Physiology
Gastrointestinal
Regarding water absorption in the large intestine, which of the following statements is INCORRECT:
Answer:
The chyme that initially enters the large intestine is isotonic, however in the colon, more water than electrolytes is absorbed, and the fluid becomes hypertonic, leading to water being absorbed against a concentration gradient. The process of water absorption is controlled by Na+/K+ ATPases located in the basolateral and lateral membranes of the epithelial cells. Na+ is pumped into extracellular spaces and tight junctions at the luminal membrane prevent the diffusion of Na+ and Cl- back from the extracellular space into the lumen. This leaves a hypertonic solution close to the lumen, causing water to follow by osmosis. The electrolytes are absorbed by a variety of mechanisms (similar to those of the small intestine), with essentially a net movement of K+ and HCO3- into the lumen of the large intestine, because of the potential difference set up by the asymmetrical absorption of Na+ and Cl- across the epithelium.Function of Large Intestine
Physiology / Gastrointestinal / Large Intestine
Last Updated: 21st April 2019
The large intestine extends from the ileocaecal valve to the anus.
Ileocecal Valve
Approximately 1.5 L of chyme enters the large intestine per day through the ileocaecal sphincter. Distension of the terminal ileum results in the opening of the ileocaecal sphincter and distension of the caecum causes it to close allowing regulation of flow into the colon to maximise the function of the large intestine, which is to concentrate faeces by absorbing water and electrolytes. The initial 1.5 L is reduced to about 150 g of faeces consisting of 100 mL of water and 50 g of solids.
Functional Role of Taenia Coli
The muscle layers of the large intestine are slightly different from those found in the rest of the GI tract. It still has a powerful circular muscle layer but the longitudinal smooth muscle layer is concentrated into three bands called the teniae coli. These bands are shorter than the circular muscle layer and gather the caecum and colon into a series of pouch-like folds called haustra.
The caecum and ascending and transverse colon are innervated by parasympathetic branches of the vagus nerve; the descending and sigmoid colon, rectum and anal canal are innervated by parasympathetic branches of the pelvic splanchnic nerves. These parasympathetic fibres innervate intramural plexuses. The sympathetic nerves via the superior mesenteric plexus, and via the inferior mesenteric and superior hypogastric plexuses, innervate the proximal and distal parts of the large intestine respectively.
Functional Anatomy of the Large Intestine. (Image by unknown [public domain], via Wikimedia Commons)
Handling of Chyme
Movement of chyme through the large intestine involves both mixing (via haustral segmental contractions) and propulsion (via peristalsis). Haustration aids the exposure of chyme to the mucosal surface and helps the reabsorption of water and electrolytes. Chyme usually remains in the colon for up to 20 h.
Parasympathetic stimulation causes segmental contraction whereas stimulation of sympathetic fibres stops colonic activity.
Water Absorption
The chyme that initially enters the large intestine is isotonic, however in the colon, more water than electrolytes is absorbed, and the fluid becomes hypertonic, leading to water being absorbed against a concentration gradient.
The process of water absorption is controlled by Na+/K+ ATPases located in the basolateral and lateral membranes of the epithelial cells. Na+ is pumped into extracellular spaces and tight junctions at the luminal membrane prevent the diffusion of Na+ and Cl- back from the extracellular space into the lumen. This leaves a hypertonic solution close to the lumen, causing water to follow by osmosis.
The electrolytes are absorbed by a variety of mechanisms (similar to those of the small intestine), with essentially a net movement of K+ and HCO3- into the lumen of the large intestine, because of the potential difference set up by the asymmetrical absorption of Na+ and Cl- across the epithelium.
Intestinal Flora
Commensal intestinal bacterial flora have a role in:
- Keeping pathogenic bacteria at bay by competing for space and nutrient
- Converting conjugated bilirubin to urobilinogen (some of which is reabsorbed and excreted in urine) and stercobilinogen which is excreted in the faeces
- The synthesis of vitamins K, B12, thiamine and riboflavin
- The breakdown of primary bile acids to secondary bile acids
- The breakdown of cholesterol, some food additives and drugs
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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 |
