The kidneys are important to the body’s production of urine, but they also play a role in regulating important components of the blood.

Blood comes to the kidneys from the abdominal aorta and inferior vena cava, the large arteries and veins that are part of the ascending aorta. Oxygenated blood is brought to the kidneys from a small branch called the renal artery.

The kidneys are highly complicated “filtration factories.” Inside each kidney, the renal artery branches into smaller and smaller parts until they make contact with the core structure of the kidney, the nephrons.

The nephrons are tiny coiled tubes of arteries, veins, and ducts. Each kidney has approximately 1 million of them. The nephrons regulate waste, water, and other materials to adjust to the body’s changing needs.

Other important parts of the kidney include:

  • Renal pyramids: cone-shaped tissues consisting of parallel segments of nephrons
  • Calyx: cup-like structures that help collect urine on its way to the bladder
  • Renal pelvis: funnel-shaped portion of the ureter in the kidney
  • Hilum: a section that is the entry point to the kidney for the renal veins, renal arteries, and ureter

At the tip of each nephron is a small sac of capillaries called the glomerulus. There, under tight pressure, materials are constantly being filtered out of the blood. The pressure in each glomerulus is about three to four times higher than in any other capillary, and each glomeruli need that pressure to actively filter contaminants out of the body.

Because of their need for high pressure, the kidneys also help control blood pressure by producing a blood vessel-constricting protein (angiotensin) that also signals the body to retain sodium and water. Both the constriction and retention help restore normal blood pressure if it dips.

The kidneys also regulate the production of oxygen-delivering red blood cells. When the kidneys don’t get enough oxygen, their distress call comes in the form of production of erythropoietin, a hormone that stimulates the bone marrow to produce more oxygen-carrying red blood cells.

After the kidneys have performed their cleansing function, the filtered blood leaves the kidneys through the renal vein, moves up the inferior vena cava, and goes back to the heart.