(i) Structure. Alveoli in the lungs and nephrons in the kidneys, both possess an elaborate network of blood capillaries.
(ii) Functioning. Alveoli purify the deoxygenated blood by removing carbaon dioxide from it and making it oxygenated by introducing oxygen in it (during the gaseous exchange). Similarly, nephrons purify the dirty blood by filtering out waste products like urea from it in the form of urine.