The elimination of digestive waste products from the body is essential to health. Excreted waste products from the intestines are known as stool or feces. Stool examination is often done for evaluation of gastrointestinal (GI) disorders such as GI bleeding, GI obstruction, obstructive jaundice, parasitic disease, dysentery, ulcerative colitis, and increased fat excretion.
Stool is composed of the following materials:
Waste residue of indigestible material (e.g., cellulose) from food eaten during the previous 4 days
Bile (pigments and salts)
Intestinal secretions
Water and electrolytes
Epithelial cells that have been shed
Bacteria
Inorganic material, mainly calcium and phosphates
Undigested or unabsorbed food (normally present in very small quantities)
The output of feces depends on a complex series of absorptive, secretory, and fermentative processes. Normal function of the colon involves three physiologic processes: (1) absorption of fluid and electrolytes, (2) contractions that churn and expose the contents to the GI tract mucosa and transport the contents to the rectum, and (3) defecation.
The small intestine is approximately 23 feet (7 m) long, and the large intestine is 4–5 feet (1.2–1.5 m) long. The small intestine degrades ingested fats, proteins, and carbohydrates to absorbable units and then absorbs them. Pancreatic, gastric, and biliary secretions exert their effects on the GI contents to prepare this material for active mucosal transport. Other active substances absorbed in the small intestine include fat-soluble vitamins, iron, and calcium. Vitamin B12, after combining with intrinsic factors, is absorbed in the ileum. The small intestine also absorbs as much as 9.5 L of water and electrolytes for return to the bloodstream. Small intestine contents (i.e., chyme, which contains fragments of proteins, droplets of fat, salt, and water) begin to enter the rectum as soon as 2–3 hours after a meal, but the process is not complete until 6–9 hours after eating.
The large intestine performs less complex functions than the small intestine. The proximal, or right, colon absorbs most of the water remaining after the GI contents have passed through the small intestine. Colonic absorption of water, sodium, and chloride is a passive process. Fecal water excretion is only about 100 mL/d. The colon mainly moves the luminal contents by seemingly random contractions of circular smooth muscle. Increased propulsive activity (i.e., peristalsis) occurs after eating. Peristaltic waves are caused by the gastrocolic and duodenocolic reflexes, which are initiated after meals and stimulated by the emptying of the stomach into the duodenum. The muscles of the colon are innervated by the autonomic nervous system. In addition, the parasympathetic nervous system stimulates movement, and the sympathetic system inhibits movement. Massive peristalsis (progressive waves of muscular contractions pushing the contents ahead) usually occurs several times a day. Resultant distention of the rectum initiates the urge to defecate. In persons with normal motility and a mixed dietary intake, normal colon transit time is 24–48 hours.