1 Describe the anatomy of the kidney

The kidneys are paired organs lying retroperitoneally against the posterior abdominal wall. Although their combined weight is only 300 g (about 0.5% of total body weight), they receive 20% to 25% of the total cardiac output. The renal arteries are branches of the aorta, originating below the superior mesenteric artery. The renal veins drain into the inferior vena cava. Nerve supply is abundant; sympathetic constrictor fibers are distributed via celiac and renal plexuses. There is no sympathetic dilator or parasympathetic innervation. Pain fibers, mainly from the renal pelvis and upper ureter, enter the spinal cord via splanchnic nerves.

On cross section of the kidney, three zones are apparent: cortex, outer medulla, and inner medulla (Figure 44-1). Eighty percent of renal blood flow is distributed to cortical structures. Each kidney contains about 1 million nephrons. Nephrons are classified as superficial (about 85%) or juxtamedullary, depending on location and length of the tubules. The origin of all nephrons is within the cortex.

Figure 44-1 This scheme depicts a short-looped and a long-looped nephron together with the collecting system (not drawn to scale). Within the cortex a medullary ray is delineated by a dashed line. 1, Renal corpuscle, including Bowman’s capsule and the glomerulus (glomerular tuft). 2, Proximal convoluted tube. 3, Proximal straight tubule. 4, Descending thin limb. 5, Ascending thin limb. 6, Distal straight tubule (thick ascending limb). 7, Macula densa located within the final portion of the neck of the thick ascending limb. 8, Distal convoluted tube. 9, Connecting tubule of the juxtamedullary nephron that forms an arcade. 10, Cortical collecting tubule. 11, Outer medullary collecting duct. 12, Inner medullary collecting duct.

(From Kriz W, Bankir I: A standard nomenclature for structures of the kidney, Am J Physiol 254:F1, 1988, with permission.)

The glomerulus and capsule are known collectively as the renal corpuscle. Each Bowman’s capsule is connected to a proximal tubule that is convoluted within its cortical extent but becomes straight limbed within the outer cortex; at this point the tubule is known as the loop of Henle. The loop of Henle of superficial nephrons descends only to the intermedullary junction, where it makes a hairpin turn, becomes thick limbed, and ascends back into the cortex, where it approaches and touches the glomerulus with a group of cells known as the juxtaglomerular apparatus. The superficial nephrons form distal convoluted tubules that merge to form collecting tubules within the cortex. The renal corpuscles of juxtamedullary nephrons are located at juxtamedullary cortical tissue. They have long loops of Henle that descend deep into the medullary tissue; the loops also reascend into cortical tissue, where they form distal convoluted tubules and collecting tubules. These nephrons (15% of the total) are responsible for conservation of water.

About 5000 tubules join to form collecting ducts. Ducts merge at minor calyces, which in turn merge to form major calyces. The major calyces join and form the renal pelvis, the most cephalic aspect of the ureter.

2 List the major functions of the kidney

Regulation of body fluid volume and composition

Acid-base balance

Detoxification and excretion of nonessential materials, including drugs

Elaboration of renin, which is involved in extrarenal regulatory mechanisms

Endocrine and metabolic functions such as erythropoietin secretion, vitamin D conversion, and calcium and phosphate homeostasis

3 Discuss glomerular and tubular function

Glomerular filtration results in production of about 180 L of glomerular fluid each day. Filtration does not require the expenditure of metabolic energy; rather it is caused by a balance of hydrostatic and oncotic forces. Glomerular filtration rate (GFR) is the most important index of intrinsic renal function. Normal GFR is 125 ml/min in males, slightly less in females.

Tubular function reduces the 180 L/day of filtered fluid to about 1 L/day of excreted fluid, altering its composition through active and passive transport. Transport is passive when it is the result of physical forces such as electrical or concentration gradients. When transport is undertaken against electrochemical or concentration gradients, metabolic energy is required, and the process is termed active.

Substances may be either resorbed or secreted from tubules and may move bidirectionally, taking advantage of both active and passive transport. The direction of transit for resorbed substances is from tubule to interstitium to blood, whereas the direction for secreted substances is from blood to interstitium to tubule. Secretion is the major route of elimination for drugs and toxins, especially when they are plasma protein bound.

4 Review the site of action and significant effects of commonly used diuretics

See Table 44-1.

TABLE 44-1 Diuretics*

DCT, Distal convoluted tubule; GFR, glomerular filtration rate.

* With the exception of osmotic diuretics, all diuretics interfere with sodium conservation.

5 Describe the unique aspects of renal blood flow and control

Renal blood flow (RBF) of about 1200 ml/min is well maintained (autoregulated) at blood pressures of 80 to 180 mm Hg. The cortex requires about 80% of blood flow to achieve its excretory and regulatory functions, and the outer medulla receives 15%. The inner medulla receives a small percent of blood flow; a higher flow would wash out solutes responsible for the high tonicity (1200 mOsm/kg) of the inner medulla. Without this hypertonicity, urinary concentration would not be possible.

Control of RBF is through extrinsic and intrinsic neural and hormonal influences; a principal goal of blood flow regulation is to maintain GFR. The euvolemic, nonstressed state has little baseline sympathetic tone. Under mild-to-moderate stress RBF decreases slightly; but efferent arterioles constrict, maintaining GFR. During periods of severe stress (e.g., hemorrhage, hypoxia, major surgical procedures) both RBF and GFR decrease secondary to sympathetic stimulation.

The renin-angiotensin-aldosterone axis also has an effect on RBF. A proteolytic enzyme formed at the macula densa of the juxtaglomerular apparatus, renin acts on angiotensinogen within the circulation to produce angiotensin I. Enzymes within lung and plasma convert angiotensin I to angiotensin II, which is a potent renal vasoconstricting agent (especially of the efferent arteriole) and a factor in the release of aldosterone. During periods of stress levels of angiotensin are elevated and contribute (along with sympathetic stimulation and catecholamines) to decreased RBF.

Prostaglandins (PGs) are also found within the kidney. PGE2 and PGE3 are intrinsic mediators of blood flow, producing vasodilation.

6 Describe the sequence of events associated with decreased renal blood flow