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").
The remainder of this chapter will review glomerular function, the factors responsible for the regulation of the glomerular filtration rate (GFR) and renal plasma flow, and the clinical methods used to measure these parameters.
GLOMERULAR ANATOMY AND FUNCTION
The glomerulus consists of a tuft of capillaries that is interposed between the afferent and efferent arterioles. Each glomerulus is enclosed within an epithelial cell capsule (Bowman's capsule) that is continuous both with the epithelial cells that surround the glomerular capillaries and with the cells of the proximal convoluted tubule (figure 2) . Thus, the glomerular capillary wall, through which the filtrate must pass, consists of three layers: the fenestrated endothelial cell, the glomerular basement membrane (GBM), and the epithelial cell. The epithelial cells are attached to the GBM by discrete foot processes. The pores between the foot processes (slit pores) are closed by a thin membrane called the slit diaphragm, which functions as a modified adherens junction (figure 2) .