Notes
Elimination
ELIMINATION:
Process of defecation (a bowel movement), in which undigested fecal waste material exits the body.
Muscular sphincters:
Regulate elimination → under both voluntary and involuntary control.
Internal anal sphincter:
Smooth muscle, involuntary control
External anal sphincter:
Skeletal muscle, voluntary control
Anatomical structures involved in defecation:
- Sigmoid colon → Rectum → Anal Canal → Anus
- Internal anal sphincter and external anal sphincter
Steps of Elimination:
- Mass movements push feces into the sigmoid colon and rectum.
- Rectal distension activates stretch receptors, which trigger the defecation reflex.
- Defecation reflex:
- Rectal smooth muscle contraction → further pushes feces from the rectum to the anal canal
- Internal anal sphincter relaxation and opening → allow feces to pass through the anal canal and exit via the anus.
- Defecation: both the external and internal anal sphincters relax.
Abdominal contraction also aids defecation → increases intra-abdominal pressure and pushes feces through the distal GI tract.
Voluntary external anal sphincter contraction prevents defecation.
The distended rectal wall relaxes and the urge to eliminate dissipates.
Subsequent mass movements reactivate rectal stretch receptors and defecation reflex initiation.
Clinical Correlation: Diverticular Disease
- Lower portion of the large intestine.
- Pouches (diverticula) form at weak areas of colon wall → form outpockets of the mucosa and submucosa → bulge through, disrupt the smooth muscle layer.
- Diverticula form following straining during a bowel movement (i.e. during constipation) → causes high pressures in the colon, weakening the colon wall.
- Smooth muscle layer thickens over time at areas of diverticula formation. The muscle contracts more strongly to eliminate feces.
Other contributors to diverticular disease are:
- Age, more common in elderly.
- Diets low in fiber, which cause hardened feces.
- Defects in GI motility.
- Defects in wall strength.
Diverticular disease = asymptomatic
Diverticulitis: diverticula become infected and progress to an inflammatory state, which causes severe pain.