Physiology
Gastrointestinal
Regarding water and electrolyte absorption in the small intestine, which of the following statements is INCORRECT:
Answer:
As the contents of the intestine are isotonic with body fluids and mostly have the same concentration of the major electrolytes, their absorption is active. Water cannot be moved directly, but follows osmotic gradients set up by the transport of ions, primarily mediated by the sodium pump. Na+/K+ ATPase located on the basolateral membrane of the epithelial cells pumps three Na+ ions from the cell in exchange for two K+ ions, against their respective concentration gradients. This leads to a low intracellular concentration of Na+ and a high intracellular concentration of K+. The low intracellular concentration of Na+ ensures a movement of Na+ from the intestinal contents into the cell down its concentration gradient by both membrane channels and transporter protein mechanisms. Na+ is then rapidly pumped again by the basolateral sodium pump. K+ leaves the cell across the basolateral membrane down its concentration gradient linked to an outward movement of Cl- against its concentration gradient (Cl- having entered the cell across the luminal membrane down its concentration gradient).Function of Small Intestine
Physiology / Gastrointestinal / Small Intestine
Last Updated: 26th July 2024
The small intestine is the main site for digestive and absorptive processes. Motility of the small intestine (via peristalsis and segmental contraction) facilitates mixing of intestinal contents, the bringing of the intestinal contents into contact with the absorptive intestinal brush border and the forward propulsion of the contents. The small intestine absorbs water, electrolytes, carbohydrates, amino acids, minerals, fats and vitamins.
Duodenal Chyme
The chyme that first enters the duodenum is acidic, hypertonic and only partially digested. There is an osmotic movement of water across the freely permeable wall which leads to the contents becoming isotonic, and addition of both bicarbonate from the pancreas and bile from the liver which neutralises the acidity, allowing further digestion of the chyme performed by the addition of enzymes from the pancreas, liver and intestine itself.
Functional Anatomy
The mucosa of the small intestine has a much greater surface area for absorption than in other parts of the gastrointestinal tract.
Factors increasing the surface area include:
- The small intestine is very long - about 5 m in length.
- The inner wall of the small intestine is covered by numerous folds of mucous membrane called plicae circulares.
- The lining of the small intestine is folded into many finger-like projections called villi.
- The surface of the villi is covered with a layer of epithelial cells which, in turn, have many small projections called microvilli that project towards the lumen of the intestine (forming the brush border).
The microvilli are covered by a glycocalyx which contains many enzymes which are involved in digestion and transport. Each villus contains a single, blind-ended lymphatic vessel called a lacteal and also a capillary network, via which nutrients are absorbed.
Functional Anatomy of the Small Intestine. (Image by OpenStax College [CC BY 3.0 , via Wikimedia Commons)
Water Absorption and the Sodium Pump
As the contents of the intestine are isotonic with body fluids and mostly have the same concentration of the major electrolytes, their absorption is active. Water cannot be moved directly, but follows osmotic gradients set up by the transport of ions, primarily mediated by the sodium pump.
Na+/K+ ATPase located on the basolateral membrane of the epithelial cells pumps three Na+ ions from the cell in exchange for two K+ ions, against their respective concentration gradients. This leads to a low intracellular concentration of Na+ and a high intracellular concentration of K+. The low intracellular concentration of Na+ ensures a movement of Na+ from the intestinal contents into the cell down its concentration gradient by both membrane channels and transporter protein mechanisms. Na+ is then rapidly pumped again by the basolateral sodium pump. K+ leaves the cell across the basolateral membrane down its concentration gradient linked to an outward movement of Cl- against its concentration gradient (Cl- having entered the cell across the luminal membrane down its concentration gradient).
These movements set up an osmotic gradient between the lumen and the blood, leading to water absorption following the movement of Na+ and Cl- across the luminal membrane. Up to 9 L of water is absorbed from the gastrointestinal tract per day, most of it from the small intestine, especially the jejunum.
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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 |
