The kidney consists of an inner renal medulla and an outer renal cortex enclosed inside a tough renal capsule. While the renal medulla contains structures known as collecting ducts and loops of Henle, the renal cortex contains renal corpuscles and convoluted tubules. Inside the renal corpuscles, blood is filtered in the first part of the process of urine formation. The convoluted tubule then reabsorbs useful minerals and nutrients from the filtered blood before it passes into the loop of Henle in the medulla.
Inside the kidney, there are millions of units known as nephrons. Each nephron contains a corpuscle, situated in the renal cortex, together with a series of tubules, some of which dip into the medulla. A renal corpuscle consists of what is called a glomerulus, which is a tiny knot of blood vessels, inside a containing structure known as Bowman's capsule. Blood flows into the glomerulus, where the blood vessels are full of tiny holes. These are too small to allow blood cells to escape but water, minerals, nutrients and other tiny molecules are able to pass through into what is known as Bowman's space.
Between the glomerulus blood vessels and Bowman's space there is a filtration membrane which helps to keep cells and proteins from leaving the glomerulus. The process occurring in the renal cortex is sometimes referred to as ultrafiltration, and the water and molecules in Bowman's space are known as filtrate. Filtrate remains in the renal cortex for the next stage of urine formation, which involves drainage into the proximal convoluted tubule.
In the proximal convoluted tubule, more useful substances are absorbed from the filtrate and returned to the blood. The cells lining the tubule are specially modified so that they can actively pump molecules from the filtrate, and some of the water follows. Next, the urine formation process moves away from the renal cortex into the medulla, where water is recovered from the filtrate inside the loop of Henle. This leads into the distal convoluted tubule, which is located in the cortex once again, and where further molecules are absorbed.
Finally, urine from the distal convoluted tubule drains into the collecting duct. While large collecting ducts are found in the medulla, smaller ones may be situated in the renal cortex. The ducts drain into a space called the renal pelvis which connects to the ureter, the tube that transports urine away from the kidney and into the bladder.