The nephron, the functional unit of the kidney, is responsible for removing waste from the body. Each kidney is composed of over one million nephrons that dot the renal cortex, giving it a granular appearance when sectioned sagittally (from front to rear).
A nephron consists of three parts: a renal corpuscle, a renal tubule, and the associated capillary network, which originates from the cortical radiate arteries. The renal corpuscle, located in the renal cortex, is composed of a network of capillaries known as the glomerulus, as well as a cup-shaped chamber that surrounds it: the glomerular or Bowman’s capsule. The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the renal circulation. Here, fluid and solutes are filtered out of the blood and into the space made by Bowman’s capsule. The Bowman’s capsule surrounds the glomerulus. Red blood cells and large proteins, such as serum albumins, cannot pass through the glomerulus under normal circumstances.
The renal tubule is a long, convoluted structure that emerges from the glomerulus. It can be divided into three parts based on function. The first part is called the proximal convoluted tubule (PCT), due to its proximity to the glomerulus. The proximal tubule is the first site of water reabsorption into the bloodstream, and the site where the majority of water and salt reabsorption takes place.
The second part is called the loop of Henle, or nephritic loop because it forms a loop (with descending and ascending limbs) that goes through the renal medulla. The loop of Henle is a U-shaped tube that consists of a descending limb and ascending limb. It transfers fluid from the proximal to the distal tubule. The descending limb is highly permeable to water but completely impermeable to ions, causing a large amount of water to be reabsorbed.
The third part of the renal tubule is called the distal convoluted tubule (DCT); this part is also restricted to the renal cortex. This last part of the nephron connects with and empties its filtrate into collecting ducts that line the medullary pyramids. The collecting ducts amass contents from multiple nephrons, fusing together as they enter the papillae of the renal medulla. The distal convoluted tubule and collecting duct is the final site of reabsorption in the nephron. Unlike the other components of the nephron, its permeability to water is variable depending on a hormone stimulus to enable the complex regulation of blood osmolarity, volume, pressure, and pH. After passage through the collecting duct, the fluid is brought into the ureter, where it leaves the kidney as urine.
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Key Points
• A nephron comprises a renal corpuscle, a renal tubule, and the associated capillary network.
• Internally, kidneys are mainly composed of over one million nephrons and an extensive network of blood vessels and capillaries.
• The glomerulus is the site in the nephron where fluid and solutes are filtered out of the blood to form a glomerular filtrate.
• The proximal and distal tubules, the loop of Henle, and the collecting ducts are sites for the reabsorption of water and ions.
• All of the glucose in the blood is reabsorbed by the proximal convoluted tubule through ion cotransport.
• The loop of Henle (sometimes known as the nephron loop) is a U-shaped tube that consists of a descending limb and ascending limb, which differ in permeability.
• The collecting duct and distal convoluted tubule are normally impermeable to water, but this is altered due to hormone stimulus during homeostasis.
Key Terms
glomerulus: a small intertwined group of capillaries within a kidney’s nephron that filters the blood to make urine.
loop of Henle: a structure in a kidney’s nephron that connects the proximal convoluted tubule to the distal convoluted tubule.
proximal convoluted tubule: involved in the resorption of sugar, sodium and chloride ions, and water from the glomerular filtrate
distal convoluted tubule: have an important role in the absorption of many ions, and in water reabsorption
Bowman’s capsule: a cup-like sack at the beginning of the tubular component of a nephron that performs the first step in the filtration of blood to form urine
collecting duct: the final site of reabsorption in the nephron