The rectum stores stool until it pushes stool out of the body during a bowel movement. The large intestine absorbs water and any remaining nutrients and changes the waste from liquid into stool. Muscles push these waste products into the large intestine. The waste products of the digestive process include undigested parts of food and older cells from the GI tract lining. The blood delivers the nutrients to the rest of the body. The walls of the small intestine absorb the digested nutrients into the bloodstream. The muscles of the small intestine mix food with digestive juices from the pancreas, liver, and intestine and push the mixture forward to help with further digestion. The muscle of the lower part of the stomach mixes the food and liquid with digestive juice. The muscle of the upper part of the stomach relaxes to accept large volumes of swallowed material from the esophagus. The stomach stores swallowed food and liquid, mixes the food and liquid with digestive juice it produces, and slowly empties its contents, called chyme, into the small intestine. As food approaches the closed sphincter, the muscle relaxes and lets food pass through to the stomach. The lower esophageal sphincter, a ringlike muscle at the junction of the esophagus and stomach, controls the passage of food and liquid between the esophagus and stomach.
Once swallowing begins, it becomes involuntary and proceeds under the control of the esophagus and brain. Pancreatic amylase and the disaccharidases finish the chemical breakdown of digestible carbohydrates. Carbohydrates are not chemically broken down in the stomach, but rather in the small intestine. When a person swallows, food pushes into the esophagus, the muscular tube that carries food and liquids from the mouth to the stomach. Carbohydrate digestion begins in the mouth with the mechanical action of chewing and the chemical action of salivary amylase. Peristalsis looks like an ocean wave traveling through the muscle as it contracts and relaxes. The movement of organ walls-called peristalsis-propels food and liquid through the GI tract and mixes the contents within each organ.
Fecal odor results from gases produced by bacterial metabolism, including skatole, mercaptans, and hydrogen sulfide.The large, hollow organs of the GI tract contain a layer of muscle that enables their walls to move. The characteristic brown color of feces are due to stercobilin and urobinin, both of which are produced by bacterial degradation of bilirubin. The bulk of fecal solids are bacteria and undigested organic matter and fiber. Normal feces are roughly 75% water and 25% solids. Mucus is an important lubricant that protects the epithelium, and also serves to bind the dehydrated ingesta to form feces. Goblet cells are abundant in the colonic epithelium, and secrete mucus in response to tactile stimuli from lumenal contents, as well as parasympathetic stimuli from pelvic nerves. For humans and cats, this does not typically contribute substantially to body energy balance, but in herbivorous species like horses, colonic absorption of volatile fatty provides the major source for energy intake. In addition to water and electrolytes, the large intestine efficiently absorbs short-chain or volatile fatty acids.
The resulting secretion of bicarbonate ions into the lumen aids in neutralization of the acids generated by microbial fermentation in the large gut. The colonic epithelium is actually more efficient at absorbing water than the small intestine and sodium absorption in the colon is enhanced by the hormone aldosterone.Ĭhloride is absorbed by exchange with bicarbonate. The mechanism responsible for generating this osmotic pressure is essentially identical to what was seen in the small intestine - sodium ions are transported from the lumen across the epithelium by virtue of the epithelial cells having very active sodium pumps on their basolateral membranes and a means of absorbing sodium through their lumenal membranes. Water, as always, is absorbed in response to an osmotic gradient.
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