The kidneys consist of a cluster of very thin-walled capillaries. Each cluster is associated with a cup-shaped end of a tube, in which filtered urine is collected. These basic filtering units of the kidneys are called nephrons. Each kidney possesses a large number of nephrons (approximately 1- 1.5 million).
Structure of a nephron
The main components of a nephron are the glomerulus, Bowman’s capsule, and a long renal tube. The blood enters the kidneys through the renal artery, which branches into many capillaries associated with the glomerulus. Water and solutes are transferred to the nephron at the Bowman’s capsule.
In the proximal tubule, some substances such as amino acids, glucose, and salts are selectively reabsorbed and unwanted molecules are added in the urine. The filtrate then moves down into the loop of Henle, in which more water is reabsorbed. From here, the filtrate moves upwards into the distal tubule, and finally to the collecting duct. This duct collects urine from many nephrons.
The urine formed in each kidney then enters a long tube called the ureter. The ureters move the urine from the kidneys to the urinary bladder. The urinary bladder is under the control of the nervous system. This helps us to control the urge to urinate.