Urine is a filtrate of the blood. The formation of urine is achieved through a process of ultrafiltration. Ultrafiltration through sieve-like capillaries sends water and small solutes to the primary urine, but proteins and cells stay in the blood unless the cardiovascular system is damaged. This is just the biologists' way of saying forming a fluid by applying a pressure to a liquid at a semi-permeable membrane. In vertebrates, the heart supplies the necessary pressure.
Capillaries are the semi-permeable membrane.
Ultrafiltration occurs at the portal to each nephron in the kidney. The nephron itself is a tubular epithelium. Two additional processes modify the urine formed by ultrafiltration, which is called the primary urine. These are reabsorption, the movement of water and solutes from the nephron lumen into peritubular capillaries, and secretion, movement of water and solutes from the peritubular capillaries into the nephron. We next viewed a drawing of a human kidney.