Renal Clearance
Renal clearance of a substance is plasma volume that completely clears a particular substance per minute.
Clearance rate of a substance
= [Urine concentration of that substance(amount∕mL)
? Urine flow rate(mL∕ min)]/
Plasma concentration of that substance(amount∕mL).
If a substance is freely filtered but not reabsorbed or secreted, the plasma clearance rate will equal the glomerular filtration rate (GFR). But no endogenous substance is completely cleared from plasma through the kidneys. For example, the glucose is freely filtered, complete reabsorption takes place usually and glucose clearance will be zero. Since 50% of filtered urea is reabsorbed, the plasma clearance of urea is 50% of GFR. The H+ is freely filtered as well as secreted but not reabsorbed. So, the plasma clearance rate of H+ is always greater than GFR.
An exogenous biologically inert substance, inulin obtained from tubers of a plant, is neither reabsorbed nor secreted but freely filtered. So, inulin clearance rate will be a measure of GFR. Determination of plasma inulin clearance requires the continuous intravenous infusion of inulin to maintain a constant plasma concentration. Because of this reason, renal clearance of an endogenous substance, creatinine (end product of muscle metabolism), is usually used clinically to give a rough estimate of GFR. Creatinine is freely filtered, not reabsorbed but slightly secreted. So, creatinine clearance is not an absolute reflection of GFR but gives a close approximation.
A substance must be freely filtered, and the remaining should be secreted for it to be completely cleared from total plasma reaching kidneys because GFR constitutes only 20% of renal plasma flow. Such a substance can only be used for the measurement of renal plasma flow. But no known substance is completely cleared from plasma.
However, para- aminohippuric acid (PAH) is the only substance that is cleared up to 90% from plasma. So, renal PAH clearance can be used to approximate renal plasma flow measurement.9.8.1 Assessment of Renal Function
The renal function tests may be classified into (1) tests which measure glomerular filtration rate and (2) tests which study the tubular function. In clinical biochemistry, certain physical and chemical constituents are checked when reporting on a urine sample (Table 9.2). The physical constituents have volume, appearance, odour, colour and specific gravity. The chemical characteristics usually checked are pH, proteins, blood, reducing sugars (glycosuria), ketone bodies, bile pigments and nonprotein nitrogen compounds like urea, creatinine and uric acid. Clearance tests are usually performed to assess the GFR. The measurement of specific gravity or osmolality indicates tubular function. The tubular efficiency for urine concentration can be evaluated using water deprivation and ADH response tests.
Table 9.2 Characteristics of urine in different species
| Parameters | Cattle | Sheep | Goat | Horse | Dog | Cat | Rabbit | Humans |
| Urine volume | 16-50 mL/kg | 10-40 mL/kg | 10-40 mL/kg | 8-30 mL/kg | 14-50 mL/kg | 18-25 mL/ kg | 20-350 mL/ kg | 1-2 L/day |
| Colour | Pale yellowdark brown yellow | Pale yellowdark brown yellow | Pale yellowdark brown yellow | Ochre | Pale yellowdark brown yellow | Yellowstrong dark yellow | Pale yellow-red brown | Colourless- umber |
| Transparency | Clear | Clear | Clear | Turbid | Clear | Clear | Clear | Clear |
| Odour | Aromatic | Indifferent aromatic | Indifferent aromatic | Aromatic | Garlicy | Sharp | n.s. | Coffee, saffron, onion |
| Specific gravity | 1.020-1.040 | 1.020-1.040 | 1.020-1.040 | 1.020-1.040 | 1.001-1.065 | 1.001-1.080 | 1.003-1.036 | 1.003-1.030 |
| pH value | 7.0-8.4 | 7.5-8.5 | 7.5-8.5 | 7.6-9.0 | 5.5-7.0 | 5.0-7.0 | 8.2 | 4.6-8 {7} |
| Protein | Negative | Negative | Negative | Negative | Negative | Negative | Negative | 0-20 mg/dL |
| Glucose | Negative | Negative | Negative | Negative | Negative | Negative | Negative | Negative |
| Ketones | Negative | Negative | Negative | Negative | Negative | Negative | Negative | Negative |
| Bilirubin | Negative | Negative | Negative | Negative | Negative- weak positive | Negative | Negative | Negative |
| Urobilinogen | Negative- weak positive | Negative- weak positive | Negative- weak positive | Negative- weak positive | Negative- weak positive | Negative- weak positive | Negative- weak positive | 0.2-1 mg/ dL |
| Blood | Negative | Negative | Negative | Negative | Negative | Negative | Negative | Negative |
| Leucocytes | Negative | Negative | Negative | Negative | Negative | Negative | Negative | 0-2/high- power field (Hpf) |
9.8.2 Diuresis
It is the increased urine formation by the kidneys resulting in excessive urination. The reasons for diuresis are increased water intake, action of diuretic drugs and certain diseases such as diabetes mellitus and diabetes insipidus.
In diabetes mellitus, the extra glucose present in the tubular fluid buildup increases osmotic pressure and causes osmotic diuresis. In diabetes insipidus, impaired water reabsorption occurs due to lack of sufficient ADH and excessive water loss through urine. The increased water loss may lead to dehydration and polydipsia.9.9