Physiology
Renal
The normal renal plasma flow in a healthy adult is about ________ :
Answer:
The normal renal plasma flow (RPF) is about 600 mL/min.Glomerular Filtration Rate
Physiology / Renal / Mechanism of Filtration
Last Updated: 26th July 2024
Glomerular Filtration Rate
The glomerular filtration rate (GFR) is the amount of ultrafiltrate produced from plasma flowing through the glomerulus per unit time.
The normal glomerular filtration rate is about 120 mL/min. The total amount filtered per day is about 180 L/day.
The normal renal plasma flow (RPF) is about 600 mL/min.
Therefore the filtration fraction (FF = GFR/RPF), the proportion of plasma that is filtered, is about 20%.
Factors Determining the GFR
The GFR is dependent on the difference between the hydrostatic and oncotic pressures in the glomerular capillaries and Bowman's capsule, as determined by Starling's equation.
The glomerular capillary pressure is greater than that elsewhere (~45 mmHg) because of the unique arrangement of afferent and efferent arterioles, with low afferent but high efferent resistances. As the pressure in the Bowman's capsule is ~10 mmHg, the net hydrostatic force driving filtration is ~35 mmHg.
This is opposed by the oncotic pressure of capillary plasma (~25 mmHg); the filtrate oncotic pressure is essentially zero (no protein).
Therefore, GFR = [Glomerular capillary hydrostatic pressure (Pc) - Bowman's capsule hydrostatic pressure (Pb)] - Capillary oncotic pressure (?c) = 35 mmHg - 25 mmHg = 10 mmHg
It should be noted that as fluid is filtered and plasma proteins are not filtered, the oncotic pressure in the capillary will rise as blood traverses the glomerulus, reducing (but not abolishing) filtration. It is this raised oncotic pressure (together with a relatively low hydrostatic pressure) that promotes reabsorption in the peritubular and vasa recta capillaries.
Factors Determining the Glomerular Filtration Rate. (Image by OpenStax College [CC BY 3.0 , via Wikimedia Commons)
Regulation of Glomerular Filtration Rate
Glomerular capillary hydrostatic pressure and thus GFR is strongly dependent on the relative resistance of the afferent and efferent arterioles. High pressure in the glomerular capillaries forces filtrate through the filtration barrier. This pressure is reduced by afferent arteriolar constriction and increased by efferent arteriolar constriction.
The glomerular filtration rate (GFR) is constant over a wide range of blood pressures (90 - 200 mmHg) because of substantial renal autoregulation, such that if systemic BP falls and renal perfusion pressure falls, GFR is maintained.
Renal disease, circulating and local vasoconstrictors, and sympathetic activation all reduce GFR, although angiotensin II preferentially constricts efferent arterioles, increasing the glomerular capillary hydrostatic pressure and thus increasing the GFR.
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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 |
