Defecation

After electrolytes and water have been absorbed, the waste material that is left passes to the rectum, leading to an increase in rectal pressure, relaxation of the internal anal sphincter, and the urge to defecate. If the urge to defecate is denied, feces are prevented from entering the anal canal by the external anal sphincter. In this case the feces remain in the rectum, and may even back up into the sigmoid colon. The defecation reflex normally occurs when the rectal pressure rises to a particular level that is determined, to a large degree, by habit. At this point, the external anal sphincter relaxes to admit feces into the anal canal.

During the act of defecation, the longitudinal rectal muscles contract to increase rectal pressure, and the internal and external anal sphincter muscles relax. Excretion is aided by contractions of abdominal and pelvic skeletal muscles, which raise the intra-abdominal pressure (this is part of Valsalva's maneuver, described in chapter 14). The raised pressure helps to push the feces from the rectum, through the anal canal, and out of the anus.

Test Yourself Before You Continue

1. Describe how electrolytes and water are absorbed in the large intestine and explain how diarrhea may be produced.

2. Identify the nature and significance of the intestinal microflora.

3. Describe the structures and mechanisms involved in defecation.

The Digestive System

Central vein

Hepatic sinusoids

Hepatic plate Bile ductule

Central vein

Hepatic sinusoids

Hepatic plate Bile ductule

Bile Canaliculi

Branch of hepatic portal vein

Branch of hepatic artery

Bile canaliculi

Bile ductule

Branch of hepatic portal vein

Branch of hepatic artery

Bile canaliculi

Bile ductule

Portal triad

■ Figure 18.20 Microscopic structure of the liver. Blood enters a liver lobule through the vessels in a portal triad, passes through hepatic sinusoids, and leaves the lobule through a central vein. The central veins converge to form hepatic veins that transport venous blood from the liver.

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Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • AMIRA
    What aids in the defecation reflex?
    7 years ago

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