Formation of urine
Urine is formed in the kidneys from blood plasma; every minute, one-quarter of the blood volume ejected by the heart passes through the kidneys. The main structural and functional unit of the kidney, responsible for urine formation, is the nephron. The human kidney contains approximately 1.2 million nephrons. The nephron consists of several sequentially connected sections located in the cortex and medulla: the glomerulus; the main, or proximal, tubule; the thin descending limb of the loop of Henle; the distal tubule; and the collecting duct.
The mechanism of urine formation consists of three main processes: 1) glomerular filtration of water and low-molecular-weight components from blood plasma to form primary urine; 2) tubular reabsorption (reabsorption into the blood) of water and essential substances from primary urine; 3) tubular secretion of ions and organic substances of endogenous and exogenous origin.