Gross Anatomy

Normally, two kidneys are located behind the peritoneum, not really in the abdominal cavity, one on either side of the spine (Fig. 68-1). The adult kidney is 4 to 5 inches (10 to 13 cm) long, 2 to 3 inches (5 to 7 cm) wide, and about 1 inch (2.5 to 3 cm) thick. It weighs about 8 ounces (250 g). The left kidney is slightly longer and narrower than the right kidney. Larger-than-usual kidneys may indicate obstruction or polycystic disease. Smaller-than-usual kidneys may indicate chronic kidney disease (CKD).

FIG. 68-1 Anatomic location of the kidneys and structures of the urinary system.

Variation in kidney shape and number is not uncommon and does not necessarily mean there is also a problem in kidney function. Some people have more than two kidneys or may have only one, large, horseshoe-shaped kidney. As long as tests of kidney function are normal, these variations are of no significance.

Several layers of tissue surround the kidney, providing protection and support. On the outer surface of the kidney is a layer of fibrous tissue called the capsule (Fig. 68-2). This capsule covers most of the kidney except the hilum, which is the area where the renal artery and nerve plexus enter and the renal vein and ureter exit. The renal capsule is surrounded by layers of fat and connective tissue.

FIG. 68-2 Bisection of the kidney showing the major structures of the kidney.

Lying beneath the renal capsule are the two layers of functional kidney tissue—the cortex and the medulla. The renal cortex is the outer tissue layer and is covered by the renal capsule. The medulla is the medullary tissue lying below the cortex in the shape of many fans. Each “fan” is called a pyramid, and there are 12 to 18 pyramids per kidney. The renal columns are cortical tissue that dips down into the interior of the kidney and separates the pyramids.

The tip, or end, of each pyramid is called a papilla. The papillae drain urine into the collecting system. A cuplike structure called a calyx collects the urine at the end of each papilla. The calices join together to form the renal pelvis, which narrows to become the ureter.

The kidneys have a rich blood supply and receive 20% to 25% of the total cardiac output. Blood flow to the kidneys varies from about 600 to 1300 mL/min. The blood supply to each kidney comes from the renal artery, which branches from the abdominal aorta. The renal artery divides into progressively smaller arteries, supplying all blood to areas of the kidney tissue (parenchyma) and the nephrons. The smallest arteries (afferent arterioles) feed the nephrons directly to form urine.

Venous blood from the kidneys starts with the capillaries surrounding each nephron. These capillaries drain into progressively larger veins, with blood eventually returned to the inferior vena cava through the renal vein.

Microscopic Anatomy

The nephron is the “working” or functional unit of the kidney, and it is here that urine is actually formed from blood. There are about 1 million nephrons per kidney, and each nephron separately makes urine from blood.

There are two types of nephrons: cortical nephrons and juxtamedullary nephrons. The cortical nephrons are short, with all parts located in the renal cortex. The juxtamedullary nephrons (about 20% of all nephrons) are longer, and their tubes and blood vessels dip deeply into the medulla. The purpose of the juxtamedullary nephrons is to concentrate urine during times of low fluid intake. The ability to concentrate urine allows for continued excretion of body wastes with less fluid loss.

Blood supply to the nephron is delivered through the afferent arteriole—the smallest, most distal portion of the renal arterial system. From the afferent arteriole, blood flows into the glomerulus, which is a series of specialized capillary loops. It is through these capillaries that water and small particles are filtered from the blood to make urine. The remaining blood leaves the glomerulus through the efferent arteriole, which is the first vessel in the venous system of the kidney. From the efferent arteriole, blood exits into one of two additional capillary systems:

Each nephron is a tubelike structure with distinct parts (Fig. 68-3). The tube begins with Bowman’s capsule, a saclike structure that surrounds the glomerulus. The tubular tissue of Bowman’s capsule narrows into the proximal convoluted tubule (PCT). The PCT twists and turns, finally straightening into the descending limb of the loop of Henle. The descending loop of Henle dips in the direction of the medulla but forms a hairpin loop and comes back up into the cortex as the ascending loop of Henle.

FIG. 68-3 Anatomy of the nephron, the functional unit of the kidney. Note that the particular nephron labeled here is a juxtamedullary nephron.

There are two segments of the ascending limb of the loop of Henle: the thin segment and the thick segment. The distal convoluted tubule (DCT) forms from the thick segment of the ascending limb of the loop of Henle. The DCT ends in one of many collecting ducts located in the kidney tissue. The urine in the collecting ducts passes through the papillae and empties into the renal pelvis.

Special cells in the afferent arteriole, efferent arteriole, and DCT are known as the juxtaglomerular complex (Fig. 68-4). These specialized cells produce and store renin. Renin is a hormone that helps regulate blood flow, glomerular filtration rate (GFR), and blood pressure. Renin is secreted when sensing cells in the DCT (called the macula densa) sense changes in blood volume and pressure. The macula densa lies next to the renin-producing cells. Renin is produced when the macula densa cells sense that blood volume, blood pressure, or blood sodium level is low. Renin then converts renin substrate (angiotensinogen) into angiotensin I. This leads to a series of reactions that cause secretion of the hormone aldosterone (Fig. 68-5). Aldosterone increases kidney reabsorption of sodium and water, restoring blood pressure, blood volume, and blood sodium levels. It also promotes excretion of potassium. (See Chapter 13 for more discussion of the renin-angiotensin-aldosterone pathway.)

FIG. 68-4 The juxtaglomerular complex showing juxtaglomerular cells and the macula densa.

FIG. 68-5 The role of aldosterone, renin substrate (angiotensinogen), angiotensin I, and angiotensin II in the renal regulation of water and sodium.

The glomerular capillary wall has three layers (Fig. 68-6): the endothelium, the basement membrane, and the epithelium. The endothelial and epithelial cells lining these capillaries are separated by pores that filter water and small particles from the blood into Bowman’s capsule. This fluid is called the “filtrate” or “early urine.”

FIG. 68-6 Glomerular capillary wall.

Regulatory Functions

The kidney processes that maintain fluid, electrolyte, and acid-base balance are glomerular filtration, tubular reabsorption, and tubular secretion. These processes use filtration, diffusion, active transport, and osmosis. (See Chapter 13 for a review of these actions.) Table 68-1 lists the functions of nephron tubules and blood vessels.

TABLE 68-1 VASCULAR AND TUBULAR COMPONENTS OF THE NEPHRON

STRUCTURE	ANATOMIC FEATURES	PHYSIOLOGIC ASPECTS
Vascular Components
Afferent arteriole	Delivers arterial blood from the branches of the renal artery into the glomerulus	Autoregulation of renal blood flow via vasoconstriction or vasodilation
		Renin-producing granular cells
Glomerulus	Capillary loops with thin, semipermeable membrane	Site of glomerular filtration
		Glomerular filtration occurs when hydrostatic pressure (blood pressure) is greater than opposing forces (tubular filtrate and oncotic pressure)
Efferent arteriole	Delivers arterial blood from the glomerulus into the peritubular capillaries or the vasa recta	Autoregulation of renal blood flow via vasoconstriction or vasodilation
		Renin-producing granular cells
Peritubular capillaries (PTCs) and vasa recta (VR)	PTCs: surround tubular components of cortical nephrons	Tubular reabsorption and tubular secretion allow movement of water and solutes to or from the tubules, interstitium, and blood
	VR: surround tubular components of juxtamedullary nephrons
Tubular Components
Bowman’s capsule (BC)	Thin membranous sac surrounding of the glomerulus	Collects glomerular filtrate (GF) and funnels it into the tubule
Proximal convoluted tubule (PCT)	Evolves from and is continuous with Bowman’s capsule	Site for reabsorption of sodium, chloride, water, glucose, amino acids, potassium, calcium, bicarbonate, phosphate, and urea
	Specialized cellular lining facilitates tubular reabsorption
Loop of Henle	Continues from PCT	Regulation of water balance
	Juxtamedullary nephrons dip deep into the medulla
	Permeable to water, urea, and sodium chloride
Descending limb (DL)	Continues from the loop of Henle	Regulation of water balance
	Permeable to water, urea, and sodium chloride
Ascending limb (AL)	Emerges from DL as it turns and is redirected up toward the renal cortex	Potassium and magnesium reabsorption in the thick segment
		Thin segment is impermeable to water
Distal convoluted tubule (DCT)	Evolves from AL and twists so the macula densa cells lie adjacent to the juxtaglomerular cells of afferent arteriole	Site of additional water and electrolyte reabsorption, including bicarbonate
		Potassium and hydrogen secretion
Collecting ducts	Collect formed urine from several tubules and deliver it into the renal pelvis	Receptor sites for antidiuretic hormone regulation of water balance

Glomerular filtration is the first process in urine formation. As blood passes from the afferent arteriole into the glomerulus, water, electrolytes, and other small particles (e.g., creatinine, urea nitrogen, glucose) are filtered across the glomerular membrane into the Bowman’s capsule to form glomerular filtrate. As the filtrate enters the proximal convoluted tubule (PCT), it is called tubular filtrate.

Large particles, such as blood cells, albumin, and other proteins, are too large to filter through the glomerular capillary walls. Therefore these substances are not normally present in the filtrate or in the final urine.

About 180 L of glomerular filtrate is formed from the blood each day. The rate of filtration is expressed in milliliters per minute. Normal glomerular filtration rate (GFR) averages 125 mL/min. If the entire amount of filtrate were excreted as urine, death would occur quickly from dehydration. Actually, only about 1 to 3 L is excreted each day as urine. The rest is reabsorbed back into the circulatory system.

The GFR is controlled by blood pressure and blood flow. The ability of the kidneys to self-regulate renal blood pressure and renal blood flow keeps GFR constant. GFR is controlled by selectively constricting and dilating the afferent and efferent arterioles. When the afferent arteriole is constricted or the efferent arteriole is dilated, pressure in the glomerular capillaries falls and filtration decreases. When the afferent arteriole is dilated or the efferent arteriole is constricted, pressure in the glomerular capillaries rises and filtration increases. Through this process, the kidney can maintain a constant GFR, even when systemic blood pressure changes. When systolic pressure drops below about 70 mm Hg, these processes cannot compensate and GFR stops.

Tubular reabsorption is the second process involved in urine formation. This reabsorption of most of the filtrate keeps normal urine output at 1 to 3 L/day and prevents dehydration. As the filtrate passes through the tubular parts of the nephron, most of the water and electrolytes is reabsorbed. Reabsorption returns particles (solutes) and water to the blood. Reabsorption occurs from the filtrate across the tubular lumen of the nephron and into the blood of the peritubular capillaries. The PCT reabsorbs about 65% of the total glomerular filtrate.

The tubules return about 99% of all filtered water back into the body (Fig. 68-7). Most water reabsorption occurs as the filtrate passes through the PCT. Water reabsorption continues as the filtrate flows down the descending loop of Henle. The thin and thick segments of the ascending loop of Henle are not permeable to water, and water reabsorption does not occur here.

FIG. 68-7 Sodium and water reabsorption by the tubules of a cortical nephron. ADH, Antidiuretic hormone; Na⁺, sodium.

The distal convoluted tubule (DCT) can be permeable to water, and some water reabsorption can occur as the filtrate continues to flow through the tubule. The membrane of the DCT may be made more permeable to water through the action of antidiuretic hormone (ADH) and aldosterone. ADH increases tubular permeability to water, allowing water to leave the tube and be reabsorbed into the capillaries. ADH is also known as vasopressin and affects arteriole constriction. Arteriole constriction alters blood pressure, which, in turn, affects the amount of fluid and solutes that exit glomeruli capillaries. Aldosterone promotes the reabsorption of sodium in the DCT. Water reabsorption occurs as a result of the movement of sodium (where sodium goes, water follows).

The ability of the kidneys to vary the volume or concentration of urine helps regulate water balance regardless of fluid intake. In this way, the healthy kidney can prevent dehydration when fluid intake is low and can prevent circulatory overload when fluid intake is excessive.

In addition to water, some particles in the tubular filtrate also are returned to the blood. This process is called tubular reabsorption and is selective. About 50% of all urea in the early urine is reabsorbed. On the other hand, no creatinine is reabsorbed.

Most sodium, chloride, and water reabsorption occurs in the PCT. The collecting ducts are the other site of sodium, chloride, and water reabsorption. Here reabsorption is caused by aldosterone. Potassium is also mostly reabsorbed in the PCT, with an additional 20% to 40% reabsorbed in the thick segment of the loop of Henle.

Bicarbonate, calcium, and phosphate are mostly reabsorbed in the PCT. Bicarbonate reabsorption helps balance acids and maintain a normal blood pH. Blood levels of calcitonin and parathyroid hormone (PTH) (see Chapters 13 and 66) control calcium balance.

The kidney reabsorbs some of the glucose filtered from the blood. However, there is a limit to how much glucose the kidney can reabsorb. This limit is called the renal threshold for glucose reabsorption or the transport maximum for glucose reabsorption. The usual renal threshold for glucose is about 220 mg/dL. This means that at a blood glucose level of 220 mg/dL or less, all glucose is reabsorbed and returned to the blood, with no glucose present in final urine. When blood glucose levels are greater than 220 mg/dL, some glucose stays in the filtrate and is present in the urine. Normally, almost all glucose and any amino acids or proteins are reabsorbed and are not present in the urine.

Tubular secretion is the third process involved in urine formation. Like glomerular filtration, it allows substances to move from the blood into the early urine. During tubular secretion, substances move from the peritubular capillaries in reverse, across capillary membranes, and into the cells that line the tubules. From the cells, these substances are moved into the urine and are excreted from the body. Potassium (K⁺) and hydrogen (H⁺) ions are some of the substances moved in this way to maintain homeostasis of electrolytes and pH.

Hormonal Functions

The kidneys produce renin, prostaglandins, bradykinin, erythropoietin, and activated vitamin D (Table 68-2). Other kidney products, such as the kinins, change kidney blood flow and capillary permeability. The kidneys also help break down and excrete insulin.

TABLE 68-2 RENAL HORMONE PRODUCTION AND HORMONES INFLUENCING RENAL FUNCTION

Renin, as discussed on p. 1468 in the Microscopic Anatomy section, assists in blood pressure control. It is formed and released when there is a decrease in blood flow, blood volume, or blood pressure through the renal arterioles or when too little sodium is present in kidney blood. These conditions are detected through the receptors of the juxtaglomerular complex.

Renin release causes the production of angiotensin II through a series of steps (see Fig. 68-5). Angiotensin II increases systemic blood pressure through powerful blood vessel constricting effects and triggers the release of aldosterone from the adrenal glands. Aldosterone increases the reabsorption of sodium in the distal tubule of the nephron. Therefore more water is reabsorbed and blood pressure is increased because of increases in blood volume. When blood flow to the kidney is reduced, this system also regulates pressures in the nephron to prevent fluid loss and maintain circulating blood volume (see also Chapter 13).

Prostaglandins are produced in the kidney and many other tissues. Those produced specifically in the kidney are prostaglandin E₂ (PGE₂) and prostacyclin (PGI₂). These substances help regulate glomerular filtration, kidney vascular resistance, and renin production. PGE₂ acts on the distal tubule and collecting duct to increase sodium and water excretion.

Bradykinin is released by the kidney in response to the presence of angiotensin II, prostaglandins, and ADH. It is a small hormone that dilates the afferent arteriole and increases capillary membrane permeability to some solutes. These actions maintain kidney blood flow and reabsorption even when other conditions cause systemic blood vessel constriction.

Erythropoietin is produced and released in response to decreased oxygen tension in the kidney’s blood supply. It triggers red blood cell (RBC) production in the bone marrow. When kidney function is poor, erythropoietin production decreases and the person becomes anemic.

Vitamin D activation occurs through a series of steps. Some of these steps take place in the skin when it is exposed to sunlight, and then more processing occurs in the liver. From there, vitamin D is converted to its active form (1,25-dihydroxy-cholecalciferol) in the kidney. It is needed to absorb calcium in the intestinal tract and to regulate calcium balance.