The nephron is the basic functional and structural unit of the kidney, and each human kidney contains from 800,000 to one million of these units. They are responsible for maintaining the concentrations of water and soluble substances in the blood and regulating blood volume, blood pressure, and the blood's pH or acidity.
This structure works by filtering the blood, reabsorbing nutrients, and excreting excess water and waste as urine. There are two types of nephrons, distinguished by their location in the kidney. Cortical nephrons are located in the renal cortex on the outside of the organ, while juxtamedullary nephrons are located deeper in the kidney, in the renal medulla.
Each nephron is made up of a renal corpuscle and a renal tubule. The renal corpuscle provides the initial filtering component, while the renal tubule is responsible for reabsorption. The corpuscle is composed of the glomerulus and Bowman's capsule. The glomerulus is a bundle of capillaries, or small, permeable blood vessels, through which oxygenated blood enters the kidneys. Excess water and waste products are collected in Bowman's capsule, which houses the glomerulus, and the rest of the blood rejoins the main bloodstream.
The renal tubule consists of the proximal tubule, the loop of Henle, and the distal convoluted tubule. Each portion is responsible for a different part of reabsorption. About two-thirds of the filtered salt and water from the renal corpuscle, along with all of the filtered organic solutes, are reabsorbed in the proximal tubule. The loop of Henle has two major portions: the descending limb and the ascending limb. The former is water-permeable, but impermeable to salt, while the later is impermeable to water. Water is removed from the tubule fluid as it passes through the descending limb of the loop of Henle, while sodium is pumped out of the fluid as it passes through the ascending limb.
The distal convoluted tubule is controlled by hormones from the endocrine system, causing it to reabsorb or excrete certain nutrients as required for the body's needs. It also regulates blood pH. After reabsorption is complete, the remaining filtrate passes out of the nephron and into the collecting duct system, which collects urine before it is excreted. Urine leaves the collecting ducts through the renal papillae, passing into the renal calyxes, then the renal pelvis, and finally entering the bladder by way of the ureter.
