All-in-One Key Stage 3 (KS3) Science – Cells Practice Test 2025: The Complete Guide to Exam Success!

The lungs are the organs responsible for breathing in humans. They play a critical role in the respiratory system by facilitating the exchange of gases—mainly oxygen and carbon dioxide—between the air and the bloodstream. When we inhale, air enters the lungs, where oxygen is absorbed into the blood, and carbon dioxide, a waste product of metabolism, is expelled when we exhale.

This process is vital for maintaining the oxygen levels necessary for cellular respiration, which produces energy for all bodily functions. The structure of the lungs, along with the diaphragm and rib cage, allows for this critical function of inhalation and exhalation. In contrast, the other options do not participate in the breathing process: the stomach is involved in digestion, the kidneys filter waste from the blood, and the heart is responsible for pumping blood throughout the body.