Fig. 33.2, right). When compared with superficial nephrons, juxtamedullary nephrons differ anatomically in two important ways: the loop of Henle is longer and extends deeper into the medulla, and the efferent arteriole forms not only a network of peritubular capillaries but also a series of accompanying vascular loops called the vasa recta.
Fig. 33.2 (left), the vasa recta descend into the medulla, where they form capillary networks that surround the collecting ducts and ascending limbs of the loop of Henle. The blood returns to the cortex via the ascending vasa recta. Although less than 0.7% of the renal blood flow (RBF) enters the vasa recta, these vessels serve important functions in the renal medulla that include (1) conveying oxygen and important metabolic substrates to support nephron function, (2) delivering substances to the nephron for secretion, (3) serving as a pathway for return of reabsorbed water and solutes to the circulatory system, and (4) concentrating and diluting urine (urine concentration and dilution are discussed in more detail in
Ultrastructure of the Glomerulus
The first step in urine formation begins with passive movement of a plasma ultrafiltrate from the glomerular capillaries (i.e., glomerulus) into Bowman’s space. The term ultrafiltration refers to this passive movement of fluid—similar in composition to plasma, except for the fact that the ultrafiltrate protein concentration is much lower than that in the plasma—from the glomerular capillaries into Bowman’s space. To appreciate this process, one must understand the anatomy of the glomerulus, which consists of a network of capillaries supplied by the afferent arteriole and drained by the efferent arteriole (
Figs. 33.5 33.6 ). During embryological development, the glomerular capillaries press into the closed end of the proximal tubule, forming Bowman’s capsule. As the epithelial cells thin on the outside circumference of Bowman’s capsule, they form the parietal epithelium (see
Fig. 33.5 ). The epithelial cells in contact with the capillaries thicken and develop into podocytes, which form the visceral layer of Bowman’s capsule (
Figs. 33.7–33.9 ). The visceral cells face outward at the vascular pole (i.e., where afferent and efferent arterioles enter and exit Bowman’s capsule) to form the parietal layer of Bowman’s capsule. The space between the visceral layer and the parietal layer is Bowman’s space, which at the urinary pole (i.e., where the proximal tubule joins Bowman’s capsule) of the glomerulus becomes the lumen of the proximal tubule.
The endothelial cells of glomerular capillaries are covered by a basement membrane surrounded by podocytes. The capillary endothelium, basement membrane, and foot processes of podocytes form the so-called filtration barrier (see
Figs. 33.5 and 33.7–33.9 ). The endothelium is fenestrated