The renal artery progressively branches into the interlobar artery, the arcuate artery, the interlobular artery, and the afferent arteriole. These arterioles lead into the glomerular capillaries, which then form the efferent arteriole. The efferent arteriole connects to the peritubular capillaries, which supply blood to the nephron. The venous system runs parallel to the arterial vessels and forms the interlobular vein, arcuate vein, interlobar vein, and renal vein. These veins course beside the ureter.

The nephron is the functional unit of the kidneys and each kidney contains approximately 1.2 million nephrons. Nephrons are hollow tubes composed of a single epithelial cell layer. A nephron consists of a renal corpuscle, proximal tubule, loop of Henle, distal tubule, and collecting duct system. The renal corpuscle includes glomerular capillaries enclosed within Bowman's capsule. The proximal tubule exits the renal corpuscle and coils before descending towards the medulla. The loop of Henle is composed of a straight portion, descending thin limb, ascending thin limb (only in nephrons with long loops of Henle), and a thick ascending limb. The macula densa, a short segment of the thick ascending limb, is located near the end of the thick ascending limb and abuts the glomerulus.

The organization of the nephron is more complex than presented here, but for simplicity, it is divided into five segments. The collecting duct system, although not part of the nephron, is considered as part of it for simplicity. The term "glomerulus" is commonly used to describe the renal corpuscle, even though it is composed of glomerular capillaries and Bowman's capsule.

The vascular system of the human kidney is organized with interlobar arteries and veins, arcuate arteries and veins, and interlobular arteries and veins. The afferent and efferent arterioles connect to the glomerular capillary networks. The loop of Henle includes the straight portion of the proximal tubule, descending thin limb, ascending thin limb, and thick ascending limb. Other components of the nephron include Bowman's capsule, cortical collecting duct, distal tubule, inner medullary collecting duct, macula densa, outer medullary collecting duct, and pelvis.
Fig. 33.2 (left), the renal artery branches progressively to form the interlobar artery, the arcuate artery, the interlobular artery, and the afferent arteriole, which leads into the glomerular capillaries. The glomerular capillaries come together to form the efferent arteriole, which leads into a second capillary network, the peritubular capillaries, which supply blood to the nephron. The vessels of the venous system run parallel to the arterial vessels and progressively form the interlobular vein, arcuate vein, interlobar vein, and renal vein, which courses beside the ureter.
Ultrastructure of the Nephron
The functional unit of the kidneys is the nephron. Each human kidney contains approximately 1.2 million nephrons, which are essentially hollow tubes composed of a single epithelial cell layer. The nephron consists of a renal corpuscle, proximal tubule, loop of Henle, distal tubule, and collecting duct system (Fig. 33.3
Fig. 33.2 ). The renal corpuscle consists of glomerular capillaries enclosed within Bowman’s capsule. The proximal tubule exits this structure and initially forms several coils, followed by a straight piece that descends toward the medulla. The next segment is the loop of Henle, which is composed of aThe organization of the nephron is actually more complicated than presented here. However, for simplicity and clarity of presentation in subsequent chapters, the nephron is divided into five segments. The collecting duct system is not actually part of the nephron. However, again for simplicity, we consider the collecting duct system part of the nephron. bAlthough the renal corpuscle is composed of glomerular capillaries and Bowman’s capsule, the term glomerulus is commonly used to described the renal corpuscle.
• Fig. 33.2 Left, Organization of the vascular system of the human kidney. 1, interlobar arteries; 1a, interlobar vein; 2, arcuate arteries; 2a, arcuate veins; 3, interlobular arteries; 3a, interlobular veins; 4, stellate vein; 5, afferent arterioles; 6, efferent arterioles; 7a, 7b, glomerular capillary networks; 8, descending vasa recta; 9, ascending vasa recta. Right, Organization of the human nephron. A superficial nephron is illustrated on the left and a juxtamedullary (JM) nephron is illustrated on the right. The loop of Henle includes the straight portion of the proximal tubule (PT), descending thin limb (DTL), ascending thin limb (ATL), and thick ascending limb (TAL). B, Bowman’s capsule; CCD, cortical collecting duct; DT, distal tubule; IMCD, inner medullary collecting duct; MD, macula densa; OMCD, outer medullary collecting duct; P, pelvis. (Modified from Kriz W, Bankir LA. Am J Physiol 1988;254:F1; and Koushanpour E, Kriz W. Renal Physiology: Principles, Structure, and Function. 2nd ed. New York: Springer-Verlag; 1986.) the straight part of the proximal tubule, the descending thin limb (which ends in a hairpin turn), the ascending thin limb (only in nephrons with long loops of Henle), and the thick ascending limb. Near the end of the thick ascending limb, the nephron passes between the afferent and efferent arterioles of the same nephron. This short segment of the thick ascending limb abutting the glomerulus is called the macula densa (see