Nephron: The functional unit of the kidney

2021-04-30 09:12 PM

Each nephron contains a cluster of glomerular capillaries called glomeruli, through which a large amount of fluid is filtered from the blood, and a long tube in which the filtered fluid is converted to urine.

Nephron's structure, function, and operation

Each human kidney contains between 800,000 and 1,000,000 nephrons, each capable of forming urine. The kidneys cannot regenerate new nephrons. Consequently, with normal kidney injury, disease, or aging, the number of nephrons gradually decreases. After age 40, the number of active nephrons typically decreases by about 10 percent every 10 years; therefore, at the age of 80, many people have 40% less active nephrons than they did when they were 40 years old. This loss is not life-threatening because adaptive changes in the remaining nephrons allow them to excrete the appropriate water, electrolytes, and waste.

Each nephron contains (1) a cluster of glomerular capillaries called glomeruli, through which large amounts of fluid are filtered from the blood, and (2) a long tube in which the filtered fluid is converted to urine.

The glomerulus contains a network of interconnected and branched glomerular capillaries that, in comparison with other capillaries, have high hydrostatic pressure (about 60 mm Hg). Glomerular capillaries are covered by epithelial cells, and the entire glomerulus is encapsulated in Bowman's capsule.

The filtered fluid from the glomerular capillaries flows into the Bowman's sac and then into the proximal tubule, located in the renal cortex. From the proximal tubule, fluid flows into the loop of Henle, this tube will dip into the kidney marrow. Each loop consists of a descending limb and an ascending limb. The walls of the lower limb and the lower end of the limb are very small upward and are therefore called the small segment of the capillary Henle. After the ascent of the loop returns the part back to the shell, its wall becomes much thicker, and it is called the thicker segment of the ascending section.

Figure. The main vessels provide blood flow to the kidneys and the microcirculation pattern of each nephron.

At the end of the ascending limb is a short segment in its wall with a plaque of specialized epithelial cells, called the macula. The macula plays an important role in controlling nephron function. In addition to the macula, the fluid enters the distal tubule, like the proximal tubule, located in the renal cortex. The distal tubule is connected by the conduit and the manifold, leading to the sheath manifold. The initial sections of 8 to 10 sheath manifolds fuse together to form a single larger manifold that runs down the marrow and becomes the canal's manifold. The manifold fuses to form gradually enlarged ducts that eventually empty into the renal pelvis through the ends of the papilla. In each kidney, there are about 250 very large collecting tubes, each collecting urine from about 4000 nephrons.

Figure. Basic tubular segments of the nephron. The relative lengths of the different tubular segments are not calculated proportionally.

Regional differences in Nephron structure: Nephron shell and marrow. Although each nephron has all of the ingredients described earlier, there are some differences, depending on the depth of the nephron in the kidney mass. Nephrons whose outer glomeruli are located are called the cortical nephrons; they have short rings of Henle that penetrate only a short space into the marrow.

 

Figure. Diagram of the relationship between blood vessels and tubular structure and differences between cortical and renal medullary nephron.

About 20 to 30 percent of nephrons have a glomerulus located deep in the kidney cortex near the medulla and are called lateral nephrons. These nephrons have long rings of Henle embedded deeply in the marrow, in some cases as far as the ends of the papilla.

The vascular structure that supplies the shell nephron is also different from the structure that supplies the myeloid nephron. For shell nephrons, the entire tubular system is surrounded by a wide network of peritoneal capillaries. As for the medullary nephron, the arterioles extend from the glomerulus down to the outer marrow and then divide into specialized peritoneal capillaries that extend downwards into the marrow, located next to each other with the Henle loops. Like the rings of Henle, the duct returns toward the shell and empties into the cortical veins. This specialized capillary network in the marrow plays an essential role in forming concentrated urine.

 

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