Defecation

Composition of feces

Normal stool consists of water and solids (about a quarter of the total mass). The solid component includes:

• Undigested elements: keratins, plant fiber, and some collagens.
• Biological component: intestinal bacteria, which account for one-third of the solid residue.
• Exogenous and endogenous inclusions: mucus, salts, and cholesterol.
• Pigments: stercobilin, which is formed from bile pigments and provides the characteristic coloration.

The amount of feces produced depends on diet, emotional state, and physical activity. In pathologies associated with impaired absorption and digestion, fats (steatorrhea), proteins (creatorrhea), and carbohydrates (amylorrhea) may be detected in the stool. Analysis of the chemical composition and consistency of stool is extremely important for assessing the functioning of the digestive system.

Functional system of defecation

The main adaptive goal of this system is the accumulation and retention of a specific volume of feces in the rectum.

The system comprises two self-regulation links:

1. Internal link is responsible for fecal formation in the colon and rectum. The most important mechanism here is the secretion and absorption of water. In addition, residual nutrients are assimilated and undigested particles are bound together by mucus coming from the upper GI tract.
2. External link is directly responsible for emptying the rectum.

Accumulation of feces and urge formation

The accumulation process is mediated by the autonomic nervous system. Sympathetic influences close the internal anal sphincter, while a decrease in parasympathetic tone relaxes the rectal wall. As a result, the rectal ampulla fills with feces.

When the bowel wall stretches to a critical level, receptors are stimulated. Signals travel via sensory fibers of the posterior roots to the spinal defecation center (at the level of the III–IV lumbar and sacral segments).

During the retention phase, the spinal center increases the tone of the sympathetic hypogastric nerves, which enhances the contraction of the internal sphincter. Simultaneously, the signal is transmitted to the brain. The hypothalamus and limbic system shape the emotional perception of the urge, while the cerebral cortex assesses the environment and triggers specific behavior to find a restroom. The process is realized on the principle of dominance, where excitation related to defecation becomes the priority for the brain.

Mechanism of the defecation act

Bowel emptying is under voluntary control. Descending cortical commands (primarily from the precentral gyrus) are directed to the spinal centers.

Reconfiguration of the autonomic balance occurs:

• The tone of the sympathetic hypogastric nerves decreases, and the internal sphincter relaxes.
• The tone of the parasympathetic pelvic nerves increases, which causes contraction of the musculature of the rectum and distal colon.

The somatic nerve system, via the pudendal nerves, ensures relaxation of the external sphincter. Fecal masses are expelled by peristaltic waves.

For effective emptying, straining is recruited: breath is held, the diaphragm, abdominal muscles, and pelvic floor muscles contract, including the levator ani muscle (m. levator ani). As a result, not only the rectum is cleared, but also the lower sections of the large intestine.

Class and exam questions

What else to study on this topic

• Pathologies: creatorrhea, steatorrhea, and amylorrhea
• Defecation disorders and their causes
• The role of conditioned reflexes in the development of the defecation habit in humans
• Afferent and efferent pathways of the defecation act

Other topics in the "Digestion" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.