Urine is made by the occurrence of three processes in the nephron: glomerular filtration, tubular resorption and tubular secretion.
In the nephron the blood carried by the afferent arteriole enters the glomerular capillary network where it is filtered. The filtration implies that part of the blood returns to the circulation through the efferent arteriole and the other part, known as the glomerular filtrate, enters the proximal tubule of the nephron. In the nephron tubules (also known as convoluted tubules) substances of the glomerular filtrate like water, ions and small organic molecules are resorbed by the cells of the tubule wall and gain again the circulation. These cells also secrete other substances inside the tubules. The urine is formed of not resorbed filtered substances and of secreted (by the tubules) substances. Urine is drained by the collecting ducts to the ureter of each kidney, then it enters the bladder and later it is discharged through the urethra.
The nephron tubules are surrounded by an extensive capillary network that collects resorbed substances and provides others to be secreted.
