Physiology
Renal
About what percentage of filtered Na+ is reabsorbed in the proximal tubule:
Answer:
Of the filtered sodium, about 65% is reabsorbed in the proximal tubule.Sodium Handling
Physiology / Renal / Mechanism of Filtration
Last Updated: 21st April 2019
Sodium is the major extracellular cation and its concentration is tightly controlled.
Na+ is freely filtered at the glomerulus, a large amount is absorbed in the proximal tubules and the loop of Henle, and the little that is left is reabsorbed in a precisely regulated manner by the distal tubules and collecting ducts to maintain accurate salt balance.
The basolateral membranes of the tubular cells contain Na+/K+ ATPases that actively pump sodium into the peritubular interstitial fluid. From here, sodium ions pass freely into the blood. The continual pumping of sodium out of the cells and its subsequent removal by the blood creates a Na+ gradient between the tubular filtrate and the cell cytoplasm. This gradient allows Na+ from the filtrate to enter the cells passively at their apical membrane, provided that suitable channels or transporters are present.
Sodium Handling in the Proximal Tubule
Of the filtered sodium, about 65% is reabsorbed in the proximal tubule.
In the early tubule, the sodium gradient drives the cotransport of sodium with bicarbonate, amino acids, glucose or other organic molecules. The Na+/H+ exchanger uses the sodium gradient to drive sodium reabsorption from the filtrate and H+ secretion into the filtrate. As carbonic anhydrase is present in the cell cytoplasm and tubular lumen, the secretion of H+ is equivalent to the reabsorption of bicarbonate. The apical secretion of H+ is balanced by the basolateral exit of bicarbonate with sodium.
Chloride concentration rises along the proximal tubule. When the positively charged sodium ions leave the lumen with neutral organic molecules, the lumen is left with a negative charge. This repels negatively charged chloride ions which leave the lumen through the paracellular route between cells. By the time the filtrate reaches the late proximal tubule, most organic molecules and bicarbonate have already been removed and sodium ions are reabsorbed mainly with chloride ions.
Sodium Handling in the Loop of Henle
About 25% of filtered sodium is reabsorbed in the the loop of Henle.
The thin descending limb is permeable to water but not to sodium, so water leaves the tubule passively to enter the hypertonic medullary interstitium. In contrast, the thin ascending limb is permeable to sodium but not to water and as there is a high tubular concentration of NaCl, both ions diffuse out.
In the thick ascending limb, the Na+/K+/2Cl- cotransporter uses the sodium gradient to actively reabsorb one sodium, one potassium and two chloride ions. As the potassium ions can re-enter the tubule via an ROMK channel, the net effect is the removal of one sodium and two chloride ions, leaving the tubular lumen positively charged. This positive potential drives the paracellular transport of positively charged ions, including sodium, potassium, calcium, magnesium and ammonium.
Sodium Handling in the Distal Nephron
The early distal tubule reabsorbs a further 5% of the filtered sodium, mainly via the Na+/Cl- symporter. As the fluid in the lumen in this portion of the nephron is negative, there is also some paracellular movement of negatively charged chloride ions.
About 2 - 5% of filtered sodium is reabsorbed in the late distal tubule and collecting ducts. Sodium reabsorption by principal cells and chloride reabsorption by intercalated cells are the final stages in sodium chloride reabsorption before urine leaves the kidney.
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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 |
