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Figure

(a) Location of the juxtaglomerular apparatus. (b) Enlargement of a section of the juxtaglomerular apparatus, which consists of the macula densa (darker yellow) and the juxtaglomerular cells. Black arrows indicate direction of blood flow. Blue arrows indicate flow of glomerular filtrate and tubular fluid.

Juxtaglomerular Apparatus

Near its origin, the distal convoluted tubule passes between the afferent and efferent arterioles and contacts them. At the point of contact, the epithelial cells of the distal convoluted tubule are quite tall and densely packed. These cells comprise a structure called the macula densa.

Close by, in the wall of the afferent arteriole near its attachment to the glomerulus, are large, vascular smooth muscle cells called juxtaglomerular cells. Together with the cells of the macula densa, they constitute the juxtaglomerular apparatus (juks"tah-glo-mer'u-lar ap"ah-ra'tus) (complex). This structure is important in regulating the secretion of renin (see chapter 13, p. 528) (fig. 20.11).

Cortical and Juxtamedullary Nephrons

Most nephrons have corpuscles located in the renal cortex near the surface of the kidney. These cortical nephrons have relatively short nephron loops that usually do not reach the renal medulla.

Another group, called juxtamedullary nephrons, have corpuscles close to the renal medulla, and their nephron loops extend deep into the medulla. Although these nephrons represent only about 20% of the total, they are important in regulating water balance (fig. 20.12).

Blood Supply of a Nephron

The cluster of capillaries that forms a glomerulus arises from an afferent arteriole whose diameter is greater than that of other arterioles. Blood passes through the capillaries of the glomerulus, then (minus any filtered fluid) enters an efferent arteriole (rather than a venule), whose diameter is smaller than that of the afferent arteriole. The greater resistance to blood flow of the efferent arteriole causes blood to back up into the glomerulus. This results

in a relatively high pressure in the glomerular capillaries compared to capillaries elsewhere.

The efferent arteriole branches into a complex network of capillaries that surrounds the renal tubule called the peritubular capillary (per"i-tu'bu-lar kap' i-ler"e) system. Blood in the system has passed through two arterioles and is under relatively low pressure (see fig. 20.9). Branches of this system that primarily receive blood from the efferent arterioles of the juxtamedullary nephrons form capillary loops called vasa recta. These loops dip into the renal medulla and are closely associated with the loops of the juxtamedullary nephrons (fig. 20.13). After flowing through the vasa recta, blood returns to the renal cortex, where it joins blood from other branches of the peritubular capillary system and enters the venous system of the kidney. Figure 20.14 summarizes the pathway that blood follows as it passes through the blood vessels of the kidney and nephron.

O Describe the system of vessels that supplies blood to the kidney.

Name the parts of a nephron.

Which structures comprise the juxtaglomerular apparatus?

Distinguish between a cortical nephron and a juxtamedullary nephron.

Describe the blood supply of a nephron.

Urine Formation

The main function of the nephrons is to control the composition of body fluids and remove wastes from the blood. The product is urine, which is excreted from the body. It contains wastes and excess water and electrolytes.

Urine formation involves three processes. In glomerular filtration, certain substances move from the glomerular capillaries into the renal tubules. In tubular reabsorption, some of these substances move back into the blood plasma. In tubular secretion, certain other substances move from the peritubular capillaries into the renal tubules. In other words, the following relationship determines the volume of substances excreted in the urine:

Proximal convoluted tubule

Glomerular

Glomerulus

Efferent arteriole

Peritubular capillaries

Glomerular

Proximal convoluted tubule

Glomerulus

Efferent arteriole

Peritubular capillaries

Interlobular Artery

Interlobular artery and vein

Afferent arteriole

Distal convoluted tubule

Collecting duct

Nephron loop

The capillary loop of the vasa recta is closely associated with the nephron loop of a juxtamedullary nephron.

Interlobular artery and vein

Afferent arteriole

Distal convoluted tubule

Renal artery

Interlobar artery

Arcuate artery

Interlobular artery

Afferent arteriole

Glomerular capillary

Efferent arteriole

Nephron loop

Collecting duct

Vasa recta and peritubular capillary

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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