DIGESTIVE SYSTEM
The digestive system consists of the GASTROINTESTINAL (GI) TRACT (which runs from mouth to anus) plus various accessory organs (i.e. Salivary glands, liver, gallbladder, and pancreas).
The function of the system is to convert ingested food into small molecules, which can be absorbed into the blood (or lymph) and distributed to the body cells.
[Fig. 15-1] The GI tract is about 5 meters long in an adult human. The LUMEN is continuous with the external environment (and thus, technically, outside the body).
[Fig. 15-2] Four processes are associated with the GI tract:
SECRETION of bile, enzymes, etc., which contributes to DIGESTION (the breakdown of ingested food) and allows ABSORPTION of the breakdown products into the blood.
These three processes occur as the material travels down the tract because of its MOTILITY.
[Fig. 15-5] Most of the fluid secreted into the GI tract is reabsorbed into the body.
Digestion begins in the MOUTH. CHEWING begins the mechanical breakdown of food, which is then mixed with SALIVA (which contains enzymes for polysaccharide breakdown, and which moistens and lubricates the food mass).
The bolus of food moves to the PHARYNX and is swallowed (a complex reflex activity). The food is then moved (via PERISTALTIC WAVES) down the ESOPHAGUS and through a sphincter into the STOMACH.
[see Figure] GI MOTILITY both moves food along the tract and mechanically mixes it.
Most of the intestinal tract is composed of single-unit smooth muscle, which contracts spontaneously.
The force of contraction is related to the number of action potentials per unit time.
[see Figure] PERISTALSIS is important in the esophagus and stomach, but SEGMENTATION is the most common motion in the intestine. However, after most of the food has been absorbed, a peristaltic MIGRATING MOTILITY COMPLEX moves the residual material to the LARGE INTESTINE.
[Fig. 15-16] The churning of the stomach continues the mechanical breakdown of food. Parietal cells secrete hydrochloric acid (HCl), which helps to sterilize the food, break it down chemically, and which converts pepsinogen to the protein-digesting enzyme, PEPSIN.
[Fig. 15-18] This slide summarizes reactions involved in HCl secretion by PARIETAL CELLS. Note the importance of carbonic anhydrase and the exchange of bicarbonate for chloride.
[Fig. 15-4] The acidic CHYME from the stomach enters the DUDODENUM (first portion of the small intestine) via the PYLORIC SPHINCTER, where it is neutralized by secretions of bile (formed in LIVER and released from GALLBLADDER) and "juice" from the PANCREAS.
BILE contains bile salts (which emulsify fats) and bile pigments (e.g. bilirubin).
PANCREATIC "JUICE" contains bicarbonate ions (which neutralize the acidic chyme) plus many enzymes important for the digestion of proteins (e.g. trypsin), fats (lipase), carbohydrates (amylase), and nucleic acids (e.g. ribonuclease).
The cells of the SMALL INTESTINE secrete salts, mucus, and a variety of digestive enzymes which contribute to the final breakdown of the chyme.
[Fig. 15-7] Most ABSORPTION occurs into the blood or lymph capillaries (lacteals) within the VILLI of the small intestine.
[Fig. 15-8] The villi (and the MICROVILLI on the surface of their epithelial cells) greatly increase the surface area for nutrient absorption.
[Fig. 15-13] Digestive processes are regulated by hormones (e.g. GASTRIN and CHOLECYSTOKININ) and neurally by both "LONG REFLEXES" involving the CNS and ANS and "SHORT REFLEXES", which do not involve the CNS. These latter involve the ENTERIC NERVOUS SYSTEM (sometimes considered as a third division of the ANS and which has been nicknamed "the little brain").
Chyme (about 1500 ml/day) from the small intestine enters the large intestine (COLON) through the ileocaecal sphincter. There is active uptake of sodium from the blood, with associated osmotic uptake of water.
Intestinal bacteria produce various gases (as fermentation byproducts) plus some vitamins (e.g. Vitamin K), which can be absorbed by the host.
Periodically, a contraction wave (MASS MOVEMENT) moves the FECES into the RECTUM, which can activate a DEFECATION REFLEX, forcing the feces out through the INTERNAL (involuntary) and EXTERNAL (voluntary) ANAL SPHINCTERS.
[see Figure] The digestion of carbohydrate begins in the mouth, where salivary amylase can break down starch to maltose. However, most starch digestion depends on pancreatic amylase. Intestinal enzymes break down the disaccharides into monosaccharides, which are then absorbed into the capillaries of the villi.
[see Figure] Protein digestion begins in the stomach with chemical digestion (via HCl) and enzymatic digestion via pepsin. Pancreatic proteases continue breakdown of proteins into peptides and intestinal peptidases break down peptides into amino acids, which are then absorbed into the capillaries.
[see Figure] Although there is some limited fat digestion (via lipase) in the mouth and stomach, most occurs in the small intestine. Bile salts emulsify fats and pancreatic lipase separates fatty acids from glycerol.
Eventually, the lipids are absorbed into the LACTEALS (not the capillaries) in the intestinal villi.
[Fig. 15-12]
[see Figure] Pancreatic lipase digests the emulsified fat droplets, forming smaller droplets of fatty acids, monoglycerides, etc. known as MICELLES. These products diffuse into the epithelial cells and are reformed into triglycerides, etc. and packaged into droplets called CHYLOMICRONS, which enter the lymph (and eventually the blood) via the lacteals.
[see Figure] The products of digestion enter the liver (via the HEPATIC PORTAL VEIN) before entering the general circulation. In the liver, the nutrients are processes (nutrients converted to storage form – glycogen, etc.). This helps maintain a more constant concentration of blood nutrients.
THE END