The Digestive System: Part C

The Digestive System: Part C
23 The Digestive System: Part C

Pancreas Location Mostly retroperitoneal, deep to the greater curvature of the stomach Head is encircled by the duodenum; tail abuts the spleen

Pancreas Endocrine function Exocrine function
Pancreatic islets secrete insulin and glucagon Exocrine function Acini (clusters of secretory cells) secrete pancreatic juice Zymogen granules of the secretory cells (Acinar) contain digestive enzymes

Small duct Acinar cells Basement membrane Zymogen granules Rough
endoplasmic reticulum (a) Figure 23.26a

Pancreatic Juice Watery alkaline solution (pH 8) neutralizes chyme and allows pancreatic secreted enzymes to work The epithelial cells lining the small pancreatic ducts secrete the electrolytes (primarily HCO3–) The bicarbonate is made in the epithelial cells – for every bicarbonate secreted a H+ is returned to the blood – thus the alkaline tide in the venous blood return from the stomach is balanced by the acidic venous blood from the pancreas

Pancreatic Juice Acinar cells produce the enzyme rich secretion
Enzymes Amylase, lipases, nucleases are secreted in active form but require ions or bile for optimal activity Proteases secreted in inactive form Protease activation in duodenum Trypsinogen is activated to trypsin by brush border enzyme enteropeptidase Procarboxypeptidase and chymotrypsinogen are activated by trypsin

Stomach Pancreas Epithelial cells Membrane-bound enteropeptidase
Trypsinogen (inactive) Chymotrypsinogen Procarboxypeptidase Trypsin Chymotrypsin Carboxypeptidase Figure 23.27

Regulation of Bile Secretion
Gallbladder contraction is stimulated by Cholecystokinin (CCK) from intestinal cells exposed to proteins and fat in chyme Vagal stimulation (minor stimulus) CKK also causes the hepatopancreatic sphincter to relax

Regulation of Bile Secretion
Bile secretion is stimulated by Bile salts in enterohepatic circulation – the more bile salts in the enterohepatic circulation the more bile is secreted. Secretin from intestinal cells exposed to HCl and fatty chyme

Regulation of Pancreatic Secretion
Bile and pancreatic secretions are regulated by the same factors (neural and hormonal) CCK induces the secretion of enzyme-rich pancreatic juice by acini Secretin causes secretion of bicarbonate-rich pancreatic juice by duct cells Vagal stimulation also causes release of pancreatic juice (minor stimulus)

Chyme entering duodenum causes release of cholecystokinin (CCK) and
Slide 1 1 Chyme entering duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. 4 Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. 5 CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum. 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 3 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes HCO3–-rich 6 During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder. Figure 23.28

1 Chyme entering duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. Figure 23.28, step 1

1 Chyme entering duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. Figure 23.28, step 2

1 Chyme entering duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 3 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes HCO3–-rich Figure 23.28, step 3

1 Chyme entering duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. 4 Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 3 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes HCO3–-rich Figure 23.28, step 4

1 Chyme entering duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. 4 Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. 5 CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum. 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 3 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes HCO3–-rich Figure 23.28, step 5

1 Chyme entering duodenum causes release of cholecystokinin (CCK) and secretin from duodenal enteroendocrine cells. 4 Bile salts and, to a lesser extent, secretin transported via bloodstream stimulate liver to produce bile more rapidly. 5 CCK (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum. 2 CCK (red dots) and secretin (yellow dots) enter the bloodstream. 3 CCK induces secretion of enzyme-rich pancreatic juice. Secretin causes HCO3–-rich 6 During cephalic and gastric phases, vagal nerve stimulation causes weak contractions of gallbladder. Figure 23.28, step 6

Digestion in the Small Intestine
Chyme from stomach contains Partially digested carbohydrates and proteins Undigested fats (minimally worked on by salivary lipase and gastric lipase)

Requirements for Digestion and Absorption in the Small Intestine
The intestine must get slow delivery of hypertonic acidic chyme from the stomach 3cc or less per peristaltic wave and 3 waves per minute so 9 cc or less per minute into small intestine from stomach If the hypertonic chyme was delivered to the small intestine too quickly it would pull in too much water from the bloodstream Additionally the chyme is quite acidic – thus ulcer formation if it enters the small intestine too fast

The small intestine only provides brush border enzymes – most digestive chemicals in the small intestine come from the liver and pancreas Delivery of bile, enzymes, and bicarbonate from the liver and pancreas Intestinal motility mixes chyme with pancreatic, bile and intestinal juices as a result of its segmentation waves The small intestine mainly uses segmentation waves – peristaltic waves begin after most of the materials have been absorbed

Motility of the Small Intestine
Segmentation Waves Make the intestinal contents appear as if they are being massaged- the chyme is moved back and forward in the lumen a few centimeters at a time by alternating contraction and relaxation of rings of smooth muscle. Initiated by intrinsic pacemaker cells located in circular muscles – but unlike stomach pacemaker cells which have only one rhythm – the pacemakers in the duodenum depolarize more frequently ( contractions per minute) than those in ileum ( 8 or 9 contractions per minute)

Mixes and moves contents slowly and steadily toward the ileocecal valve – giving plenty time to complete digestion and absorption The intensity of the waves is altered by long and short reflexes The segmentation waves wane in the late intestinal (fasting) phase after most of the small intestinal contents have been absorbed Once the segmentation waves wane the peristaltic waves begin and a result of secretion of motilin from the duodenal mucosae

Peristalsis Initiated by motilin in the late intestinal phase As the motilin blood level rises peristaltic waves are initiated in the proximal duodenum every 90 – 120 minutes and sweep slowly along the intestines – dying out in approximately 2 feet from its initiation area. The next wave starts distal to the previous wave thus termed the MMC ( migrating motility complex) A complete trip from duodenum to ileum takes approximately two hours The process repeats itself thus meal remnants, bacteria, sloughed off mucosal cells and debris are moved to the large intestine

This housekeeping function is critical for preventing the overgrowth of bacteria that migrate from the large intestine. As food enters the stomach with the next meal, peristalsis is replaced by segmentation

The local enteric neurons coordinate intestinal motility and it depends on which neurons are activated or inhibited Cholinergic sensory neurons may activate the myenteric plexus Causes contraction of the circular muscle proximally and of longitudinal muscle distally Forces chyme along the tract

Most of the time the ileocecal sphincter is closed. Two mechanisms open it The stomach initiates a gastroileal reflex – a long reflex that enhances the force of segmentation in the ileum Gastrin increases the motility of the ileum and relaxes the ileocecal valve Ileocecal valve flaps close when chyme exerts backward pressure

Microvilli Absorptive cell (b) Figure 23.3b

(a) Peristalsis: Adjacent segments of alimentary
From mouth (a) Peristalsis: Adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally. Figure 23.3a

Large Intestines (Colon)
Approximately twice the diameter of the small intestines – 3 inches wide Approximately 5 feet long Function (absorb most of the remaining water from indigestible food residues and temporarily store the residues before elimination as feces)

Functions of the Large Intestine
Vitamins, water, and electrolytes are reclaimed Major function is propulsion of feces toward the anus Colon is not essential for life

Large Intestine Unique features Teniae coli
Three bands of longitudinal smooth muscle in the muscularis Haustra Pocketlike sacs caused by the tone of the teniae coli Epiploic appendages Fat-filled pouches of visceral peritoneum

Large Intestine Regions Cecum (pouch with attached vermiform appendix)
Colon Rectum Anal canal

External anal sphincter (a)
Right colic (hepatic) flexure Left colic (splenic) flexure Transverse mesocolon Transverse colon Epiploic appendages Superior mesenteric artery Descending colon Haustrum Ascending colon Cut edge of mesentery IIeum Teniae coli IIeocecal valve Sigmoid colon Cecum Vermiform appendix Rectum Anal canal External anal sphincter (a) Figure 23.29a

Colon Ascending colon and descending colon are retroperitoneal Transverse colon and sigmoid colon are anchored via mesocolons (mesenteries)

Greater omentum Transverse colon Transverse mesocolon Descending colon
Jejunum Mesentery Sigmoid mesocolon Sigmoid colon Ileum (c) Figure 23.30c

Liver Lesser omentum Pancreas Stomach Transverse mesocolon Duodenum
Transverse colon Mesentery Greater omentum Jejunum Ileum Visceral peritoneum Parietal peritoneum Urinary bladder Rectum (d) Figure 23.30d

Rectum and Anus Rectum Anal canal Sphincters
Three rectal valves stop feces from being passed with gas Anal canal The last segment of the large intestine Sphincters Internal anal sphincter—smooth muscle External anal sphincter—skeletal muscle

Rectal valve Rectum Hemorrhoidal veins Levator ani muscle Anal canal
External anal sphincter Internal anal sphincter Anal columns Pectinate line Anal sinuses Anus (b) Figure 23.29b

Large Intestine: Microscopic Anatomy
Mucosa of simple columnar epithelium except in the anal canal (stratified squamous) Abundant deep crypts with goblet cells Extensive mucus eases passage of feces and protects the intestinal wall from irritating acids and gases released by resident bacteria in the colon Low folds give anal columns and anal sinuses are between the folds – the sinuses exude mucus when defecate

The horizontal tooth-shaped line that parallels the inferior margins of the anal sinuses is called the pectinate line. Superior to this line, the mucosa is innervated by visceral sensory fibers and is relatively insensitive to pain. The area inferior to this line is innervated by somatic sensory fibers – thus very sensitive to pain. Superficial venous plexuses of the anal canal form hemorrhoids if inflamed

Bacterial Flora 10 million different types
Enter from the small intestine or anus Metabolize some host products (mucin, heparin, and hyaluronic acid) Ferment some indigestible carbohydrates (cellulose, xylan, and others Release irritating acids and a mixture of gases (dimethyl sulfide, H2, N2, CH4, and CO2) Dimethyl sulfide is quite odorous About 500 ml of gas is produced each day Synthesize Vitamin B complex vitamins and Vitamin K

Most bacteria exist peacefully with their host in the large intestine – but an elegant system keeps them from breaching the mucosal barrier The epithelial cells of the gut mucosa respond to specific bacterial components by releasing chemicals that recruit immune cells, particularly dendritic cells into the mucosa. The dendritic cells pry open the tight junctions between the epithelial cells and send extensions into the lumen of sample the microbial antigens They then migrate to the nearby lymphoid follicles (MALT) where they present antigens to T cells. An IgA antibody response restricted to the gut lumen is triggered that prevents the bacteria from straying into tissues deep to the mucosa.

Motility of the Large Intestine
Haustral contractions (occur every 30 minutes or so) Slow segmenting movements that occur mainly in the transverse and descending colon Haustra sequentially contract in response to distension

Mass Movements Long slow-moving but powerful contractions that move over large areas of the colon- three to four times a day and force the contents towards the rectum. Typically, they occur during or just after eating, which indicates the presence of food in the stomach activates the gastrocolic reflex in the colon. Of the 500 cc of fluid entering the cecum only about 150cc becomes feces.

Motility of the Large Intestine
Gastrocolic reflex Initiated by presence of food in the stomach Activates three to four slow powerful peristaltic waves per day in the colon (mass movements)

Mass movements force feces into rectum
Defecation Mass movements force feces into rectum Distension initiates spinal defecation reflex Parasympathetic signals Stimulate contraction of the sigmoid colon and rectum Relax the internal anal sphincter Conscious control allows relaxation of external anal sphincter

Distension, or stretch, of the rectal walls due to movement
Impulses from cerebral cortex (conscious control) Distension, or stretch, of the rectal walls due to movement of feces into the rectum stimulates stretch receptors there. The receptors transmit signals along afferent fibers to spinal cord neurons. 1 Sensory nerve fibers Voluntary motor nerve to external anal sphincter A spinal reflex is initiated in which parasympathetic motor (efferent) fibers stimulate contraction of the rectal walls and relaxation of the internal anal sphincter. 2 Sigmoid colon Stretch receptors in wall Involuntary motor nerve (parasympathetic division) Rectum External anal sphincter (skeletal muscle) Internal anal sphincter (smooth muscle) If it is convenient to defecate, voluntary motor neurons are inhibited, allowing the external anal sphincter to relax so that feces may pass. 3 Figure 23.31

Chemical Digestion Catabolic Enzymatic Hydrolysis

Chemical Digestion and Absorption of Carbohydrates
Digestive enzymes Salivary amylase, pancreatic amylase, and brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase)

Bonding Carbohydrate monomers together
Monosaccharides bond together by the removal of a water molecule (dehydration synthesis) to form a covalent bond between the two monosaccharides known as a “glycosidic bond” When bond two monosaccharides together termed a disaccharide, when join 3 – 10 together termed an Oligosaccharide – more than 10 together – termed a polysaccharide

Sucrose – table sugar (glucose alpha 1,2 to fructose)
Common Disaccharides Sucrose – table sugar (glucose alpha 1,2 to fructose) Maltose – in beer (glucose alpha 1,4 to glucose) Lactose – in milk (galactose beta 1, 4 to glucose)

Amylose Alpha 1,4 Linkages (Linear)

Amylopectin Alpha 1,4 and Alpha 1,6 (Branching)

Alpha 1,4 and many Alpha 1,6

Humans Cannot Break the Beta Bond in Cellulose (Fiber) so it adds bulk to the diet

Digestive enzymes Salivary amylase, pancreatic amylase, and brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase) The amylases (salivary, pancreatic and brush border type) break alpha 1, 4 linkages The dextrinase brush border enzyme breaks alpha 1,6 linkages The lactase breaks the Beta 1,4 between glucose and galactose Maltase breaks the alpha 1,4 between glucose and glucose Sucrase breaks the alpha 1,2 bond between glucose and fructose

Secondary active transport (cotransport) with Na+ Facilitated diffusion of some monosaccharides Enter the capillary beds in the villi Transported to the liver via the hepatic portal vein

Small intestine Oligosaccharides and disaccharides Small intestine
Carbohydrate digestion Enzyme(s) and source Site of action Foodstuff Path of absorption • Glucose and galactose are absorbed via cotransport with sodium ions. Starch and disaccharides Salivary amylase Mouth • Fructose passes via facilitated diffusion. Pancreatic amylase Small intestine Oligosaccharides and disaccharides • All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Brush border enzymes in small intestine (dextrinase, glucoamylase, lactase, maltase, and sucrase) Small intestine Lactose Maltose Sucrose Galactose Glucose Fructose Figure (1 of 4)

Protein Digestion and Absorption
Dietary Proteins (125 grams a day) Enzyme Proteins in Gut (15 – 25 grams) Proteins in sloughed mucosal cells (15 – 25 grams)

R group still free

Chemical Digestion and Absorption of Proteins
Enzymes: pepsin in the stomach Pancreatic proteases Trypsin, chymotrypsin, and carboxypeptidase Brush border enzymes Aminopeptidases, carboxypeptidases, and dipeptidases Absorption of amino acids is coupled to active transport of Na+ and in the cases of Dipeptides and Tripeptide – coupled to H+

Peptidases (enzymes that break the peptide bond)
Exopeptidases – break the end amino acids off at the N- terminal (Aminopeptidase) or C- Terminal (Carboxypeptidase- from pancreas and brush border) Endopeptidases – break peptide bonds within protein Pepsin – from chief cells in stomach breaks peptide bonds between tyrosine and phenylalanine Trypsin (from pancreas) cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine, except when either is followed by proline Chymotrypsin (from pancreas) - preferentially cleaves peptide amide bonds where the carboxyl side of amide bond (the S1 position) is a tyrosine, tryptophan, or phenylalanine Dipeptidase – from brush border splits dipeptides

Amino acids of protein fragments Lumen of intestine
Brush border enzymes Apical membrane (microvilli) Pancreatic proteases Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxypeptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase) of mucosal cells. 1 Na+ Absorptive epithelial cell Na+ The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side (transcytosis). 2 Amino acid carrier The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts. 3 Active transport Capillary Passive transport Figure 23.33

• Amino acids are absorbed by cotransport with sodium ions. Protein
Protein digestion Enzyme(s) and source Site of action Foodstuff Path of absorption • Amino acids are absorbed by cotransport with sodium ions. Protein Pepsin (stomach glands) in presence of HCl Stomach • Some dipeptides and tripeptides are absorbed via cotransport with H+ and hydrolyzed to amino acids within the cells. Large polypeptides Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) Small intestine + Small polypeptides, small peptides • Amino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Brush border enzymes (aminopeptidase, carboxypeptidase, and dipeptidase) Small intestine Amino acids (some dipeptides and tripeptides) Figure (2 of 4)

Chemical Digestion and Absorption of Lipids
Pre-treatment—emulsification by bile salts Enzymes—pancreatic lipase Absorption of glycerol and short chain fatty acids Absorbed into the capillary blood in villi Transported via the hepatic portal vein

Chemical Digestion and Absorption of Lipids
Absorption of monoglycerides and fatty acids Cluster with bile salts and lecithin to form micelles Released by micelles to diffuse into epithelial cells Combine with proteins to form chylomicrons Enter lacteals and are transported to systemic circulation

Large fat globules are emulsified
(physically broken up into smaller fat droplets) by bile salts in the duodenum. 1 Bile salts Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa. 2 Fat droplets coated with bile salts Micelles made up of fatty acids, monoglycerides, and bile salts Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons. 3 Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph. 4 Epithelial cells of small intestine Lacteal Figure 23.34

Short Chain Fatty Acids
Passage of short chain fatty acids is quite different from what we have described. These fat breakdown products do no depend on the presence of bile salts or micelles, are not recombined to form triglycerides within the intestinal lumen cells, and simply diffuse into the portal blood for distribution.

• Fatty acids and monoglycerides enter the intestinal cells via
Fat digestion Enzyme(s) and source Site of action Foodstuff Path of absorption Unemulsified fats • Fatty acids and monoglycerides enter the intestinal cells via diffusion. Emulsification by the detergent action of bile salts ducted in from the liver Small intestine • Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells, and the resulting chylomicrons are extruded by exocytosis. Pancreatic lipases Small intestine • The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct. Monoglycerides and fatty acids Glycerol and fatty acids • Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein. Figure (3 of 4)

Chemical Digestion and Absorption of Nucleic Acids
Enzymes Pancreatic ribonuclease and deoxyribonuclease – break into nucleotides. Intestinal brush border enzymes (nucleosidases and phosphatases) Absorption Active transport Transported to liver via hepatic portal vein

Nitrogenous Bases

Nucleic acid digestion Enzyme(s) and source Site of action Foodstuff
Path of absorption Nucleic acids • Units enter intestinal cells by active transport via membrane carriers. Pancreatic ribonuclease and deoxyribonuclease Small intestine • Units are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein. Brush border enzymes (nucleosidases and phosphatases) Small intestine Pentose sugars, N-containing bases, phosphate ions Figure (4 of 4)

Vitamin Absorption In small intestine
Fat-soluble vitamins (A, D, E, and K) are carried by micelles and then diffuse into absorptive cells – Thus to get maximal absorption fat soluble vitamins – need to eat some fat containing food Water-soluble vitamins (vitamin C and B vitamins) are absorbed by diffusion or by passive or active transporters. Vitamin B12 binds with intrinsic factor, and is absorbed by endocytosis in the terminal ileum

Vitamin Absorption In large intestine Vitamin K and B vitamins from bacterial metabolism are absorbed

Electrolyte Absorption
Mostly along the length of small intestine Iron and calcium are absorbed mainly in duodenum Na+ is coupled with absorption of glucose and amino acids Ionic iron is stored in mucosal cells with ferritin K+ diffuses in response to osmotic gradients Ca2+ absorption is regulated by vitamin D and parathyroid hormone (PTH)

Na+/K+ pump in basal membrane of mucosal epithelial cells sets up gradient
Na+ helps monosaccharides get absorbed (glucose and galactose) and the amino acids The anions generally follow Na+ Chloride is actively transported out of the lumen – and particularly by a HCO3- exchange transporter in the terminal intestine K+ follows behind water – as follows leaves the intestinal lumen it creates a high concentration gradient for K+ - so K+ is then pulled by the osmotic gradient – thus if for some reason water is not passively absorbed properly – K+ is loss from the body and some is even pulled in the lumen from the interstitial space

Iron Absorption 1 Iron is brought into the cell through an active transport process involving the protein DMT-1 (divalent metal transporter-1), which is expressed on the apical surface of enterocytes in the initial part of the duodenum. DMT-1 is not specific to iron, and can transport other metal ions such as zinc, copper, cobalt, manganese, cadmium or lead.

Iron Absorption 2 Once inside the enterocyte, there are two fates for iron: (1) It may leave the enterocyte and enter the body via the basolateral transporter known as ferroportin. (2) It can be bound to ferritin, an intracellular iron-binding protein. For the most part, iron bound to ferritin in the enterocyte will remain there. This iron will be lost from the body when the enterocyte dies and is sloughed off from the tip of the villus.

Iron 3 Iron that enters the body from the basolateral surface of the enterocyte is rapidly bound to transferrin, an iron-binding protein of the blood. Transferrin delivers iron to red blood cell precursors, that take up iron bound to transferrin via receptor-mediated endocytosis. Normally, the capacity of transferrin to bind iron in the plasma greatly exceeds the amount of circulating iron. The transferrin saturation (percent of transferrin occupied by iron) is measured to determine if an individual has an excessive load of iron in the body. The normal transferrin saturation is in the range of 20-45%.

Water Absorption 95% is absorbed in the small intestine by osmosis Net osmosis occurs whenever a concentration gradient is established by active transport of solutes Water uptake is coupled with solute uptake

Malabsorption of Nutrients
Causes Anything that interferes with delivery of bile or pancreatic juice Damaged intestinal mucosa (e.g., bacterial infection)

Malabsorption of Nutrients
Gluten-sensitive enteropathy (celiac disease) Gluten damages the intestinal villi and brush border Treated by eliminating gluten from the diet (all grains but rice and corn)

Developmental Aspects
In the third week Endoderm has folded and foregut and hindgut have formed Midgut is open and continuous with the yolk sac Mouth and anal openings are nearly formed In the eighth week Accessory organs are budding from endoderm

Lung bud Stomodeum Brain Foregut Oral membrane Site of liver
Stomach Site of liver development Heart Bile duct Yolk sac Midgut Spinal cord Dorsal pancreatic bud Cloacal membrane Gall- bladder Hindgut Cystic duct Duodenum Body stalk Ventral pancreatic bud Proctodeum Endoderm (a) (b) Figure 23.35

Fetal nutrition is via the placenta, but the GI tract is stimulated to mature by amniotic fluid swallowed in utero The newborn’s rooting reflex helps the infant find the nipple; the sucking reflex aids in swallowing

During old age GI tract activity declines, absorption is less efficient, and peristalsis is slowed Diverticulosis, fecal incontinence, and cancer of the GI tract

Stomach and colon cancers rarely have early signs or symptoms
Metastasized colon cancers frequently cause secondary liver cancer Prevention Regular dental and medical examination

The Digestive System: Part C

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