Liver and Gallbladder

• Liver is one of the body’s most important organs and has many metabolic and regulatory roles. The digestive function is to produce bile for export to the duodenum.

• Bile is stored in the gall bladder• Bile emulsifies fats physically breaks fats into tiny

particles so they are more accessible to digestive enzymes

Composition of Bile

• Yellow-green alkaline solution• Contains bile salts, bile pigments,

cholesterol, triglycerides, phospholipids, and a variety of electrolytes.

• Only bile salts and phospholipids aid in the digestive process.

Composition of Bile

• Bile salts emulsify fats distributes fat throughout the water intestinal contents, just as a dish detergent breaks up a pool of fat drippings in a roasting pan.

• As a result, large fat globules are physically separated into millions of small, more accessible fatty droplets that provide a large surface area for fat-digesting enzymes to work on.

• Bile salts also help with fat and cholesterol absorption

Composition of Bile

• Bilirubin chief bile pigment. A waste product of the heme of hemoglobin formed during the breakdown of RBCs.

• It is metabolized by bacteria in the small intestine and one of the products is stercobilin, which gives the brown color.

• When bile is absent, feces are gray-white in color and have fatty streaks because essentially no fats are digested or absorbed.

Pancreas

• Soft, tadpole-shaped gland that extends across the abdomen.

• Produces enzymes (collectively called pancreatic juice) that break down all categories of foodstuffs.

• Pancreatic juice (pH ~8)consists of water, enzymes and electrolytes. High pH helps to neutralize acidic chyme.

Pancreas

• Enzymes:– Trypsinogen activated to trypsin by a brush

border enzyme– Amylase Carbohydrates– Lipases fats– Amylase and lipase are secreted in active form but– require ions or bile be present for optimal activity

Large Intestine

• Frames the small intestine on 3 sides

• Extends from the ileocecal valve to the anus.

• Lumen diameter ~7cm (2 ¾ in)• Length 1.5 meter (5 ft)

Large Intestine

• Major function is to absorb most of the remaining water from the indigestible food, stores the residue temporarily, and then eliminate them from the body in the form of feces.

• Because the colon doesn’t secrete digestive enzymes, chemical digestion officially ends in the small intestine.

Large Intestine

• Cecum (“blind pouch”) – lies below the ileocecal valve. First part of the large intestine.

Large Intestine

• Appendix – • attached to the

cecum. • Contains lymphoid

tissue and plays an important role in body immunity

Large Intestine

• Ascending colon –• travels up the right side

of the abdominal cavity• Transverse colon -• travels across the

abdominal cavity

Large Intestine• Descending colon • travels down the

left side of the abdominal cavity

• Sigmoid colon • when colon enters

the pelvis • s-shaped • travels transverse

Large Intestine• Rectum – • travels down, just in

front of the sacrum• Anal canal – • last portion of large

intestine• about 3 cm long. • Has the sphincters that

open and close the anus during defecation

Large Intestine

• Since most food is absorbed before reaching the large intestine, there are no circular folds, no villi, and virtually no cells that secrete digestive enzymes.

Large Intestine

• The mucosa is thicker, and abundant crypts are deeper and there are lots of goblet cells that produce mucus.

• The mucus eases the passage of feces and

protects the intestinal wall from irritating acids and gases released by resident bacteria

Large Intestine - Bacterial Flora

• Most bacteria entering from the small intestine are dead, but some are still alive.

• These and the bacteria that enter the GI tract from the anus, these constitute the bacterial flora of the large intestine over 700 species!

Large Intestine - Bacterial Flora• Metabolize some proteins and ferment some

indigestible carbohydrates (cellulose, xylan, and others), releasing irritating acids and a mixture of gases (including dimethyl sulfide, H2, N2, CH4, and CO2).

• Some are quite odorous • (dimethyl sulfide)!

Large Intestine - Bacterial Flora• About 500 ml of gas (flatus) is produced each

day. More when certain carbohydrate rich foods (beans) are eaten.

• Bacteria also synthesize B complex vitamins and most of the vitamin K that the liver needs to make some of the clotting proteins.

• Also found in the large intestine are viruses and protozoans. Of these at least 20 are known pathogens.

Large Intestine - Bacterial Flora• Most enteric bacteria are

commensals (no harm and no benefit to the host)

• that coexist peacefully with their host as long as they remain in the lumen.

• Bacteria break down and metabolize the residual complex carbohydrates further, adding much to their own nutrition but essentially nothing to ours.

Large Intestine - Motility

• Haustral contractions– Slow segmenting

movements mainly occuring in the transverse and descending colon.

– Last about 1 minute and occur every 30 minutes or so

Large Intestine - Motility• Mass movements– Long, slow-moving, powerful contractile waves

that move over large areas of the colon 3-4 times daily and force the contents towards the rectum.

– Typically occurs during or just after eating.

– Bulk, or fiber, in the diet increases the strength of colon contraction and softens the stool, allowing the colon to act like a well-oiled machine.

Homeostatic Imbalance - Diverticulosis• When the diet lacks bulk, and the

volume of the residues in the colon is small, the colon narrows and its contractions become more powerful, increasing the pressure on its walls.

• This promotes the formation of diverticula, small herniations of the mucosa through the colon walls.

• Most commonly occurs in the sigmoid colon. Affects over half of people over the age of 70.

Defecation• The semisolid product delivered to the rectum

is called feces or stool, contains undigested food residues, mucus, sloughed-off epithelial cells, millions of bacteria, and just enough water to allow for smooth passage.

• The rectum is usually empty, but when mass movements force the feces into it, the defecation reflex is triggered.

Homeostatic Imbalance

• Watery stools• Results from any condition that rushes food

residue through the large intestine before that organ has had sufficient time to absorb the remaining water.

• Prolonged diarrhea may result in dehydration and electrolyte imbalance.

Homeostatic Imbalance

• When food remains in the colon for extended periods

• too much water is absorbed and the stool becomes hard and difficult to pass.

• May result from a lack of fiber in the diet, improper bowel habits (holding it), lack of exercise, emotional upset, or laxative abuse