Anatomy of the kidneys

The kidneys are two bean-shaped organs in the retroperitoneal spaces at the level of the twelfth thoracic to third lumbar vertebrae. The right kidney is slightly lower than the left. Each kidney weighs approximately 150 g. The notched portion of the kidney is called the hilum, which is where the ureter, the renal vein, and the renal artery enter the kidney (Fig. 13-1). The kidney is divided into an outer cortex and an inner medulla.

FIG. 13-1 Anatomy of renal and urinary systems. A, Urinary system. B, Cross section of a kidney.

Blood is supplied to each kidney by a renal artery arising from each side of the abdominal aorta. The rate of blood flow through both kidneys of a man who weighs 70 kg is approximately 1200 mL/min, or approximately 21% of the cardiac output. As the renal artery enters the kidney at the hilum, it divides into the interlobar arteries. Branches from the interlobar arteries divide into afferent arteries that supply the capillaries of the nephrons. The capillaries form the efferent arterioles and divide to form the peritubular capillaries that help to supply the nephron. (Figs. 13-2 and 13-3).

FIG. 13-2 Functional nephron.

(From Hall J: Guyton and Hall textbook of medical physiology, ed 12, Philadelphia, 2011, Saunders.)

FIG. 13-3 Arterial supply to kidney. Soon after entering the hilum of the kidney, the renal artery divides into several anterior and posterior branches. Branches divide into interlobar arteries, which give off arcuate arteries that course between cortex and medulla.

The nephron is the functional unit of the kidney. Together the kidneys contain approximately 2.4 million closely packed nephrons. Each nephron consists of a glomerulus, a proximal convoluted tubule, a loop of Henle, a distal convoluted tubule, and collecting ducts. The blood enters the afferent arteriole and goes into the glomerulus located in the cortex. The glomerulus is a compact network of capillaries encased in a double-layered capsule, the Bowman capsule. Filtered blood flows out of the glomerulus into the efferent arteriole. The portion of the blood that is filtered drains into the proximal convoluted tubule. The renal tubules begin in the Bowman capsule. The pressure gradient, caused by renal artery blood flow, forces fluid to leave the glomerulus and enter the Bowman capsule. The filtrate flows into the proximal convoluted tubule, which is still in the cortex of the kidney, and then into the loop of Henle. The proximal loop of Henle is thick walled, but becomes thin at the distal segment in the medulla of the kidney. The filtrate then flows into the distal convoluted tubule, located in the cortex of the kidney, and passes into the collecting ducts. The collecting ducts traverse the cortex to the medulla, where they merge into the renal pelvis by way of the renal calyces. In the collecting ducts, the filtrate is termed urine.

The renal pelvis is a wide, funnel-shaped structure composed of the calyces draining the kidney. The pelvis drains into the ureter, which leads to the bladder.

Kidney physiology

Urine is formed by processes of filtration, reabsorption, and secretion. Filtration occurs as the blood passes through the glomerulus. The force of filtration is a pressure gradient that pushes fluid through the glomerular membrane. Approximately 180 L of water every 24 hours along with other substances is filtered out of plasma by the glomeruli (Table 13-1). Blood cells and heavy particles including proteins are retained in the blood because they are too large to pass through the glomerular epithelium. The presence of red blood cells or protein in the urine usually indicates a pathologic process in the kidney.

Table 13-1 Particles Filtered by the Glomerulus, Reabsorbed by the Tubules, and Excreted in the Urine

Reabsorption occurs in the proximal and distal tubules. Approximately 99% of the water filtered by the glomeruli is reabsorbed. Many substances in the water are reabsorbed with active or passive transport. Active transport requires energy for movement of the substance across the membrane. Passive transport can be regarded as simple diffusion that does not require energy.

Important constituents of body fluids—substances such as glucose, amino acids, sodium, potassium, calcium, and magnesium—are almost entirely reabsorbed. Certain substances are reabsorbed in limited quantities, such as urea and phosphate, and consequently appear in the urine. In a healthy individual, creatinine is the only filtered substance not reabsorbed and entirely secreted, allowing creatinine to serve as an indicator of glomerular filtration ability. The last process in the formation of urine is secretion. Various substances, including hydrogen and potassium ions, are secreted directly into the tubular fluid through the epithelial cells that line the renal tubules. Secretion plays an important role in promoting the body’s acid-base balance.

Regulation of kidney function

The formation of urine and the reabsorption of substances needed for body function are aided by three physiologic mechanisms: the countercurrent mechanism, autoregulation, and hormonal control.

Hormonal control

Secretion of antidiuretic hormone (ADH) by the posterior pituitary gland is affected by plasma osmolality. When the blood becomes hypertonic, ADH is secreted and water is retained by the kidneys. If the blood is hypotonic, less ADH is formed, causing the kidneys to reabsorb less water and increasing urine formation. ADH acts on the distal tubules and collecting tubules by altering permeability to water.

The juxtaglomerular complex is a group of cells, located just before the glomerulus and in close proximity to the distal tubule, which contain granules of inactive renin. Renin is released in response to reduced arterial blood pressure entering the afferent arteriole of the kidney or a low concentration of sodium in the distal tubule. The released renin acts as an enzyme to convert angiotensinogen to angiotensin I. Angiotensin I is converted to angiotension II by the angiotensin-converting enzyme, which leads to angiotensin II. Angiotensin II stimulates the adrenal cortex to release aldosterone. Aldosterone signals the kidney tubules to increase the reabsorption of sodium and retention of water. Because the renin-angiotensin system causes this reabsorption of water and sodium, it plays a role in the control of arterial blood pressure and is important in the conservation of sodium and control of fluid volume in hypotensive states (Fig. 13-4).

FIG. 13-4 Renin-angiotensin-vasoconstrictor mechanism for arterial pressure control.

(From Hall J: Guyton and Hall textbook of medical physiology, ed 12, Philadelphia, 2011, Saunders.)

Regulation of body homeostasis

The kidneys play a significant role in regulation of body fluids. The kidneys adjust blood volume and influence extracellular fluid volume, extracellular fluids, electrolytes, and ions. Homeostasis is maintained when the kidneys remove waste products and toxic substances.

The kidneys regulate blood volume through ADH, aldosterone, and the renin-angiotensin mechanism. When the circulating blood volume is excessive, the cardiac output and arterial pressure increases, resulting in greater pressure in the renal artery and the kidney afferent arteriole. Water and sodium are excreted. If the patient is hypovolemic, the kidneys reabsorb fluid to return the blood volume to normal limits.

The extracellular fluid volume is controlled indirectly by the kidneys as blood volume is controlled. The relative ratio of the extracellular fluid volume to blood volume depends on the physical properties of the circulation and of the interstitial spaces, including compliances and dynamics. The kidney maintains the osmolality of the extracellular fluid mainly by regulating the extracellular sodium concentration. Extracellular sodium controls 90% to 95% of the effective osmotic pressure of extracellular fluid. The extracellular concentration of other electrolytes, including potassium, calcium, magnesium, and phosphate ions, is also under renal control.