelvic cavities, and cover internal organs like the heart. The term parietal (pah-ri'e-tal) refers to the wall of the body cavity, while the term visceral (vis'er-al) pertains to the internal organs. Therefore, parietal membranes line the interior of the thoracic and abdominopelvic cavities, and visceral membranes cover the organs.
Serous membranes consist of a layer of simple squa­mous epithelium overlying a layer of connective tissue. They secrete a watery fluid that keeps the membranes lu­bricated. Serous membranes support the internal organs and tend to compartmentalize the large thoracic and ab­dominopelvic cavities. This helps to hinder the spread of any infection.
In the thorax, the pleural membranes are serous membranes that line the thoracic cavity and then double back to cover the lungs. The parietal pleura lines the thoracic walL while the visceral pleura adheres to the surface of the lungs. A well-known infection of these mem­branes is called pleurisy. A serous membrane is part of the pericardium, a covering for the heart.
In the abdomen, the interior wall and organs are lined by a serous membrane called the peritoneum (per"i-to­ne'um). The parietal peritoneum lines the abdominopelvic cavity, and the visceral peritoneum covers the organs. The peritoneum comes together to form a double-layered mesentery (mes'en-ter"e) that supports the visceral organs. The greater omentum is a double-layered peritoneum that covers the intestines, and the lesser omentum is a double­layered peritoneum that runs between the stomach and the liver.
Peritonitis is an infection of the peritoneum. Peritonitis is likely if an inflamed appendix bursts before it removed.
Synovial Membranes 
Synovial (sl-no've-al) membranes line freely movable joint cavities and are composed of connective tissues, They secrete synovial fluid into the joint cavity; this fluid lubri­cates the ends of the bones so that they can move freely. In rheumatoid arthritis, the synovial membrane becomes in­flamed and grows thicker. Fibrous tissue then invades the joint and may eventually become bony so that the bones of the joint are no longer capable of moving.
Meninges 
The meninges (me'-nin jez) are membratnes found within th dorsal caivty? They are composed only of con­nective tissue and serve as a protective covering for the brain and spinal cord. Meningitis is a life-threatening infec­tion of the meni nges.
Cutaneous Membrane 
The cutaneous (ku-ta'ne-us) membrane, or skin, forms the outer covering of the body. It consists of a thin outer layer of stratified squamous epithelium attached to a thicker un­derlying layer of connective tissue.

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Human body and Body Tissues

Types of body tissues
Body Tissues 
A tissue is composed of similarly specialized cells that per­form a common function in the body. The tissues of the human body can be categorized into four major types: epithelial tissue, which covers body surfaces and lines body cavities; connective tissue, which binds and supports body parts; muscular tissue, which is specialized for contraction; and nervous tissue, which responds to stimuli and transmits impulses from one body part to another.
Epithelial Tissue 
Epithelial (ep "y -the'le-al) tissue, also called epithelium, forms a continuous layer, or sheet, over the entire body surface and most of the body's inner cavities. On the exter­nal surface, it protects the body from drying out, injury, and bacterial invasion. On internal surfaces, epithelial tissue may be specialized for other functions, in addition to protection. For example, in the respiratory tract, epithe­lial tissue sweeps up impurities by means of cilia, while along the digestive tract, it secretes mucus, which protects the organs of the digestive tract from the digestive en­zymes. Epithelial tissue also efficiently absorbs molecules from kidney tubules because of fine, cellular extensions called microvilli.
The three main types of epithelial tissue are squamous, cuboidal, and columnar. Squamous epithelium is composed of flat cells and lines the lungs and blood ves­sels. Cuboidal epithelium has cube-shaped cells and lines the kidney tubules. Columnar epithelium has pillar- or column-shaped cells, with nuclei usually located near the bottom, and is found lining the digestive tract.
An epithelium may have microvilli (tiny extensions from the cells) or cilia, depending on its particular func­tion. For example, the oviducts are lined by ciliated colum­nar cells that propel the egg toward the uterus, or womb.
An epithelium may also be simple or stratified. Simple means that the cells occur in a single layer. Stratified means that the cells exist as layers piled one over the other. The nose, mouth, esophagus, anal canal, and vagina are all lined by stratified squamous epithelium. The outer layer of skin is also stratified squamous epithelium, but the cells are reinforced by keratin, a protein that provides strength. Pseudostratified epithelium appears to be layered; how­ever, true layers do not exist because each cell touches the baseline. Pseudostratified ciliated columnar epithelium lines the air passages of the respiratory system, includ­ing the nasal cavities and the trachea and its branches. Mucus-secreting goblet cells are scattered among the cili­ated epithelial cells. A surface covering of mucus traps foreign particles, and upward ciliary motion carries the mucus to the back of the throat, where it may be either swallowed or expectorated.
An epithelium like pseudostratified columnar epithe­lium that seaetes a product is described as glandular. A gland can be composed of a single epithelial cell as in the case of the mucus-secreting goblet cells found within the columnar epithelium lining the digestive tract, or it can have many cells. Glands that seaete their products into ducts are called exocrine glands (for exam­ple, salivary glands and sweat glands), and those that se­aete directly into the bloodstream are called endocrine glands (for example, the pituitary gland and the thyroid gland).
Epithelial tissue is classified according to cell shape, vvhich may be squamous, cuboidal, or columnar. The cells may be stratified and/or ciliated, and the tissue may be glandular.


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Types of Body Tissues - Connective Tissue, Fibrous Connective Tissue and Loose Connective Tissue

Connective Tissue,  Fibrous Connective Tissue and  Loose Connective Tissue
Connective Tissue
Connective tissue binds structures together, provides sup­port and protection, fills spaces, produces blood cells, and stores fat. The body uses this stored fat for energy, insula­tion, and organ protection. As a rule, connective tissue cells are widely separated by a noncellular matrix that varies in consistency from solid to semifluid to fluid. Whereas the functional and physical properties of epithelial tissues are derived from the characteristics of cells, connective tissue properties are largely derived from the characteristics of the matrix.
The matrix may have fibers of three types. White fibers contain collagen, a substance that gives the fibers flexibility and strength. Yellow fibers contain elastin, which while not as strong as collagen, is more elastic. Reticular fibers are very thin, highly branched, collagenous fibers that form delicate supporting networks.
Loose Connective Tissue 
Loose (aerolar) connective tissue binds structures together. The cells of this tissue are mainly fibroblasts­large, star-shaped cells that produce extracellular fibers. In loose connective tissue, the fibroblasts are located some distance from one another and are separated by a jellylike matrix that contains many white and yellow fibers. The white fibers occur in bundles and are strong and flexible. The yellow fibers form highly elastic networks that return to their original length after stretching. Loose connective tissue commonly lies beneath an epithelium. In certain in­stances, the epithelium and its underlying connective tis­sue form a body membrane.
Adipose tissue  is a type of loose connective tis­sue in which the fibroblasts enlarge and store fat and there is limited matrix. The fibroblasts of reticular connective tissue are called reticular cells, and the matrix contains only retic­ular fibers. This tissue, also called lymphoid tissue, is found in lymph nodes, the spleen, thymus, and red bone marrow. These organs are a part of the immune system because they store and/or produce white blood cells, particularly lym­phocytes. All types of blood cells are produced in red bone marrow.
Fibrous Connective Tissue 
Fibrous connective tissue has a matrix produced by fi­broblasts that contain closely packed bundles of white col­lagenous fibers. This type of tissue has more specific functions than does loose connective tissue. For ex­ample, fibrous connective tissue is found in tendons, which connect muscles to bones, and ligaments, which connect bones to other bones at joints. Tendons and liga­ments take a long time to heal following an injury because their blood supply is relatively poor.
Loose and fibrous connective tissues. vvhich bind body parts together. differ according to the type and abundance of fibers in the matrix.

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Types of Body Tissues - Cartilage, Bone and Blood tissues

Cartilage, Bone and Blood tissues
Cartilage Tissues
In cartilage (kar'ti-lii), the cells (chondrocytes), which lie in small chambers called lacunae (lah-ku'ne), are sepa­rated by a matrix that is solid yet flexible. Unfortunately, because this tissue lacks a direct blood supply, it heals very slowly. The three types of cartilage are classified according to the type of fiber in the matrix.
Hyaline cartilage is the most common type of cartilage. The matrix, which contains only very fine col­lagenous fibers, has a glassy, white, opaque appearance. This type of cartilage is found in the nose, at the ends of the long bones and ribs, and in the supporting rings of the windpipe. The fetal skeleton is also made of this type of cartilage, although the cartilage is later replaced by bone.
Elastic cartilage has a matrix containing many elastic fibers, in addition to collagenous fibers. For this reason, elastic cartilage is more flexible than hyaline cartilage. Elas­tic cartilage is found, for example, in the framework of the outer ear.
Fibrocartilage has a matrix containing strong collage­nous fibers. This type of cartilage absorbs shock and re­duces friction between joints. Fibrocartilage is found in structures that withstand tension and pressure, such as the pads between the vertebrae in the backbone and the wedges found in the knee joint.
Bone Tissues
Bone is the most rigid of the connective tissues. It has an extremely hard matrix of mineral salts, primarily calcium salts, deposited around protein fibers. The miner­als give bone rigidity, and the protein fibers provide elastic­ity and strength, much as steel rods do in reinforced concrete. The outer portion of a long bone contains compact bone. In compact bone, bone cells (called osteocytes) are located in lacunae that are arranged in a concentric cylin­der called a Haversian system (osteon). Haversian systems form around tiny tubes called central canals, which con­tain nerve fibers and blood vessels. The blood vessels bring the nutrients that allow bone to renew itself. The nutrients can reach all of the cells because minute canals (canaliculi) containing thin processes of the osteocytes connect the os­teocytes with one another and with the central canals.
The ends of a long bone contain spongy bone, which has an entirely different structure. Spongy bone contains numerous bony bars and plates separated by irregular spaces. Although lighter than compact bone, spongy bone is still designed for strength. Like braces used for support in buildings, the solid portions of spongy bone follow lines of stress. Blood cells are formed within red marrow found in spongy bone at the ends of certain long bones.
Cartilage and bone are support tissues. Cartilage is more flexible than bone because the matrix is rich in protein, rather than the mineral salts found in bone.
Blood Tissues
Blood is a cormective tissue in which the cells are separated by a liquid matrix called plasma. Collec­tively, the blood cells are called formed elements. Blood cells are of two types: red blood cells (erythrocytes), which carry oxygen, and white blood cells (leukocytes), which aid in fighting infection. Also present are platelets, which are important to the initiation of blood clotting. Platelets are not complete cells; rather, they are fragments of giant cells found in the bone marrow.
In red bone marrow, cells called stem cells continually divide to produce new cells that mature into the different types of blood cells. The rate of cell division is high.
Blood is unlike other types of connective tissue in that the intercellular matrix (that is, plasma) is not made by the cells of the tissue. Plasma is a mixture of different types of molecules that enter blood at various organs.
Blood is a connective tissue in which the matrix is plasma.

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Structure and Functions Body Tissues - Muscular and Nervous Tissues

Muscular Tissue and Nervous Tissue
Muscular Tissue 
Muscular tissue is composed of muscle fibers that contain actin and myosin filaments, whose interaction accounts for the muscle contraction. Muscle contraction, in turn, ac­counts for movement. Three types of muscular tissue are found in the body: skeletal, smooth, and cardiac muscle.
Skeletal muscle is attached to the bones of the skeleton and functions to move body parts. Skeletal muscle fibers are cylindrical and run the length of a muscle. They are multinucleated, with the nuclei appearing just in­side the plasma membrane. The fibers also have character­istic light and dark bands perpendicular to the length of the cell. These bands give the muscle a striated appearance. Skeletal muscle is under conscious or voluntary control and contracts faster than any other muscle type.
Smooth muscle is so named because it lacks striations. The spindle-shaped cells that make up smooth muscle are not under voluntary control and are said to be involuntary. Smooth muscle, which is found in the viscera (intestine, stomach, and so on) and in blood vessels, con­tracts more slowly than skeletal muscle, but can remain contracted for a longer time. The cells tend to form layers in which the thick middle portion of one cell is opposite the thin ends of adjacent cells. Consequently, the nuclei form an irregular pattern in the tissue.
Cardiac muscle, which is found only in the heart, is responsible for the heartbeat. Cardiac muscle seems to combine features of both smooth and skeletal muscle. Cardiac muscle has striations like those of skeletal muscle, but the contraction of the heart is involuntary for the most part. Cardiac muscle fibers also differ from skele­tal muscle fibers in that they are branched and seemingly fused, one with the other, so that the heart appears to be composed of one large, interconnecting mass of muscle cells. Actually, however, cardiac muscle fibers are separate and individual but they are bound, end-to-end, at interca­lated disks, areas of folded plasma membrane between the cells.
Muscular tissue contains actin and myosin filaments. These form a striated pattern in skeletal and cardiac muscle. but not in emooth muscle. Cardiac and smooth muscle are under Involuntary control. Skeletal muscle is under voluntary control.
Nervous Tissue 
Nervous tissue, found in the brain and spinal cord, con­tains specialized cells called neurons that conduct nerve impulses. A neuron (nu'ron) has three parts: (1) a dendrite conducts signals to the cell body; (2) the cell body contains the nucleus and most of the cytoplasm of the neu­ron; and (3) the axon generally conducts nerve impulses away from the cell body.
Long axons are called fibers. Outside the brain and spinal cord, fibers are bound together by connective tissue to form nerves. Nerves conduct impulses from sense or­gans to the spinal cord and brain, where the phenomenon called sensation occurs. They also conduct nerve impulses away from the spinal cord and brain to the muscles, caus­ing the muscles to contract.
In addition to neurons, nervous tissue contains neu­roglial (nu-rog'le-al) cells. These cells maintain the tissue by supporting and protecting the neurons. Schwann cells are neuroglial cells that encircle all long nerve fibers that are out­side of the brain or spinal cord. Each Schwann cell encircles only a small section (1 mm) of a nerve fiber. The gaps be­tween Schwann cells are called the nodes of Ranvier. A nerve impulse moves from node to node. Collectively, the Schwann cells give nerve fibers a protective layer of fatty in­sulation called a myelin sheath. Because the myelin sheath is white, all nerve fibers appear to be white.
Nervous tissue contains conducting cells called neurons. Neurons have processes called axons and dendrites. Outside the brain and spinal cord. these long axons (fibers) are found in nerves.

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Adrenal Cortex and its disorders

Adrenal Cortex and its Disorders
Adrenal Cortex 
The adrenal cortex produces two major types of hor­mones: (1) the glucocorticoids, which help regulate the level of glucose in the blood; and (2) mineralocorticoids, which help regulate the level of minerals in the blood. It also secretes a small amount of male sex hormones and a small amount of female sex hormones in both sexes; that is, the adrenal cortex produces both male and female sex hormones in males, and both male and female sex hor­mones in females.
Glucocorticoids 
Cortisol is responsible for the greatest amount of gluco­corticoid activity. Cortisol promotes the hydrolysis of mus­cle protein to amino acids, which enter the blood. This leads to a higher blood glucose level when the liver con­verts these amino acids to glucose. Cortisol also favors me­tabolism of fatty acids rather than carbohydrates. In opposition to insulin (a pancreatic hormone to be dis­cussed shortly), cortisol raises the blood glucose level. Cor­tisol also counteracts the inflammatory response that leads to joint pain and swelling in arthritis and bursitis. The ad­ministration of cortisol aids these conditions because it re­duces inflammation (see the Medical Focus reading on this page).
Cortisol. a glucocorticoid secreted by the adrenal cortex, raises the blood glucose level.
Minera locorticoids 
Aldosterone (al" dos'ter-on) is the most important of the mineralocorticoids. These hormones maintain the elec­trolyte (ion) concentration in blood and, therefore, other body fluids. Aldosterone's primary target organ is the kid­ney, where it promotes renal absorption of sodium (Na+) and renal excretion of potassium (K+). The levels of sodium and potassium in the blood are critical for nerve conduction and muscle contraction; in fact, cardiac failure may result from too iowa level of potassium.
The secretion of mineralocorticoids is not under the control of the anterior pituitary. When the blood volume and blood sodium level is low, the kidneys secrete the en­zyme renin. Renin converts the plasma protein an­giotensinogen to angiotensin 1, which is changed to angiotensin II by a converting enzyme found in the lungs. Angiotensin II stimulates the adrenal cortex to release al­dosterone. The effect of this system, called the renin­angiotensin-aldosterone system, is to raise the blood vol­ume and pressure in two ways. First, angiotensin II constricts the arterioles directly, and second, aldosterone causes the kidneys to reabsorb sodium. When the blood sodium level rises, water is reabsorbed, and blood volume and pressure are maintained.
Two other hormones play a role in the homeostatic maintenance of blood volume.Anti­diuretic hormone (ADH) helps increase blood volume by causing the kidney to reabsorb water. Also, when the atria of the heart are stretched due to increased blood volume, cardiac cells release a hormone called atrial natriuretic (a'tre-al na"tre-u-ret'ik) hormone (ANH), which inhibits renin secretion by the kidneys and aldosterone secretion from the adrenal cortex. The effect of ANH, therefore, is to cause sodium excretion-that is, natriuresis. When sodium is excreted, so is water, and therefore, blood volume and blood pressure decrease.
Aldosterone. a mineralocorticoid secreted by the adrenal cortex. and ADH raise blood volume by causing the kidneys to reabsorb Na+ and vvater. Their action is
opposed by ANH. and in this vvay. normal blood volume is • maintained.
Disorders of the Adrenal Cortex 
Addison Disease A person with a low level of adrenal cor­tex hormones due to hyposecretion develops Addison disease. Typically, symptoms include a peculiar bronzing of the skin. Because the lack of cortisol results in a poten­tially severe drop in blood glucose level, the individual is highly susceptible to any kind of stress due to an insuffi­cient energy supply. Even a mild infection can cause death. Due to the lack of aldosterone, the blood sodium level is low, and the person experiences low blood pres­sure and possibly severe dehydration. Left untreated, Ad­dison disease can be fatal.
Cushing Syndrome A person with a high level of adrenal cortex hormones due to hypersecretion develops Cushing syndrome. Excess cortisol causes a tendency toward dia­betes mellitus, a decrease in muscular protein, and an in­crease in subcutaneous fat. Because of these effects, the person usually has an obese trunk, while the arms and legs remain normal. Due to the high level of sodium in the blood, the blood is basic (pH greater than normal), hypertension occurs, and there is edema of the face, which gives it a moonlike shape. Masculinization may oc­cur in women due to oversecretion of the adrenal male sex hormone.
Addison disease is due to adrenal cortex hyposecretion. Cushing syndrome is due to adrenal cortex hypersecretion.
Hormone secretion and Aging
Although hormone-secreting glands shrink with age, their performance is often unaffected. Thyroid disorders and diabetes are the most important endocrine problems sig­nificantly affecting health and function. Both hypothyroidism and hyperthyroidism are seen in the elderly. Graves' disease, which results from hyperthyroidism, causes symptoms of cardiovascular disease, increased body temperature, and apathy. In addition, there may be a weight loss of as much as 20 pounds, depression, and mental confusion. Hypothyroidism (myxedema) may fail to be diagnosed because the symptoms of hair loss, skin changes, and mental deterioration may be attributed simply to the process of aging.
The true incidence of type II diabetes among the el­derly is unknown. Its symptoms can be confused with those of other medical conditions that are present. Type II diabetes is associated with being overweight and often can be controlled by a proper diet.

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What are the Effects of Aging on Hormone Secretion?

Hormone secretion and Aging
Although hormone-secreting glands shrink with age, their performance is often unaffected. Thyroid disorders and diabetes are the most important endocrine problems sig­nificantly affecting health and function. Both hypothyroidism and hyperthyroidism are seen in the elderly. Graves' disease, which results from hyperthyroidism, causes symptoms of cardiovascular disease, increased body temperature, and apathy. In addition, there may be a weight loss of as much as 20 pounds, depression, and mental confusion. Hypothyroidism (myxedema) may fail to be diagnosed because the symptoms of hair loss, skin changes, and mental deterioration may be attributed simply to the process of aging.
The true incidence of type II diabetes among the el­derly is unknown. Its symptoms can be confused with those of other medical conditions that are present. Type II diabetes is associated with being overweight and often can be controlled by a proper diet.
Endocrine Glands and Hormones 
Other glands in the body also produce hormones, and we will discuss two of these. The pineal (pin' e-al) gland is a cone-shaped gland located in the roof of the brain's third ventricle. It is smaller than the pituitary gland and decreases in size as a person ages. In the adult, it becomes a thickened strand of fibrous tissue.
The pineal gland secretes the hormone melatonin, par­ticularly at night, which is believed to regulate daily rhythms, such as a person's sleep pattern. An injection of melatonin can induce sleep. It also inhibits the secretion of the go­nadotropic hormones FSH and LH; therefore, excessive
amounts of melatonin inhibit the ovarian and uterine cycles.
The thymus is a lobular gland in the upper thoracic cavity. This organ reaches its largest size and is most active during childhood. With aging, the organ gets smaller and becomes fatty. Certain white blood cells, called T (for thymus) lymphocytes, originate in the bone marrow but must pass through the thymus to reach maturity. The thymus produces various hormones called thymosins, which aid the differentiation of T lymphocytes and may stimulate immune cells in general. There is hope that thy­mosins will prove helpful in patients suffering from AIDS (acquired immune deficiency syndrome).
The pineal gland secretes melatonin. which is believed to regulate daily rhythms. The thymus gland secretes thymosins necessary to immunity.
Hormones Not Associated with Glands 
Even organs that are not usually considered to be endocrine glands have been found to secrete hormones. As discussed earlier, the heart produces atrial natriuretic hormone (ANH), which helps regulate blood volume and pressure by promoting renal excretion of sodium and water. ANH is a peptide that is released not only by the atria but also by the aortic arch, the ventricles, the lungs, and the pituitary gland in response to increases in blood pressure. The stomach and the small intestine produce pep­tide hormones that help regulate digestive secretions.
A number of different types of organs and cells secrete peptide growth factors, which cause an increase in certain cells. Peptide growth factors are like hormones in that they act on cells that have specific receptors to receive them. Some, including lymphokines and blood cell growth fac­tors, are released into blood; others diffuse to nearby cells. Other growth factors include:
Platelet-derived growth factor, which is released from platelets and many other cell types. It helps in wound healing and causes an increase in the number of fibro­blasts, smooth muscle cells, and certain cells of the nervous system.
Epidermal growth factor and nerve growth factor, which stimulate the cells indicated by their names as well as many others.
Tumor angiogenesis factor, which stimulates the forma­tion of capillary networks and is released by tumor cells. One treatment for cancer is to prevent the activity of this growth factor.
Prostaglandins (PG) are produced by cells and act on tissues or cells in the immediate vicinity. They are active in very small quantities and have diverse actions that affect such processes as nervous system function, blood flow in the kidneys, pregnancy, and the inflammation of arthritis.
Sometimes, prostaglandins have contrary effects. For exam­ple, one type helps prevent the formation of blood clots, while another helps bring about the formation of blood clots. Also, a large dose of PG may have an effect opposite that of a small dose. Therefore, standardizing PG therapy is difficult, and in most instances, the therapy is still consid­ered experimental.
Prostaglandins stimulate the inflammatory response.Drugs such as ibuprofen, aspirin, and acetaminophen, which block the synthesis of prostaglandins, are therefore anti-inflammatory drugs useful in relieving bursitis, arthri­tis, tennis elbow, and similar conditions.
Various growth factors stimulate cell production. Prostaglandins are only active locally and have many varied effects.

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Testes - Ovaries and Male-Female sex Hormones

Testes and Ovaries 
The sex organs are the testes, located in the male scrotum, and the ovaries, located in the female pelvic cavity. The testes produce the androgens (for example, testosterone), which are the male sex hormones, and the ovaries produce estrogen and progesterone, the female sex hormones. The hypothalamus and pituitary gland control the hormonal secretions of these organs in the same manner as described for the thy­roid gland.
Testosterone 
The male sex hormone, testosterone, is essential for the normal development and functioning of the male sex or­gans. It is also necessary for the maturation of sperm.
Greatly increased testosterone secretion at the time of puberty stimulates the growth of the penis and the testes. Testosterone also brings about and maintains the male sec­ondary sex characteristics that develop at the time of pu­berty, such as a beard, axillary (underarm) hair, and pubic hair. It prompts the larynx and vocal cords to enlarge, caus­ing the voice to change. Testosterone is responsible for the greater muscular strength of males, which is why some ath­letes take supplemental amounts of anabolic steroids, which are either testosterone or similar chemicals. But testos­terone also promotes closure of the epiphyses of long bones and therefore stops growth. Testosterone also causes sebaceous and sweat glands in the skin to secrete; therefore, it is largely respon­sible for acne and body odor. Another effect of testosterone activity is pattern baldness. Genes for baldness probably are inherited by both sexes, but baldness is seen more often in males because of the presence of testosterone.
Testosterone is believed to be largely responsible for the sex drive. It may even contribute to the supposed ag­gressiveness of males.
Estrogen and Progesterone 
The female sex hormones, estrogen and progesterone, have many effects on the body. In particular, estrogen secreted a the time of puberty stimulates the growth of the uterus and the vagina. Estrogen is necessary for egg maturation and is largely responsible for female secondary sex characteristics, such as female body hair and fat distribution. In general, fe­males have a more rounded appearance than males because of a greater accumulation of fat beneath the skin. Also, the pelvic girdle enlarges in females so that the pelvic cavity has a larger relative size compared to males; this means that fe­males have wider hips. Estrogen also promotes closure of the epiphyses of long bones and therefore stops growth. Both es­trogen and progesterone are also required for breast develop­ment and regulation of the uterine cycle, which includes monthly menstruation (discharge of blood from the uterus).
The androgens, primarily testosterone, are the male sex hormones produced by the testes. Estrogen and progesterone are the female sex hormones produced by the ovaries. The sex hormones maintain the sex organs and the secondary sex characteristics.

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Composition and Functions of Pancreas

Pancreas 
The pancreas is a long, soft organ that lies transversely in the abdomen between the kidneys and near the duodenum of the small intestine. It is composed of two types of tissues: One produces and secretes the digestive juices that are carried by the ducts to the small intestine, and the other, called the pancreatic islets (of Langer­hans), produces and secretes the hormones insulin and glucagon directly into the blood. Insulin is secreted by beta cells, and glucagon is secreted by alpha cells.
Insulin is secreted when the blood glucose level is high, which usually occurs immediately after eating. In­sulin has three different actions: (1) It stimulates all cells, and in particular fat, liver, and muscle cells, to absorb and metabolize glucose; (2) it stimulates the liver and muscles to store glucose as glycogen; and (3) it promotes the buildup of fats and proteins, and inhibits their use as an energy source so that they will be available during leaner times. As a result of its activities, insulin lowers the blood glucose level.
Glucagon is secreted from the pancreas between meals, and its effects are opposite those of insulin. Glucagon stimulates the breakdown of glycogen and raises the blood glucose level.
The pancreas produces and secretes the hormones insulin and glucagon. Insulin lovvers the blood glucose level. while glucagon raises the blood glucose level.

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What do you know about Thyroid and Parathyroid Glands

Thyroid and Parathyroid Glands 
The thyroid gland is located in the neck and is attached to the trachea just inferior to the larynx. The parathyroid glands are embedded in the posterior surface of the thyroid gland.
Thyroid Gland 
The thyroid gland is composed of a large number of folli­cles, each a smalL spherical structure made of thyroid cells and filled with stored thyroxine. Thyroxine, one hormone produced by the thyroid gland, occurs in two forms. Thy­roxine is usually secreted as T4 (tetraiodothyronine), which contains four iodine atoms, but eventually this form is converted to T3 (triiodothyronine), the active form of the hormone. Iodine, which is required for thyroxine production, is actively transported into the thyroid gland, where its concentration may be as much as 25 times that found in the blood. If iodine is lacking in the diet, the thy­roid gland enlarges, producing a simple goiter. The use of iodized salt helps prevent such a condition.
When the level of thyroxine in the blood is low (called hypothyroidism), the anterior pituitary pro­duces TSH, which stimulates the thyroid. If iodine is not present in the thyroid, the level of thyroxine will remain the same, and TSH will continue to stimulate the thyroid. The result is hypertrophy of the gland, called a simple goiter.
Thyroxine increases the metabolic rate. It does not have one target organ; instead, it stimulates most of the cells of the body to metabolize at a faster rate. For example, it causes more glucose to be broken down.
Failure of the thyroid to develop properly results in a condition called cretinism (kre'tI-nizm). Cretins are short and stocky, and have had extreme hypothyroidism since childhood or infancy. Thyroxine therapy can initiate growth, but unless treatment is begun within the first two months of birth, mental retardation results. Hypothy­roidism in adults produces the condition known as myxedema (mik"se-de'mah), which is characterized by lethargy, weight gain, loss of hair, slower pulse rate, low­ered body temperature, and thickness and puffiness of the skin. The administration of adequate doses of thyroxin re­stores normal function and appearance.
In the case of hyperthyroidism, or Graves' (gravz) disease, the thyroid gland is enlarged and overactive, causing a goiter to form and the eyes to protrude because of edema in eye socket tissues and swelling of extrinsic eye muscles. This type of goiter is called exophthalmic (ek"sof-thal'mik) goiter. The patient usually becomes hyperactive, nervous, and irritable, and suffers. from insomnia. Hyperthyroidism can also be caused by a thyroid tumor, which is usually detected as a lump during physical examination. The treatment for hyperthyroidism is surgery in combination with administration of radioac­tive iodine. The prognosis for most patients is excellent.
Calcitonin 
In addition to thyroxine, the thyroid gland also produces the hormone calcitonin (kal"sI-to'nin), which helps reg­ulate the calcium level in the blood and opposes the ac­tion of parathyroid hormone. (The interaction of these two hormones is discussed in the next section.) Calci­tonin lowers blood calcium by increasing the buildup of bone.
The anterior pituitary produces TSH, a hormone that promotes the production of thyroxine by the thyroid. Thyroxine, which increases metabolism, can affect the 4imtire body, as exemplified by cretinism and myxedema. The thyroid also produces calcitonin, which lowers the blood calcium level.
Parathyroid Glands 
Many years ago, the four, small parathyroid glands were sometimes mistakenly removed during thyroid surgery. Parathyroid hormone (PTH), the hormone produced by the parathyroid glands, causes the calcium (Ca2+) level in the blood to increase and the phosphate (HPO4 -2) level to decrease. PTH promotes bone breakdown and calcium re­tention by the kidneys, and activates vitamin D, which, turn, stimulates the absorption of calcium from the intes­tine. It also promotes the kidneys' excretion of phosphate in unne.
Parathyroid hormone inhibits the activity of os­teoblasts and promotes the activity of osteoclasts in bone, thereby raising the blood calcium level. Calcitonin has the opposite effect, and therefore, the homeostatic balance of calcium in the blood is achieved through the action of both hormones.
If insufficient parathyroid hormone is produced, the blood calcium level drops, resulting in tetany (tet'ah-ne). In tetany, the body shakes from continuous muscle con­traction. The effect is actually brought about by increased excitability of the nerves, which fire spontaneously and without rest. Calcium plays an important role in both ner­vous conduction and muscle contraction. It is also neces­sary to blood clotting.
PTH maintains a high blood calcium level by promoting calcium absorption in the intestine, calcium retention by the kidneys, and bone breakdown. These actions are opposed by calcitonin produced by the thyroid gland.
Adrenal Glands 
The adrenal glands, as their name implies (ad means near; renal means kidneys), lie atop the kidneys. Each consists of an outer portion, called the cortex, and an inner portion, called the medulla. These portions, like the anterior and posterior pituitaries, have no connection with one another.
The hypothalamus exerts control over the activity of both portions of the adrenal glands. It can initiate nerve impulses that travel by way of the brain stem, spinal cord, and sympathetic nerve fibers to the adrenal medulla, which then secretes its hormones. The hypothalamus, by means of corticotrophin-releasing hormone, controls the anterior pituitary's secretion of adrenocorticotropic hormone (ACTH), which, in turn, stimulates the adrenal cortex. Stress of all types, including both emotional and physical trauma, prompts the hypothalamus to stimulate the adre­nal glands to release hormones.
Adrenal Medulla 
The adrenal medulla, which is under the control of the sympathetic division of the autonomic nervous system, produces epinephrine (ep"i-nefrin), also called adrena­line, and norepinephrine (nor"ep-i-nefrin), also called noradrenaline. These hormones are responsible for the "fight-or-flight" reaction that occurs in times of emer­gency. Epinephrine and norepinephrine bring about these reactions:

• Blood glucose level rises, and the metabolic rate in­creases. 
• Bronchioles dilate, and breathing rate increases. 
• Blood vessels to the digestive tract constrict; those to the skeletal muscles dilate. 
• Cardiac muscle contraction is more forcefuL and heart rate increases. 

The adrenal medulla releases epinephrine and norepinephrine into the bloodstream. helping the body cope vvith situations that seem to threaten survival.

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What to know about Hypothalamus and Pituitary Glands

Hypothalamus and Pituitary Gland 
The hypothalamus, located beneath the thalamus in the lower walls and floor of the third ventricle of the brain, helps regulate the body's internal environment. For ex­ample, the hypothalamus helps control heart rate, body temperature, and water balance, as well as the activity of the pituitary gland.
The pituitary gland is small-about 1 centimeter in diameter-and lies just inferior to the hypothalamus. It has two portions: (1) the anterior pituitary, or hypophysis, and (2) the posterior pituitary.
Posterior Pituitary 
The posterior pituitary is connected to the hypothalamus by means of a stalklike structure. The hormones released by the posterior pituitary are made by neurosecretory cells in the hypothalamus. The hormones then migrate through axons that terminate in the posterior pituitary.
Antidiuretic (an"ti-di"u-ret'ik) hormone (ADH), also called vasopressin, promotes the reabsorption of water from the kidneys, thereby preventing dehydration. The hypo­thalamus is believed to contain cells that are sensitive to blood solute concentrations. When these cells detect that the blood lacks sufficient water, ADH is produced by spe­cial neurosecretory cells and is transported by their fibers to the posterior pituitary, where it is released. As the blood becomes more dilute, the hormone ceases to be produced and released.
Inability to produce ADH causes diabetes insipidus (watery urine), in which a person produces copious amounts of urine with a resultant loss of electrolytes from the blood. The condition can be corrected by the adminis­tration of ADH.
Oxytocin is another hormone made in the hypothala­mus and released by the posterior pituitary. Oxytocin causes the uterus to contract and can be used to artificially induce labor. It also stimulates the release of milk from the breast when a baby is nursing.
Anterior Pituitary 
The hypothalamus controls the anterior pituitary by pro­ducing hypothalamic-releasing and release-inhibiting hormones, which are transported to the anterior pitu­itary by the blood within a portal system. Each of these hypothalamic hormones causes the anterior pituitary ei­ther to secrete or to stop secreting a specific hormone. The anterior pituitary produces several different hor­mones.
Growth hormone (GH), or somatotropin, produced by the anterior pituitary, affects the physical appearance dramatically since it determines an individual's size and height. If little or no GH is secreted by the anterior pituitary during childhood, a person can become a pituitary dwarf, characterized by perfect proportions but small stature. If too much GH is secreted, a person can become a giant. Giants usually have poor health, primarily because GH has a secondary effect on the blood sugar level pro­moting an illness called diabetes mellitus.
GH is produced in greatest quantities during child­hood and adolescence, when most body growth is occur­ring, but is still produced (though in lower quantities) in adults to aid in continued protein synthesis and normal cell division and replacement. If GH production increases in an adult after full height has been obtained, only the bones of the jaw, eyebrow ridges, nose, fingers, and toes re­spond. When these bones begin to grow, the person ac­quires a slightly grotesque look, with huge fingers and toes. This condition is called acromegaly.
Prolactin (PRL) is produced by the anterior pituitary only after childbirth. It causes the mammary glands in the breasts to develop and produce milk.
The anterior pituitary also secretes the hormones that follow.

• Since these hormones have an effect on other en­docrine glands, the anterior pituitary is sometimes called the master gland.
• Thyroid-stimulating hormone (TSH), which stimu­lates the thyroid to produce thyroxine 
• Adrenocorticotropic (ad-re"no-kor"te-ko-trop'ik) hormone (ACTH), which stimulates the adrenal cor­tex to produce and secrete hormones 
• Gonadotropic (gon"ah-do-trop'ik) hormones, which stimulate the gonads-the testes in males and the ovaries in females-to secrete sex hormones 

A three-tiered relationship exists between the hypo­thalamus, anterior pituitary, and other endocrine glands. The hypothalamus produces releasing hormones that con­trol the anterior pituitary, which produces hormones that control the thyroid, adrenal cortex, and gonads.

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