Just seeing if you're paying attention!

GE 345: Week 5

The GI Tract

GI/Alimentary Tract provides water, electrolytes, and nutrients to the body:

Moves food through the tract
Secretes digestive juices to break down food
Absorbs digestive products, water and electrolytes
Blood circulation through GI organs carries away absorbed substances
Controlled by nervous and hormonal systems

General Design

Long tube consisting of several muscular and mucosal layers, and an inner epithelial lining. Each muscle layer is a branching lattice of smooth muscle bundles. An action potential anywhere in the muscle mass generally travels in all directions, causing widespread contraction.

Tonic contractions (continuous) control segment pressure, sphincters
Rhythmic contractions have phasic GI tract functions as mixing, peristalsis

Nervous Control

Intrinsic control: enteric nervous system. May be controlled by sympathetic or parasympathetic systems.

Parasympathetic stim (from eating, anticipation): increases enteric activity, and therefore most GI functions.
Sympathetic stim inhibits GI activity. Norepinephrine inhibits smooth muscle contraction, and inhibits enteric neurons.

Enteric neurons secrete neurotransmitters, many of whose functions are not yet known. Acetylcholine typically sitmulates GI activity. Norepinephrine inhibits.

Blood Flow

Part of the splanchnic circulation. Includes blood flow through the gut, spleen, pancreas and liver. All blood though gut, spleen and pancreas flows immediately to the liver through the portal vein, where bacteria and other particles are removed, and nonfat, water souble nubrients absorbed from the gut are stored and processed, before the blood returns to the general circulation.

Blood flow is related to level of local activity. During nutrient absorption blood flow increases. Possibly controlled by the hormones which influence gut activity, kinins (kallidin and bradykinin) which are powerful vasodilators, and decreased oxygen concentration in the gut from increased metabolic rate.

Parasympathetic stimulation increases blood flow to the GI tract as it increases glandular secretions. Thought to be more from secretions than parasympathetic stimulation. Sympathetic stimulation causes intense vasoconstriction. Flow returns to normal after a few minutes via an autoregulatory escape mechanism -- ischemia triggers local vasodilator mechanisms.

Movements in the GI Tract

The tract moves to either propel or mix the contents.

Peristalsis (propulsion): moves food by forming a contractile ring which moves forward, pushing the contents ahead of it. Stimulated by gut distension.
Mixing: different in different parts of the tract. Peristaltic waves can be blocked by a sphincter, so the wave churns the contents. Or, local constrictive contractions can occur every few centimeters for a few seconds, then occur at other points, "chopping" the contents.

Secretions

Digestive enzymes and mucus (lubricates and protects tract) are secreted throughout the alimentary tract. Digestive secretions are formed in response to the presence of food in the tract. In some parts of the tract, these secretions can vary based on the type of food present. Most secretory glands lie in the walls of the tract, although a few are outside.

Secretory substances are synthesized in the ER and Golgi complexes, and discharged into the cytoplasm in secretory vesicles, which remains stored until extrusion through the cell's surface.

Digestion

Proteins, carbohydrates and fats are all digested in a process called hydrolysis, in which hydrogen and hydroxyl ions derived from water split the molecules with the assistance of enzymes.

In the normal human diet, carbohydrate sources are mainly the disaccharides sucrose (cane sugar) and lactose (in milk), and polysaccharides, commonly known as starches, which are found in most foods. Starches are first hydrolyzed to the disaccharide maltose, which, along with lactose and sucrose, is hydrolyzed into the monosaccharides glucose, galactose, and fructose in the small intestine.

Triglyceride, a glycerol nucleus with three fatty acids, is the most common fat in the diet. Common diets also include some phospholipids, cholesterol and cholesterol esters. Phospholipids and cholesterol esters contain fatty acid, and are considered fats themselves. Cholesterol is a sterol compound with no fatty acid, but does exhibit some physical and chimical characteristics of fats, is derived from fats, and is metabolized similarly to fats, so is considered a fat from a dietary standpoint. During fat digestion, fatty acids are separated from glycerol.

Dietary proteins are derived from meats and vegetables, and are digested in the stomach and upper part of the small intestine and broken down into their constituent amino acids.