Defecation is a reflex which results in the expulsion of feces and is normally under voluntary control in adult. In infants, it is purely a reflex action. The desire for defecation is felt when the finally formed feces enter the rectum. Continuous escape or dribbling of feces is prevented by presence of two sphincters.
a. Internal anal sphincter:
Formed of smooth muscle supplied by parasympathetic and sympathetic stimulation, the former relaxes it while the latter contracts the sphincter. Thus it is involuntary.
b. External anal sphincter is formed by the skeletal muscle fibers and supplied by pudendal nerve which is somatic and is, therefore, activity is under voluntary control.
Defecation reflex has following:
i. Stimulus:
Distension of rectal wall by the feces.
ii. Receptors:
Stretch receptors in the mucosa of rectum. These receptors can distinguish between fluid and gas.
iii. Afferents:
Pelvic splanchnic (parasympathetic) nerves.
iv. Center:
S2, S3 and S4 segments of spinal cord controlled by impulses from the higher centers.
v. Efferent:
Parasympathetic pelvic splanchnic nerves.
vi. Effectors:
Smooth muscle of rectum and internal anal sphincter.
vii. Response:
Contraction of the rectum and relaxation of internal anal sphincter.
When the desire of defecation is felt, the individual can voluntarily postpone the same by the act the contractions of the external anal sphincter, if the time and place are not socially acceptable. This voluntary control is lacking in infants. They develop the habit and control by training. External anal sphincter can withstand pressure up to 55 mm Hg.
Once the act of defecation is postponed, it remains inhibited for some time. When the time and place are socially acceptable, the act of defecation can be carried out by straining efforts. This consists of a deep inspiration followed by forced expiration against closed glottis—Valsalva maneuver.
During straining, the downward movement of diaphragm and contraction of abdominal muscle and voluntary relaxation of external anal sphincter help in the act of defecation. Levator ani also contracts pulling the anal canal up over the fecal matter so that the fecal matter can pass through.
Gas in the Intestine:
Gas in the intestine is in the form of O2, N2, CO2, H2, H2S, and CH4.
O2, N2, CO2 are respiratory gases of the swallowed air.
Others are formed by bacterial action on foodstuff. Some of these are absorbed while others are expelled through canal as flatulence. When there is large bowel obstruction, the patient does not pass feces as well as flatulence.
Effect of Colectomy:
Sometimes most of the large intestine has to be removed especially in cancer. In that case, the ileum is made to open to the exterior through anterior abdominal wall. The individual can still survive provided his diet and water electrolyte balance are properly maintained (ileostomy).
Constipation:
Diminished frequency of defecating is called constipation. The frequency of defecation varies from individual to individual. It is usually due to faulty habit, sometimes due to diseases of the colon. Chronic constipation leads to loss of appetite, abdominal distention. Constipation is the most important symptom of intestinal obstruction (pain, vomiting, and constipation).
Hirschsprung’s Disease or Megacolon:
In this congenital disease, a short segment of distal colon becomes spastic due to absence of ganglion cells in submucous and myenteric plexus. Colon above the segment is dilated.
Important symptom will be constipation.
Treatment:
Surgery which includes resection of spastic segment and continuity between normal proximal and distal colon is re-established by anastomosis.
Diarrhea:
Diarrhea is increased frequency of defecation, usually due to infection. It leads to loss of water and electrolytes resulting in dehydration and electrolyte imbalance. This will become a serious problem if diarrhea is severe. Loss of K+ in particular is detrimental to the cardiac function.
Vomiting or Emesis:
Vomiting is the expulsion of gastric contents to the exterior through the esophagus and mouth.
Sequence of events during vomiting is as follows:
1. Nausea
2. Increased salivation
3. Sweating
4. Tachycardia
5. Rapid, deep, irregular breathing
6. Retching: Uncoordinated contractions of inspiratory muscle that is diaphragm moves down during expiration.
7. Glottis is closed.
8. Soft palate contracts and blocks the nasopharynx to prevent the vomitus from entering the nose.
9. Powerful contractions of pyloric antrum with simultaneous relaxation of body of stomach, cardiac contents travel upwards through the relaxed stomach and esophagus into the mouth. The downward movement of the diaphragm and powerful contractions of abdominal muscles also have a compressing effect on the stomach to aid in the vomiting processes.
Vomiting is a reflex and the vomiting center is situated in the reticular formation of medulla.
Impulses for vomiting can arise not only from stomach but also from small intestine, kidney, uterus, heart (in myocardial infarction) and vestibular apparatus (inner ear—responsible for motion sickness).
Some of the drugs, like morphine, digitalis, can also cause vomiting by stimulating the vomiting center through a group of neurons in the medulla called chemoreceptor trigger zone.
Agents which cause vomiting are called emetics and the agents that suppress the same are called antiemetics.