Composition of Feces
Normal stool consists of water and solid substances (about a quarter of the total mass). The solid component includes:
- Undigested elements: keratins, plant fiber, and a portion of 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 derived from bile pigments and provides the characteristic color.
The amount of feces produced depends on diet, emotional state, and physical activity. In pathologies associated with malabsorption and impaired digestion, feces may contain fats (steatorrhea), proteins (creatorrhea), and carbohydrates (amylorrhea). Analysis of stool chemical composition and consistency is crucial for assessing digestive system function.
Functional System of Defecation
The primary adaptive goal of this system is the accumulation and retention of a specific volume of feces in the rectum.
The system features two self-regulation loops:
- Internal loop is responsible for stool formation in the large intestine and rectum. The key mechanism here is the secretion and absorption of water. In addition, nutrient residues are absorbed, and indigestible particles are bound by mucus coming from the upper gastrointestinal tract.
- External loop is directly responsible for emptying the rectum.
Fecal Accumulation 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 rectal wall is stretched to a critical level, receptors are stimulated. Signals travel via sensory fibers of the dorsal 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 sympathetic hypogastric nerves, which enhances the contraction of the internal sphincter. Simultaneously, the signal is transmitted to the brain. The hypothalamus and limbic system form the emotional perception of the urge, while the cerebral cortex evaluates the environment and initiates specific behavior to find a suitable location. The process operates on the principle of dominance, where excitation related to defecation becomes a priority for the brain.
Mechanism of the Defecation Act
Bowel evacuation is under voluntary control. Descending cortical commands (primarily from the precentral gyrus) are directed to the spinal centers.
Autonomic balance shifts:
- The tone of the sympathetic hypogastric nerves drops, and the internal sphincter relaxes.
- The tone of the parasympathetic pelvic nerves increases, causing contraction of the musculature of the rectum and distal large intestine.
The somatic nervous system, via the pudendal nerves, ensures the relaxation of the external sphincter. Fecal matter is expelled by peristaltic waves.
For efficient evacuation, straining (Valsalva maneuver) is engaged: breath is held, the diaphragm, abdominal muscles, and pelvic floor muscles—including m. levator ani—contract. Consequently, not only the rectum but also the lower sections of the large intestine are emptied.