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 metabolites and regulates blood pH levels.
The renal arteries branch quickly into very small capillaries that make up the glomerulus of the nephron. The blood pressure within the capillaries squeezes liquid into the Bowman's capsule. This process is known as ultra-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.
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.
The loop of Henle descends from the cortex into the medulla, and then ascends back from the medulla to the cortex. A countercurrent mechanism involving the loop of Henle in the medulla allows reabsorption of water and salt. By this point, only 6% of original water and 4% of original salt remains.
The distal tubule is similar to the 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.
The collecting 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 part of the 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 collecting duct are also permeable to urea, allowing some of it to enter the medulla of the kidney, thus maintaining its high ion concentration (which is very important for the nephron).