- Alveoli: Tiny air sacs in the lungs where gas exchange happens. Oxygen moves from the air in the alveoli into the blood, and carbon dioxide moves from the blood into the alveoli to be exhaled.
- Nephrons: Tiny filtering units in the kidneys that clean the blood. They remove waste products and extra water, forming urine. Both alveoli and nephrons have a large surface area and thin walls to make the exchange process efficient.
