Gross Anatomic Features of the Kidney
The kidneys are bean-shaped organs that lie in the retroperitoneal cavity of the body. In sagittal section, the kidneys have three main regions (Fig. 6-1): (1) The cortex is the outer region, located just under the kidney capsule. (2) The medulla is a central region, divided into an outer medulla and an inner medulla. The outer medulla has an outer stripe and an inner stripe. (3) The papilla is the innermost tip of the inner medulla and empties into pouches called minor and major calyces, which are extensions of the ureter. The urine from each kidney drains into a ureter and is transported to the bladder for storage and subsequent elimination.
Figure 6–1 Sagittal and coronal sections of the kidney.
Structure of the Nephron
The functional units of the kidney are nephrons. Each kidney contains approximately 1 million nephrons (Fig. 6-2). A nephron consists of a glomerulus and a renal tubule. The glomerulus is a glomerular capillary network, which emerges from an afferent arteriole. Glomerular capillaries are surrounded by Bowman’s capsule (or Bowman’s space), which is continuous with the first portion of the nephron. Blood is ultrafiltered across the glomerular capillaries into Bowman’s space, which is the first step in urine formation. The remainder of the nephron is a tubular structure lined with epithelial cells, which serve the functions of reabsorption and secretion.
Figure 6–2 Segments of a superficial and a juxtamedullary nephron.
The nephron or renal tubule comprises the following segments (beginning with Bowman’s space): the proximal convoluted tubule, the proximal straight tubule, the loop of Henle (which contains a thin descending limb, a thin ascending limb, and a thick ascending limb), the distal convoluted tubule, and the collecting ducts. Each segment of the nephron is functionally distinct, and the epithelial cells lining each segment have a different ultrastructure (Fig. 6-3). For example, the cells of the proximal convoluted tubule are unique in having an extensive development of microvilli, called a brush border, on their luminal side. The brush border provides a large surface area for the major reabsorptive function of the proximal convoluted tubule. Other correlations between cell ultrastructure and function will be emphasized throughout the chapter.
Figure 6–3 Schematic diagram of a nephron. Ultrastructural features are shown for major segments of the nephron.
There are two types of nephrons, superficial cortical nephrons and juxtamedullary nephrons, which are distinguished by the location of their glomeruli. The superficial cortical nephrons have their glomeruli in the outer cortex. These nephrons have relatively short loops of Henle, which descend only into the outer medulla. The juxtamedullary nephrons have their glomeruli near the corticomedullary border. The glomeruli of the juxtamedullary nephrons are larger than those of the superficial cortical nephrons and, accordingly, have higher glomerular filtration rates. The juxtamedullary nephrons are characterized by long loops of Henle that descend deep into the inner medulla and papilla and are essential for the concentration of urine.
Blood enters each kidney via a renal artery, which branches into interlobar arteries, arcuate arteries, and then cortical radial arteries. The smallest arteries subdivide into the first set of arterioles, the afferent arterioles. The afferent arterioles deliver blood to the first capillary network, the glomerular capillaries, across which ultrafiltration occurs. Blood leaves the glomerular capillaries via a second set of arterioles, the efferent arterioles, which deliver blood to a second capillary network, the peritubular capillaries. The peritubular capillaries surround the nephrons. Solutes and water are reabsorbed into the peritubular capillaries, and a few solutes are secreted from the peritubular capillaries. Blood from the peritubular capillaries flows into small veins and then into the renal vein.
The blood supply of superficial cortical nephrons differs from that of juxtamedullary nephrons. In the superficial nephrons, peritubular capillaries branch off the efferent arterioles and deliver nutrients to the epithelial cells. These capillaries also serve as the blood supply for reabsorption and secretion. In the juxtamedullary nephrons, the peritubular capillaries have a specialization called the vasa recta, which are long, hairpin-shaped blood vessels that follow the same course as the loop of Henle. The vasa recta serve as osmotic exchangers for the production of concentrated urine.