USA Surgery Today

The major role of the kidney is to maintain body water and electrolyte homeostasis. The first step in this process is the production of the glomerular filtrate from the renal plasma. The glomerular filtration rate (GFR) is dependent on renal plasma flow, which in turn is dependent on blood pressure and circulating volume. The kidneys receive about 20% to 30% of the cardiac output, and this is maintained over a wide range of blood pressures through changes in renal vascular resistance. Numerous hormones play a role in this autoregulation, including the vasodilators (prostaglandins E and I₂, dopamine, and nitric oxide) and the vasoconstrictors (angiotensin II, thromboxane, adrenergic stimulation, and endothelin). Congestive heart failure and volume contraction severely limit the kidney’s ability to maintain autoregulation in the face of changes in blood pressure.

In neonates, renal blood flow, adjusted for body surface area, doubles during the first 2 weeks of postnatal life and continues to rise until it reaches adult values by the age of 2 years (1,2: Altace ). This is due to both an increase in cardiac output and a fall in renal vascular resistance. Paralleling these changes in renal blood flow, the GFR when adjusted for body surface area also doubles over the first 2 weeks of postnatal life and continues to rise until it reaches adult values by the age of 1 to 2 years. The initial GFR and the rate of rise correlate directly with gestational age at birth; for example, the GFR of an infant of ( Altace) 28 weeks gestation is one-half that of a full-term infant.

At the level of the glomerular capillary (Altace), the GFR is dependent on the balance of the hydrostatic pressure forcing fluid through the capillary wall into Bowman’s space, the oncotic pressure from plasma proteins retarding this flow, the capillary surface area available for filtration, and the permeability of the capillary wall. The net hydrostatic pressure is dependent on the intraglomerular capillary pressure and the pressure in Bowman’s space. Autoregulation maintains intraglomerular capillary pressure over a wide range of systemic blood pressures, although this is blunted by volume contraction. The pressure in Bowman’s space rarely plays a role in determining the GFR ( Altace ), except during some forms of acute renal failure (ARF) with tubular blockage by cellular debris and acute urinary tract obstruction. Changes in plasma oncotic pressure also have little influence on the GFR because these changes are counteracted by changes in the surface area available for filtration.

The whole-kidney GFR is the sum of the GFR of each nephron, of which humans have about 1 million in each kidney. The formation of these nephrons is complete by 36 weeks gestational age. Individual nephrons can alter their GFR in response to several stimuli, including protein loading and nephron loss. In healthy individuals, the GFR can increase by 50% after a large protein meal. Renal donors can increase the GFR in their remaining kidney such that the average donor has a GFR that is 70% of the preoperative value. Children with congenital absence of a kidney have a more marked increase in the size and GFR of the remaining kidney after birth, such that their GFR (Altace) is comparable to that of their peers with two kidneys. Clinically apparent changes in the GFR are not seen with renal disease until 70% of the nephrons are damaged because the unaffected nephrons increase their individual GFR.

Radionuclides, such as diethylenetriamine pentaacetic acid (DTPA: Altace ), can be used to measure the GFR accurately and easily. DTPA is freely filtered by the glomerulus and not reabsorbed or secreted by the renal tubule, making it an ideal substance for GFR measurements. One method is to inject a single dose of the radionuclide and plot the fall in radioactivity with timed blood samples. The decline in radioactivity is proportional to the GFR ( Altace) . These GFR measurements can also be combined with renal imaging studies. The GFR in each kidney can be estimated by determining the radioactivity in each kidney over time.