Question

What is residual volume? How much is it in a normal human adult?

Answer 1

Residual volume is the amount of air that is still in a person's lungs after fully exhaling. Doctors use tests to check an individual's residual air volume to check how well the lungs are functioning. It's normal to possess some air remaining after exhaling to keep the lungs from collapsing.

Complete answer:
The volume of air left in lungs and respiratory passage after even the severest expiratory effort is claimed to be residual volume. Its normal value is about 1500 ml.
Lung volumes and lung capacities ask the quantity of air within the lungs at different phases of the respiratory cycle. The typical total lung capacity of an adult human male is about 6 litres of air.
Residual volume is measured by: A gas dilution test. This test measures the entire amount of air the lungs can hold [total lung volume]. For this test, an individual sits inside an airtight booth called a plethysmograph and breathes through a mouthpiece while pressure and air flow measurements are collected.
The residual volume functions to keep the alveoli open even after maximum expiration. In healthy lungs, the air that creates up the residual volume is employed for continual gas exchange to occur between breaths.
It is the quantity of air remaining within the lungs after maximal exhalation.

Note:
During the breathing cycle, the residual volume of air in lungs provides the advantage because it prevents the lungs from collapsing. Also, thanks to the presence of residual volume, there's sufficient time for oxygen to be absorbed and for the $CO_2$ to be released during breathing.