The kidneys regulate the composition and volume of bodily fluids by filtering waste and excess water from the blood. This process starts when blood enters either kidney through the renal arteries (“renal” means having to do with the kidneys). It then passes through microscopic tubes called nephrons, which do the actual filtering. From there, the filtered blood leaves the kidney through the renal veins. The waste and excess water removed by the nephrons become urine, which flows through a structure called the renal pelvis, out of the kidney and into the ureter.
The diagram below is a cross section of a kidney.
Most nephrons are found in the renal cortex, which is the outer layer of the kidneys, but they also reach into the inner layer, known as the renal medulla. A fibrous layer called the renal capsule encases the whole kidney.
Nephrons are made up of five parts, each of which plays an important role in the filtration of blood and formation of urine. They can be seen in the diagram below (numbers 3 and 4 are together one part).
The filtration process begins when blood enters the spherical Bowman’s capsule, which contains a tuft of capillaries called a Glomerulus. These capillaries have membranes that let smaller materials like water molecules, minerals, and waste pass through, but not larger materials like proteins and cells.
The Bowman’s capsule channels the water, minerals, and waste that have been removed from the blood into the proximal convoluted tubule. At this point, wanted materials like water and minerals are absorbed back into the blood.
The remaining waste then enters the loop of Henle, which has two parts: The descending loop of Henle and the ascending loop of Henle. This is the part of the nephron that leaves the renal cortex and enters the renal medulla before returning back to the cortex.
Next, the waste enters the distal convoluted tubule before flowing into the collecting ducts, which carry it to the renal pelvis and the ureter, at which point it leaves the kidney as urine.