Rectum, Anus, and Defecation

Rectum

The rectumThe final section of the large intestine, where feces are stored before elimination. is a “fecesSolid waste material formed in the large intestine and expelled through the anus. waiting room.” The rectum is an expandable organ. This histological picture on the right displays the rectal-anal junction. It shows large folds in the rectum. These folds can expand to accommodate more feces waiting to leave the body. There are baroreceptors in the rectum that feedback to the brain as it fills. The rectum’s epithelia is very similar to the epithelium of the large intestine. It contains simple columnar cellsThe basic structural and functional units of life. with even more mucus cells scattered among them.

Anus

Similar to the folds of the rectum that allow for expansion, the anusThe terminal opening of the digestive tract through which feces are expelled. has mucosal columns. These columns and sinusesAir-filled spaces in the skull that lighten it and help humidify air. allow for expansion as large feces pass through. When feces press on the sinuses they are prompted to secrete extra mucus that would aid and defecation. This is why straining in defecation can lead to excess mucus production. Large feces can also press upon superficialNear the surface of the body. veinsBlood vessels that return deoxygenated blood to the heart (except pulmonary veins, which carry oxyge in the anus. Continued pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. can result in distended veins which are hemorrhoids. Hemorrhoids can be present on the internal anal tissue or the external anal tissue. Despite the location, anal tissue is made of stratified squamous epithelium. External anal tissue is slightly more keratinized than internal anal tissue. This ensures that cells can be lost as feces scrape them off the anus as they pass through it.

Defecation

Defecation occurs very much like urination and involves 2 distinct sphincters. The internal anal sphincter, made of smooth muscle, is involuntary, while the external sphincter, made of skeletal muscle, is voluntary. Loss of control over these sphincters can occur without proper care.  The process begins when a baroreceptor from either the anus or the rectum feeds back to the central nervous systemComposed of the brain and spinal cord; integrates and processes information. . An unconscious reflex returns to the anus. It then affects the internal anal sphincter, allowing it to relax. This puts the onus of containment solely on the external anal sphincter that you control. When you send the message to relax the external anal sphincter then feces are capable of leaving the body.