Porth's Essentials of Pathophysiology, 4e

602

Kidney and Urinary Tract Function

U N I T 7

constancy of the internal environment are reabsorbed into the bloodstream while other, unneeded materials are secreted into the tubular filtrate for elimination. Nephrons can be roughly grouped into two catego- ries. Approximately 85% of the nephrons originate in the superficial part of the cortex and are called corti- cal nephrons (Fig. 24-4B). They have short, thick loops of Henle that penetrate only a short distance into the medulla. The remaining 15% are called juxtamedullary nephrons. They originate deeper in the cortex and have longer and thinner loops of Henle that penetrate the entire length of the medulla. The juxtamedullary neph- rons are largely concerned with urine concentration. The nephrons are supplied by two capillary systems, the glomerulus and peritubular capillary network (see Fig. 24-4A). The glomerulus is a unique, high-pressure capillary filtration system located between two arteri- oles—the afferent and the efferent arterioles. The peri- tubular capillaries originate from the efferent arteriole. They are low-pressure vessels that are adapted for reab- sorption rather than filtration. These capillaries sur- round all portions of the tubules, an arrangement that permits rapid movement of solutes and water between the fluid in the tubular lumen and the blood in the capil- laries. In the deepest part of the renal cortex, the efferent arterioles continue as long, thin-walled looping vessels called the vasa recta. The vasa recta accompany the long loops of Henle in the medullary portion of the kidney to assist into the exchange of substances flowing in and out of that portion of the kidney. The peritubular capil- laries rejoin to form the venous channels through which blood leaves the kidneys and empties into the inferior vena cava. The Renal Corpuscle The renal corpuscle, commonly called the glomerulus , consists of a compact tuft of capillaries, with a cen- tral region of mesangial cells and surrounding matrix, encased in a thin double-layered capsule called Bowman capsule . The inner or visceral layer of the capsule envel- ops the capillaries of the glomerulus and the external or parietal layer forms the outer wall of the capsule. Blood flows into the glomerular capillaries through the affer- ent arterioles and flows out through the efferent arte- rioles, which leads to a second capillary network, the peritubular capillaries, that surrounds the renal tubules. Fluid and particles from the blood are filtered through the capillary wall into a fluid-filled space between the visceral and parietal layers of Bowman capsule, called Bowman space. The portion of the blood that is filtered into the capsule space is called the glomerular filtrate (Fig. 24-5A). The glomerular capillary wall consists of a thin layer of endothelial cells, a glomerular basement membrane, and a surrounding layer of visceral epithelial cells of Bowman capsule (Fig. 24-5B). The endothelium of the glomerular capillary, which interfaces with blood as it moves through the capillary, contains many small perfo- rations, called fenestrations. These fenestrations allow for the free passage of water, and of small particles such

Proximal tubule

Efferent arteriole

Bowman space

Afferent arteriole

A

Mesangial cell

Epithelial podocytes

Basement membrane

Glomerular capsule

Bowman space

Endothelial cell

Mesangial matrix

B

Glomerular capillary

Blood

Endothelial cells with fenestrations

Podocyte

Foot process of podocyte

Ultrafiltrate

Bowman capsule

Filtration slit Glomerular basement membrane

C

as sodium, potassium, and glucose, but prevent the pas- sage of red blood cells, white blood cells, or platelets. In addition to their role as a filtration barrier, the endothe- lial cells synthesize a number of vasoactive substances such as nitric oxide (a vasodilator) and endothelin-1 (a vasoconstrictor) that control renal blood flow. FIGURE 24-5. Renal corpuscle. (A) Structures of the glomerulus. (B) Cross-section of the glomerular membrane showing the position of endothelium, basement membrane, and mesangial cells. (C) Structures of the glomerulus in relation to the filtration process.