The Digestive System

Chapter 23Anatomy of the Digestive

System – Part 3

Small Intestine

• The body’s major digestive organ• Digestion is completed and virtually all absorption

occurs.• Starts at the pyloric sphincter and extends to the

ileocecal valve.• Longest portion of the digestive tract 7-13 feet

while alive and ~20 feet in a cadaver. Changes are because of loss of muscle tone when deceased.

• Diameter ranges from 2.5 – 4 cm (1-1.6 in)

Duodenum

• Literally means “12 finger widths long”• About 10 inches long.• Only (and last) place where digestive juices

enter.

Duodenum• Shortest section but

contains:– Bile duct delivers bile

from liver– Main pancreatic duct

carries pancreatic juice from pancreas

– Hepatopancreatic ampulla where the two connect and then open into the duodenum via the major duodenal papilla

Jejunum and Ileum

• Jejunum (“empty”) 8 ft• Ileum (“twisted) 12 ft• Hang in sausage like coils in the central and

lower part of the abdominal cavity• Highly adapted for nutrient absorption. Three

structures increase the surface area to the size of a tennis court (250 m2).

Jejunum and Ileum

• Plicae circulares deep, permanent folds of the mucosa and submucosa. These folds force chyme to spiral through the lumen, slowing its movement and allowing time for full nutrient absorption

Jejunum and Ileum

• Villi – “tufts of hair” finger like projections of the mucosa (1mm), that give it a velvety texture (like the nap of a towel). Foodstuffs absorbed here. Are large and leaflike in the duodenum (most active absorption) and gradually narrow and shorten along the length of the small intestine

Jejunum and Ileum• microvilli tiny projections of the plasma

membrane of absorptive cells of the mucosa. Give the surface a fuzzy appearance called the brush border. The plasma membranes bear enzymes referred to as brush border enzymes that complete the digestion of carbohydrates and proteins

Jejunum and Ileum

• Most absorption occurs in the proximal part of the small intestines so the Plicae circulatures, villi and microvilli decrease in number towards the distal end.

Intestinal Juice

• 1-2 liters of intestinal juice are secreted daily.• Major stimulus for its production is the

distention or irritation of the intestinal mucosa by acidic chyme.

• Slightly alkaline (7.4-7.8)• Largely water, but also contains some mucus.

Fairly enzyme poor because intestinal enzymes are limited to the bound enzymes on the brush border.

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