Renel Function

The kidneys' principal role is the elimination of waste material and the regulation of the volume and composition of body fluid. The kidneys have a unique system involving the free ultrafiltration of water and non-protein-bound low-molecular-weight compounds from the plasma and the selective reabsorption and/or excretion of these as the ultrafiltrate passes along the tubule.

An essential feature of renal function is that a large volume of blood - 25% of cardiac output or approximately 1300 mL per minute - passes through the two million glomeruli.

A hydrostatic pressure gradient of approximately 10 mmHg (a capillary pressure of 45 mmHg minus 10 mmHg of pressure within Bowman's space and 25 mmHg of plasma oncotic pressure) provides the driving force for ultrafiltration of virtually protein-free and fat-free fluid across the glomerular capillary wall into Bowman's space and so into the renal tubule.

The ultrafiltration rate varies with age and sex but is approximately 120-130 mL/min per 1.73 m 2 surface area in adults. This means that each day ultrafiltration of 170-180 L of water and unbound small-molecular-weight constituents of blood occurs. The 'need' for this high filtration rate relates to the elimination of compounds present in relatively low concentration in plasma (e.g. urea).

If these large volumes of ultrafiltrate were excreted unchanged as urine, it would be necessary to ingest huge amounts of water and electrolytes to stay in balance. This is avoided by the selective reabsorption of water, essential electrolytes and other blood constituents, such as glucose and amino acids, from the filtrate in transit along the nephron.

Thus, 60-80% of filtered water and sodium are reabsorbed in the proximal tubule along with virtually all the potassium, bicarbonate, glucose and amino acids.

Further water and sodium chloride are reabsorbed more distally, and fine tuning of salt and water balance is achieved in the distal and collecting tubules under the influence of aldosterone and antidiuretic hormone (ADH). The final urine volume is thus 1-2 L daily. Calcium, phosphate and magnesium are also selectively reabsorbed in proportion to the need to maintain a normal electrolyte composition of body fluids.

The urinary excretion of some compounds is more complicated. For example, potassium is freely filtered at the glomerulus, almost completely absorbed in the proximal tubule, and excreted in the distal tubule and collecting ducts.

An important clinical consequence of this is that the ability to eliminate unwanted potassium is less dependent on GFR than is the elimination of urea or creatinine. Other compounds filtered and reabsorbed or excreted to a variable extent include urate and many organic acids, including many drugs or their metabolic breakdown products.

The more tubular secretion of a compound occurs, the less dependent is elimination on the GFR; penicillin and cefradine are examples of compounds secreted by the tubules.