The kidneys sit within either side of the posterior portion of the ribcage. They are supplied with blood by the renal artery, which branches off of the descending aorta, and they deliver deoxygenated blood back to the heart via the renal vein, which branches from the inferior vena cava. The kidney itself is made up of four distinct parts: an outer renal cortex, an inner renal medulla, and a renal pelvis which descends into a long tube called the ureter. The ureter transports kidney filtrate to the bladder.
The mouth of the ureter flares open to create the renal pelvis, which has a cup-shaped extension to receive urine and other filtrate from the renal tissue. Blood from the renal artery is moved into the renal cortex, in which afferent arterioles deposit their contents into basket-like receptacles called glomeruli (singular: glomerulus). The glomerulus filters red blood cells and other high-molecular weight molecules from the general filtrate, which is mostly water and urea, which is then moved further down the renal tubules. Renal tubules, which consist of a proximal convoluted tubule, a Loop of Henle (located in the renal medulla), and a distal convoluted tubule, move filtrate through the kidney, where vital substances like sugar can be reabsorbed into the bloodstream. Any fluid that is not reabsorbed is dumped into the renal pelvis, where it moves down the ureter into the urinary bladder.