Physiology of the Gastrointestinal System III: Elimination

Elimination begins with digestion, where food is broken down for absorption, and indigestible parts are expelled.

The last stage of digestion occurs in the large intestine.

Here, water and electrolytes are absorbed, transforming unabsorbed minerals, undigested food, bacteria, bile pigments, and exfoliated epithelial cells into feces.

Gut microbes are essential in breaking down indigestible substances, producing vitamins, and metabolizing bile and amino acids.

Additionally, mucus secretion aids in feces movement and mucosal protection.

Elimination is regulated by slow colonic movements initiated by gastrocolic and duodenocolic reflexes in response to food in the stomach and duodenum.

These reflexes stimulate the colonic peristalsis, moving stool toward the rectum.

The rectum stores stool until it is stretched by fecal matter, triggering defecation reflexes.

This reflex involves rectal contraction and relaxation of the internal anal sphincter, accompanied by voluntary relaxation of the external anal sphincter.

Furthermore, the Valsalva maneuver consciously increases abdominal pressure, assisting in defecation.