Nephron

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Nephron of the kidney

A nephron is the basic structural and functional unit of the kidney. It is a tiny tubule consisting of a cluster of capillaries called the glomerulus, surrounded by a hollow bulb known as Bowman's capsule. Bowman's capsule leads into a long, convoluted tubule consisting of four sections: the proximal tubule, loop of Henle, distal tubule, and collecting duct. The collecting ducts empty into the central cavity of the kidney, the renal pelvis, which connects to the ureter. Each human kidney has about a million nephrons.

The basic function of the nephron is to regulate water and soluble substances (especially ions) in the body by filtering it all out first, reabsorbing what should be kept and excreting the rest. This is a function vital to supporting human life.

The nephron filters the blood by several processes concentrating the filtrates, reabsorbing ions (such as sodium, potassium, calcium, hydrogen, bicarbonate, chloride, and ammonium ions), solutes (such as glucose, amino acids, phosphates, and so on) according to the body's needs under hormonal control, or from anti-diuretic hormones, aldosterone, parathyroid hormone, atrial-natriuretic peptide and others. In this process urine is produced, and in doing so, eliminates wastes from the body, regulates blood volume and blood pressure, regulates the levels of important electrolytes and metabolytes and regulates blood pH levels.

Table of contents

Bowman's Capsule

The renal arteries branch quickly into very small capillaries that make up the glomerulus of the nephron. Because the capillaries are so small, the blood pressure is great and the liquid is squeezed into the Bowman's capsule. This process is known as filtration. The solids, including the blood cells, platelets, and large proteins, stay behind. The liquid, known as glomerular filtrate, contains water, glucose, salt, amino acids, and urea.

Proximal Tubule

The valuable molecules, water, glucose, salt, and amino acids are reabsorbed by the blood. The filtrate is now known as urine. The glomerular filtrate has lost about 75% of the original water content and volume.

Loop of Henle

THe loop of Henle moves urine from the cortex (outside region) into the medulla. The descending loop is permeable to water, but not to salt. In addition, the medulla has high concentration of salt. As a result, the urine travelling through the descending loop loses water. The ascending loop is just the opposite: it is permeable to salt but not water. Since the urine is now high in salt, the salt diffuses. By this point, only 6% of original water and 4% of original salt remains.

Distal Tubule

The distal tubule is similar to proximal tubule. Salt is pumped out and water follows by osmosis. After travelling through the distal tubule, only 3% of water remains, and the remaining salt content is negligible. A third process (in addition to filtration and reabsorption) takes place: secretion. Hydrogen and potassium ions are actively transported from the blood into the urine to be excreted.

Collection duct

The collection duct goes deep into medulla, and as much as ? of the remaining water can be reabsorbed as the urine travels through the collecting duct. But in reality, the amount of water reabsorbed is dependent on the permeability of the walls of the duct, regulated by ADH secreted by the posterior pituitary gland. The more ADH, the more permeable the ducts, the more water reabsorbed; the less ADH, the less permeable the ducts, the less water reabsorbed. Dehydration results in an increase in ADH, while water sufficiency results in low ADH allowing for diluted urine. Lower portions of the collection duct are also permeable to urea, allowing some of it to enter the medulla of the kidney, thus maintaining its high ion concentration (very important for the nephron).

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