A&P Course Objectives - Exam 4 - Digestion
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- What are three major categories of food?
- Food can be categorized as carbohydrates, fats, nucleic acids and proteins with nucleic acids being relatively minor components.
- What are the breakdown products derived from carbohydrates, proteins, nucleic acids and fats?
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Carbohydrates are further categorized as polysaccharides, such as starch and glycogen; disaccharides such as sucrose, lactose and maltose; and simple sugars such as glucose, fructose and galactose.
Proteins are constructed from amino acids linked by peptide bonds and are broken by proteases into individual amino acids.
Lipids are generally triglycerides coposed of one molecule of glycerol and three molecules of fatty acid.
Nucleic acids are digested into five-carbon sugars and bases.
All of digestion and metabolism is a process of rendering these four classes of molecules into smaller and smaller parts until carbohydrates are converted to simple sugars, proteins are converted to amino acids, lipids are converted to monoglycerides, glycerol and fatty acids and nucleic acids are converted to pentose sugars, phosphate ion and the component bases.
The function of digestion is to render the four food categories into their component parts and absorb them for use in the body, either as a form of energy, or as building blocks for the generation of new building blocks. - What is meant by the terms motility, secretion and absorption?
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Motility, secretion and absorption are three fundamental concepts of digestion. An understanding of these three aspects of digestion from the mouth to the large intestine provide important insights into the function of the digestive system.
- Motility is defined as agitation and movement of digesta through the system.
- Secretion is defined as the addition of enzymes or cofactors necessary for digestion.
- Absorption is defined as the uptake of products of digestion. - What is the role of motility, secretion and absorption in the mouth?
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The first step in digestion is ingesting food and chewing. Chewing can be considered as a form of motility and is the first step in breaking food into smaller pieces, which is accomplished mechanically by the action of the muscles of mastication and teeth.
Secretion in the mouth consists of saliva, mucous and amylase. Saliva is produced by the salivary glands, which are classic exocrine glands, and are primarily under control of the parasympathetic nervous system. Saliva is mostly water. The secretory cells, often called acinar cells, are responsible for secreting saliva. The concentration of ions in saliva shortly after it is produced by the acinar cells is similar to that in the interstitial fluid. Saliva is formed by transporting sodium, potassium, chloride and bicarbonate ions across the acinar cells, and water follows. Most of the ions are resorbed by the duct epithelial cells so that the ion concentration in saliva is normally more dilute than the ion concentration in the interstitial fluid. Of these ions, potassium is higher in the saliva than in the interstitial fluid.
With the ingestion of food, the rate of saliva production increases and the concentration of salts also increases because the duct epithelial cells have a shorter period of time to resorb the ions before the saliva enters the mouth.
In addition to wetting food with saliva, exocrine glands, primarily located in the sublingual salivary gland, produce mucous. The purpose of the mucous is to lubricate the ingesta so that it can more easily be swallowed. Mucous is rich in polysaccharides, but its mechanism of production is not completely clear.
Salivary amylase, which is screted in saliva, is an enzyme that converts starch, a complex carbohydrate, to dissaccharides and oligosaccharides.
Swallowing is a reflex with the control residing in the brain stem. The upper one third is skeletal muscle and the lower two-thirds is smooth muscle. The upper third is under control of the somatic motor nuerons in the brain stem whereas the lower two thirds is under control of the parasympathetic system. Swallowing centers in the brain stem sending signals to both the skeletal and smooth muscle causing contraction in a wave-like fashion that propels food from the mouth to the stomach by a mechanism called peristalsis.
The cardiac sphincter is a region of circular muscle that controls the movement of food from the esophagus into the stomach. - What are the roles of the stomach in digestion? How are they related to motility, absorption and secretion?
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The stomach is a holding vat where ingesta is mechanically broken into smaller pieces and mixed with water. Motility is one of the most important functions of the stomach. The three layers of smooth muscle churn the stomach contents and help to reduce digesta to small pieces suspended in a fluid called chyme.
There are muscular valves at either end of the stomach: the cardiac sphincter opens in response to swallowing and admits food from the esophagus into the stomach, and the pyloric sphincter is a muscular valve that when closed prevents movement of digesta from the stomach into the small intestine, but when open admits chyme from the stomach into the small intestine. - What are the names of the layers of the stomach and the remainder of the digestive system? What is the composition of each layer? Why is the organization of the longitudinal and circular smooth muscle layers important in the stomach and the remainder of
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Throughout the digestive system, the cellular layers are organized into a predictable pattern:
- The mucosa is composed of the layer of epithelial cells, usually columnar epithelium, that line the tube of the digestive system. The epithelial cells are attached via a basal lamina to the underlying lamina propria, which is a type of connective tissue. Beneath the lamina propria lies a layer of smooth muscle called the muscularis mucosae. Taken together, the epithelium, the lamina propria and the smooth muscle layer are known as the mucosa. Included within the epithelial layer of the mucosa are glands.
- The submucosa is a thick layer of connective tissue, which in the stomach is rich in elastic fibers to accomodate the distention of the stomach after a large meal. Blood vessels and lymphatic vessels run through the submucosa.
Two or more layers of smooth muscle lie external to the submucosae, and are known collectively as the muscularis externa. The more internal layer of smooth muscle is a circular layer and the more external layer is the longitudinal layer. In the stomach, there is an additional layer, the oblique layer, that aids in churning and movement of the digesta in the stomach.
There are two neuronal networks known as the enteric nervous system. One network lies between the submucosa and the muscularis externa, and the other layer lies between the circular and longitudinal muscle layers. These networks, in concert with the autonomic nervous system control motility, and to some extent, secretion with the digestive system.
External to the muscularis externa lies the serosa. For the digestive system, the serosa is the mesentery. The serosa is an epithelial cell layer known as the mesothelium. These cells have their basilar regions adjacent to the longitudinal muscle layer and their apical regions facing the peritoneal cavity. - Why is motility so important in the stomach?
- Motility is one of the most important functions of the stomach. The three layers of smooth muscle churn the stomach concents and help to reduce digesta to small pieces suspended in fluid called chyme.
- How is the movement of material in and out of the stomach controlled? What are the names of the sphincters?
- There are muscular valves at either end of the stomach. The cardiac sphincter opens in response to swallowing and admist food from the espophagus into the stomach. The pyloric sphincter is a muscular valve that when closed prevents movement of digesta from the stomach into the small intestine, but when open admits chyme from the stomach into the small intestine.
- What are the major secretory cells in the stomach? Where are they located? What is the role of each?
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Cells of the glandular epithelium in the mucosa of the stomach are responsible for secretion of acid, pepsinogen, histamine and gastrin. Cells of the gastric gland are located in regions called gastric pits which are extensions of the columnar epithelium of the stomach mucosa.
Of these, acid and pepsinogen are products of exocrine glands whose products are secreted into the lumen of the stomach. Gastrin is an endocrine hormone that helps regulate acid secretion.
The secretory cells of the stomach are the pariental cells that make and secrete acid, the chief cells that make and secrete pepsinogen, a precursor molecule for the protease pepsin, G cells that secrete the hormone gastrin and enteroendocrine cells that are located near the parietal cells and secrete histamine that functions locally on parietal cells in a paracrine-like fashion.
Secretory cells in the gastric glands. The parietal cells, chief cells and enteroendocrine cells are found near one another in regions of the stomach mucosa called gastric glands. The openings of the gastric glands into the lumen of the stomach are called gastric pits and are formed by surface epithelial cells whose primary function is to secrete mucosa.
The role of parietal cells is to produce hydrochloric acid that is secreted into the lumen of the stomach. The parietal cells contain carbonic anhydrase, an enzyme that rapidaly converts CO2 and water into carbonic acid (H2CO3), which then rapidly dissociates into H+ ions and bicarbonate (HCO3-) ions. The hydrogen ion is pumped oput of the parietal cell into the lumen of the stomach and the bicarbonate ion is released into the blood. After a large meal, the movement of H+ ions into the stomach and the movement of bicarbonate into the blood can cause the blood to become slightly alkaline. This is called the alkaline tide.
The release of H+ from the parietal cells is controlled by the action of acetycholine released from parasympathetic postganglionic neuron axon terminals, gastrin released from G cells and histamine released from enteroendocrine cells. The parietal cells have recepts on their basolateral surfaces. Through specific recepts in the basilar membrane and associated second messenger systems, acetylcholine, gastrin and histamine regulate the release of H+ from the parietal cells. Gastrin acts like a true hormone in that it is released from G cells and enters the blood. When it arrives at the parietal cell, it binds to recepts and induces the release of H+. Gastrin release is stimulated by a rising pH in the stomach and the presence of amino acids and peptides in the lumen of the stomach.
Besides gastrin, the release of H+ from the parietal cells is under the control of histamine which is released from the enteroendocrine cells and binds to an unusual histamine receptor, the H2 receptor. this receptor is almost exclusively on the parietal cells, and because of its restricted distribution, molecules that specifically block this receptor can be used to prevent acid release from the parietal cells. Cimetidine is a common antacid that works as an inhibitor of the H2 histidine receptor.
The normally low pH of the stomach, which may become less than 1.0, helps to kill bacteria that enter the digestive system with food. It also helps to digest proteins and plant material.
The chief cells secrete pepsinogen, a precursor to the protease pepsin. The pepsinogen itself is inactive, but becomes active when exposed to the low pH of the stomach lumen and when a portion of the enzyme is removed by active pepsin in the stomach. The control of pepsinogen release is similar to the control of acid release from the parietal cells. - How are the epithelial cells of the mucosa replaced?
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In addition to these secretory cells, the gastric pits contain stem cells that are precursor cells to the epithelial cells of the stomach lining. As would be expected in the harsh environment of the stomach, epithelial cells lining the stomach last for only a few days before being replaced by cells that originate in the gastric pits. The secretory cells of the gastric pits have a much longer life span that the epithelial cells lining the stomach.
The stomach mucosa protects itself from the effects of low pH and the proteolytic enzymes by maintaining a 200 um thick coat of mucous covering the epithelial cells. - Describe cephalic and gastric control of acid secretion.
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The cephalic response is the response that occurs at the sight or smell of food. Our stomachs similarly respond with the release of acid from the parietal glands.
With food in the stomach, the stomach becomes distended, which causes the secretion of acid by the parietal cells and pepsinogen by the chief cells. One signal that increases acid secretion is a drop in the pH of the stomach contents when food arrives and buffers some of the acid. In response, the parietal cells secrete acid and the chief cells secrete pepsinogen. The conversion of pepsinogen to pepsin results from effects of both low pH and the effects of pepsin in the stomach lumen. Pepsin converts pepsinogen to pepsin by cleaving a protein from the pepsinogen molecule to convert it to an active enzyme. - Describe the flow of blood through the liver. What are sinusoids?
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Blood flowing through the vessels of the submucosa is delivered to the liver through the hepatic portal system. The capillaries in the villi empty into a venous system that delivers blood rich in amino acids and monosaccharides to the liver. In addition to the blood from the intestines, the liver receives oxygen-rich blood from the hepatic artery. The blood from the hepatic artery and from the hepatic portal system mixes in the liver.
The hepatic portal vein and the hepatic artery empty into a series of sinusoids, which are the capillaries of the liver that carry the blood to the central vein. The sinusoids are lined by fenestrated endothelial cells that form a porous barrier between the blood and the hepatocytes adjacent to the sinusoid.
Kupffer cells, which are specialized macrophages, are located in the sinusoids, and act to monitor blood from the intestine for bacteria and other pathogens. - What do hepatocytes do?
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The hepatocytes that comprise the liever are organized into liver lobules, each about two millimeters in diameter in the shap of a hexagon. The central vein, which drains the liver lobule, is located at the center of the hexagon, and a branch of the hepatic portal vein, a branch of the hepatic artery, and a branch of the bile duct are located at the corner of the side of the hexagon.
In addition to many other functions, the hepatocytes produce bile and secrete it into the caniculi, which are blind avenues that run parallel to the sinusoids and empty into the bile ducts at each corner of the hexagon lobule. As the bile is produced, it flows in the opposite direction of the blood in the sinusoids.
The hepatocytes are metabolically active. They remove glucose from the blood in the sinusoids, and convert it into glycogen for storage. They remove amino acids from the blood in the sinusoids, and make proteins, especially the plasma protein, albumin. They remove toxins from the blood in the sinusoids and detoxify them. They remove ammonia from the blood and convert it to urea, which is excreted in the kidneys. - What is bile? What are its major constituents? How does it function to emulsify fat in the small intestine?
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The digestion of fat begins with the emulsification of fats by bile acids that are produced in the liver. When chyme from the stomach arrives in the small intestine, it arrives as a bolus of fat that needs to be emulsified into smaller spheres.
Bile salts are constructed from a molecule of cholesterol or phosphytidyl choline and a molecule of glycine and taurine. These molecules are amphipathic and form an interface between the lipid and aqueous environment. The bile acids act like detergents to increase the surface area of the fat in the aqueous environment by breaking the fat into many small globules so that the fat is accessible to digestion by pancreatic lipase. Bile acids are synthesized in the liber and stored in the gall bladder. - What is the significance of the blood flow in the hepatic portal system?
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Blood flowing through vessels of the submucosa is delivered to the liver by the hepatic portal system. The capillaries in the villi empty into a venous system that delivers blood rich in amino acids and monosaccharides to the liver.
In addition to the blood from the intestines, the liver receives oxygen-rich blood from the hepatic artery. The blood from the hepatic cartery and the hepatic portal system mixes in the liver. - Where is bile stored? How is it released into the duodenum?
- The bile acids are synthesized in the liver and stored in the gall bladder before they are released into the duodenum. The release of bile is under the control of the hormone cholecystokinin (CCK). When fat and peptide-laden chyme arrives in the duodenum, secretory cells in the intestinal mucosa release CCK to signal the release of bile into the duodenum. The effect of the CCK is to cause muscle contraction in the gall bladder to empty the gall bladder into the duodenum.
- What are the two main cells of the endocrine pancreas? What does each secrete?
- There are two sets of scattered endocrine cells embedded in the pancreas: the beta cells and the alpha cells. The beta cells product insulin while the alpha cells product glucagon. Both hormones are important for the metabolism of glucose.
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If a wire in the lumen of the intestine were to penetrate through the wall into the abdominal cavity, in which order would the tissue layers of the intestine be penetrated?
a. mucosa, serosa, muscularis, submucosa
b. mucosa, submucosa, mu - b. mucosa, submucosa, muscularis, serosa
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What is a chylomicron?
a. a particle composed of several types of lipids that is synthesized in intestinal epithelial cells
b. a large ball of cholesterol that forms in the gall bladder and sometimes blocks the common bile duct
c. a - a. a particle composed of several types of lipids that is synthesized in intestinal epithelial cells
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Secretin is a hormone producted by ___ that stimulates the pancredas to secrete large quantities of ____.
a. pancreas, bicarbonate
b. liver, bile
c. intestinal endocrine cells, bicarbonate
d. intestinal endocrine cells, hydrochl - c. intestinal endocrine cells, bicarbonate
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Trypsin is an enzyme that "chops up" ____ and is secreted (as typsinogen) by the ____.
a. proteins, pancreas
b. proteins, liver
c. cellulose, salivary glands
d. lipoproteins, liver
e. glycogen, pancreas - a. proteins, pancreas
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Two things must be present in the lumen of the small intestine for efficient digestion and absorption of dietary fat: lipase and ____.
a. bile salts
b. albumin
c. trypsin
d. hydrochloric acid
e. bilirubin - a. bile salts
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All absorption of glucose and amino acids occurs in the:
a. stomach
b. large intestine
c. small intestine
d. small and large intestines
e. stomach and small intestines - c. small intestine
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An animal or person with pancreatic exocrine insufficiency (failure to secrete adequate quantities of pancreatic enzymes) would have difficulty digestion:
a. proteins only
b. starch and proteins, but not fats
c. fats and starch, but - e. fats, proteins and starch
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Accumlation of ____ in the spaces between intestinal epithelial cells is directly responsible for establishing the osmotic gradient that allows water absorption.
a. sodium ions
b. chylomicrons
c. potassium ions
d. calcium ions - a. sodium ions
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The hormone leptin is syntehsized and secreted prdominately from ____ and affects the ____ to suppress food intake.
a. fat, hypothalamus
b. liver, cerebral cortex
c. hypothalamus, liver
d. fat, adrenal gland
e. liver, hypot - a. fat, hypothalamus
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All mammals have at least some ability to digest dietary cellulose. What is the source of the cellulases that allow this process?
a. salivary ceullases
b. microbes living in the lumen of the digestive tract
c. cellulases are present - b. microbes living in the lumen of the digestive tract.
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Which of the following enzymes would you expect to be more activ ein an environment where the pH was 4.0 (quite acidic)? What is the substrate for this enzyme?
a. amylase, nuetral fat
b. trypsin, neutral fat
c. enterkinase, triglycer - e. pepsin, protein
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During the so-called "cephalic phase" of gastric activity, the stomach is mildly stimulated to secrete and undergo smooth muscle contractions. What is stimulating the stomach in this situation?
a. the liver
b. the brain
c. - b. the brain
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What is peristalsis?
a. a froce that allows water to diffuse across biologic membranes to balance solute concentrations
b. a metabolic pathway in which fatty acids are broken down to provide energy
c. a pattern of smooth muscle contr - c. a pattern of smooth muscle contractions that propel food down the digestive tube
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If you were describing the life of a mature absorptive epithelial cell in the small intestine, which of the following statements would be most accurate?
a. it resides in the crypts, absorbs nutrients for only a few hours, then dies and is repl - d. it resides on the tips of the villi, lives for only a few days, dies and is replaced by new absorptive cells coming from the crypts
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Lactose cannot be absorbed in the small intestine. Rather it must be hydrolized to form ____ and ____, both of which can be absorbed
a. frutose and galactose
b. fructose and maltose
c. glucose and galactose
d. glucose and sucros - c. glucose and galactose
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Predict the effect of receiving an injection of a large overdose of insulin.
a. body temperature would decrease 2-3 degrees
b. large amounts of glucose would be lost into urine
c. dramatically increased concentration of blood glucose - d. dramatically lowered concentration of blood glucose (hypoglycemia)
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In which major segment of the digestive tube would you find only stratified squamous epithelium?
a. esophagus
b. duodenum
c. ileum
d. colon
e. jejunum - a. esophagus
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Which of the following enzymes is most important in allowing you to digest and absorb a starch-rich food like a potato?
a. amylase
b. pepsin
c. bile salts
d. gastrin
e. trypsin - a. amylase
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How are amino acids absorbed from the lumen of the digestive tube across the plasma membrane and into absorptive epithelial cells?
a. they bind to a transporter protein i the membrane that also binds and transports sodium
b. they are solu - a. they bind to a transporter protein i the membrane that also binds and transports sodium
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The vascular system of the liver is different from that o falmost all other organs. Which of the following vascular situtations is seen in the liver?
a. much of the blood coming into the liver is venous blood
b. blood leaving the liver fl - a. much of the blood coming into the liver is venous blood
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Which of the following is a hormone produced by cells in the pancrease that among other things stimulates accelerated breakdown of hepatic glycogen stores?
a. insulin
b. epinephrine
c. gastrin
d. glucagon
e. somatostatin - d. glucagon
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Several very commonly prescribed drugs suppres acid production from the ____ cells in the stomach by blocking the receptor for ____.
a. parietal, histamine
b. chief, histamine
c. G cells, pepsin
d. chief, secretin
e. pariet - a. parietal, histamine
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In which of the following segments of the digestive system would you find villi and crypts?
a. esophagus
b. stomach
c. colon
d. jejunum
e. pancreas - d. jejunum
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Cortisol, a steroid homrone secreted from the ____ helps to keep blood glucose concentrations up by stimulating ____ in the liver.
a. pancreas, gluconeogenesis
b. adrenal gland, glycogenolysis
c. anterior pituitary, glycolosis
d - d. adrenal gland, gluconeogenesis
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A molecule of glucose is absorbed across the epithelium in the small intestine. What blood vessel will that molecule of glucose flow through before it reaches the sinusoids in the liver?
a. vena cava
b. hepatic artery
c. central vein - e. portal vein
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The major pancreatic and common bile ducts empty into ____.
a. colon
b. sinusoids
c. stomach
d. duodenum
e. ileum - d. duodenum
- You get a chance to watch an abdominal exploratory surgery and are examining the patient's gallbladder. The physician injects the patient with something and the gallbladder contracts noticeably expressing bile into the duodenum. Which of the following wa
- d. cholecystokinin