Physiology
Renal
Where in the nephron is most water reabsorbed:
Answer:
The main function of the proximal tubule is reabsorption. The proximal tubule reabsorbs most glucose, amino acids, phosphate and bicarbonate together with about 60 - 70% of filtered Na+, K+, Ca2+, Cl-, urea and water.Proximal Tubule
Physiology / Renal / Mechanism of Filtration
Last Updated: 26th July 2024
The proximal tubule continues from the renal corpuscle. The wall of the proximal tubule is composed of a single layer of columnar epithelial cells which interdigitate extensively at the basal surface increasing their surface area and are connected by tight junctions at their luminal surface limiting diffusion through gaps between cells. The luminal surface of each cell is made up of millions of microvilli, forming a dense brush border that increases the surface area available for reabsorption of tubular filtrate.
The main function of the proximal tubule is reabsorption. The proximal tubule reabsorbs most glucose, amino acids, phosphate and bicarbonate together with about 60 - 70% of filtered Na+, K+, Ca2+, Cl-, urea and water.
Sodium
The concentration of Na+ in the filtrate is ~ 140 mmol/L, but is much lower in the cytosol of epithelial cells, which is also negatively charged. Tubular Na+ is thus absorbed into the epithelial cells of the proximal tubule down its electrochemical gradient, providing the driving force for the secondary transport of other substances. About 80% of the Na+ is reabsorbed via the Na+/H+ antiporter (this secretion of H+ is critical for HCO3- reabsorption) and some is coupled with the transport of other substances e.g. glucose, amino acids and phosphate ions.
Na+ is removed from tubular cells into the interstitial fluid by Na+/K+ ATPase pumps on the basolateral membrane, thus maintaining the electrochemical gradient for reabsorption of further Na+. However only about 20% of transported Na+ diffuses into the capillaries, as there is a significant back flux into the tubule via paracellular pathways.
Water
Water is reabsorbed by osmosis via both transcellular and paracellular pathways, following absorption of Na+ and HCO3- as the osmolality of the peritubular interstitial fluid increases and the osmolality of the tubular fluid decreases. The reabsorption of water increases tubular concentration of Cl-, K+, Ca2+ and urea which are therefore reabsorbed into the peritubular space passively down their concentration gradients, largely via paracellular pathways, although the route for Ca2+ may be transcellular.
Glucose
Glucose is reabsorbed by cotransport with Na+ across the apical membrane of epithelial cells and then diffuses out of the cells into the peritubular interstitium.
Function of the Proximal Tubule. (Image by OpenStax College [CC BY 3.0 , via Wikimedia Commons)
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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 |
