The Digestive and Endocrine Systems, Exercises of Animal Anatomy and Physiology

Download The Digestive and Endocrine Systems and more Exercises Animal Anatomy and Physiology in PDF only on Docsity! The Digestive and Endocrine Systems ASSIGNMENT 6 Read in your textbook, Clinical Anatomy and Physiology for Veterinary Technicians, pages 358–377, 436, and 474–475. Then read Assignment 6 in this study guide. Introduction The endocrine system involves the secretion of chemicals called hormones by glands within the body. These chemicals control bodily functions, often at locations very distant from the gland that secreted the chemical. The opposite of endocrine is exocrine, which involves the secretion of sub- stances into spaces outside the body. The glands in the skin and gastrointestinal tract are examples of exocrine organs. (The lumen of the intestine is a space that technically isn’t “within” the body, because it’s continuous with the outside environment via the mouth and anus.) The pancreas is unique in being both an endocrine gland and an exocrine gland. The endocrine function is the secretion of substances such as insulin, which metabolizes sugar. The exocrine function involves the secretion of digestive enzymes into the duodenum. Many endocrine organs exist throughout the body (see Table 15-2 on page 360 of your textbook). While they’re all classified as endocrine glands, their anatomy and functions aren’t necessarily similar or even remotely related. Therefore, there isn’t really a grand organizational scheme to this sys- tem as there is with some of the other body systems. The glands are considered together only because their means of secretion is similar. All endocrine glands secrete hormones in the form of proteins that travel via the blood to the target organ for that particular hormone. The hormone causes changes in the activity, and in some cases the structure, of the target organ. Hormones 93 L e s s o n 3 L e s s o n 3 Animal Anatomy and Physiology 294 collectively coordinate the physiology of nearly all activities in the body. In this way, the endocrine system is related to the nervous system. A comparison of the functions of the nerv- ous and endocrine systems can be found on page 359 of your textbook (see Table 15-1). Some endocrine glands have distinct structures that separate them from other organs. Examples include the pituitary gland, the adrenal glands, the thyroid gland, and the parathyroid gland. Other endocrine cells are incorporated in the tissue of other organs. Examples include the insulin- producing cells of the pancreas, certain cells of the brain and kidneys, and the endocrine sex glands. Many endocrine glands are somewhat self-regulating via a process called negative feedback. Hormone secretion raises the blood level of that hormone or affects the level of specific substances in the blood. The endocrine cells detect the altered levels of the hormone or substance that they monitor and decrease the secretion of their hormone. Breakdown in this self-regulation can lead to disorders of endocrine secretion. Pituitary Gland and Hypothalamus A ventral protrusion of the brain called the pituitary gland (also called the hypophysis) has a multitude of functions, many of which involve control of other endocrine glands. The pituitary gland consists of two parts: the anterior pituitary and the posterior pituitary. Each contains different types of cells that secrete different substances. The posterior pituitary, also called the neurohypophysis, is an outgrowth of nervous tissue from the hypothalamus in the brain. The epithelial tis- sue that comprises the anterior pituitary wraps around the posterior pituitary gland. Two hormones are released by the posterior pituitary gland: antidiuretic hormone (ADH) and oxytocin. ADH is also called vasopressin and acts to improve water uptake in the renal collecting duct by increasing its permeability to water. This ensures the body doesn’t excrete too much water and become dehydrated. ADH also causes some smooth-muscle contrac- tion that can lead to blood vessel constriction and a resulting Lesson 3 97 outer layer of tissue called the adrenal cortex and an inner core of tissue called the adrenal medulla. The adrenal medulla secretes epinephrine and norepinephrine. The adrenal cortex secretes mineralocorticoids, including aldos- terone; a group of hormones known as glucocorticoids; and some sex hormones, particularly androgen and estrogen. Aldosterone stimulates the uptake of sodium and water as well as the excretion of potassium in the renal tubules. It’s important in regulation of blood pressure. Glucocorticoid hormones are a type of steroid that has many metabolic effects throughout the body. These hormones primarily affect carbohydrate metabolism via effects on starch metabolism in the liver and regulation of insulin. The adrenal medulla is derived from nervous tissue. It’s con- trolled by the sympathetic nervous system. Epinephrine and norepinephrine work to stimulate the body in response to a threat, so the animal can fight and overcome the threat or flee and escape it. The heart and lungs are stimulated to work faster, so the muscles have plenty of oxygen supply. Digestion is halted temporarily so blood and oxygen can be diverted to the muscles—after all, digestion won’t be needed anymore if the animal is eaten by a lion. Endocrine Pancreas We discussed the pancreas in some detail in the unit on the digestive system, where its exocrine function was examined. The pancreas is also an endocrine organ, secreting several hormones into the blood. Scattered among the exocrine pan- creatic cells, and located near blood vessels, are clumps of cells called the islets of Langerhans. The primary hormones secreted by the pancreatic islet cells are insulin and glucagon. Insulin enables cells, especially liver, muscle, and fat cells, to take glucose from the blood to be used for generating energy for cellular functions. Insulin also inhibits synthesis of glucose from starch stored in the liver. Glucagon opposes many of the effects of insulin by decreasing uptake of glucose by cells. Glucagon also stimulates the synthesis of glucose from glycogen in the liver. Animal Anatomy and Physiology 298 The pancreas also secretes the hormone somatostatin. This hormone has many effects, but it primarily inhibits secretion of insulin and glucagon. Problems associated with endocrine pancreatic function are relatively common. The most common disorder is diabetes mellitus. Diabetes mellitus is a deficiency of insulin secretion, or in some cases, a lack of responsiveness to the presence of insulin. The cause of diabetes mellitus, in most cases, is damage to the beta cells in the islets of Langerhans. Lack of insulin results in cellular inability to take in glucose and leads to hyperglycemia (excessive levels of glucose in the blood). Weight gain or weight loss, increased appetite, and increased thirst and urination are all signs of diabetes mellitus. Gonads One of the many functions of the reproductive organs, the testes and ovaries, is production and secretion of sex hor- mones. Within the testes, hormones from the anterior pituitary stimulate the continuous production of androgens by clumps of specialized cells. Androgens are a group of hormones involved in the development of male secondary sex characteristics. Cells in the ovaries produce the female sex hormones on a cyclical timetable when stimulated by the hormones FSH and LH from the anterior pituitary. The ovaries actually produce two groups of hormones, the estrogens and the progestins. The estrogens function to prepare the female for breeding. The primary progestin, progesterone, is involved in preparing the uterus for pregnancy. Miscellaneous Endocrine Organs Various other hormone-producing cells exist in tissues throughout the body. These include cells in the kidneys, stomach, small intestine, thymus, placenta, and pineal body. A variety of organs also produce the hormone-like compounds known as prostaglandins. These compounds are organized into nine groups, designated by the letters A through I, and act within the area where they’re produced. Lesson 3 99 Kidneys Cells in the kidneys secrete the hormone erythropoietin. This hormone is released in response to tissue hypoxia and results in stimulation of the bone marrow to produce red blood cells. Stomach Cells in the stomach produce the hormone gastrin, which acts to stimulate the production of digestive enzymes in the stomach. Small Intestine The small intestine contains cells that secrete the hormones secretin and cholecystokinin (CCK). These hormones stimulate gallbladder contraction and the release of other digestive enzymes. Placenta and Thymus The placenta and thymus are endocrine organs active only during specific life-cycle stages. The placenta encloses the fetus during pregnancy and forms the interface between the fetal and maternal circulations. It secretes the hormone chorionic gonadotropin, which functions in maintenance of pregnancy. Small amounts of sex hormones are also produced in placental tissue. The thymus gland is active in infancy and secretes the hormones thymosin and thymopoietin. These hormones act to stimulate the develop- ment of T-lymphocytes and strengthen the immune system. Pineal Body The small pineal body is seated within a fluid-filled space in the third ventricle of the brain. The cells in the pineal body secrete the hormone melatonin. The function of melatonin isn’t well understood, but it appears to play a role in regulat- ing sleep and wakefulness. Secretion of melatonin may be influenced in part by the amount and duration of sunlight in the environment. Animal Anatomy and Physiology 2102 ASSIGNMENT 7 Read in your textbook, Clinical Anatomy and Physiology for Veterinary Technicians, pages 264–281, 436–439, and 465–469. Then read Assignment 7 in this study guide. Introduction Your body is constantly using energy, even when you’re at rest. Your cells use energy to carry out the normal functions of protein synthesis, cell maintenance and repair, and their particular functions. On a larger scale, processes like breath- ing, pumping of the heart, maintenance of normal levels of substances within the body, and elimination of waste prod- ucts are vital to life. These processes continue while you’re sleeping. Because your body can’t manufacture energy, it must obtain that energy from outside sources. In all animals, this energy comes from food. Food also provides the body with fresh raw materials for the growth, maintenance, and repair of body structures. The digestive system deals with the intake, physical breakdown, chemical digestion, and Self-Check 6 13. The most common thyroid disorder in the dog is _______. 14. The primary hormone produced by the kidney is _______. 15. List two hormones produced by the adrenal medulla. __________________________________________________________ __________________________________________________________ Check your answers with those on page 133. Lesson 3 103 absorption of food, along with the elimination of the waste products created by this process. The digestive system also eliminates certain toxic substances and secretes hormones it uses to regulate itself. The digestive system is also known as the digestive tract, the gastrointestinal (GI) tract, the alimentary canal, or simply the gut. Numerous organs are involved in the processes of diges- tion and absorption of nutrients (see Figure 11-1 on page 265 of your textbook). Anything within the GI tract is actually located outside the body. The GI tract begins with the mouth and ends at the anus. For materials to pass into the body from this tube, they must pass through the lining epithelium. The specific structure and physiology of the digestive system varies among different species. Herbivores (plant-eating animals, such as cattle, sheep, and goats), carnivores (meat- eating animals, such as cats), and omnivores (animals that eat plant material and meat, such as pigs and humans) all vary in the specific mechanisms of intake, digestion, and absorption of nutrients. Some species of animals have sim- ple, single stomachs (monogastric animals), while others have complex organs designed to mix and ferment food (ruminants). The basic functions of the GI tract are 1. Prehension—Grasping food with the lips or teeth 2. Mastication—Mechanical grinding and breaking down of food (a.k.a. chewing) 3. Digestion—Chemical breakdown of food 4. Absorption—Movement of nutrients and water into the body 5. Elimination—Removal of waste materials Structure of the GI tract The intestinal tract is suspended from the dorsal body wall by the mesentery. These sheets of connective tissue also con- tain blood vessels, lymph vessels, and nerves that supply the GI tract. Animal Anatomy and Physiology 2104 The walls of the GI tract contain four layers of tissue (see Figure 11-2 on page 266 of your textbook). 1. Mucosa—Consists of epithelium and some loose connec- tive tissue that lines the tube 2. Submucosa—Dense connective tissue layer located just below the mucosa 3. Muscle layer—Thick muscles outside the submucosa 4. Serosa—Consists of a thin, tough layer of connective tissue The digestive tract contains both stratified squamous epithe- lium and simple columnar epithelium. The mouth, pharynx, esophagus, and anus are lined with stratified squamous epithelium. The stomach and intestines are lined thin simple columnar epithelium. Muscles of the digestive tract include both skeletal and smooth muscle. The mouth, pharynx, cra- nial area of the esophagus, and the external anal sphincter contain skeletal muscle under voluntary control. The remain- der of the digestive tract walls contains involuntary smooth muscle. Peristaltic contractions of the muscles within the walls of the intestines are responsible for movement of mate- rials through the GI tract while segmental contractions result in mixing of the food within the tract (see Figure 11-3 on page 267 of your textbook). Oral Cavity Your mouth, or oral cavity, consists of the lips, teeth and gums, tongue, oropharynx (ventral digestive passageway), and associated salivary glands. Your lips are a zone of transition from the skin of the face to the mucous membrane (a general term denoting the surface of an organ lubricated by moisture) lining your gums and the inside of your cheeks. The lips have small salivary glands that help moisten their surface and soften and partially digest food in the mouth. Several layers of muscle help the lips grab food and retain food and water within the mouth. These muscles allow us to whistle and kiss. Lesson 3 107 3. Premolars—Grind food into smaller particles for swallowing 4. Molars—Grind food into smaller particles for swallowing You’ll find all four types of teeth in carnivores, horses, and pigs. Ruminants lack upper incisors and all canine teeth. The teeth are arranged in two arches called dental arcades, one on the upper jaw and one on the lower jaw. Refer to your textbook for the dental formula for each species (page 269, Table 11-1). As just mentioned, ruminants lack upper incisors; instead, a dental pad consisting of hardened mucous membrane tissue is present in the rostral upper jaw opposite the lower inci- sors. The teeth of each species are adapted to the type of diet eaten by that species. For example, dogs and cats are carni- vores that need to cut and tear their food; thus incisors and canine teeth are important. Cows, on the other hand, eat plant material that must be ground up before being swal- lowed, so the molars and premolars are their most prominent teeth. Pigs and people are omnivores, eating both animal and plant material, so all types of teeth are present and equally prominent in the mouth. Tongue The tongue is a muscular organ used for manipulating food within the mouth. It includes taste buds, which allow you to taste food. The interior of the tongue is composed of many interlocking muscle bundles. These allow the tongue to move in many directions. The muscle fiber bundles are held together by connective tissue. Scattered throughout the con- nective tissue are blood vessels, nerves, and small accessory salivary glands. The tongue’s surface is covered with squamous epithelial tis- sue just like the rest of the mouth, but on the surface are small projections called papillae (singular: papilla) that help the tongue control the food. Some papillae are specially modified to carry the taste buds. In cats, the papillae are especially prominent (if you touch the top of a cat’s tongue, you’ll note it feels like sandpaper) and are used like the bristles of a brush for grooming. Animal Anatomy and Physiology 2108 Salivary Glands Salivary glands are located throughout the oral cavity—in the tongue, on the inside of the lips and cheeks, below the tongue, and in the walls of the pharynx. Most of these glands are very small, consisting of a cluster of cells that produce saliva and a small tube called a duct. The duct carries the saliva from the secreting cells to the oral cavity’s surface. These smaller glands, called accessory salivary glands, secrete saliva in a continuous manner to keep the surface of the mouth moist. A few glands are much larger and more complex in structure; these glands secrete the majority of saliva in response to the presence of food or other stimuli (usually substances that have a bad taste). These reactions are controlled by the nervous system. Three pairs of the larger salivary glands exist: 1. The parotid salivary gland is located in the angle between the ear and the back of the lower jawbone. 2. The submandibular, or just mandibular, salivary gland is located just inside and forward of the parotid salivary gland between the caudal borders of the mandibles. 3. Sublingual salivary glands, located beneath the tongue, may actually be a pair or cluster of salivary glands. Saliva contains water; electrolytes (sodium, potassium, chloride); mucin (a proteinaceous material that gives saliva a slimy feel); and in omnivorous species, amylase and lysozyme. Amylase is an enzyme that helps begin the process of digesting certain sugars in the food. Lysozyme is an enzyme that has some antibacterial activity. Saliva performs the following functions: n Moistens food to make it easier to chew and form a bolus n Keeps the lining of the oral cavity moistened n Washes particles over the taste buds for the sensation of taste n Kills some, but not many, bacteria Lesson 3 109 Pharynx The pharynx is a muscular walled area that opens to both the digestive system and the respiratory system. The pharynx opens into the nasal cavity, the oral cavity, the esophagus, and the trachea. The pharynx serves as a conduit to trans- port food and liquid from the oral cavity to the esophagus. The pharynx also transports air from the mouth or nasal cavity to the trachea. Monogastric Stomach and Intestine The stomach is similar to the esophagus in basic tubular anatomy, but the similarity ends there. The stomach begins the process of digestion, mixes the food with digestive fluids, and serves as a holding area that delivers its contents into the small intestine in a controlled fashion over several hours. The stomach differs from the esophagus in three important ways: 1. The stomach can expand to a much larger diameter than the esophagus, so the stomach’s capacity to store food is much greater. 2. The muscular layers of the stomach differ from those of the esophagus, because the stomach must not only eventually propel food in the aborad direction, but it must also churn and mix the food being stored in it. Accordingly, the stomach has a third layer of muscle in the muscularis (the other layers being the circular and longitudinal muscles) called the oblique muscle layer. The oblique muscle fibers lie at an angle to the long axis of the stomach. When they contract, the stomach contracts at oblique angles that help mix the food in the stomach. 3. The mucosa of the stomach consists of a variety of glan- dular epithelial structures that secrete digestive fluids, mucus, and acid and absorb a small amount of the materials in the stomach. Dogs, cats, pigs, and horses have a type of stomach called a simple stomach. The mucosa of simple stomachs contains many folds called rugae. The rugae flatten out as the stomach expands to accommodate more food. Animal Anatomy and Physiology 2112 Digestive Processes The digestive process must break chemical bonds between molecules in ingested food to retrieve the simplest form of the nutrient. The process by which the carbon bonds are broken is called hydrolysis, and is the most basic process of diges- tion. All forms of digestion use hydrolysis, but the means by which hydrolysis is achieved varies with the type of nutrient being digested. Enzymes are proteins produced throughout the body. They perform a variety of functions involving the manipulation of other molecules. In the digestive system, enzymes are the key to the chemical digestion of food and are produced by glandular tissue in various parts of the digestive system, including the salivary glands, stomach, intestine, and pancreas. Digestive enzymes recognize specific molecules or types of molecules in food and act upon those molecules to perform hydrolysis. The three categories of enzymes in the digestive tract are 1. Proteases—Break down proteins 2. Lipases—Break down fat molecules 3. Amylases—Break down carbohydrates Within each enzyme category are numerous specific enzymes. Some enzymes are potentiating enzymes. These enzymes break a chemical bond on another enzyme to convert that enzyme to an active form. An example of a potentiating enzyme is enterokinase, an enzyme produced by the intestine. Enterokinase’s only known function is to break a chemical bond on the pancreatic enzyme trypsinogen, an inactive enzyme, to convert it to trypsin, an active enzyme. Trypsin in turn breaks a chemical bond on chymotrypsinogen, an inac- tive enzyme, to convert it to chymotrypsin, an active enzyme. Trypsin is both a potentiating enzyme and a digestive enzyme because it also acts to break down protein molecules. Without digestive enzymes, the body wouldn’t be able to digest food, and the food wouldn’t benefit the body. Lesson 3 113 Digestion can be either a luminal process or a membrane process. Luminal digestion occurs in the lumen of the stom- ach or intestine. Digestive enzymes mix with the food and acid or alkaline fluids to digest free-floating food particles in the lumen. Membrane digestion occurs only in the intestine and involves digestion of food particles on the surface of the mucosa. Enzymes are present in the cell membrane of the enterocytes lining the mucosa. These enzymes digest food particles that come in contact with the mucosa but have no effect on particles floating free in the lumen. The majority of digestion and absorption of food in dogs, cats, and pigs occurs in the small intestine. The presence of villi and microvilli, as well as the extreme length of the small intestine (several feet in length even in the smallest dog or cat), all serve to increase the surface area of intestine exposed to ingested food. This large surface area in the small intestine facilitates the absorption and digestion of food. Large Intestine Although the anatomy of the large intestine varies widely depending on an animal’s diet, this organ has the same basic features in all domestic animals. The large intestine has three main parts: 1. Colon 2. Rectum 3. Anus The cecum is a blind sac projecting from the colon that technically isn’t part of the large intestine. Structure The large intestine is similar in structure to the small intes- tine, with a tubular arrangement and villi. The enterocytes of the large intestine have an absorptive function but not a digestive function, and there are crypts where new entero- cytes are produced as older cells are shed. Animal Anatomy and Physiology 2114 Goblet cells are present in increasingly larger numbers as you travel down the colon; these cells secrete mucus for lubrica- tion of the developing stool. The terminal portion of the large intestine is the rectum, where stool is stored temporarily until defecation occurs. The anus is the opening of the gastroin- testinal tract for elimination of stool. At the junction of the rectum and anus, the mucosa changes from glandular villi to flattened squamous epithelial tissue similar to that found on the skin. A large-muscle sphincter surrounds the anus to help retain the stool before defecation. In some cases, nerv- ous control of this sphincter deteriorates and the animal becomes incontinent, unable to retain stool. Function The large intestine functions mostly as a site for absorption of water and electrolytes (e.g., sodium, chloride) from the ingested material, which is converted to waste material (i.e., feces). The exception to this rule is the horse, which has a very large cecum and a large colon, both of which serve as sites for fermentation very much like the rumen in ruminant animals that we’ll cover in the next section. Horses and simi- lar species are thus sometimes known as hindgut fermenters, because fermentation occurs in the large intestine (i.e., the “hindgut”). The cecum and large intestine in these species absorb the products of fermentation along with digested bacteria and protozoa. Sodium is actively reabsorbed in the large intestine of all species, which facilitates the absorption of water and chlo- ride. Mucus is secreted by goblet cells within the mucosa, and this mucus helps lubricate the feces as it passes through the colon, anus, and rectum. The rectum serves only as a waste storage site until the feces are ready to be evacuated, and the anus contains a muscle sphincter that controls fecal evacuation. Contractions of the colon and rectum triggered by distention of the colon are under the influence of the nervous system. When the colon becomes full, the stretching of its wall is detected by various nerve sensors, and this stimulates a reflex series of muscle contractions in the colon as well as relaxation of the anal sphincter, thus emptying the colon. Lesson 3 117 Another important function of the hepatocytes is to secrete bile into the bile ducts, where it’s carried to the gallbladder or duodenum. The hepatic artery and the portal vein carry substances to the hepatocytes, whereas the bile duct carries substances away from the hepatocytes. With so many functions, it’s understandable that a severely damaged liver can cause many serious and sometimes fatal consequences. Among the problems faced by patients with liver failure are exposure to toxins that affect the brain, infec- tions, poor digestion and absorption of fats and vitamins, and an inability to store and use long-term energy sources such as glycogen. Gallbladder and Bile Ducts Exiting from the liver on its caudal surface is the hepatic duct. This duct is the collective continuation of the millions of small bile ducts in the portal triads. The hepatic duct carries bile from the liver to the gallbladder and duodenum. The gallbladder is closely associated with the liver both anatomi- cally and functionally. It’s a relatively simple, muscular, thin-walled sac that sits tucked between the lobes of the liver on the right side. When empty, the gallbladder mucosa has many folds; when filled with bile, the folds flatten out. A small duct, called the cystic duct, connects the gallbladder to the hepatic duct. This connection serves as means for bile to enter and leave the gallbladder. The common bile duct starts where the hepatic duct and cystic duct join. This duct ends where the bile duct empties into the duodenum at the major duodenal papilla, where the pancreatic duct empties. Thus, the common bile duct must pass through, or very near, the pancreas before entering the duodenum. The gallbladder serves as a storage site for bile; while the bile is stored, water is reabsorbed from it by the gallbladder, making the bile more concentrated. Under the right condi- tions after eating, the gallbladder wall contracts, forcing bile down the common bile duct to the duodenum. Bile aids in the digestion and absorption of fats in the diet. Horses don’t have gallbladders. Animal Anatomy and Physiology 2118 Food entering the duodenum also stimulates the production of two hormones from endocrine cells interspersed with other cells in the mucosa: secretin and cholecystokinin (CCK). Secretin and CCK both stimulate secretion of pancreatic enzymes. CCK stimulates contraction of the gallbladder to release bile into the duodenum. Both the pancreatic secretions and bile enter the duodenum through the major duodenal papilla. The pancreas produces the proenzymes trypsinogen and chymotrypsinogen. The enzyme enterokinase on the intestinal wall activates trypsinogen to trypsin, which in turn converts chymotrypsinogen to chymotrypsin. Both of these enzymes break down proteins and peptides into amino acids. The pancreas also produces lipase, which breaks down fats into triglycerides, fatty acids, and glycerol, and amylase, which breaks down starches into sugars. This enzymatic digestion occurs in the lumen and is called luminal digestion. Similar enzymes on the mucosal surface of the enterocytes exist to carry out membrane digestion into the basic compo- nents of each nutrient. Bile helps break fat globules into smaller fat globules, a process known as emulsification. Emulsification increases the surface area of fat exposed to lipases, increasing the speed and efficiency of fat digestion. Contractions of segments of the intestinal muscularis pro- duce peristaltic mixing motions that blend the contents so there’s increased contact between enzymes and the specific materials they digest. Absorption Process As digestion is occurring, the intestine absorbs the products of digestion. The anatomic unit most active in the absorption process is the villus, whose surface contains many entero- cytes possessing microvilli to absorb digestive products. Microscopically, the center of the villus has an array of arteries that supply the villus with oxygen and nutrients, capillaries that capture absorbed food products from the enterocytes, and veins that carry the absorbed products from the villi to the rest of the body. In the very center of the villus is a tubular structure called a lacteal, which absorbs certain Lesson 3 119 fats in the diet and carries them to the general circulation via lymphatic vessels. The actual absorption of the digested food occurs at the cellular level in the enterocytes. Absorption occurs in the enterocyte via either passive diffusion or active transport. Different molecules are absorbed in different ways, depending on the molecule’s size, configuration, and electrical charge. Water is absorbed strictly via diffusion, often following other molecules, because the movement of other molecules changes the concentration of water inside the cell membrane. Water can actually move out of the enterocyte if the lumen’s con- centration is very high relative to the enterocyte’s cytoplasm. Electrolytes are absorbed either via diffusion or active trans- port, depending on the specific ion. Proteins are absorbed primarily in the form of amino acids via active transport. Both sugar and protein transport are linked to sodium trans- port; as sodium is transported into the cell, sugar or protein molecules also enter the cell. Sugars, proteins, and certain ions passively diffuse through the other side of the enterocyte into the extracellular fluid surrounding all cells and are then picked up by the capillaries to be transported to the general circulation. Fats are digested into fatty acids, which don’t dissolve well in water. To make them soluble in the intestinal fluid, bile provides these molecules with a coating that’s electrically charged on the outside and neutral on the inside. The exte- rior charge makes the complex, called a micelle, dissolve in water. When a micelle comes in contact with the enterocyte’s surface, fatty acids are released. Fatty acids are very soluble in the cell membrane’s lipid, so fatty acids can cross the cell membrane via diffusion. Once in the enterocyte, the fatty acids are converted to triglycerides. Triglycerides are transported with cholesterol and phospho- lipids through the enterocyte to the other side, where the triglycerides are released into the extracellular fluid. Triglycerides are absorbed from the extracellular fluid into the lacteals and from there are eventually transported to the general circulation. A few of the shorter fatty-acid chains are relatively soluble in water and aren’t converted into trigyl- cerides by the enterocytes. They’re absorbed directly into the blood rather than entering the lymphatic system. Animal Anatomy and Physiology 2122 replace the abomasum, followed by surgical attachment of the abomasal wall to the abdominal wall to prevent future displacement, is required. An interesting structure can be found in the ruminant stom- ach. This structure, called the gastric groove, consists of the following three parts: 1. Reticular groove (or esophageal groove), which is in the reticulum 2. Omasal groove, which is in the omasum 3. Abomasal groove, which is in the abomasum These grooves can nearly completely close in infant rumi- nants to form a tubular structure that transports liquids directly to the abomasum from the esophagus. This is impor- tant because infant ruminants don’t ingest fibrous plant material; instead, they ingest milk from the mother. Therefore, fermentation is unnecessary, and the rumen needs to be bypassed. Milk is delivered directly to the abomasum, where it’s properly digested. As the ruminant matures and begins eating a fibrous plant diet, the gastric groove stops closing so food can enter the rumen for fermentation. Birds, Reptiles, and Amphibians Birds have high energy demands and therefore have anatomic and physiologic adaptations to allow rapid absorption of nutrients from food. The beaks of birds vary considerably depending on diet and foraging strategies. The mouth may contain an enlargement that serves as a pouch for temporary food storage. In many species, the esophagus expands to form a structure called the crop. The crop func- tions to store, lubricate, and regulate the passage of food. The stomach of birds consists of a glandular portion (proventriculus) and a muscular portion (gizzard). The process of chemical digestion begins in the proventriculus. The giz- zard is comprised of bands of striated muscles that grind food. In some species, paired sacs called ceca are located at the junction of the small and large intestine. Excretions from these sacs are released several times per week and appear dark brown and moist. The GI tract ends at the cloaca. The Lesson 3 123 anterior section receives excrement from the intestine, and the posterior sections stores the excrement. Waste is elimi- nated through the vent. The structure and function of digestive tract organs in reptiles and amphibians is highly variable. Carnivorous, omnivorous, and herbivorous reptiles all exist. Snakes and crocodilians are strict carnivores, whereas all three categories are represented by the lizards and chelonians. All adult amphibians are carnivores as well. Some species have very specific feeding habits. Accessory organs may also be present that are used for both tracking and capture of prey. Now, review the material you’ve learned in this study guide as well as the assigned pages in your textbook for Assignments 6–7. Once you feel you understand the material, complete Self-Check 7. Then check your answers with those provided at the end of this study guide. If you’ve missed any answers, or you feel unsure of the material, review the assigned pages in your textbook and this study guide. When you’re sure that you completely understand the information presented in Assignments 6–7, complete your examination for Lesson 3. Animal Anatomy and Physiology 2124 Self-Check 7 1. Name the four types of teeth found in most domestic animals. __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ 2. The salivary gland that’s located in the angle between the ear and the back of the lower jawbone is the _______. 3. In a newborn ruminant, the _______ diverts milk from the esophagus to the abomasum, bypassing the rumen. 4. List the four compartments of the ruminant stomach in order from the esophagus to the small intestine. __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ (Continued) 127 1. The cells in the pancreas that secrete glucagon are called _______ cells. A. gamma C. delta B. beta D. alpha 2. Which of the following is caused by glucocorticoids? A. Increased blood glucose level B. Decreased gluconeogenesis C. Stimulated cartilage growth D. Worsened rheumatoid arthritis 3. The _______ functions as both an endocrine and exocrine organ. A. hypothalamus C. pituitary B. pancreas D. thyroid gland 4. Which of the following is secreted by the pineal body? A. Melatonin C. Pinealatonin B. Melanin D. Thymosine E x a m in a tio n E x a m in a tio n Lesson 3 The Digestive and Endocrine Systems When you feel confident that you have mastered the material in Lesson 3, go to http://www.takeexamsonline.com and submit your answers online. If you don’t have access to the Internet, you can phone in or mail in your exam. Submit your answers for this examination as soon as you complete it. Do not wait until another examination is ready. Questions 1–25: Select the one best answer to each question. EXAMINATION NUMBER 39652400 Whichever method you use in submitting your exam answers to the school, you must use the number above. For the quickest test results, go to http://www.takeexamsonline.com Examination, Lesson 3128 5. The _______ are endocrine organs active only during specific life-cycle stages. A. kidneys and pancreas B. hypophysis and pineal body C. ovary and testes D. placenta and thymus 6. Which of the following produces aldosterone, glucocorticoids, and androgen? A. Parathyroid glands C. Pituitary gland B. Adrenal glands D. Kidneys 7. Cholecystokinin is secreted by the A. stomach. C. small intestine. B. pancreas. D. adrenal cortex. 8. Which of the following is secreted by the adenohypophysis? A. PTH C. ADH B. Insulin D. TSH 9. Hormone secretion is usually controlled by _______ systems. A. target C. portal B. cortical D. negative feedback 10. The pituitary gland is also known as the A. hypophysis. C. lesser hypothalamus. B. parahypophysis. D. portalpituitary. 11. Adrenocorticotropic hormone (ACTH) production A. is also known as growth hormone. B. can be released quickly via stimulation of the hypothalamus by other parts of the brain. C. is produced by the posterior pituitary. D. can trigger oogenesis. 12. In most animals, ovulation occurs when which hormone reaches its peak? A. Follicle-stimulating hormone (FSH) B. Adrenocorticotropic hormone (ACTH) C. Luteinizing hormone (LH) D. Thyroid-stimulating hormone (TSH) 13. A deficiency of antidiuretic hormone (ADH) in the body causes the disease A. diabetes insipidus. C. hypoadrenocorticism. B. diabetes mellitus. D. hyperadrenocorticism. Examination, Lesson 3 129 14. Which of the following best describes calcitonin? A. It’s released by the parathyroid gland. B. It functions to prevent hypercalcemia. C. It functions to prevent hypocalcemia. D. It’s released by the adrenal medulla. 15. Horses are unlike dogs and cats in that a large amount of digestion and absorption in these hindgut fermenters occurs in the A. rectum. C. jejunum. B. cecum. D. ileum. 16. Which structure is the main organ responsible for removing bacteria and toxins from the blood? A. Pancreas C. Gallbladder B. Cecum D. Liver 17. Which ruminant compartment is the smallest? A. Omasum C. Rumen B. Reticulum D. Abomasum 18. The outer layer of the crown of canine and feline teeth is composed of which material? A. Cement C. Dentine B. Enamel D. Pulp 19. Which compartment of the ruminant stomach is functionally most similar to the simple stomach of carnivores? A. Rumen C. Abomasum B. Reticulum D. Omasum 20. Which structure prevents backflow of duodenal contents into the stomach? A. Pylorus C. Fundus B. Antrum D. Cardia 21. Instead of upper incisors, ruminants have A. occlusion. C. retained deciduous teeth. B. dental pads. D. strong gingiva. 22. Without bile, the dog would have trouble digesting which type of nutrient? A. Proteins C. Lipids B. Carbohydrates D. Cellulose Self-Check 3 1. cleavage 2. blastocyst 3. amniotic fluid 4. parturition 5. oxytocin 6. Colostrum 7. capacitation 8. cotyledonary 9. zonary 10. morula Self-Check 4 1. c 2. d 3. a 4. e 5. b 6. sulcus, gyrus 7. Sympathetic and parasympathetic nervous systems 8. Pia mater, arachnoid, and dura mater 9. The midbrain, the pons, and the medulla oblongata 10. Neurotransmitters Self-Check 5 1. c 2. e 3. a 4. b Self-Check Answers132 5. d 6. The external ear, the middle ear, and the inner ear 7. The malleus, the incus, and the stapes 8. proprioception 9. aqueous chamber, vitreous chamber 10. The lens, suspensory ligament, and the ciliary body Self-Check 6 1. g 2. i 3. f 4. e 5. h 6. d 7. c 8. a 9. j 10. b 11. TSH-releasing factor 12. Two hormones that influence the blood calcium level are parathyroid hormone (parathormone, PTH) and calcitonin. 13. hypothyroidism 14. erythropoietin 15. Two hormones produced by the adrenal medulla are epinephrine and norepinephrine. Self-Check Answers 133 Self-Check 7 1. Incisors, canines, premolars, and molars 2. parotid gland 3. gastric groove 4. Reticulum, rumen, omasum, and abomasum 5. c 6. d 7. e 8. a 9. b 10 c Self-Check Answers134