The Digestive System
What are villi and microvilli?
(villi) fingerlike projections from the epithelial lining of the SI (microvilli) hairlike projections on each cell of every villus
What is bile composed of?
-Bile salts (formed from cholesterol) -Bile pigments (bilirubin) -Phospholipids (lecitin) -Cholesterol -Water -Na+, K+, Ca2+, Cl-, and HCO3-
The stomach secretes what six products?
-HCl (kills microbes, denatures proteins, and converts pepsinogen into pepsin) -pepsinogen (cleaved in the stomach to pepsin, an enzyme that partially digests proteins) -mucus (protects mucosa) -Bicarbonate (protects mucosa) -water (dissolves and dilutes ingested material) -intrinsic factor (required for normal absorption of B12)
For each of the cell types below, list the major secretions of the cell and the functions of these secretions. Mucous cell Chief cell Parietal cell G-cell
-Mucous cell: mucus: protects lining of stomach, increases pH (bicarb) -Chief cell: pepsinogen: digests proteins, activated by H+ -Parietal cell: HCl, intrinsic factor: HCl decreases pH, kills microbes, denatures proteins, carries out some chemical digestion; intrinic factor: absorption of vitamin B12 -G-cell: gastrin: increases HCl production, increases gastric motility
(Pro)carboxypeptidases A and B
-produced in the acinar cells of the pancreas -functions in the duodenum where it hydrolyzes terminal peptide bond at carboxy end -activated by enteropeptidase
For each of the following substances, determine whether it is a digestive enzyme or a hormone and briefly summarize its functions. sucrase secretin dipeptidase CCK enteropeptidase
-sucrase: enzyme: brush-border enzyme; breaks down sucrose into monosaccharides -secretin: hormone: increases pancreatic secretions especially bicarb; reduces HCl decretion; decreases motility -dipeptidase: brush-border enzyme; breaks down dipeptides into free amino acids -CCK: hormone: recrutis secretion from gallbladder and pancreas; promotes satiety -enteropeptidase: enzyme: activates trypsinogen, which initiates an activation cascade
absorption of lipids in small intestine
1) absorbed into epithelial cells by diffusion (don't need transporters because lipids are nonpolar, easily move across cell membrane) 2) diffuse into intestinal capillaries
What are the three main energy sources?
1. carbohydrates (1st line of energy) 2. fats (2nd line of energy) 3. proteins (can be used but usually not...last resort)
Gastric glands have how many cell types? What are they?
3; mucous cells, chief cells, and parietal cells
How long is the small intestine?
6 meters
Maltase
A digestive enzyme that breaks maltose into glucose.
triacylglycerol
A lipid consisting of three fatty acids linked to one glycerol molecule; also called a fat or triglyceride.
stomach
A muscular and elastic sac that serves mainly to store food, break it up mechanically, and begin chemical digestion of proteins and fat.
Gallbladder
A muscular sac attached to the liver that secretes bile and stores it/concentrates it until needed for digestion
Esophagus
A muscular tube that connects the mouth to the stomach.
What is the esophagus?
A muscular tube that connects the mouth to the stomach.
Rectum
A short tube at the end of the large intestine where waste material is compressed into a solid form before being eliminated
Dissacharides
A sugar containing two monosaccharides Sucrose, Lactose, Maltose
Glycerol
A three-carbon alcohol to which fatty acids are covalently bonded to make fats and oils.
bile duct
A tube that carries bile from the liver and gallbladder to the intestine
Dipeptidases
A type of protease which hydrolyse the bond between the two amino acids of a dipeptide. Dipeptidases are membrane-bound, being part of the cell-surface membrane of the epithelial cells lining the ileum.
Fat soluble vitamins are
A, D, E, K
What are the general functions of the hormones involved in feeding?
ADH and aldosterone trigger sensation of thirst, encouraging fluid consumption. Glucagon, secreted by the pancreas, and ghrelin, secreted by the stomach and pancreas, stimulate feelings of hunger.. Leptin and cholecystokinin stimulate feelings of satiety.
Several hormones are involved with feeding behavior. Name them.
ADH, aldosterone, glucagon, ghrelin, leptin, and cholecystokinin
primary active transport
Active transport in which ATP is hydrolyzed, yielding the energy required to transport an ion or molecule against its concentration gradient.
Differentiate between aminopeptidase and dipeptidases.
Aminopeptidase is a peptidase secreted by glands in the duodenum that removes the N-terminal AA from a protein. Depeptidases cleave the peptide bonds of DIPEPTIDES to release free amino acids.
Glycogen
An extensively branched glucose storage polysaccharide found in the liver and muscle of animals; the animal equivalent of starch.
What is a zymogen?
An inactive enzyme precursor
What is found at the center of each villus?
At the middle of each villus there is a capillary bed for the absorption of water-soluble nutrients and a lacteral, a lymphatic channel tat takes up fats for transport into the lymphatic system
water soluble vitamins
B vitamins and vitamin C
What is the cecum?
Beginning of large intestine; an outpocketing that accepts fluid exiting the small intestine through the ileocecal valve and is the site of attachment of the appendix
What are pancreatic juices?
Bicarbonate-rich alkaline secretions containing many digestive enzymes that work on all three classes of biomolecules. It contains a variety of enzymes: pancreatic amylase, pancreastic peptidases (trypsinogen, cymotrypsinogen, and carboxypeptidases A and B), pancreatic lipase, and nucleases.
How do bile and pancreatic lipase work together to digest fats?
Bile accomplishes mechanical digestion of fats, emulsifying them and increasing their surface area. Pancreatic lipase accomplishes chemical digestion of fats, breaking their ester bonds.
Where is bile synthesized? Where is bile stored? Where does bile carry out its digestive function?
Bile is synthesized in the liver, stored in the gallbladder, and serves its function in the duodenum
What are chylomicrons?
Chylomicrons are tiny fatty droplets composed of triglycerides, small amounts of phospholipids, cholesterol, free fatty acids, and some protein.
Describe the pathophysiology of cirrhosis.
Cirrhosis is scarring of the liver, and this scar tissue builds up, creating increases resistance within the portal vein, resulting in portal hypertension. This causes a backup of fluid within the portal system, resulting in swollen veins in the digestive system, especially the esophagus, which may rupture and cause life-threatening bleeding. This often manifests as hematemesis, or vomiting of blood. Cirrhosis also causes bleeding disorders because of production of clotting factors is disrupted. The inability to properly dispose of ammonia results in increased ammonia in the blood, which affects mentation. Finally, cirrhosis may also cause hepatocellular carcinoma, or cancer of the hepatocytes.
The human digestive tract has specialized section with different functional roles. The most basic functional distinction is between digestion and absorption. Differentiate between the two.
Digestion involves the breakdown of food into its constituent organic molecules: starches and other carbohydrates into monosaccharies, lipids into free fatty acids and glycerol, and proteins into amino acids. Absorption involves the transport of products of digestion from the digestive tract into the circulatory system for distribution to the body's tissues and cells.
hepatic portal circulation
Drains nutrient-rich blood from digestive organs Delivers it to the liver for processing
The pancreatic peptidases (trypsinogen, chymotrypsinogen, and carboxypeptidase A and B) are released in their zymogen form, but once activated are responsible for protein digestion. What is the master switch for these enzymes?
Enteropeptidase (enterokinase) is the master switch. It converts trypsinogen into tryspin, which can activate the other zymogens, and also activates procarboxypeptidases A and B.
The oral cavity only plays a role in mechanical digestion. T or F?
False. It plays a role in both mechanical and chemical digestion of food.
Mechanical and chemical digestion take place in the esophagus. T or F?
False. No mechanical or chemical digestion takes place in the esophagus, except for the continued enzymatic activity initiated in the mouth by salivary enzymes. Thus, digestion that occurs prior to the entrance of the bolus into the stomach is minimal compared to the digestion that occurs in the stomach and small intestine.
A large amount of chemical digestion occurs in the mouth. T or F?
False. The amount of chemical digestion in the mouth is minimal because the food does not stay in the mouth for long.
The large intestine and small intestine do not contain many bacteria. T or F?
False. The large intestine -and even the small intestine-is home to many different species of bacteria. In fact, 30% of the dry matter in stool consists of bacteria. Most of these bacteria are anaerobes, but the cecum is also home to many aerobic bacteria.
The salivary glands are only innervated by the parasympathetic nervous system. T or F?
False. The parasympathetic nervous system is responsible for promoting salivation; however, the sympathetic nervous system has some input into the glands as well. The sympathetic nervous system increases the viscosity of saliva, which is why dry mouth and even a tacky sensation in the mouth occurs during a fight-or-flight response.
The brain and spinal cord completely regulate the enteric nervous system. T or F?
False. The system can function independently of the brain and spinal cord, although it is heavily regulated by the autonomic nervous system.
Differentiate how water-soluble vitamins and fat-soluble vitamins are absorbed in the small intestine.
Fat-soluble vitamins dissolve directly into chylomicrons to enter the lymphatic circulation; therefore, failure to digest and absorb fat properly, which can be due to pathologies in the liver, gallbladder, pancreas, or small intestine, may lead to deficiencies in of fat-soluble vitamins. The water-soluble vitamins are taken up, along with water, amino acids, and carbohydrates, across the endothelial cells of the small intestine, passing directly into the plasma.
What happens in gastroesophageal reflux disease?
Food and acid reflux into the lower esophagus due to the weakened LES. The acid eventually erodes the mucosa so there is less protection. As food and acid continue to reflux, they irritate the less-protected mucosa, stimulating pain receptors. These receptors localize pair poorly, so a general burning sensation is felt in the lower chest-leading to the common term heartburn
emesis (vomiting)
Forcible emptying of gastric, and occasionally, intestinal contents
secondary active transport
Form of active transport which does not use ATP as an energy source; rather, transport is coupled to ion diffusion down a concentration gradient established by primary active transport.
What does the stomach use to digest food?
HCl and enzymes, creating a fairly harsh environment
Parietal cells secrete
HCl and intrinsic factor
How are the muscles that control voiding similar to the muscles that control feces?
In both cases, there is an internal sphincter under autonomic control (internal urethral sphincter and internal anal sphincter) and an external sphincter under somatic control (external urethral sphincter and external anal sphincter)
What is the anus composed of?
Internal and external muscular sphincters
Peristalsis
Involuntary waves of muscle contraction that keep food moving along in one direction through the digestive system.
What is gastric juice?
It a combination of secretion from the chief cells and parietal cells of the gastric glands
Why is the hepatic portal system important?
It allows the nutrient-rich blood to be processed by the liver before draining into the inferior vena cava. For example, the liver takes up excess sugar to create glycogen, the storage form of glucose, and stores fats as triacylglycerols. The liver can also reverse these processes, producing glucose for the rest of the body glycogenolysis and gluconeogenesis and mobilizing fats in lipoproteins. The liver can also then detoxify endogenous compounds (those made in the body) and exogenous compounds (those brought in from the environment). For example, the liver converts ammonia, a toxic waste product of amino acid metabolism, into urea, which can be excreted by the kidneys. The liver also detoxifies and metabolizes alcohol and medications. Some drugs actually require activation by the enzymes of the liver. In addition, some drugs cannot be taken orally because modification of these drugs by the liver renders them inactive.
How does a dipeptidase release free amino acids.
It breaks the bond of dipeptides, which only have two amino acids. If you break the bond between the only two amino acids, they become free
What happens if we don't have a particular disaccharidase in the brush border?
It causes an inability to break down the corresponding disaccharide. Then bacteria in the intestines are able to hydrolyze that disaccharide, producing methane gas as a byproduct. The undigested disaccharides can also have an osmotic effect, pulling water into the stool and causing diarrhea. This is why people who are lactose intolerant have symptoms of bloating, flatulence, and possibly diarrhea after ingesting dairy products.
Describe the size of the large intestine relative to the small intestine.
It has a larger diameter but a shorter length
why is mastication an important part of digestion?
It helps to increase the surface area-to-volume ratio of the food, creating more surface for enzymatic digestion as it passes through the gut tube. It also moderates the size of food particles entering the lumen of the alimentary canal; food particles too large create an obstruction risk in the tract.
What is the epiglottis and what does it do?
It is a cartilaginous structure that folds down to cover the laryngeal inlet during swallowing to prevent food from entering the larynx.
What is the enteric nervous system? What does it do?
It is a collection of one hundred million neurons that govern the function of the GI system. These neurons are present in the walls of the digestive tract and trigger peristalsis, or rhythmic contractions of the gut tube, in order to move materials through the system.
What occurs in celiac disease?
It is a disease that results from an immune reaction against gluten, a protein found in grains, especially wheat. In this condition, the immune system develops antibodies against certain components of gluten. These antibodies then cross-react with elements of the small intestine, causing damage to the mucosa. This results in diarrhea and discomfort. Sometimes, this condition also results in malabsorptive syndromes, including the inability to absorb fat and fat-soluble vitamins. Contrary to popular belief, celiac disease and gluten sensitivity are immune conditions, but not true allergies.
What is secretin and what does it do?
It is a hormone that is released when *chyme arrives in the duodenum and is too acidic.* It secretes bicarbonate. It is a peptide hormone that causes pancreatic enzymes to be released into the duodenum. It also regulates the pH of the digestive tract by reducing HCl secretion from parietal cells and increasing bicarbonate secretion from the pancreas. Secretin is also an enterogastrone, a hormone that slows motility through the digestive tract.. Slowing of motility allows increased time for digestive enzymes to act on chyme-especially fats.
What is intracellular digestion?
It is a part of metabolism that involves the oxidation of glucose and fatty acids for energy. Our diets do not consist of pure glucose and fatty acids; rather, these substances must be extracted from our food
Explain what occurs in Zollinger-Ellison syndrome.
It is a rare disease resulting from a gastrin-secreting tumor (gastrinoma). Typically, this tumor is found in the pancreas. The excess gastrin leads to excessive HCl production by parietal cells. One of the most common signs of the syndrome is the presence of intractable ulcer disease.
What is the function of enteropeptidase (enterokinase)?
It is enzyme critical for the activation of trypsinogen, a pancreatic protease, to trypsin, which then initiates an activation cascade. It can also activate procarboxypeptidases A and B to their active forms.
Where is the chemical digestion of carbohydrates initiated and where does it end?
It is initiated in the mouth with salivary amylase and is completed in the SI with pancreatic amylase.
The sympathetic division of the enteric nervous system is involved in what?
It is involved in inhibition of activities caused by parasympathetic stimulation. The fact that so often we feel sleepy and lethargic after eating a big meal is due, in part, to parasympathetic activity. ON the other hand, during periods of high sympathetic activity, blood flow is decreased to the digestive tract, and gut motility slows significantly.
The parasympathetic division of the enteric nervous system is involved in what?
It is involved in stimulation of digestive activities, increasing secretions from exocrine glands and promoting peristalsis.
What is pancreatitis usually caused by?
It is usually caused by gallstones or excessive consumption of alcohol. Regardless of the cause, pancreatitis results from premature activation of pancreatic enzymes and autodigestion of the pancreatic tissue. This is a very painful condition that may result in a long hospital stay and long-term consequences such as diabetes and the reduced digestion of proteins and fats.
Why is the acidic environment of the stomach important?
It kills most harmful bacteria (with the exception of Helicobacter pylori, infection with which is usually asymptomatic but can cause inflammation, ulcers, and certain gastric cancers). The acidic environment also helps to denature proteins and can break down some intramolecular bonds that hold food together
Where does the alimentary canal start and end?
It runs from the mouth to the anus and is sectioned off by sphincters (circular smooth muscles around the canal that contract to allow compartmentalization of function).
What is the function of the digestive system?
It takes complex foods-composed of polysaccharides, fats, and proteins-and turns these large marcomolecules into smaller, simpler monosaccharides, fatty acids, and amino acids. These simpler substances are then absorbed from the gut, transported to the tissues by the circulatory system, and used by cells.
What is jaundice wand why does it occur?
Jaundice is yellowing of the skin or sclera. So, bile is composed of bile salts, pigments, and cholesterol. Bile salts are amphipathic molecules that can emulsify fat in the digestive system. The major pigment in bile is bilirubin, which is a byproduct of the breakdown of hemoglobin. Bilirubin travels to the liver, where is conjugated (attached to a protein) and secreted into the bile for excretion. if the liver is unable to process or excrete bilirubin (due to liver damage, excessive RBC destruction, or blockage of the bile ducts) jaundice may occur.
Where is the stomach located?
LUP of the abdominal cavity, underneath the diaphragm
Explain how the mouth plays a role in mechanical digestion.
Mechanical digestion in the mouth involves the breaking up of large food particles into smaller particles using the teeth, tongue, and lips. This process is called mastication (chewing).
Differentiate between mechanical digestion and chemical digestion.
Mechanical digestion is the physical breakdown of large food particles into smaller food particles, but DOES NOT INVOLVE BREAKING CHEMICAL BONDS. Chemical digestion is the enzymatic cleavage of chemical bonds, such as the peptide bonds of proteins or the glycosidic bonds of starches.
What is the difference between mechanical and chemical digestion?
Mechanical digestion, such as chewing, physically breaks down food into smaller pieces. Chemical digestion involves hydrolysis of bonds and breakdown of food into smaller biomolecules
What are micelles?
Micelles are tiny droplets created when the hydrophilic heads of a set of lipids react with the water and form hydrogen bonds and the hydrophobic tails interact with themselves
facilitated diffusion
Movement of specific molecules across cell membranes through protein channels
How can NSAIDs potentially affect the digestive system?
One side effect is the disruption of mucus production within the stomach. This leaves the mucosa of the stomach unprotected, and irritation results. This irritation can be in the form of gastritis, inflammation of the mucosa of the stomach, or gastric ulcers, deep wounds in the lining of the stomach.
CCK also promotes the secretion of pancreatic juices into the duodenum. What are pancreatic juices and why are they important?
Pancreatic juices are a complex mix of several enzymes in a bicarbonate-rick alkaline solution. The bicarbonate helps to neutralize acidic chyme, as well as provide an ideal working environment for the digestive enzymes which are most active around pH 8.5. Pancreatic juices contain enzymes that can digest all three types of nutrients.
Chyme
Partially digested, semiliquid food mixed with digestive enzymes and acids in the stomach.
Aminopeptidase
Restriction enzyme that cuts to the right of amino terminal
lower esophageal sphincter (LES)
Ring of muscles between the esophagus and stomach; cardiac sphincter
What two main enzymes are found in saliva? What do these enzymes do?
Saliva contains salivary amylase (ptyalin), which digests starch into smaller sugars (maltose and dextrin); and lipase, which digests fats
What enzymes are found in saliva and what are their functions?
Saliva contains salivary amylase and lipase. Salivary amylase hydrolyzes starch into smaller sugars (maltose and dextrins), while lipase catalzyes the hydrolysis of lipids.
How are fats absorbed into the blood from the small intestine?
Short-chain fatty acids will follow the SAME PROCESS as carbohydrates and amino acids by diffusing directly into the intestinal capillaries. These fatty acids do not require transporters because they are nonpolar, so they can easily traverse the cellular membrane. Larger fats, glycerol, and cholesterol move separately into the intestinal cell but then reform into triglycerides. The triglycerides and esterified cholesterol molecules are packaged into chylomicrons. Rather than entering the bloodstream, chylomicrons enter the lymphatic circulation through lacteals, small vessels that form the beginning of the lymphatic system.. These lacteals converge and enter the venous circulation at the thoracic duct in the base of the neck, which empties into the left subclavian vein.
What is the appendix? What is its function?
Small tail-like projection that is connected to the cecum; it was once thought to be vesitgial. Recent evidence, however, suggests that it may have a role in warding off certain bacterial infections and repopulating the large intestine with normal flora after episodes of diarrhea. Inflammation of the appendix (appendicits) is a surgical emergency; in fact, it is the most common reason for an unscheduled surgery in the US.
Why is the gallbladder a common site of cholesterol or bilirubin stone formation?
Stone formation is nothing more than precipitation of a solid once its concentration has reached the K(sp) of the compound. This is common in the gallbladder because bile is concentrated there. This painful condition causes inflammation of the gallbladder. The stones may also travel into the bile ducts and get stuck in the biliary tree. In some cases, stones can get caught just before entering the duodenum, resulting blockage of not only the biliary tree, but the pancreatic duct as well, causing pancreatitis.
Where is swallowing initiated?
Swallowing is initiated in the muscles of the oropharynx, which constitute the upper esophageal sphincter. Peristalsis squeezes, pushes, and propels the bolus toward the stomach. As the bolus approaches the stomach, a muscular ring known as the LES relaxes and opens to allow the passage of food.
Why is the presence of bacteria in the colon a symbiotic relationship?
The bacteria are provided with a steady source of food, and the byproducts produced by the bacteria are beneficial to humans. For example, bacteria in the gut produce vitamin K, which is essential for the production of clotting factors, and biotin (vitamin B7), which is a coenzyme for many metabolic enzymes.
Beginning with the mouth, where does food travel?
The digestive tract begins with the oral cavity (mouth) followed by the pharynx, a shared pathway for both food entering the digestive system and air entering the respiratory system. From the pharynx, food enters the esophagus, which transports it to the stomach. From the stomach, food travels to the large intestine, and then to the large intestine. Finally, waste products of digestion enter the rectum, where feces are stored until an appropriate time of release.
Describe the different functions performed by the three segments of the small intestine?
The duodenum is responsible for the majority of chemical digestion and has some minor involvement in absorption; however, most of the absorption in the small intestine takes place in the jejunum and ileum.
What is the main function of the colon?
The extraction of water and salts from feces; the small intestine actually absorbs much more water than the colon, so the colon primarily concentrates the remaining material to form feces. Too little or too much water absorption cause diarrhea or constipation
Gluconeogenesis
The formation of glucose from noncarbohydrate sources, such as amino acids.
large intestine
The last section of the digestive system, where water is absorbed from food and the remaining material is eliminated from the body
What is the thoracic duct?
The main channel through which lymph from the majority of the body is returned to the venous system.
What is the anus?
The opening for defecation (expelling of stool).
What effect does the parasympathetic nervous system have on the digestive system? What effect does the sympathetic nervous system have?
The parasympathetic nervous system increases secretions from all of the glands of the digestive system and promotes peristalsis. The sympathetic nervous system slows peristalsis
small intestine
The part of the digestive system in which most chemical digestion takes place (duodenum, jejunum, ileum)
Once chyme enters the duodenum, what happens?
The presence of chyme in the duodenum causes the release of brush-border enzymes like disaccharidases (maltase, isomaltase, lactase, and sucrase) and peptidases (including dipeptidase). These brush border enzymres are present on the luminal surface of cells lining the duodenum and break down dimers and trimers of biomolecules into absorbable monomers. The duodenum also secretes enteropeptidase, which is involved in the activation of other digestive enzymes from the accesory organs of digestion. Finally, it secretes hormones like secretin and cholecystokinin into the blood stream.
What triggers salivation?
The presence of food in the oral cavity triggers a neural circuit that ultimately leads to increased parasympathetic stimulation of these glands.It can also be triggered by signals that food is near, such as smell or sight
What is extracellular digestion?
The process by which the nutrients are obtained from food occurs within the lumen of the alimentary canal is known as extracellular digestion. This is technically "outside" the body, because the lumen of the GI tract communicates directly with the outside of the word.
What is the rectum?
The rectum is where the feces collect at the end of the large intestine; it consists of indigestible material, water, bacteria (E. coli and others) and certain digestive secretions that are not reabsorbed (enzymes and some bile)
Describe the correlation between pH and enzyme activity.
The stomach has an acidic environment. The duodenum has a basic environment. Pepsin is active in the stomach and works best at pH 2. Pancreatic enzymes are active in the duodenum and work best at pH 8.5.
Why is the mucosa of the stomach quite thick?
The stomach used HCl and enzymes to digest food, creating a fairly harsh environment. Therefore, its mucosa is quite thick to prevent autodigestion.
endothelial cells
The thin layer of cells that line the interior surface of all blood vessels. Only one cell thick in capillaries.
The top third of the esophagus is composed of _______, the bottom third is composed of __________, and the middle third is a mix of both. What does this mean terms of nervous control?
The top third is composed of skeletal muscle. The bottom third is composed of smooth muscle. This mean the top of the esophagus is under somatic (voluntary) motor control, the bottom and most of the rest of the GI tract is under autonomic (involuntary) control
What are the two circulatory vessels in a villus? What biomolecules are absorbed into each?
The two circulatory vessels are capillaries and lacteals. The capillary absorbs water-soluble nutrients like monosaccharides, amino acids, short-chain fatty acids,
fundus of stomach
The upper left portion of the stomach, which has a curved, dome-shaped appearance.
Stomach: absorption organ or digestion organ?
There are few substances that are absorbed directly from the stomach (such as alcohol and aspirin), but the stomach is mainly an organ of digestion
How are simple sugars, such as glucose, fructose, and galactose, and amino acids absorbed into the blood from the small intestine?
They are absorbed secondary active transport and facilitated diffusion into the epithelial cells lining the small intestine. Then, these substances move across the epithelial cell membrane into the intestinal capillaries. Blood is constantly passing by the epithelial cells, carrying the carbohydrate and amino acid molecules away. This creates a concentration gradient such that the blood always has a lower concentration of monosaccharides and amino acids than inside the epithelial cells. Thus, simple carbohydrates and amino acids diffuse from the epithelial cells into the capillaries. The absorbed molecules then go to the liver via the hepatic portal circulation.
What are bile salts?
They are cholesterol derivatives that function in fat emulsification and absorption. They are not enzymes and therefore do not directly perform chemical digestion (the enzymatic cleavage of chemical bonds). However, bile salts serve an important role in the mechanical digestion of fats and ultimately facilitate the chemical digestion of lipids. Bile salts have hydrophobic and hydrophilic regions, allowing them to serve as a bridge between aqueous and lipid environments. In fact, bile salts are much like the common soaps and detergents we use to wash our hands, clothes , and dishes. in the small intestine, bile salts emulsify fats and cholesterol into micelles. Without bile, fats would spontaneously separate out of the aqueous mixture in the duodenum and would not be accessible to pancreatic lipase, which is water-soluble. In addition, these micelles increase the surface area of the fats, increasing the rate at which lipase can. Ultimately, proper fat digestion depends on both bile and lipase. Bile gets the fats into the solution and increases their surface area by placing them micelles (mechanical digestion). Then, lipase, can come into to hydrolyze the ester bonds holding the lipids together (chemical digestion).
How are pancreatic juices transferred to the duodenum?
They are transferred via a duct system that runs along the middle of the pancreas. Like all exocrine cells, the acinar cells secrete their products into ducts (specifically the pancreatic duct) which then empty into the duodenum
The pyloric glands contain which cells?
They contain G-cells that secrete gastrin, a peptide hormone. Gastrin induces the parietal cells in the stomach to secrete more HCl and signals the stomach to contract, mixing its contents. The digestion of solid food in the stomach results in an acidic, semifGastrinluid mixture known as chyme. The combined mechanical and chemical digestive activities of the stomach result in a significant increase in the surface area of food particles, so when the chyme reaches the small intestine, the absorption of nutrients from it can be maximized.
Describe the endocrine functions of the pancreas.
They include the release of insulin, glucagon, and somatostatin-peptide hormones necessary for the maintenance of proper blood sugar levels. The hormonal function is limited to cells in islets of Langerhans scattered throughout the organ
The gastric glands respond to signals from what?
They respond to signals from the vagus nerve of the parasympathetic nervous system, which is activated by the brain in response to the sight, taste, and smell of food
What is the function of parietal cells in the stomach?
They secrete HCl, which cleaves pepsinogen to pepsin. They also secrete intrinsic factor, a glycoprotein involved in the proper absorption of vitamin B12.
What do chief cells contribute to gastric juice?
They secrete pepsinogen. This is the inactive, zymogen form of pepsin, a proteolytic enzyme. Hydrogen ions in the stomach, secreted by parietal cells as HCl, cleave pepsinogen to pepsin. Pepsin digests proteins by cleaving peptide bonds near aromatic amino acids, resulting in short peptide fragments. Because pepsin is activated by the acidic environment, it follows that pepsin is most active at a low pH. This is a unique characteristic among human enzymes, as most human enzymes are most active at physiological pH.
What is the function of the tongue?
To move food in the mouth, swallowing and chewing. It also contains thousands of taste buds which are nerve-like cells that connects into the brain which transmits taste signals. It forms the food into a bolus, which is forced back to they pharynx and swallowed.
The pancreas has exocrine and endocrine functions. T or F?
True
What causes emesis?
Under normal circumstances, peristalsis proceeds down the digestive tract. However, certain factors such as exposure to chemicals, infectious agents, physical stimulation in the posterior pharynx, and even cognitive stimulation, can lead to reversal of peristalsis in the process of emesis.
Vibrio cholera causes a severe infection in the intestines, leading to massive volumes of watery diarrhea-up to 20 liters per day. Given these symptoms, does cholera likely impact the small intestine or the large intestine?
While the large intestine's main function is to absorb water, the small intestine actually absorbs a much larger volume of water. Thus, massive volumes of watery diarrhea are more likely to arise from infections in the small intestine than in the large intestine.
Is water absorbed in the small intestine? If so, how?
Yes, the small intestine also absorbs water. Much of the water in chyme is actually the result of secretions. The average person may consume up to two liters of fluid per day, but secretions into the upper gastrointestinal tract may total up to seven liters of fluid per day. In order to maintain proper fluid levels within the body, much of this fluid must reabsorbed by osmosis. As solutes are absorbed into the bloodstream, water is drawn with them, eventually reaching the capillaries. Water passes not only TRANSCELLULARLY (across the cell membrane), but also paracellularly (squeezing between cells) to reach the blood.
What is bile?
a bitter greenish-brown alkaline fluid that aids digestion by emulsifying fats and is secreted by the liver and stored in the gallbladder. It emulsifies large fat droplets when secreted into the small intestine via the bile duct.
intrinsic factor
a gastric secretion that combines with vitamin B12 so that the vitamin can be absorbed
Enterogastrone
a hormone secreted by the small intestine that decreases gastric secretions and motility
What is the ileocecal valve?
a sphincter that controls the flow of material from the ileum into the cecum
What is the capacity of the stomach?
about two liters
What is the main function of the large intestine?
absorb water
how do pancreatic juices get into duodenum?
acinar cells secrete products into ducts - empty into duodenum thru major and minor duodenal papillae
The bulk of the pancreas is made of exocrine cells called
acinar cells that produce pancreatic juices
The liver also synthesizes several plasma proteins, such as albumin and clotting factors. What are their functions?
albumin is a protein that maintains plasma oncotic pressure and serves as a carrier for many drugs and hormones. Clotting factors are proteins used during blood coagulation
Salivary amylase (ptyalin)
an enzyme in saliva that breaks the complex starch polysaccharide into maltose (a disaccharide)
Lactase
an enzyme that catalyzes the hydrolysis of lactose to glucose and galactose.
Sucrase
an enzyme that catalyzes the hydrolysis of sucrose into glucose and fructose
What are the divisions of the colon?
ascending, transverse, descending, sigmoid
Sam has a dysfunctional epiglottis. What is he at risk for?
aspiration of food and choking
What are the two ways in which the liver communicates with the digestive system?
bile ducts and hepatic portal vein. Bile ducts connect the liver with the gallbladder and small intestine. Bile is produced in the liver and travels down these bile ducts where it may be stored in the gallbladder or secreted into the duodenum. The liver also receives all blood draining from the abdominal portion of the digestive tract through the hepatic portal vein.
What are the major components of bile?
bile salts, which emulsify fats, pigments (especially bilirubin, from the breakdown of hemoglobin), and cholesterol
brush border enzymes
break down double sugars into simple sugars and complete protein digestion
What do peptidases do?
break down proteins
Glycogenolysis
breakdown of glycogen to glucose
What does pancreatic lipase do?
breaks down fats into free fatty acids and glycerol
Pancreatic amylase does what?
breaks down large polysaccharides into small disaccharides and is therefore responsible for carbohydrate digestion
What substances are secreted by the duodenum?
brush border enzymes (disaccharidases, peptidases), enteropeptidase, secretion, and CCK
disaccharidases
brush-border enzymes that break down maltose, isomaltose, lactose, and sucrose into monosaccharides
procarboxypeptidases A and B are converted to...
carboxypeptidase A and B by trypsin
What are the three divisions of the large intestine?
cecum, colon, and rectum
What are the three sections of the large intestine, in order?
cecum, colon, rectum
body of stomach
central portion of the stomach
What are some causes of liver cirrhosis?
chronic alcohol consumption, hep C, autoimmune hepatitis, and fatty liver disease
sphincter
circular ring of muscle that constricts a passage or closes a natural opening
Lipoproteins
clusters of lipids associated with proteins that serve as transport vehicles for lipids in the lymph and blood
lesser curvature of stomach
concave medial surface of the stomach; internal curvature of the stomach
pancreatic duct
conducts pancreatic juice from the pancreas to the small intestine
enteropeptidase (enterokinase)
converts trypsinogen to trypsin
greater curvature of stomach
convex lateral surface of the stomach
The duodenum is primarily involved in
digestion
Isomaltase
disaccharidase/ brush boarder enzyme digests isomaltose into two glucose molecules
Where does digestion end?
duodenum
What are the three sections of the small intestine, in order?
duodenum, jejunum, and ileum
What are the three segments of the small intestine?
duodenum, jejunum, ileum
Chemical digestion in the mouth relies on what?
enzymes from saliva produced by the the three pairs of salivary glands.
The chemical digestion of which nutrients begins in the mouth?
fats and carbohydrates
Where are gastric glands located?
fundus and body of stomach
What are the four main anatomical division of the stomach?
fundus, body, antrum, and pylorus. The fundus and body contain mostly gastric glands. The antrum and pylorus contain mostly pyloric glands.
The mucosa of the stomach contains...
gastric and pyloric glands
Gastrin
hormone secreted in the stomach that stimulates secretion of HCl and increases gastric motility
Where is the gallbladder located?
inferior surface of the liver
What is pancreatitis?
inflammation of the pancreas
To supply the body with nutrients, we must do what?
ingest food
rugae of stomach
interior folds of the stomach that allow it to expand when full
How does saliva aid mechanical digestion?
it moistens and lubricates food
Accesory organs of the digestive system
liver, gallbladder, pancreas
antrum of stomach
lower portion of the stomach
What are the two types of digestion?
mechanical and chemical
Digestions begins in the
mouth and continues in the stomach and the first part of the small intestine (duodenum)
pylorus of stomach
narrowed bottom part of stomach
What are the three parts of the pharynx?
nasopharynx (behind the nasal cavity), oropharynx (at the back of the mouth), and the laryngopharynx (above the vocal cords)
Trace the path of food through the body, starting with ingestion and ending with excretion of feces.
oral cavity -> pharynx -> esophagus -> SI -> LI -> rectum -> anus
The accessory organs of digestion originate as...
outgrowths of endoderm from the gut tube during development
Digestion is a complex process that requires the release of enzymes not only from the cells directly lining the alimentary canal, but also from what organs? What are these organs collectively called?
pancreas, liver, and gallbladder; they are called accessory organs of digestion
All of the glands in the body are innervated by the _________ except _________.
parasympathetic nervous system; sweat glands
What are the three salivary glands?
parotid, submandibular, sublingual
emulsification
physical process of breaking up large fat globules into smaller globules, thereby increasing the surface area that enzymes can use to digest the fat
Where are vitamins absorbed?
primarily in the small intestine
What are the functions of the liver?
processing and synthesis of nutrients (glycogenesis and glycogenolysis, storage and mobilization of fats, gluconeogenesis), production of urea, detoxification of chemicals, production of bile, synthesis of albumin, and synthesis of clotting factors
gastric glands of stomach
produce gastric juice, which may be composed of a combination of mucus, hydrochloric acid, intrinsic factor, pepsinogen, and a variety of hormones.
What is gluten?
protein found in certain grains such as wheat, barley, and rye
biliary tree
provides the channels through which bile is transported between the liver, gallbladder, and duodenum
Where are pyloric glands located?
pylorus and atrum of stomach
Upper esophageal sphincter (UES)
region of the upper part of the esophagus that opens to allow a mass of food to enter the esophagus
vestigial structures
remnant of a structure that may have had an important function in a species' ancestors, but has no clear function in the modern species.
pyloric sphincter
ring of muscle that guards the opening between the stomach and the duodenum
What is cirrhosis of the liver?
scarring of the liver and formation of fibrous tissue in the liver
external anal sphincter
skeletal muscle, voluntary
Absorption of nutrients primarily occurs where
small intestine, especailly jejunum and ileum
accessory pancreatic duct
smaller duct that empties directly into duodenum
internal anal sphincter
smooth muscle, involuntary
lacteals
specialized lymph vessels in the small intestine that absorb fat into the bloodstream
The jejunum and ileum are primarily involved in
the absorption of nutrients
What is the pharynx?
the cavity that leads from the mouth and posterior nasal cavity to the esophagus. It connects not only to the esophagus, but also to the larynx, which is a part of the respiratory tract
Food that leaves the stomach through the pyloric sphincter enters...
the duodenum
CCK is secreted in response to what? What is its function?
the entry of chyme (specifically amino acids and fat in the chyme) into the duodenum. This peptide hormone stimulates the release of both bile and pancreatic juices and also acts in the brain, where it promotes satiety.
Pharynx
the membrane-lined cavity behind the nose and mouth, connecting them to the esophagus.
The bile duct system merges with...
the pancreatic duct before emptying into the duodenum
What would happen if the G-cells did not secrete gastrin?
the parietal cells in the stomach would less HCl and the stomach not contract to mix its contents
In addition to the digestive tract, which organs help in the digestion of food and what do they do?
the salivary glands, pancreas, liver, and gallbladder help to provide the enzymes and lubrication necessary to aid in the digestion of food
alimentary canal
the whole passage along which food passes through the body from mouth to anus. It includes the esophagus, stomach, and intestines.
What is the function of mucous cells?
they produce the bicarbonate-rich mucus that protects the muscular wall from the harshly acidic (pH=2) and proteolytic environment of the stomach
When does the gallbladder contract and push bile into the biliary tree?
upon release of CCK
Where is the liver located?
upper right quadrant of the abdomen
What is a key feature of GERD?
weakness in the LES which causes classic heartburn symptoms after eating
