Functions of the Digestive System

1. Ingestion. Food must be placed into the mouth before it can be acted on; this is an active, voluntary process called ingestion.
2. Propulsion. If foods are to be processed by more than one digestive organ, they must be propelled from one organ to the next; swallowing is one example of food movement that depends largely on the propulsive process called peristalsis (involuntary, alternating waves of contraction and relaxation of the muscles in the organ wall).
3. Food breakdown: mechanical digestion. Mechanical digestion prepares food for further degradation by enzymes by physically fragmenting the foods into smaller pieces, and examples of mechanical digestion are: mixing of food in the mouth by the tongue, churning of food in the stomach, and segmentation in the small intestine.
4. Food breakdown: chemical digestion. The sequence of steps in which the large food molecules are broken down into their building blocks by enzymes is called chemical digestion.
5. Absorption. Transport of digested end products from the lumen of the GI tract to the blood or lymph is absorption, and for absorption to happen, the digested foods must first enter the mucosal cells by active or passive transport processes.
6. Defecation. Defecation is the elimination of indigestible residues from the GI tract via the anus in the form of feces.

Anatomy of the Digestive System

Organs of the Alimentary Canal

Mouth

• Lips. The lips (labia) protect its anterior opening.
• Cheeks. The cheeks form its lateral walls.
• Palate. The hard palate forms its anterior roof, and the soft palate forms its posterior roof.
• Uvula. The uvula is a fleshy finger-like projection of the soft palate, which extends inferiorly from the posterior edge of the soft palate.
• Vestibule. The space between the lips and the cheeks externally and the teeth and gums internally is the vestibule.
• Oral cavity proper. The area contained by the teeth is the oral cavity proper.
• Tongue. The muscular tongue occupies the floor of the mouth and has several bony attachments- two of these are to the hyoid bone and the styloid processes of the skull.
• Lingual frenulum. The lingual frenulum, a fold of mucous membrane, secures the tongue to the floor of the mouth and limits its posterior movements.
• Palatine tonsils. At the posterior end of the oral cavity are paired masses of lymphatictissue, the palatine tonsils.
• Lingual tonsil. The lingual tonsils cover the base of the tongue just beyond.

Pharynx

• Oropharynx. The oropharynx is posterior to the oral cavity.
• Laryngopharynx. The laryngopharynx is continuous with the esophagus below; both of which are common passageways for food, fluids, and air.

Esophagus

• Size and function. About 25 cm (10 inches) long, it is essentially a passageway that conducts food by peristalsis to the stomach.
• Structure. The walls of the alimentary canal organs from the esophagus to the large intestine are made up of the same four basic tissue layers or tunics.
• Mucosa. The mucosa is the innermost layer, a moist membrane that lines the cavity, or lumen, of the organ; it consists primarily of a surface epithelium, plus a small amount of connective tissue (lamina propria) and a scanty smooth muscle layer.
• Submucosa. The submucosa is found just beneath the mucosa; it is a soft connective tissue layer containing blood vessels, nerve endings, lymph nodules, and lymphaticvessels.
• Muscularis externa. The muscularis externa is a muscle layer typically made up of an inner circular layer and an outer longitudinal layer of smooth muscle cells.
• Serosa. The serosa is the outermost layer of the wall that consists of a single layer of flat serous fluid-producing cells, the visceral peritoneum.
• Intrinsic nerve plexuses. The alimentary canal wall contains two important intrinsic nerve plexuses- the submucosal nerve plexus and the myenteric nerve plexus, both of which are networks of nerve fibers that are actually part of the autonomic nervous system and help regulate the mobility and secretory activity of the GI tract organs.

Stomach

• Location. The C-shaped stomach is on the left side of the abdominal cavity, nearly hidden by the liver and the diaphragm.
• Function. The stomach acts as a temporary “storage tank” for food as well as a site  for food breakdown.
• Cardiac region. The cardiac region surrounds the cardioesophageal sphincter, through which food enters the stomach from the esophagus.
• Fundus. The fundus is the expanded part of the stomach lateral to the cardiac region.
• Body. The body is the midportion, and as it narrows inferiorly, it becomes the pyloric antrum, and then the funnel-shaped pylorus.
• Pylorus. The pylorus is the terminal part of the stomach and it is continuous with the small intestine through the pyloric sphincter or valve.
• Size. The stomach varies from 15 to 25 cm in length, but its diameter and volume depend on how much food it contains; when it is full, it can hold about 4 liters (1 gallon) of food, but when it is empty it collapses inward on itself.
• Rugae. The mucosa of the stomach is thrown into large folds called rugae when it is empty.
• Greater curvature. The convex lateral surface of the stomach is the greater curvature.
• Lesser curvature. The concave medial surface is the lesser curvature.
• Lesser omentum. The lesser omentum, a double layer of peritoneum, extends from the liver to the greater curvature.
• Greater omentum.  The greater omentum, another extension of the peritoneum, drapes downward and covers the abdominal organs like a lacy apron before attaching to the posterior body wall, and is riddled with fat, which helps to insulate, cushion, and protect the abdominal organs.
• Stomach mucosa. The mucosa of the stomach is a simple columnar epithelium composed entirely of mucous cells that produce a protective layer of bicarbonate-rich alkaline mucus that clings to the stomach mucosa and protects the stomach wall from being damaged by acid and digested by enzymes.
• Gastric glands. This otherwise smooth lining is dotted with millions of deep gastric pits, which lead into gastric glands that secrete the solution called gastric juice.
• Intrinsic factor. Some stomach cells produce intrinsic factor, a substance needed for the absorption of vitamin b12 from the small intestine.
• Chief cells. The chief cells produce protein-digesting enzymes, mostly pepsinogens.
• Parietal cells. The parietal cells produce corrosive hydrochloric acid, which makes the stomach contents acidic and activates the enzymes.
• Enteroendocrine cells. The enteroendocrine cells produce local hormones such as gastrin, that are important to the digestive activities of the stomach.
• Chyme. After food has been processed, it resembles heavy cream and is called chyme.

Small Intestine

• Location. The small intestine is a muscular tube extending from the pyloric sphincter to the large intestine.
• Size. It is the longest section of the alimentary tube, with an average length of 2.5 to 7 m (8 to 20 feet) in a living person.
• Subdivisions. The small intestine has three subdivisions: the duodenum, the jejunum, and the ileum, which contribute 5 percent, nearly 40 percent, and almost 60 percent of the small intestine, respectively.
• Ileocecal valve. The ileum meets the large intestine at the ileocecal valve, which joins the large and small intestine.
• Hepatopancreatic ampulla. The main pancreatic and bile ducts join at the duodenum to form the flasklike hepatopancreatic ampulla, literally, the ” liver-pacreatic-enlargement”.
• Duodenal papilla. From there, the bile and pancreatic juice travel through the duodenal papilla and enter the duodenum together.
• Microvilli. Microvilli are tiny projections of the plasma membrane of the mucosa cells that give the cell surface a fuzzy appearance, sometimes referred to as the brush border; the plasma membranes bear enzymes (brush border enzymes) that complete the digestion of proteins and carbohydrates in the small intestine.
• Villi. Villi are fingerlike projections of the mucosa that give it a velvety appearance and feel, much like the soft nap of a towel.
• Lacteal. Within each villus is a rich capillary bed and a modified lymphatic capillary called a lacteal.
• Circular folds. Circular folds, also called plicae circulares, are deep folds of both mucosa and submucosa layers, and they do not disappear when food fills the small intestine.
• Peyer’s patches. In contrast, local collections of lymphatic tissue found in the submucosa increase in number toward the end of the small intestine.

Large Intestine

• Size. About 1.5 m (5 feet) long, it extends from the ileocecal valve to the anus.
• Functions. Its major functions are to dry out indigestible food residue by absorbing water and to eliminate these residues from the body as feces.
• Subdivisions. It frames the small intestines on three sides and has the following subdivisions: cecum, appendix, colon, rectum, and anal canal.
• Cecum. The saclike cecum is the first part of the large intestine.
• Appendix. Hanging from the cecum is the wormlike appendix, a potential trouble spot because it is an ideal location for bacteria to accumulate and multiply.
• Ascending colon. The ascending colon travels up the right side of the abdominal cavity and makes a turn, the right colic (or hepatic) flexure, to travel across the abdominal cavity.
• Transverse colon. The ascending colon makes a turn and continuous to be the transverse colon as it travels across the abdominal cavity.
• Descending colon. It then turns again at the left colic (or splenic) flexure, and continues down the left side as the descending colon.
• Sigmoid colon. The intestine then enters the pelvis, where it becomes the S-shaped sigmoid colon.
• Anal canal. The anal canal ends at the anus which opens to the exterior.
• External anal sphincter. The anal canal has an external voluntary sphincter, the external anal sphincter, composed of skeletal muscle.
• Internal involuntary sphincter. The internal involuntary sphincter is formed by smooth muscles.

Accessory Digestive Organs

Teeth

• Function. The teeth tear and grind the food, breaking it down into smaller fragments.
• Deciduous teeth. The first set of teeth is the deciduous teeth, also called baby teethor milk teeth, and they begin to erupt around 6 months, and a baby has a full set (20 teeth) by the age of 2 years.
• Permanent teeth. As the second set of teeth, the deeper permanent teeth, enlarge and develop, the roots of the milk teeth are reabsorbed, and between the ages of 6 to 12 years they loosen and fall out.
• Incisors. The chisel-shaped incisors are adapted for cutting.
• Canines. The fanglike canines are for tearing and piercing.
• Premolars and molars. Premolars (bicuspids) and molars have broad crowns with round cusps ( tips) and are best suited for grinding.
• Crown. The enamel-covered crown is the exposed part of the tooth above the gingivaor gum.
• Enamel. Enamel is the hardest substance in the body and is fairly brittle because it is heavily mineralized with calcium salts.
• Root. The outer surface of the root is covered by a substance called cementum, which attaches the tooth to the periodontal membrane (ligament).
• Dentin. Dentin, a bonelike material, underlies the enamel and forms the bulk of the tooth.
• Pulp cavity. It surrounds a central pulp cavity, which contains a number of structures (connective tissue, blood vessels, and nerve fibers) collectively called the pulp.
• Root canal. Where the pulp cavity extends into the root, it becomes the root canal, which provides a route for blood vessels, nerves, and other pulp structures to enter the pulp cavity of the tooth.

Salivary Glands

• Parotid glands. The large parotid glands lie anterior to the ears and empty their secretions into the mouth.
• Submandibular and sublingual glands. The submandibular and sublingual glands empty their secretions into the floor of the mouth through tiny ducts.
• Saliva. The product of the salivary glands, saliva, is a mixture of mucus and serous fluids.
• Salivary amylase. The clear serous portion contains an enzyme, salivary amylase, in a bicarbonate-rich juice that begins the process of starch digestion in the mouth.

Pancreas

• Location. The pancreas is a soft, pink triangular gland that extends across the abdomen from the spleen to the duodenum; but most of the pancreas lies posterior to the parietal peritoneum, hence its location is referred to as retroperitoneal.
• Pancreatic enzymes. The pancreatic enzymes are secreted into the duodenum in an alkaline fluid that neutralizes the acidic chyme coming in from the stomach.
• Endocrine function. The pancreas also has an endocrine function; it produces hormones insulin and glucagon.

Liver

• Location. Located under the diaphragm, more to the right side of the body, it overlies and almost completely covers the stomach.
• Falciform ligament. The liver has four lobes and is suspended from the diaphragm and abdominal wall by a delicate mesentery cord, the falciform ligament.
• Function. The liver’s digestive function is to produce bile.
• Bile. Bile is a yellow-to-green, watery solution containing bile salts, bile pigments, cholesterol, phospholipids, and a variety of electrolytes.
• Bile salts. Bile does not contain enzymes but its bile salts emulsify fats by physically breaking large fat globules into smaller ones, thus providing more surface area for the fat-digesting enzymes to work on.

Gallbladder

• Location. The gallbladder is a small, thin-walled green sac that snuggles in a shallow fossa in the inferior surface of the liver.
• Cystic duct. When food digestion is not occurring, bile backs up the cystic duct and enters the gallbladder to be stored.

Physiology of the Digestive System

Activities Occurring in the Mouth, Pharynx, and Esophagus

• Physical breakdown. First, the food is physically broken down into smaller particles by chewing.
• Chemical breakdown. Then, as the food is mixed with saliva, salivary amylase begins the chemical digestion of starch, breaking it down into maltose.
• Stimulation of saliva. When food enters the mouth, much larger amounts of saliva pour out; however, the simple pressure of anything put into the mouth and chewed will also stimulate the release of saliva.
• Passageways. The pharynx and the esophagus have no digestive function; they simply provide passageways to carry food to the next processing site, the stomach.

• Deglutition. Deglutition, or swallowing, is a complex process that involves the coordinated activity of several structures (tongue, soft palate, pharynx, and esophagus).
• Buccal phase of deglutition. The first phase, the voluntary buccal phase, occurs in the mouth; once the food has been chewed and well mixed with saliva, the bolus (food mass) is forced into the pharynx by the tongue.
• Pharyngeal-esophageal phase. The second phase, the involuntary pharyngeal-esophageal phase, transports food through the pharynx and esophagus; the parasympathetic division of the autonomic nervous system controls this phase and promotes the mobility of the digestive organs from this point on.
• Food routes. All routes that the food may take, except the desired route distal into the digestive tract, are blocked off; the tongue blocks off the mouth; the soft palate closes off the nasal passages; the larynx rises so that its opening is covered by the flaplike epiglottis.
• Stomach entrance. Once food reaches the distal end of the esophagus, it presses against the cardioesophageal sphincter, causing it to open, and food enters the stomach.

Activities of the Stomach

• Gastric juice. Secretion of gastric juice is regulated by both neural and hormonal factors.
• Gastrin. The presence of food and a rising pH in the stomach stimulate the stomach cells to release the hormone gastrin, which prods the stomach glands to produce still more of the protein-digesting enzymes (pepsinogen), mucus, and hydrochloric acid.
• Pepsinogen. The extremely acidic environment that hydrochloric acid provides is necessary, because it activates pepsinogen to pepsin, the active protein-digesting enzyme.
• Rennin. Rennin, the second protein-digesting enzyme produced by the stomach, works primarily on milk protein and converts it to a substance that looks like sour milk.
• Food entry. As food enters and fills the stomach, its wall begins to stretch (at the same time as the gastric juices are being secreted).
• Stomach wall activation. Then the three muscle layers of the stomach wall become active; they compress and pummel the food, breaking it apart physically, all the while continuously mixing the food with the enzyme-containing gastric juice so that the semifluid chyme is formed.

• Peristalsis. Once the food has been well mixed, a rippling peristalsis begins in the upper half of the stomach, and the contractions increase in force as the food approaches the pyloric valve.
• Pyloric passage. The pylorus of the stomach, which holds about 30 ml of chyme, acts like a meter that allows only liquids and very small particles to pass through the pyloric sphincter; and because the pyloric sphincter barely opens, each contraction of the stomach muscle squirts 3 ml or less of chyme into the small intestine.
• Enterogastric reflex. When the duodenum is filled with chyme and its wall is stretched, a nervous reflex, the enterogastric reflex, occurs; this reflex “puts the brakes on” gastric activity and slows the emptying of the stomach by inhibiting the vagus nerves and tightening the pyloric sphincter, thus allowing time for intestinal processing to catch up.

Activities of the Small Intestine

• Digestion. Food reaching the small intestine is only partially digested; carbohydrate and protein digestion has begun, but virtually no fats have been digested up to this point.
• Brush border enzymes. The microvilli of small intestine cells bears a few important enzymes, the so-called brush border enzymes, that break down double sugars into simple sugars and complete protein digestion.
• Pancreatic juice. Foods entering the small intestine are literally deluged with enzyme-rich pancreatic juice ducted in from the pancreas, as well as bile from the liver; pancreatic juice contains enzymes that, along with brush border enzymes, complete the digestion of starch, carry out about half of the protein digestion, and are totally responsible for fat digestion and digestion of nucleic acids.
• Chyme stimulation. When chyme enters the small intestine, it stimulates the mucosa cells to produce several hormones; two of these are secretin and cholecystokininwhich influence the release of pancreatic juice and bile.
• Absorption. Absorption of water and of the end products of digestion occurs all along the length of the small intestine; most substances are absorbed through the intestinal cell plasma membranes by the process of active transport.
• Diffusion.  Lipids or fats are absorbed passively by the process of diffusion.
• Debris. At the end of the ileum, all that remains are some water, indigestible food materials, and large amounts of bacteria; this debris enters the large intestine through the ileocecal valve.

• Peristalsis. The net effect is that the food is moved through the small intestine in much the same way that toothpaste is squeezed from the tube.
• Constrictions. Rhythmic segmental movements produce local constrictions of the intestine that mix the chyme with the digestive juices, and help to propel food through the intestine.

Activities of the Large Intestine

• Metabolism. The “resident” bacteria that live in its lumen metabolize some of the remaining nutrients, releasing gases (methane and hydrogen sulfide) that contribute to the odor of feces.
• Flatus. About 50 ml of gas (flatus) is produced each day, much more when certain carbohydrate-rich foods are eaten.
• Absorption. Absorption by the large intestine is limited to the absorption of vitamin K, some B vitamins, some ions, and most of the remaining water.
• Feces. Feces, the more or less solid product delivered to the rectum, contains undigested food residues, mucus, millions of bacteria, and just enough water to allow their smooth passage.

• Haustral contractions. The movements most seen in the colon are haustral contractions, slow segmenting movements lasting about one minute that occur every 30 minutes or so.
• Propulsion. As the haustrum fills with food residue, the distension stimulates its muscle to contract, which propels the luminal contents into the next haustrum.
• Mass movements. Mass movements are long, slow-moving, but powerful contractile waves that move over large areas of the colon three or four times daily and force the contents toward the rectum.
• Rectum. The rectum is generally empty, but when feces are forced into it by mass movements and its wall is stretched, the defecation reflex is initiated.
• Defecation reflex. The defecation reflex is a spinal (sacral region) reflex that causes the walls of the sigmoid colon and the rectum to contract and anal sphincters to relax.
• Impulses. As the feces is forced into the anal canal, messages reach the brain giving us time to make a decision as to whether the external voluntary sphincter should remain open or be constricted to stop passage of feces.
• Relaxation. Within a few seconds, the reflex contractions end and rectal walls relax; with the next mass movement, the defecation reflex is initiated again.

Functions of the Integumentary System

1. Protection. The skin protects deeper tissues from mechanical damage (bumps), chemical damage (acids and bases), ultraviolet radiation (damaging effects of sunlight), bacterial damage, thermal damage (heat or cold), and desiccation (drying out).
2. Temperature regulation. The skin aids in body heat loss or heat retention as controlled by the nervous system.
3. Elimination. The skin aids in the secretion of urea and uric acid through perspiration produced by the sweat glands.
4. Synthesizer. Synthesizes vitamin D through modified cholesterol molecules in the skin by sunlight.
5. Sensation. The integumentary system has sensory receptors that can distinguish heat, cold, touch, pressure, and pain.

Anatomy of the Integumentary System

Structure of the Skin

Epidermis

• Composition. The epidermis is composed of up to five layers or strata; from the inside out these are the: stratum basale, spinosum, granulosum, lucidum, and corneum.
• Epithelial tissue. Like all other epithelial tissues, the epidermis is avascular; that is, it has no blood supply of its own.
• Keratinocytes. Most cells of the epidermis are keratinocytes (keratin cells), which produce keratin, the fibrous protein that makes the epidermis a tough protective layer.
• Stratum basale. The deepest layer of the epidermis, the stratum basale, lies closest to the dermis and is connected to it along a wavy a borderline that resembles corrugated cardboard; this basal layer contains epidermal cells that receive the most adequate nourishment via diffusion of nutrients from the dermis.
• Stratum spinosum. As the epidermal layers move away from the dermis and become part of the more superficial layers, the stratum spinosum.
• Stratum granulosum. Upon reaching the stratum granulosum, the layers become flatter and increasingly full of keratin.
• Stratum lucidum. Finally, they die, forming the clear stratum lucidum; this latter epidermal layer is not present in all skin regions, it occurs only where the skin is hairless and extra thick, that is, on the palms of the hands and soles of the feet.
• Stratum corneum. The outermost layer, the stratum corneum, is 20 to 30 cells layers thick but it accounts for about three-quarters of epidermal thickness; it rubs and flakes off slowly and steadily as the dandruff familiar to everyone; then, this layer is replaced by cells produced by the division of the deeper stratum basale cells.
• Cornified cells. The shinglelike dead cell remnants, completely filled with keratin, are referred to as cornified or horny cells.
• Keratin. Keratin is an exceptionally tough protein; its abundance in the stratum corneum allows that layer to provide a durable “overcoat” for the body, which protects deeper cells from the hostile external environment.
• Melanin. Melanin, a pigment that ranges in color from yellow to brown to black, is produced by special spider-shaped cells called melanocytes, found chiefly in the stratum basale.
• Melanosomes. As the melanocytes produce melanin, it accumulates within them in membrane-bound granules called melanosomes; these granules then move to the ends of the spidery arms of the melanocytes, where they are taken up by nearby keratinocytes.

Dermis

• Major regions. The dense (fibrous) connective tissue making up the dermis consists of two major regions- the papillary and reticular regions.
• Papillary layer. The papillary layer is the upper dermal region; it is uneven and has peglike projections from its superior surface called dermal papillae, which indent the epidermis above and contain capillary loops which furnish nutrients to the epidermis; it also has papillary patterns that form looped and whorled ridges on the epidermal surface that increase friction and enhance the gripping ability of the fingers and feet.
• Reticular layer. The reticular layer is the deepest skin layer; it contains blood vessels, sweat and oil glands, and deep pressure receptors called Pacinian corpuscles.
• Collagen. Collagen fibers are responsible for the toughness of the dermis; they also attract and bind water and thus help to keep the skin hydrated.
• Elastic fibers. Elastic fibers give the skin its elasticity when we are young, and as we age, the number of collagen and elastic fibers decreases and the subcutaneous tissue loses fat.
• Blood vessels. The dermis is abundantly supplied with blood vessels that play a role in maintaining body temperature homeostasis; when body temperature is high, the capillaries of the dermis becomes engorged, or swollen, with heated blood, and the skin becomes reddened and warm; if the environment is cool, blood bypasses the dermis capillaries temporarily, allowing internal body temperature to stay high.
• Nerve supply. The dermis also has a rich nerve supply; many of the nerve endings have specialized receptor end-organs that send messages to the central nervous systemfor interpretation when they are stimulated by environmental factors.

Appendages of the Skin

Cutaneous Glands

• Exocrine glands. The cutaneous glands are all exocrine glands that release their secretions to the skin surface via ducts and they fall into two groups: sebaceous glands and sweat glands.
• Sebaceous (oil) glands. The sebaceous, or oil, glands are found all over the skin, except on the palms of the hands and the soles of the feet; their ducts usually empty into a hair follicle; the product of the sebaceous glands, sebum, is a mixture of oily substances and fragmented cells, and it is a lubricant that keeps the skin soft and moist and prevents the hair from becoming brittle.
• Sweat glands. Sweat glands, also called sudoriferous glands, are widely distributed in the skin, and there are two types: eccrine and apocrine.
• Eccrine glands. The eccrine glands are far more numerous and are found all over the body; they produce sweat, a clear secretion that is primarily water plus some salts, vitamin C, trace of metabolic wastes, and lactic acid; the eccrine glands are also a part of the body’s heat regulating equipment.
• Apocrine glands. Apocrine glands are largely confined to the axillary and genital areas of the body; they are usually larger than eccrine glands and their ducts empty into hair follicles; their secretion contain fatty acids and proteins, as well as all substances present in eccrine secretion; they begin to function during puberty under the influence of androgens, and they also play a minimal role in thermoregulation.

Hair and Hair Follicles

• Hairs. A hair, produced by a hair follicle, is a flexible epithelial structure.
• Root. The part of the hair enclosed in the follicle is the root.
• Shaft. The part projecting from the surface of the scalp or skin is called shaft.
• Formation. The hair is formed by division of a well-nourished stratum basale epithelial cells in the matrix (growth zone) of the hair bulb at the inferior end of the follicle.
• Composition. Each hair is made up of a central core called the medulla surrounded by a bulky cortex layer.
• Cuticle. The cortex is enclosed by an outermost cuticle formed by a single layer of cells that overlap one another like shingles on the roof; this arrangement helps to keep the hairs apart and keeps them from matting; the cuticle is the most heavily keratinized region; it provides strength and helps keep the inner hair layers tightly compacted.
• Hair pigment. Hair pigment is made by melanocytes in the hair bulb, and varying amounts of different types of melanin combine to produce all varieties of hair color from pale blond to pitch black.
• Hair follicles. Hair follicles are actually compound structures.
• Epidermal sheath. The inner epidermal sheath is composed of epithelial tissue and forms the hair.
• Dermal sheath. The outer dermal sheath is actually dermal connective tissue; this dermal region supplies blood vessels to the epidermal portion and reinforces it.
• Papilla. Its nipplelike papilla provides the blood supply to the matrix in the hair bulb.
• Arrector pili. Small bands of smooth muscle cells -arrector pili- connect each side of the hair follicle to the dermal tissue; when these muscles contract, the hair is pulled upright, dimpling the skin surface with “goosebumps”.

Nails

• Parts. Each nail has a free edge, a body (visible attached portion), and a root(embedded in the skin).
• Nail folds. The borders of the nail are overlapped by skin folds, called nail folds.
• Cuticle. The thick proximal nail fold is commonly called the cuticle.
• Nail bed. The stratum basale of the epidermis extends beneath the nail as the nail bed.
• Nail matrix. Its thickened proximal area, the nail matrix, is responsible for nail growth.
• Color. Nails are transparent and nearly colorless, but they look pink because of the rich blood supply in the underlying dermis.
• Lunula. The exception to the pinkish color of the nails is the region over the thickened nail matrix that appears as a white crescent and is called the lunula.”

Physiology of the Integumentary System

Development of Skin Color

• Melanin. The amount and kind (yellow, reddish brown, or black) of melanin in the epidermis.
• Carotene. The amount of carotene deposited in the stratum corneum and subcutaneous tissue; carotene is an orange-yellow pigment abundant in carrots and other orange, deep yellow, or leafy green vegetables; the skin tends to take on a yellow-orange cast when the person eats large amounts of carotene-rich foods.
• Hemoglobin. The amount of oxygen-rich hemoglobin in the dermal blood vessels.
• Emotions. Emotions also influence skin color, and many alterations in skin color signal certain disease states.
• Redness or erythema. Reddened skin may indicate embarrassment, fever, hypertension, inflammation, or allergy.
• Pallor or blanching. Under certain types of emotional stress, some people become pale; pale skin may also signify anemia, low blood pressure, or impaired blood flow into the area.
• Jaundice or a yellow cast. An abnormal yellow skin tone usually signifies a liverdisorder in which excess bile pigments are absorbed into the blood, circulated throughout the body, and deposited in body tissues.
• Bruises or black-and-blue marks. Black-and-blue marks reveal sites where blood has escaped from circulation and has clotted in tissue spaces; such clotted blood masses are called hematomas.

Hair Growth Cycle

• Anagen. Anagen is the active phase of hair; the cells in the root of the hair are dividing rapidly; a new hair is formed and pushes the club hair (a hair that has stopped growing or is no longer in the anagen phase) up the follicle and eventually out.
• Catagen. The catagen phase is a transitional stage; growth stops and the outer root sheath shrinks and attaches to the root of the hair.
• Telogen. Telogen is the resting phase; during this phase, the hair follicle is completely at rest and the club hair is completely formed.

Nail Growth

• Germinal matrix. It is found on the ventral floor of the nail fold; the nail is produced by gradient parakeratosis , then cells near the periosteum of the phalanx duplicate and enlarge (macrocytosis); newly formed cells migrate distally and dorsally in a column toward the nail; cells meet resistance at established nail, causing them to flatten and elongate as they are incorporated into the nail; it initially retains nuclei (lunula); more distal cells become nonviable and lose nuclei.
• Sterile matrix. The area of the sterile matrix is distal to the lunula and it has a variable amount of nail growth; it contributes squamous cells, aiding in nail strength and thickness and it has a role in nail plate adherence by linear ridges in the sterile matrix epithelium.
• Dorsal roof of the nail fold. The nail is produced in a similar manner as the germinal matrix, but the cells lose nuclei more rapidly and it imparts shine to the nail plate.

Functions of the Blood

Components of Blood

Physical Characteristics and Volume

• Color. Depending on the amount of oxygen it is carrying, the color of blood varies from scarlet (oxygen-rich) to a dull red (oxygen-poor).
• Weight. Blood is heavier than water and about five times thicker, or more viscous, largely because of its formed elements.
• pH. Blood is slightly alkaline, with a pH between 7.35 and 7.45.
• Temperature. Its temperature (38 degrees Celsius, or 100.4 degrees Fahrenheit) is always slightly higher than body temperature.

Plasma

• Dissolved substances. Examples of dissolved substances include nutrients, salts (electrolytes), respiratory gases, hormones, plasma proteins, and various wastes and products of cell metabolism.
• Plasma proteins. Plasma proteins are the most abundant solutes in plasma; except for antibodies and protein-based hormones, most plasma proteins are made by the liver.
• Composition. The composition of plasma varies continuously as cells remove or add substances to the blood; assuming a healthy diet, however, the composition of plasma is kept relatively constant by various homeostatic mechanisms of the body.

Formed Elements

Erythrocytes

• Anucleate. RBCs differ from other blood cells because they are anucleate, that is, they lack a nucleus; they also contain a very few organelles.
• Hemoglobin. Hemoglobin, an iron bearing protein, transports the bulk of oxygen that is carried in the blood.
• Microscopic appearance. Erythrocytes are small, flexible cells shaped like biconcave discs- flattened discs with depressed centers on both sides; they look like miniature doughnuts when viewed with a microscope.
• Number of RBCs. There are normally about 5 million cells per cubic millimeter of blood; RBCs outnumber WBCs by about 1000 to 1 and are the major factor contributing to blood viscosity.
• Normal blood. Clinically, normal blood contains 12-18 grams of hemoglobin per 100 milliliters (ml); the hemoglobin content is slightly higher in men (13-18 g/dl) than in women (12-16 g/dl).

Leukocytes

• Number of WBCs. On average, there are 4,000 to 11,000 WBC/mm3 , and they account for less than 1 percent of total body volume.
• Body defense. Leukocytes form a protective, movable army that helps defend the body against damage by bacteria, viruses, parasites, and tumor cells.
• Diapedesis. White blood cells are able to slip into and out of the blood vessels- a process called diapedesis.
• Positive chemotaxis. In addition, WBCs can locate areas of tissue damage and infection in the body by responding to certain chemicals that diffuse from the damaged cells; this capability is called positive chemotaxis.
• Ameboid motion. Once they have “caught the scent”, the WBCs move through the tissue spaces by ameboid motion (they form flowing cytoplasmic extensions that help move them along).
• Leukocytosis. A total WBC count above 11, 000 cells/mm3 is referred to as leukocytosis.
• Leukopenia. The opposite condition, leukopenia, is an abnormally low WBC count.
• Granulocytes. Granulocytes are granule-containing WBCs; they have lobed nuclei, which typically consist of several rounded nuclear areas connected by thin strands of nuclear material, and includes neutrophils, eosinophils, and basophils.
• Neutrophils. Neutrophil are the most numerous of the WBCs; they have a multilobed granules and very fine granules that respond to acidic and basic stains; neutrophils are avid phagocytes at sites of acute infection, and are particularly partial to bacteria and fungi.
• Eosinophils. Eosinophils have blue red nucleus that resembles an old-fashioned telephone receiver and sport coarse, lysosome-like, brick-red cytoplasmic granules; their number increases rapidly during allergies and infections by parasitic worms or entering via the skin.
• Basophils. Basophils, the rarest of the WBCs, contain large, histamine-containing granules that stain dark blue; histamine is an inflammatory chemical that makes blood vessels leaky and attracts other WBCs to the inflammatory site.
• Agranulocytes. The second group of WBCs, the agranulocytes, lack visible cytoplasmic granules; their nuclei are closer to the norm- that is, they are spherical; they are spherical, oval, or kidney-shaped; and they include lymphocytes and monocytes.
• Lymphocytes. Lymphocytes have a large, dark purple nucleus that occupies most of the cell volume; they tend to take up residence in lymphatic tissues, where they play an important role in the immune response.
• Monocytes. Monocytes are the largest of the WBCs; when they migrate into the tissues, they transform into macrophages with huge appetites; macrophages are very important in fighting chronic infections.
• Platelets. Platelets are not cells in the strict sense; they are fragments of bizarre multinucleate cells called megakaryocytes, which pinch off thousands of anucleate platelet “pieces” that quickly seal themselves off from surrounding fluids; platelets are needed for the clotting process that occurs in plasma when blood vessels are ruptured or broken.

Hematopoiesis

• Hemocystoblast. All the formed elements arise from a common type of stem cell, the hematocystoblast.
• Descendants of hemocystoblasts. The hemocystoblast forms two types of descendants- the lymphoid stem cell, which produces lymphocytes, and the myeloid stem cell, which can produce all other classes of formed elements.

Formation of Red Blood Cells

• Life span. As they age, RBCs become more rigid and begin to fragment, or fall apart, in 100 to 120 days.
• Lost RBCs. Lost cells are replaced more or less continuously by the division of hemocystoblasts in the red bone marrow.
• Immature RBCs. Developing RBCs divide many times and then begin synthesizing huge amounts of hemoglobin.
• Reticulocyte. Suddenly, when enough hemoglobin has been accumulated, the nucleus and most organelles are ejected and the cell collapses inward; the result is the young RBC, called a reticulocyte because it still contains some rough endoplasmic reticulum (ER).
• Mature erythrocytes. Within 2 days of release, they have rejected the remaining ER and have become fully functioning erythrocytes; the entire developmental process from hemocystoblast to mature RBC takes 3 to 5 days.
• Erythropoietin. The rate of erythrocyte production is controlled by a hormone called erythropoetin; normally a small amount of erythropoeitin circulates in the blood at all times, and red blood cells are formed at a fairly constant rate.
• Control of RBC production. An important point to remember is that it is not the relative number of RBCS in the blood that controls RBC production; control is based on their ability to transport enough oxygen to meet the body’s demands.

Formation of White Blood Cells and Platelets

• Colony stimulating factors and interleukins. These colony stimulating factors and interleukins not only prompt red bone marrow to turn out leukocytes, but also marshal up an army of WBCs to ward off attacks by enhancing the ability of mature leukocytes to protect the body.
• Thrombopoeitin.  The hormone thrombopoeitin accelerates the production of platelets, but little is known about how that process is regulated.

Hemostasis

• Vascular spasms occur. The immediate response to blood vessel injury is vasoconstriction, which causes that blood vessel to go into spasms; the spasms narrow the blood vessel, decreasing blood loss until clotting can occur.
• Platelet plug forms. Injury to the lining of vessels exposes collage fibers; platelets adhere to the damaged site and platelet plug forms.
• Coagulation events occur. At the same time, the injured tissues are releasing tissue factor (TF), a substance that plays an important role in clotting; PF3, a phospholipid that coats the surfaces of the platelets, interacts with TF, vitamin K, and other blood clotting factors; this prothrombin activator converts prothrombin, present in the plasma, to thrombin, an enzyme; thrombin then joins soluble fibrinogen proteins into long, hairlike molecules of insoluble fibrin, which forms the meshwork that traps RBCs and forms the basis of the clot; within the hour, the clot begins to retract, squeezing serum from the mass and pulling the ruptured edges of the blood vessel closer together.

Blood Groups and Transfusions

Human Blood Groups

• Antigen. An antigen is a substance that the body recognizes as foreign; it stimulates the immune system to release antibodies or use other means to mount a defense against it.
• Antibodies. One person’s RBC proteins will be recognized as foreign if transfused into another person with different RBC antigens; the “recognizers” are antibodies present in the plasma that attach to RBCs bearing surface antigens different from those on the patient’s (blood recipient’s) RBCs.
• Agglutination. Binding of the antibodies causes the foreign RBCs to clump, a phenomenon called agglutination, which leads to the clogging of small blood vessels throughout the body.
• ABO blood groups. The ABO blood groups are based on which of two antigens, type A or type B, a person inherits; absence of both antigens results in type O blood, presence of both antigens leads to type AB, and the presence of either A or B antigen yields type A or B blood.
• Rh blood groups. The Rh blood groups are so named because one of the eight Rh antigens (agglutinogen D) was originally identified in Rhesus monkeys; later the same antigen was discovered in human beings; most Americans are Rh+ (Rh positive), meaning that their RBCs carry the Rh antigen.
• Anti-Rh antibodies. Unlike the antibodies of the ABO system, anti-Rh antibodies are not automatically formed and present in the blood of Rh- (Rh-negative) individuals.
• Hemolysis. Hemolysis (rupture of RBCs) does not occur with the first transfusion because it takes time for the body to react and start making antibodies.

Blood Typing

• Blood typing of ABO blood groups. When serum containing anti-A or anti-B antibodies is added to a blood sample diluted with saline, agglutination will occur between the antibody and the corresponding antigen.
• Cross matching. Cross matching involves testing for agglutination of donor RBCs by the recipient’s serum and of the recipient’s RBCs by the donor serum;
• Blood typing for Rh factors. Typing for the Rh factors is done in the same manner as ABO blood typing.