How The Kidney Works

by Stephen Z. Fadem, M.D., FACP

Blood is pumped from the heart through large blood vessels to the kidneys. The kidneys are about the size a fist, and lie in the back. They are partly protected by the lower ribs.

Each kidney contains about a million filters.

Inside the kidney the blood vessels continue to divide until they become so small that they can no longer hold water. These tiny vessels are known as capillaries. The glomerulus is the filtering unit and contains the leaky capillary. The filtrate collects into a sac called Bowmans capsule and drains into the proximal tubule.

The tiny capillaries filter minerals, wastes and water but retain red cells, proteins and large molecules. This process is known as filtration. The proteins that are not filtered remain in the capillary and create an oncotic pressure due to osmosis. Filtration depends upon the surface area of the filter and the permeability of the membrane that surrounds the capillary. It also depends upon the systemic pressure and its counter, the pressure caused by osmosis.

This animation simulates the wall of the capillary, magnified around 40,000 times. Red cells and proteins cannot cross the membrane, but small minerals cross with ease.

The filtered blood is returned to the body through the veins. The fluid and its substances must now travel through a long and winding tubular pathy, first through the cortex, or outer portion of the kidney, then the medulla, the deep portion. There the tubule makes a sharp hairpin turn and travels back up to the cortex and its parent glomerulus. After looping through the cortex it connects to a collecting tubule and makes its final descent. The nephron is the combination of the filtering unit (glomerulus), its tubule and collecting duct. Together, they filter the blood, process the filtrate and make urine.