The kidneys lie against the back muscles in the upper abdomen at about the level of the last thoracic and first three lumbar vertebrae. The right kidney is slightly lower than the left to accommodate the liver. Each kidney is firmly enclosed in a membranous renal capsule made of fibrous connective tissue. In addition, there is a protective layer of fat called the adipose capsule around the organ. An outermost layer of fascia (connective tissue) anchors the kidney to the peritoneum and abdominal wall. The kidneys, as well as the ureters, lie posterior to the peritoneum. Thus, they are not in the peritoneal cavity but rather in an area known as the retroperitoneal space.
Blood Supply to the Kidney
The kidney’s blood supply is illustrated in Figure 18-2. Blood is brought to the kidney by a short branch of the abdominal aorta called the renal artery. After entering the kidney, the renal artery subdivides into smaller and smaller branches, which eventually make contact with the functional units of the kidney, the nephrons. Blood leaves the kidney by vessels that finally merge to form the renal vein, which carries blood into the inferior vena cava for return to the heart.
Structure of the Kidney
The kidney is a somewhat flattened organ about 10 cm (4 inches) long, 5 cm (2 inches) wide, and 2.5 cm (1 inch) thick (Fig. 18-3). On the medial border there is a notch called the hilum, where the renal artery, the renal vein, and the ureter connect with the kidney. The lateral border is convex (curved outward), giving the entire organ a bean-shaped appearance. The kidney is divided into two regions: the renal cortex and the renal medulla (Fig. 18-3). The renal cortex is the kidney’s outer portion. The renal medulla contains the tubes in which urine is formed and collected. These tubes form a number of cone-shaped structures called renal pyramids. The tips of the pyramids point toward the renal pelvis, a funnel-shaped basin that forms the upper end of the ureter. Cuplike extensions of the renal pelvis surround the tips of the pyramids and collect urine; these extensions are called calyces (singular, calyx). The urine that collects in the pelvis then passes down the ureters to the bladder.
Figure 18-2 Blood supply and circulation of the kidney.
Figure 18-3 Longitudinal section through the kidney showing its internal structure (left) and an enlarged diagram of nephrons (right). Each kidney contains more than 1 million nephrons.
The Nephron As is the case with most organs, the most fascinating aspect of the kidney is too small to be seen with the naked eye. This basic unit, which actually does the kidney’s work, is the nephron (Fig. 18-4). The nephron is essentially a tiny coiled tube with a bulb at one end. This bulb, known as the glomerular (Bowman) capsule, surrounds a cluster of capillaries called the glomerulus (pl., glomeruli). Each kidney contains about 1 million nephrons; if all these coiled tubes were separated, straightened out, and laid end to end, they would span some 120 kilometers (75 miles)! Figure 18-5 is a microscopic view of kidney tissue showing several glomeruli, each surrounded by a glomerular capsule. This figure also shows sections through the tubular portions of the nephrons.
Figure 18-4 A nephron and its blood supply. The nephron regulates the proportions of water, waste, and other materials according to the body’s constantly changing needs. Materials that enter the nephron can be returned to the blood through the surrounding capillaries.
A small blood vessel, the afferent arteriole, supplies the glomerulus with blood; another small vessel, called the efferent arteriole, carries blood from the glomerulus. When blood leaves the glomerulus, it does not head immediately back toward the heart. Instead, it flows into a capillary network that surrounds the nephron’s tubular portion. These peritubular capillaries, are named for their location. The tubular portion of the nephron consists of several parts. The coiled part leading from the glomerular capsule is called the proximal convoluted tubule (PCT, or just proximal tubule). The tubule then uncoils to form a hairpin-shaped segment called the loop of Henle. The first part of the loop, which carries fluid toward the medulla, is the descending limb (see Fig. 18-4). The part that continues from the loop’s turn and carries fluid away from the medulla, is the ascending limb. Continuing from the ascending limb, the tubule coils once again into the distal convoluted tubule (DCT, or just distal tubule), so called because it is farther along the tubule from the glomerular capsule than is the PCT. The distal end of each tubule empties into a collecting duct, which then continues through the medulla toward the renal pelvis. The glomerulus, glomerular capsule, and the proximal and distal convoluted tubules of the nephron are within the renal cortex. The loop of Henle and collecting duct extend into the medulla (see Fig. 18-3).
Figure 18-5 Microscopic view of the kidney.
The Juxtaglomerular (JG) Apparatus The first portion of the DCT curves back toward the glomerulus to pass between the afferent and efferent arterioles (Fig. 18-6). At the point where the DCT makes contact with the afferent arteriole, there are specialized cells in each that together make up the juxtaglomerular (JG) apparatus. The JG apparatus helps to regulate kidney function. When blood pressure falls too low for the kidneys to function effectively, cells in the wall of the afferent arteriole secrete the enzyme renin, which raises blood pressure by a mechanism described later.
Figure 18-6 Structure of the juxtaglomerular (JG) apparatus. Note how the distal convoluted tubule contacts the afferent arteriole (right). Cells in these two structures make up the JG apparatus.