The blood flow to the kidneys averages 20 percent of the cardiac output. In terms of flow per 100 g weight, the renal blood flow (RBF) is four times greater than that to the liver or exercising muscle and eight times that of coronary blood flow. Blood enters the kidney through the renal arteries and passes through serial branches (interlobar, arcuate, interlobular) before entering the glomeruli via the afferent arterioles. The portion of the plasma not filtered across the glomerular capillary wall then leaves the glomeruli via the efferent arterioles and enters the postglomerular capillaries. In the cortex, these capillaries run in apposition to the adjacent tubules, although not necessarily to the tubule segments from the same glomerulus . In addition, branches from the efferent arterioles of the juxtamedullary glomeruli enter the medulla and form the vasa recta capillaries (figure 1). Blood returns to the systemic circulation through veins similar to the arteries in name and location.
The renal circulation affects urine formation in the following ways:
●The rate of glomerular filtration is an important determinant of solute and water excretion.
●The peritubular capillaries in the cortex return reabsorbed solutes and water to the systemic circulation and can modulate the degree of proximal tubular reabsorption and secretion (see "Chapter 3A: Cell model for proximal transport").
●The vasa recta capillaries in the medulla return reabsorbed salt and water to the systemic circulation and participate in the countercurrent mechanism, permitting the conservation of water by the excretion of a hyperosmotic urine (see "Chapter 4B: Countercurrent mechanism").