Human Digestion: Defecation, Vomiting, Saliva Synthesis

Posted on Feb 28, 2025 in Biology

Defecation

Defecation is a reflex involving the rectum and anus, triggered by the parasympathetic nervous system (PSNS), but subject to voluntary control. The rectum, a muscular tube 12-15 cm in length, is normally empty. When a mass movement forces stool into the rectum, the person feels the urge to defecate.

The anal canal is closed by two sphincters:

  • Internal anal sphincter (IAS): Composed of smooth muscle, contracted by the sympathetic nervous system (SNS) via noradrenaline (NA). It is involuntary.
  • External anal sphincter (EAS): Innervated by the pudendal nerve (from sacral segments), allowing for voluntary control.

The process unfolds as follows:

  1. Rectal filling by mass movement stretches the rectal wall.
  2. Mechanoreceptors transmit information to the cortex and sacral segments.
  3. Mechanoreceptors cause rapid relaxation of the internal sphincter.
  4. The EAS is engaged. Depending on whether conditions are appropriate, it will either relax or voluntarily contract further.
  5. If the EAS relaxes, reflex adaptation occurs: the full rectum relaxes its wall, reducing pressure, and the IAS returns to its normal contracted state. Material moves to the anal canal.
  6. When the rectum is full and the IAS relaxes, the material passes through. Anal sampling occurs via sensory receptors in the cortex, allowing for discrimination between solid and liquid.
  7. The local reflex involves contraction of the rectum and IAS. The defecation reflex is a long reflex involving sacral segments. Efferent motor fibers are stimulated by the PSNS and act on the rectum and EAS.
  8. Expulsion of fecal material is aided by voluntary contractions of the diaphragm and abdominal wall muscles, as well as closure of the glottis.

Vomiting

Vomiting is the sudden and forceful expulsion of the contents of the stomach, and sometimes the duodenum, through the mouth. It is often preceded by anorexia and nausea, along with autonomic responses like salivation, pallor (due to vasoconstriction), sweating, dizziness, and tachycardia. Retching usually precedes vomiting. Breathing is inhibited, the larynx closes, and the soft palate rises to close the nasopharynx, preventing inhalation of vomited material.

The process:

  1. The stomach and pyloric sphincter relax.
  2. Duodenal contraction reverses the normal pressure gradient, allowing intestinal contents to enter the stomach.
  3. The diaphragm and abdominal wall contract powerfully.
  4. The gastroesophageal sphincter relaxes, and the pylorus closes.

The reflex is coordinated by a portion of the reticular formation (RF) in the dorsal spinal cord. Afferent impulses reach this region from the pharynx, gastrointestinal tract, liver, gallbladder, cortex, and semicircular canals of the inner ear. Motor impulses responsible for vomiting are transmitted from the vomiting center via cranial nerves V, VI, IX, X, and XII.

Vomiting serves as a protective mechanism to expel toxic substances. However, prolonged vomiting can lead to metabolic alkalosis.

Saliva Synthesis

Saliva synthesis occurs at the level of the intercalated tubules, involving water, electrolytes, and enzymes. Acinar cells produce a primary isotonic fluid similar to plasma, which exits into the lobar ducts.

In the lobar ducts, reabsorption (R) of Na+ and Cl- into capillaries and secretion (S) of K+ and HCO3- occur via active transport. Electrolytes are *not* accompanied by water movement, resulting in hypotonic saliva. The final electrolyte content depends on the secretion rate: slower secretion leads to more hypotonic saliva.

Saliva synthesis is regulated by the autonomic nervous system (ANS), with the parasympathetic nervous system (PSNS) being predominant.