Kidneys

A pair of fist-sized, bean-shaped organs at the back of the abdomen — the body’s blood filters.

Structure

An outer cortex and inner medulla packed with about a million tiny filtering units (nephrons), draining into a central collecting space.

What they do

• Filter the blood, removing wastes and excess substances.
• Fine-tune water, salt, and acid balance.
• Help regulate blood pressure.
• Release hormones — one that drives red blood cell production, and the active form of vitamin D.

Key idea

The kidneys do far more than make urine — they continuously set the precise composition of the body’s internal fluid.