Digestive System 3

• Spongy gland posterior to greater curvature of stomach
• Head encircled by duodenum, tail on far left, body in between
• Endocrine and exocrine gland: Insulin/glucagon and pancreatic juice

• Alkaline mixture of water, enzymes, enzyme precursors, electrolytes
• Bicarbonate buffers stomach acid
• Flows down pancreatic duct and gets into duodenum with bile through hepatopancreatic sphincter
• Also gets into duodenum through accessory pancreatic duct (bypass sphincter)

Digests Starch

Digests Fat

Break down DNA and RNA

• Break down proteins
• Secreted as enzyme precursors to avoid breaking down acinar cells themselves

• released from parasympathetic nervous system during cephalic phase
• Stimulates acini to secrete enzymes

• Released from duodenum when fatty chyme is detected
• Stimulates acini to secrete enzymes and relaxes hepatopancreatic sphincter

• Released from duodenum when acidic chyme arrives
• Stimulates liver and pancreas to secrete more sodium bicarbonate to raise pH

• Site of almost all chemical digestion and nutrient absorption
• Longest part of the digestive tract (2.4-4.5 m long)
• Smaller diameter than large intestine

• Begins at pyloric sphincter
• Forms arch
• Ends at duodenojejunal flexure
• Receives chyme, pancreatic juice, bile
• Site of stomach acid neutralization
• Fats emulsified by bile acids
• Pancreatic enzymes continue chemical digestion

• Begins at duodenojejunal flexure
• 40% of the post-duodenal small intestine
• Rich blood supply
• Site of most digestion and nutrient absorption

• largest part of the SI
• Thin walls, not as much blood supply
• Ends at ileocecal junction
• Ileocecal valve regulates entry of food residue into large intestine

Surface area maximized for effective absorption

• Large folds in intestinal wall
• Occur from duodenum to middle of ileum
• Cause chyme to flow in spiral path and have more contact with mucosa

• Fingerlike projections
• Covered by absorption and goblet (mucous cells)
• Capillaries in villi absorb most of the nutrients

• Projections on each absorptive cell (villi on villi)
• Increase absorptive surface area
• Brush border enzymes in plasma membrane carry out final stages of digestion

• Pores that open into tubular glands between villi
• Secrete 1-2 L of intestinal juice per day
• Secrete it in response to acid, chyme, and intestinal distension
• Contains water, mucus, and a little bit of enzymes

• 3 Functions of intestinal contractions
• Mix chyme with intestinal juice, bile, and pancreatic juice
• Churn chyme and bring it into contact with mucosa for nutrient absorption
• Move residue towards large intestine

• Parts of SI walls contract to form segments
• Purpose is for mixing and churning
• Pacemaker cells in muscularis externa control segmentation

Wave-like contractions that moves food residue toward large intestine

Carbohydrates, proteins, lipids, vitamins, minerals

• Begins in mouth, salivary amylase breaks some starch down into oligosaccharides
• Pancreatic amylase breaks remaining starch down in small intestine
• Brush border enzymes convert oligosaccharides into glucose

• Plasma membrane of absorptive cells has transport proteins to absorb monosaccharides
• 80% of absorbed sugar is glucose
• Monosaccharides absorbed by capillaries in the villus, then go to liver via hepatic portal vein

• Many adults lack lactose enzyme in the brush border
• Lactose passes into large intestine undigested
• Increases osmolarity of intestinal contents, which leads to water retention and diarrhea
• Bacteria in colon digest lactose via fermentation and produce gas
• Yogurt and cheese have bacteria that break down lactose

• Pepsin begins protein breakdown in stomach
• Other enzymes continue digestion in the SI
• Brush border enzymes continue digestion to form amino acids

Very similar to that of carbohydrates

• Lingual lipase from salivary glands begins lipid digestion (actually occurs in stomach)
• Gastric lipase continues lipid digestion in stomach
• Fat enters duodenum as large globules
• Emulsified by bile
• Broken up by segmentation
• Pancreatic lipase continues to digest lipids in small intestine

• Absorption of lipids depends on micelles in bile
• In the duodenum micelles absorb fat soluble vitamins, cholesterol, free fatty acids, monoglycerides
• Micelles transport lipids to surface of absorptive cells
• Lipids released and diffuse through plasma membrane
• Micelles recycled
• Free fatty acids and monoglycerides synthesized into triglycerides, packaged into vesicles, secreted into lymph (dumped into bloodstream)

• Vitamins not digested (absorbed as-is)
• Fat-soluble vitamins absorbed with other lipids
• Vitamins A, D, E, K
• Not absorbed at all if they are not ingested with fat
• Wat-soluble vitamins absorbed by simple diffusion in SI
• B Complex, C
• B12 absorbed only if intrinsic factor is available

• Absorbed along small intestine
• Na+ and Cl- absorbed constantly
• Iron absorbed as needed
• Stimulated by liver hormone
• Calcium absorbed as needed
• Parathyroid hormone stimulates vitamin D synthesis by liver
• Vitamin D increases ability of duodenum to absorb calcium

• Digestive tract receives about 9 L of water/day (most from GI secretions)
• 8 L absorbed by SI, .8 L absorbed by LI, .2 L voided in feces
• Diarrhea occurs when LI doesn't absorb enough water
• Occurs if feces passes too quickly because of irritated intestine
• Occurs if feces contains high solute concentration like lactose
• Constipation occurs when feces moves too slowly, too much water is absorbed, and feces hardens

Absorbs water and salts out of feces before elimination

Regions: Cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal

Longitudinal fibers of muscularis externa

Pouches in the colon caused by taenia coli

• No circular folds of villi
• Intestinal crypts: glands in lamina propia
• Lots of goblet cells secreting mucus

• About 800 kinds of bacteria in large intestine
• Ferment cellulose and other undigested carbs
• Produces 7-10 L gas/day, most reabsorbed and about 500 ml/day eliminated as flatus
• Produce some sulfur compounds that produce the distinctive smell of feces
• Help synthesize vitamins B and K

• Large intestine takes 12-24 hours to reduce food residue to feces
• Reabsorbs water and electrolytes
• Feces consists mostly of water, but also some bacteria, undigested fiber, fat, mucus, dead epithelial cells

• Haustral contractions (equivalent of segmentation) occurs every 30 minTriggered by distention when food enters
• Mass movements (equivalent of peristalsis) occurs 1-3 times/day
• Triggered when food enters stomach and duodenum

• Extremely prevalent digestive disorder
• Intestines either produce very strong, long contractions or exact opposite
• If overactive: food reside is pushed through too quickly, causing gas and diarrhea
• If underactive: residue progresses slowly and too much water is reabsorbed, constipation and intestinal distention result
• Most people control it with strict diet
• No caffein, alcohol, carbonation, lactose etc
• Stress is a major contributor

• Passes through levator ani muscle and pelvic floor
• Terminates in anus
• Anal columns and sinuses secrete mucus and lubricant during defecation

• Smooth muscle of muscularis externa
• (Involuntary)

• Skeletal muscle
• (Voluntary)

• Stretching of rectum stimulates defecation reflexes
• External anal sphincter must be voluntarily relaxed

Within myenteric plexus and causes muscularis to contract and intrinsic sphincter to relax

Activates parasympathetic neurons to cause muscle contraction and internal sphincter relaxation