digestive.txt

breaks up food we ingest (take into our mouths) into particles small enough to pass through the wall of the SMALL INTESTINE and on into the cells of our body.

cavity inside the GI tract through which food passes and is processed

layer of the GI tract wall that comes into contact with food in the lumen, has three sub-layers, the epithelium, lamina propria, and muscularis mucosae

mucosal layer in contact with the lumen. The epithelium is the only layer of the GI tract that actually interacts with food and has only one, thin cell layer to allow for quick absorption of nutrients

between the epithelium and muscularis mucosae, contains blood and lymphatic vessels through which nutrients can enter and then be transported to the rest of the body

deepest layer of the mucosa, is a smooth muscle layer

located between the mucosa and muscularis. The submucosa is a connective tissue pathway for blood vessels, lymphatic vessels, and glands where they are most protected

composed of smooth muscle layers. The inner CIRCULAR MUSCLE LAYER runs perpendicular to the length of the GI tract organ. The outer LONGITUDINAL MUSCLE LAYER runs parallel to the length of the GI tract organ.

covers the outside of most GI tract organs. This moist, slippery membrane decreases friction as GI tract organs rub against adjacent structures.

• - covers GI tract organs and the abdominopelvic cavity in which many of these organs reside.
• - also secretes fluids that decrease friction as GI tract organs slide past one another.

breaks down carbohydrates into small sugars called DISACCHARIDES

chewing

Incisors have a flat, chiseled edge for cutting food like biting into an apple. Cuspids have a pointed edge for tearing and shredding food. Premolars and molars have larger, flattened surfaces for grinding and crushing food.

anchors the lesser curvature of the stomach to the liver.

hangs down like an apron between the anterior abdominal wall and the intestines. Because the greater omentum is a major fat storage area for your body, its weight helps anchor the stomach as the stomach moves and is sometimes referred to as the "fatty apron."

swallowing

tongue prepares food for deglutition by forming it into a wet, rounded mass called a bolus (a compact, wet mass slides down the throat easier than dry, individual pieces of scratchy Doritos corn chips).

(1) During peristalsis the circular muscle layer behind the moving bolus contracts, pinching the GI tract much like a ring getting smaller in size, which squeezes the bolus forward into the next part of the GI tract. (2) At the same time, the longitudinal muscle layer in front of the bolus contracts, shortening and generating a bulge that receives the moving bolus. If only the circular muscle layer contracted, the bolus could get squished backward as easily as forward. Instead, the longitudinal muscle layer concurrently creates a wider region that forces the bolus to move forward. (3) These sequential contractions of pinching and shortening repeats along the length of the GI tract moving ingested food from the pharynx to the stomach. Peristalsis is like squeezing a toothpaste tube to move the toothpaste toward the opening of the tube.

serves as a temporary storage tank for ingested food and prepares food for treatment by the small intestine. The most important digestive activity of the stomach is to begin the chemical breakdown of proteins.

folds running the length of the stomach. Rugae aid absorption by increasing the surface area of the mucosa and allow MIXING WAVES

MECHANICAL DIGESTION. During mixing waves, the three layers of the muscularis contract in different directions and generates an agitator action among the rugae much as the blades of a washing machine swing back and forth to agitate clothes suspended in wash water. Mixing waves physically breaks up larger food particles into smaller pieces and mixes the bolus with gastric juice. The resulting soupy, acidic mixture is called CHYME

resulting soupy, acidic mixture from mixing waves in the stomach

contains the stomach secretions hydrochloric acid and pepsin

gives gastric juice an extremely acidic pH of about 2. The resulting high acidity of gastric juice activates PEPSIN, an enzyme. Once activated pepsin, breaks proteins (large amino acid chains) down into peptides (small amino acid chains).

(1) Pepsin is secreted in an inactive form. It is not converted to the active form until it comes into contact with hydrochloric acid out in the lumen of the stomach. (2) The stomach has a thick mucus coating over its mucosa that acts like a shield to protect the stomach wall from destruction by hydrochloric acid and pepsin. This mucus coating is so thick that it interferes with absorption through the stomach wall, hence stomach absorption is limited to water, alcohol, and a few drugs.

burping

When the stomach contains too much gas, contractions of the stomach muscularis expel the excess gas upward into the esophagus with so much force that gastric juice can be forced into the esophagus. The caustic hydrochloric acid in gastric juice can irritate the mucosa of the esophagus

major site of chemical digestion and nutrient absorption. Within its twisted passageways, digestion of usable food is completed and virtually all absorption occurs, allowing nutrients to begin their journey into our body cells. The small intestine utilizes peristalsis to move undigested materials along its length.

pinches the intestine into compartments, which mixes chyme with digestive juices for their chemical digestion and absorption. Unlike peristalsis, segmentation does not result in a net movement of chyme. (1) During segmentation, the circular muscle layer pinches off the small intestine into compartments, or “segments”, which separates larger masses of chyme. (2) The circular layer pinches to even smaller small intestine segments, further breaking up undigested food and moving it back and forth like an agitator in a washing machine. (3) By repeatedly dividing undigested food particles and sloshing them back and forth, they become thoroughly mixed with digestive juices.

Pancreatic juice, brush border enzymes, and bile accomplish

enzyme that digests starches (complex sugar chain molecules). Digestion of starches begins in the mouth by the enzyme SALIVARY AMYLASE and continues in the duodenum by pancreatic amylase.

two enzymes in pancreatic juices that digest proteins, and the enzyme LIPASE to digest lipids.

• S = sodium bicarbonate
• P = pancreatic amylase
• L = lipase
• T = trypsin
• C = chymotrypsin

the process of breaking large lipid molecules apart into tinier lipid molecules that are small enough to stay suspended in water. Bile makes lipid molecules so tiny that they mix with watery pancreatic juice. The enzyme lipase can now come in contact with the suspended lipid molecules to digest them.

deep, permanent folds of the mucosa and submucosa that extend all the way around the inner surface of the small intestine. Circular folds force chyme to spiral through the lumen, slowing its movement to allow sufficient time for maximum absorption of nutrients. Figure 23.18a. The surface of each circular fold has VILLI, small fingerlike projections of the mucosa that give it a velvety texture