1.8 Defaecation

Defecation is the final step in the digestive process. It is the act of expelling feces from the digestive tract through the anus. The process differs between infants and adults in terms of voluntary control.

Definition

Defecation: Defecation (also spelled defaecation) is the term for the act of expelling feces from the digestive tract via the anus. It is a reflex action that involves the coordinated contraction and relaxation of muscles in the rectum and anal sphincters.

Main Idea

Defecation is the elimination of undigested waste materials from the body. In infants, the defecation reflex is involuntary — the anal sphincter relaxes automatically. In adults, the defecation reflex can be voluntarily controlled — a person can initiate or inhibit defecation by contracting or relaxing the external anal sphincter.

Core Concept

• Defecation = elimination of feces through the anus
• Infants = involuntary defecation reflex
• Adults = voluntary control of defecation
• Internal anal sphincter = involuntary
• External anal sphincter = voluntary
• Rectum = stores feces until elimination
• Defecation reflex = triggered by stretching of the rectal wall

Detailed Explanation

Defecation Reflex in Infants

• In infants, the defecation reflex is involuntary.
• This means that the anal sphincter relaxes automatically to allow stool to pass.
• This reflex is particularly active in the first week of life.
• It often occurs shortly after feeding.
• The infant has no voluntary control over defecation.
• The reflex is triggered by the filling of the rectum.
• The internal and external anal sphincters relax automatically.

Key Points:

Defecation Reflex in Adults

• In adults, the defecation reflex can be voluntarily controlled.
• When it is appropriate, a person can initiate the defecation reflex.
• This is done by:
• This action increases the internal pressure.
• The increased pressure forces the feces into the rectum.
• When the rectum is filled, its wall is distended.
• The defecation reflex is triggered.
• As a result:

• The defecation reflex persists only for a few minutes and quickly dies.
• A person usually can inhibit defecation voluntarily by keeping the external sphincter contracted.
• This allows the person to delay defecation until an appropriate time and place.

Key Points:

Comparison of Defecation Reflex in Infants and Adults

Intermediate Level

Mechanism of Defecation in Adults

Step-by-Step Process:

• Feces move from the sigmoid colon into the rectum.
• The rectum fills and its wall stretches.

2. Stretch receptors activated: - Stretch receptors in the rectal wall are stimulated.

• Sensory neurons send signals to the spinal cord and brain.
• The person becomes aware of the need to defecate.

• The person takes a deep breath.
• The person contracts the abdominal muscles.
• This increases intra-abdominal pressure.
• Feces are forced into the rectum.

• Peristaltic waves in the descending colon and sigmoid colon are stimulated.
• The internal anal sphincter relaxes (involuntary).

• The external anal sphincter is relaxed voluntarily.
• The puborectalis muscle relaxes.

• Feces are forced out through the anal canal.
• The process is complete.

• If it is not appropriate to defecate, the person can:
• Keep the external anal sphincter contracted.
• The reflex dies after a few minutes.
• Feces remain in the rectum until the next reflex.

Role of Sphincters

Role of Muscles

Factors Affecting Defecation

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Defecation is the elimination of feces through the anus.
• 2. In infants, the defecation reflex is involuntary.
• 3. In adults, the defecation reflex can be voluntarily controlled.
• 4. The internal anal sphincter is involuntary.
• 5. The external anal sphincter is voluntary.
• 6. The defecation reflex is triggered by stretching of the rectal wall.
• 7. Mass peristalsis moves feces into the rectum.
• 8. The reflex persists only for a few minutes.
• 9. Adults can inhibit defecation by keeping the external sphincter contracted.
• 10. Deep breath and abdominal contraction help initiate defecation.
• 11. Peristaltic waves in the descending colon are stimulated.
• 12. The process involves both intrinsic and extrinsic reflexes.
• 13. Higher centers in the brain control voluntary defecation.
• 14. Toilet training requires awareness and sphincter control.
• 15. Constipation and diarrhea are common defecation disorders.

Remember

• - Defecation = elimination of feces through anus
• - Infants = involuntary defecation reflex
• - Adults = voluntary control
• - Internal anal sphincter = involuntary
• - External anal sphincter = voluntary
• - Rectum = stores feces
• - Defecation reflex = triggered by rectal stretching
• - Mass peristalsis = moves feces into rectum
• - Reflex duration = few minutes
• - Initiation = deep breath + abdominal contraction
• - Inhibition = keep external sphincter contracted
• - Toilet training = 2–3 years

Common Mistakes

• 1. Thinking infants have voluntary control — Infants have involuntary defecation.
• 2. Believing adults cannot control defecation — Adults have voluntary control.
• 3. Confusing internal and external anal sphincters — Internal is involuntary; external is voluntary.
• 4. Thinking the defecation reflex lasts indefinitely — It lasts only a few minutes.
• 5. Believing feces are stored in the colon — Feces are stored in the rectum.
• 6. Forgetting the role of abdominal muscles — They help increase pressure during defecation.
• 7. Mixing up constipation and diarrhea — Constipation is difficult defecation; diarrhea is frequent, watery stools.
• 8. Thinking defecation is purely reflexive — In adults, it is under voluntary control.
• 9. Confusing mass peristalsis with regular peristalsis — Mass peristalsis is stronger and moves feces toward the rectum.
• 10. Believing toilet training is automatic — It requires awareness and sphincter control.

Quick Revision

Defecation is the act of expelling feces from the digestive tract through the anus. In infants, the defecation reflex is involuntary — the anal sphincter relaxes automatically, especially shortly after feeding. In adults, the defecation reflex can be voluntarily controlled. When appropriate, a person can initiate defecation by holding a deep breath and contracting the abdominal muscles, which forces feces into the rectum. The rectum stretches, triggering the defecation reflex. Peristaltic waves in the descending colon are stimulated, and the internal and external anal sphincters relax. Feces are forced out. The reflex persists only for a few minutes. A person can inhibit defecation voluntarily by keeping the external sphincter contracted. The internal anal sphincter is involuntary; the external anal sphincter is voluntary.

60-Second Revision

• Defecation = elimination of feces through anus
• Infants = involuntary defecation reflex
• Adults = voluntary control
• Internal anal sphincter = involuntary
• External anal sphincter = voluntary
• Rectum = stores feces
• Defecation reflex = triggered by rectal stretching
• Mass peristalsis = moves feces into rectum
• Initiation = deep breath + abdominal contraction
• Inhibition = keep external sphincter contracted
• Reflex duration = few minutes
• Warning: Infants have no voluntary control; adults can inhibit defecation

NEXT TOPIC

• Chapter:
• 1. Digestive System of Man

• Topic:
• 1.9 STORAGE AND METABOLIC ROLE OF LIVER

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