The renal tubule of every nephron is closely associated with two capillary beds: the glomerulus and the peritubular capillaries. The glomerulus, in which the capillaries run in parallel, is specialized for filtration. It differs from all other capillary beds in the body in that it is both fed and drained by arterioles-the afferent arteriole and the efferent arterioles, respectively.
The afferent arterioles arise from the cortical radiate arteries that run through the renal cortex. The blood pressure in the glomerulus is extraordinarily high for a capillary bed because arterioles are high-resistance vessels and the afferent-arteriole has a larger diameter than the efferent. This high blood pressure easily forces fluid and solutes out of the blood into the glomerular capsule. Most of the filtrating filtrate (99%) is reabsorbed by the renal tubule cells and returned to the blood in the peritubular capillary beds.
The peritubular capillaries arise from the efferen arterioles draining the glomeruli. These capillaries cling closely to adjacent renal tubules and empty into nearby venules. They are low-pressure, porous capillaries that readily absorb solutes and water from the tubule cells as these substances are reclaimed from the filtrate.
The efferent arterioles serving the juxtamedullary nephrons tend not to break up into meandering peritubular capillaries. Instead they form bundles of long straight vessels called vasa recta that extend deep into the medulla paralelling the longest loops of Henle. The thin -walled vasa recta play an important role in forming concentrated urine.
In summary, the microvasculature of the nephrons consists of two capillary beds separated by intervening efferent arterioles. The first capillary bed (glomerulus) produce the filtrate. The second (peritubular capillaries) reclaims most of that filtrate.
Blood flowing through the renal circulation encounters high resistance, first in the afferent and then in the efferent arterioles. As a result, renal blood pressure declines from approximately 95mmHg in the renal arteries to 9 mmHg or less in the renal veins. The resistance of the afferent arterioles protects the glomeruli from large fluctuations in systemic blood pressure. Resistance in the efferent arterioles reinforces the high glomerular pressure and reduces the hydrostatic pressure in the peritubular capillaries.