digestive pt 2

Cecum

blind pouch, very first part of large intestine

Rectum

two sphincters

Absorption of Water

- 9 L water, most from GI tract secretions, enter small intestine > 95% absorbed in the small intestine by osmosis > Most of rest absorbed in large intestine - Net osmosis occurs if concentration gradient established by active transport of solutes - Water uptake coupled with solute uptake

Pancreatic Juice

1200-1500 ml/day Watery alkaline solution (pH 8) neutralizes chyme and provides optimal environment for activity of intestinal and pancreatic enzymes Electrolytes (primarily HCO3-) > bicarbonate ion z Enzymes Amylase, lipases, nucleases secreted in active form but require ions or bile for optimal activity Proteases (protein digesting enzymes) secreted in inactive form, then activated in the duodenum

Absorption of Nucleic Acids

Absorption Active transport across epithelium > into bloodstream

Absorption of Lipids

Absorption of monoglycerides and fatty acids - Cluster with bile salts and lecithin to form micelles > also contain cholesterol and fat soluble vitamins, more easily diffused between microvilli - Released by micelles to diffuse into epithelial cells - Combined with lecithin, phospholipids, cholesterol, & coated with proteins to form chylomicrons - Enter lacteals; transported to systemic circulation > enters the lymphatic stream for distribution in the lymph - Hydrolyzed to free fatty acids and glycerol by lipoprotein lipase of capillary endothelium > Cells can use for energy or stored fat in adipose tissue - completed by the ileum Absorption of short chain fatty acids Diffuse into portal blood for distribution

Digestion in the Small Intestine

Chyme from stomach contains partially digested carbohydrates and proteins, and undigested fats 3-6 hours in small intestine Most water absorbed ~ All nutrients absorbed Small intestine, like stomach, no role in ingestion or defecation

Absorption of Protein

Amino acids transported by several types of carriers - Most coupled to active transport of Na+ Dipeptides and tripeptides actively absorbed by H+-dependent cotransport; digested to amino acids within epithelial cells Enter capillary blood by diffusion

micelles

Bile salts wrap around these in the gut, helping them to become water soluble; helps absorption of lipids; contain cholesterol and fat soluble vitamins along w/lipids

Malabsorption of Nutrients

Causes: - Anything that interferes with delivery of bile or pancreatic juice - Damaged intestinal mucosa (e.g., bacterial infection; some antibiotics)

Valsalva's maneuver

Closing of glottis, contraction of diaphragm and abdominal wall muscles > increased intra-abdominal pressure Levator ani muscle contracts > anal canal lifted superiorly > feces leave body

Chemical Digestion

Digestion Catabolic; breakdown macromolecules into monomers small enough for absorption Enzymes Intrinsic and accessory gland enzymes break down food Hydrolysis Water is added to break bonds ENDS IN SMALL INTESTINE

Bacterial Flora

Enter from small intestine or anus; most are dead but not all of them are Colonize colon Synthesize B complex vitamins and vitamin K (that liver requires) Metabolize some host-derived molecules (mucin, heparin, hyaluronic acid) Ferment indigestible carbohydrates like cellulose Release irritating acids and gases (~500 ml/day)

Digestion of Nucleic Acids

Enzymes - Pancreatic ribonuclease and deoxyribonuclease break DNA and RNA into their nucleotide monomers - Brush border enzyme nucleosidases and phosphatases break down nucleotides into their free bases, pentose sugars, phosphate ions

Epiploic appendages

Fat-filled pouches of visceral peritoneum in large intestine

gastrocolic reflex

Food hits the stomach, increases motility in the colon and increases frequency of mass movements.

Absorption of Carbohydrates

Glucose and galactose from the breakdown of starch - Secondary active transport (cotransport) with Na+ > epithelial cells - Move out of epithelial cells by facilitated diffusion > capillary beds in villi Fructose - Facilitated diffusion to enter and exit cells

Gluten-sensitive enteropathy (celiac disease)

Immune reaction to gluten Gluten causes immune cell damage to intestinal villi and brush border Treated by eliminating gluten from diet (all grains but rice acorn)

Absorption of Vitamins

In small intestine - Fat-soluble vitamins (A, D, E, and K) carried by micelles; diffuse into absorptive cells - Water-soluble vitamins (vitamin C and B vitamins) absorbed by diffusion or by passive or active transporters. - Vitamin B12 (large, charged molecule) binds with intrinsic factor, and is absorbed by endocytosis In large intestine - Vitamin K and B vitamins from bacterial metabolism are absorbed

Segmentation

Most common motion of small intestine Initiated by intrinsic pacemaker cells Mixes/moves contents toward ileocecal valve takes around 2 hours, repeats itself for what is left of food and bacteria to all be swept into large intestine; to prevent bacteria of small intestine and to prevent movement of bacteria in large intestine to move backwards slower to allow time for proper absorption of nutrients to the body parasympathetic inc; sympathetic dec

pancreas

Location Mostly retroperitoneal, lies deep to greater curvature of stomach Head encircled by duodenum; tail abuts spleen insulin, glucagon, and digestive enzymes secretes bicarbonate ions to juice to balance acidity HCl in stomach; H+ released into blood

Defecation reflex

Mass movements force feces toward rectum Distension initiates spinal defecation reflex Parasympathetic signals - Stimulate contraction of sigmoid colon and rectum - Relax internal anal sphincter - Conscious control allows relaxation of external anal sphincter Muscles of rectum contract to expel feces Assisted by Valsalva's maneuver

Haustral contractions

Most contractions of colon Slow segmenting movements Haustra sequentially contract in response to distension slow segmenting movements lasting about one minute every 30 minutes. mix residue, which aids in water absorption.

Absorption of Electrolytes

Most ions actively along length of small intestine

Anal canal

Narrowing of rectum and opening to outside

Digestion of Carbohydrates

Only monosaccharides can be absorbed Monosaccharides absorbed as ingested - Glucose, fructose, galactose Digestive enzymes - Salivary amylase, pancreatic amylase, and brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase) - Break down disaccharides sucrose, lactose, maltose; polysaccharides glycogen and starch *starts in mouth w/salivary amylase and ends in the small intestine w/brush border enzymes

Haustra

Pocketlike sacs caused by tone of teniae coli in large intestine

mass movements

Powerful peristaltic contractions that occur a few times each day in the colon that forc feces towars the rectum are called

Digestive Processes in the Large Intestine

Residue remains in large intestine 12-24 hours No food breakdown except by enteric bacteria Vitamins (made by bacterial flora), water, and electrolytes (especially Na+ and Cl-) reclaimed Major functions - propulsion of feces to anus; defecation Colon not essential for life

Starch digestion

Salivary amylase (saliva) > oligosaccharides atpH 6.75-7.00 Pancreatic amylase (small intestine) > breaks down any that escaped salivary amylase > oligosaccharides Brush border enzymes (dextrinase, glucoamylase, lactase, maltase, sucrase) > oligosaccharides > monosaccharides

Digestion of Proteins

Source is dietary, digestive enzymes, mucosal cells; digested to amino acid monomers Begins with pepsin in stomach at pH 1.5-2.5 - Inactive in high pH of duodenum Pancreatic proteases - Trypsin, chymotrypsin, and carboxypeptidase - into peptides Brush border enzymes - Aminopeptidases, carboxypeptidases, and dipeptidases - into amino acid products *starts in the stomach with pepsinogen from chief cells is activated to pepsin > acidic conditions break proteins into polypeptides and small amounts of free amino acids > moves to small intestine > pancreatic and brush border enzymes further break them into smaller peptides and final amino acid products

hemorrhoids

Swollen, painful superficial venous plexuses of anal canal; often a result of constipation

Digestion of Lipids

The small intestine is essentially the only site of lipid digestion bc the pancreas is the only significant source of fat digesting enzymes Pre-treatment emulsification by bile salts; Does not break bonds, just weakens the bond between them (the fat molecules), allowing the fat molecules to be more widely dispersed and exposed to pancreatic enzymes takes large fat globule and pulls it apart to make small fat molecule Enzymes—pancreatic lipases > Fatty acids and monoglycerides small intestine is sole site of lipid digestion bc it is were lipases are found 1. Bile salts in the duodenum emulsify large fat globules (physically break them up into smaller fat droplets). 2. 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. 3. Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other fatty substances and proteins to form chylomicrons. 4. Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals and are carried away from the intestine in lymph.

Large Intestine

Thicker mucosa of simple columnar epithelium (suited for absorption) except in anal canal (stratified squamous to withstand abrasion) No circular folds, villi, digestive secretions bc absorption has been done Abundant deep crypts with goblet cells absorb most of the remaining water form the food, storing residues temporarily and then emptying them

Teniae coli

Three bands of longitudinal smooth muscle in muscularis (ribbons of the colon); found at the end of large intestine

ileocecal valve

a sphincter muscle joins ileum to the large intestines cecum

bile salts

absorption role of the ileum is to reclaim what?

pancreatic islets

endocrine: secrete insulin and glucagon in pancreas

peristalsis

esophagus and stomach uses what kind of movement?

Zymogen granules

exocrine: What are the cellular structures in the pancreas called that store proteolytic enzymes?

acini cells

exocrine: What cells of the pancreas produce the digestive enzymes/pancreatic juice?

actively; HCO3-

how is Cl- absorbed in the small intestine? what is exchanged?

facilitated diffusion/osmotic gradients

how is K+ absorbed in the small intestine?

sugars and amino acids move through epithelial cells unchanged, but lipids are are not water soluble

how is absorption of lipids different than absorption of sugars and amino acids?

active absorption of glucose and amino acids

how is na+ absorbed in the small intestine?

chylomicrons

lipoproteins formed in the cells lining the small intestine following absorption of fats. they are made in the small intestinal cells and transpost dietary lipids to the liver; Combined with lecithin, phospholipids, cholesterol, & coated with proteins w/lipids

1. Slow delivery of acidic, hypertonic chyme > to prevent low blood volume (water would be lost if large amounts of chyme were rushed into the small intestine) 2. Delivery of bile, digestive enzymes, and bicarbonate ions from liver and pancreas

main function of small intestine is absorption; but needs other things imported from pancreas or liver In order for food to be digested and absorbed in the small intestine there must be:

Appendix

masses of lymphoid tissue, plays a role in immunity, twisted, location for bacteria to grow and multi if it is too twisted, becomes appendicitis

the duodenum

where are Iron and calcium are absorbed?

Absorption

~ All food; 80% electrolytes; most water absorbed in small intestine - Most prior to ileum - Ileum reclaims bile salts Most absorbed by active transport > blood - Exception - lipids thru passive transport via lymphatic fluid