Table of Contents
Definition of Nephron
Nephron – The microscopic functional unit of the kidney. Each kidney contains more than a million nephrons, each of which extends from the renal cortex into the renal medulla in fairly linear fashion.
Two elements make up the nephron: the renal tubules and the renal corpuscle. The renal corpuscle contains the GLOMERULUS, the coiled network of capillaries that bring BLOOD into the nephron, and Bowman’s capsule, the podlike structure that encases the glomerulus.
The pressure of the blood as it enters the glomerulus forces molecules of water, electrolytes, and other substances through the thin glomerular wall into Bowman’s capsule. This mixture, called filtrate, collects in the capsule and drains into the renal tubule. Each segment of the tubule reabsorbs different substances from the filtrate as it passes through them. A second network of capillaries separate from the glomerulus, the peritubular capillaries, entwines the renal tubule to allow the reabsorbed materials to reenter the blood circulation.
The first portion of the tubule to exit Bowman’s capsule, the proximal tubule (also called the proximal convoluted tubule), runs along the renal corpuscle, heading inward toward the renal medulla though it remains within the renal cortex. The proximal tubule reabsorbs about two thirds of the sodium and two thirds of the water the filtrate contains, and reabsorbs calcium when vitamin D is present.
The next segment, the loop of Henle, drops deep into the renal medulla, makes a sharp loop, and rises back up into the renal cortex in somewhat of a hairpin appearance. Different portions of the loop of Henle reabsorb sodium, potassium, chloride, magnesium, calcium, and water.
The loop of Henle plays a significant role in the concentration and dilution of the URINE, and is the target of some types of diuretic medications. The distal tubule (also called the distal convoluted tubule) continues up through the renal cortex and wraps around the renal corpuscle, ultimately joining with the collecting tubule (also called the collecting duct).
The distal tubule reabsorbs sodium and bicarbonate and secretes potassium. The final segment of the renal tubule is the collecting tubule, which funnels the remaining filtrate toward the renal pelvis for excretion via the URETER as urine. Only water reabsorption takes place from the collecting tubule.
For further discussion of the nephron within the context of the urinary system’s structure and function please see the overview section “The Urinary System.”