The kidney is responsible for maintaining fluid balance within the body. The basic structural and functional units of the kidneys are the nephrons. Each nephron is made of intricately interwoven capillaries and drainage canals to filter wastes, macromolecules, and ions from the blood to urine. The approximately 1 million nephrons in each human kidney form 10-20 cone-shaped tissue units called renal pyramids that span both the inner and outer portions of the kidney, the renal medulla and renal cortex.
There are two main parts of a nephron: the renal corpuscle and renal tubule.
Renal Corpuscle Structure
The renal corpuscle is the initial filtering component of the nephron and is made up of two structures known as the glomerulus and Bowman’s capsule. The Bowman’s capsule is a double membrane that cups the glomerulus. The glomerulus is a capillary extension, a small network of thin blood vessels, receiving blood from the renal circulation. The glomerular filtration rate is a measure of kidney function.
Renal Tubule Structure
Renal tubules are a duct system beginning at the Bowmen’s capsule in the cortex, looping through the renal medulla, and returning to the cortex to connect to the collecting duct system. Each renal tubule is divided into a proximal tubule, loop of Henle, and distal convoluted tubule. The proximal tubular has a brush border, that is microvilli, that increases the surface area for absorption. The distal convoluted tubule meets the afferent arteriole of the corpuscle as it joins the collecting tubule; this is called the macula densa.
Nephron Function
The blood is filtered and urine formed by the actions of the nephrons. In each nephron, high pressure in the glomerulus pushes water and small dissolved materials into the extravascular space of the Bowman’s capsule and into the tubule. The proximal tubule reabsorbs water, salts, glucose, and amino acids to maintain electrolyte levels in the body. The interstitium of, that is the tissue space surrounding, the loop of Henle concentrates salts that will be excreted in the urine, creating a concentration gradient in the medulla. The limbs of Henle’s loop are permeable to particular ions (descending, water and some urea; thin ascending, general ions; medullary thick ascending – sodium, potassium, chloride), with the cortical thick ascending limb draining into the distal convoluted tubule. The distal tubule contains cells specialized in active transport and maintains urine and blood pH levels, particularly through the regulation of sodium and potassium.
Fluid then passes from the distal tubule to the collecting ducts, a tubule system that can become permeable or impermeable to water depending on the body’s needs. Ultrafiltration also occurs in the cortex in the cortical collecting ducts, which is regarded by some anatomy references as not being a portion of the nephron, and by others as being the final portion of the nephron. The urine then passes from the collecting ducts through the drainage system of the kidney to the ureters and bladder for urination.
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