Physiology
Gastrointestinal
Regarding defaecation, which of the following statements is CORRECT:
Answer:
Colonic mass movement describes the intense contraction that begins halfway along the transverse colon and pushes the intestinal contents in the proximal colon towards the rectum. It occurs shortly after a meal due to distension of the stomach and duodenum as part of the gastrocolic reflex and if faeces is present in the rectum, stimulates the urge to defecate. Distention of the rectum causes firing of afferent cholinergic parasympathetic fibres. The internal sphincter is made up of circular smooth muscle innervated by the autonomic fibres, and the more distal external sphincter is composed of striated muscle innervated by motor fibres from the pudendal nerve. During defaecation, relaxation of pelvic muscles straightens the rectum.Defecation Reflex
Physiology / Gastrointestinal / Large Intestine
Last Updated: 30th May 2023
Gastrocolic reflex
The gastrocolic reflex is the physiological reflex that controls the peristalsis of the gastrointestinal tract. It involves an increase in motility of the colon in response to stretch in the stomach and the byproducts of digestion in the small intestine. Colonic mass movement describes the intense contraction that begins halfway along the transverse colon and pushes the intestinal contents in the proximal colon towards the rectum. It occurs shortly after a meal due to distension of the stomach and duodenum as part of the gastrocolic reflex and if faeces is present in the rectum, stimulates the urge to defecate.
Gastroileal reflex
The gastroileal reflex works with the gastrocolic reflex to stimulate the urge to defecate. The gastroileal reflex is stimulated by the presence of food in the stomach and gastric peristalsis. Initiation of the reflex causes peristalsis in the ileum and the opening of the ileocecal valve (which allows the emptying of the ileal contents into the colon for compaction). This in turn stimulates colonic peristalsis and an urge to defecate.
Defecation Reflex
The internal and external anal sphincters usually keep the anal canal closed and are controlled both reflexly and voluntarily. The internal sphincter is made up of circular smooth muscle innervated by the autonomic fibres, and the more distal external sphincter is composed of striated muscle innervated by motor fibres from the pudendal nerve.
When a critical mass of faeces is forced into the rectum, the desire for defaecation is experienced.
Distention of the rectum causes firing of afferent cholinergic parasympathetic fibres which are transmitted to spinal control centres resulting in contraction of the rectum, relaxation of the internal anal sphincter and initially contraction of the external anal sphincter. This initial contraction is soon followed by a reflex relaxation of the external sphincter initiated by an increase in the peristaltic activity in the sigmoid colon and pressure in the rectum.
This reflex relaxation of the external anal sphincter can be overridden by higher brain centre activity, leading to voluntary control over the sphincter which can delay the expulsion of faeces. The prolonged distension of the rectum then leads to a reverse peristalsis, which empties the rectum into the colon and removes the urge to defecate until the next mass movement and/or a more convenient time.
When a person decides to defecate i.e. when the external sphincter is voluntarily relaxed, an increase in intra-abdominal pressure must be achieved to expel faeces. Thus a breath is taken in and the glottis closes over the trachea, the respiratory muscles contract on lungs filled with air and the abdominal muscles contract which increases both the intrathoracic and intra-abdominal pressures, the pelvic floor muscles relax which straightens the rectum and faeces can be expelled.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
