Physiology
Renal
Regarding renal handling of urea, which of the following statements is CORRECT:
Answer:
Urea is freely filtered at the glomerulus. In the proximal tubule, about 50% of the filtered urea is reabsorbed passively, mainly through paracellular routes. Urea is secreted into the ascending limb from the medullary interstitium down its concentration gradient, increasing tubular urea concentration. The distal tubule, cortical collecting ducts and the outer medullary ducts are impermeable to urea. As it continues along the nephron, tubular urea therefore becomes more concentrated as further solutes and water are reabsorbed, and thus in the inner medullary collecting ducts (which are permeable to urea), urea is passively reabsorbed by urea transporters (activated by ADH) into the medullary interstitium. The recycling of urea between the medullary collecting ducts and the ascending limb plays an important role in the creation of a hypertonic medullary interstitium, accounting for about half of the medullary interstitial osmolality that drives water reabsorption from the descending limb and medullary collecting duct. ADH increases the permeability of the inner medullary collecting ducts to urea, further increasing the medullary osmolality and allowing further concentration of urine.Renal Handling of Water and Urea
Physiology / Renal / Mechanism of Filtration
Last Updated: 21st April 2019
Renal Handling of Water
The kidney regulates body water and sodium content in parallel to maintain body volume and osmolality (normally 285 - 295 mOsmol/kg).
In the glomerulus, water and ions are freely filtered. As the filtrate moves along the tubules, ions are reabsorbed and water follows by osmosis. Water reabsorption is influenced by the water permeability of the tubular epithelium and the osmotic gradient across the epithelium.
Proximal tubule:
The proximal tubule is highly water permeability and about 65% of filtered water is reabsorbed here by osmosis, following the reabsorption of ions.
Loop of Henle:
In the loop of Henle, the descending limb is permeable to water, but not ions, whereas the ascending limb is permeable to ions but not water. Sodium and chloride are transported out of the thick ascending limb into the medullary interstitium. This raises the osmolality of the interstitium which promotes water movement out of the descending limb. Within the loop, the transport of water and ions is separated with reabsorption of about 25% of filtered sodium and chloride but only 10% of filtered water, producing a dilute urine and a hypertonic medullary interstitium.
Distal tubule:
The distal convoluted tubule has low water permeability and does not reabsorb water. However, reabsorption of ions further dilutes the tubular fluid.
Collecting ducts:
The hypotonic urine passes down the collecting ducts, where water permeability is controlled by antidiuretic hormone (ADH).
Role of Urea
Urea is freely filtered at the glomerulus. In the proximal tubule, about 50% of the filtered urea is reabsorbed passively, mainly through paracellular routes.
Urea is secreted into the ascending limb from the medullary interstitium down its concentration gradient, increasing tubular urea concentration.
The distal tubule, cortical collecting ducts and the outer medullary ducts are impermeable to urea. As it continues along the nephron, tubular urea therefore becomes more concentrated as further solutes and water are reabsorbed, and thus in the inner medullary collecting ducts (which are permeable to urea), urea is passively reabsorbed by urea transporters (activated by ADH) into the medullary interstitium.
The recycling of urea between the medullary collecting ducts and the ascending limb plays an important role in the creation of a hypertonic medullary interstitium, accounting for about half of the medullary interstitial osmolality that drives water reabsorption from the descending limb and medullary collecting duct. ADH increases the permeability of the inner medullary collecting ducts to urea, further increasing the medullary osmolality and allowing further concentration of urine.
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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 |
