Defecation: The Physiology of Waste Elimination

Process Overview

Defecation is the final step of digestion, eliminating undigested food, bacteria, cells, and waste as feces.

Key Stages

1. Colonic transit: Indigestible residue enters the colon; water and electrolytes are absorbed, forming stool.
2. Rectal storage: Feces accumulate in the rectal ampulla, triggering stretch receptors.
3. Defecation reflex: Parasympathetic activation (S2–S4) → internal anal sphincter relaxation + external anal sphincter voluntary control.
4. Expulsion: Valsalva maneuver increases intra-abdominal pressure; puborectalis muscle relaxes, straightening the anorectal angle.

Normal Parameters

• Frequency: 3 times/day to 3 times/week
• Consistency: Bristol Stool Scale types 3–4 (smooth, soft)
• Volume: ~100–250 g/day

Clinical Significance

Alterations in frequency, consistency, blood, mucus, or pain signal digestive disorders (IBS, IBD, infection, obstruction, malignancy). Chronic constipation or diarrhea warrants medical evaluation.