Massachusetts Comprehensive Assessment System (MCAS) Biology Practice Test

Which two gases are primarily exchanged between the respiratory and circulatory systems?

1. Nitrogen and oxygen

2. Carbon dioxide and nitrogen

3. Carbon dioxide and oxygen

4. Oxygen and hydrogen

The correct answer is: Carbon dioxide and oxygen

The primary gases exchanged between the respiratory and circulatory systems during the process of respiration are carbon dioxide and oxygen. In the lungs, oxygen is inhaled and diffuses from the air into the bloodstream, where it is bound to hemoglobin in red blood cells for transport throughout the body. Simultaneously, carbon dioxide, which is a waste product of cellular metabolism, diffuses from the blood into the alveoli in the lungs to be exhaled. This exchange process occurs due to differences in partial pressures of these gases—oxygen moves from an area of higher partial pressure (in the alveoli) to an area of lower partial pressure (in the blood), while carbon dioxide moves from higher partial pressure in the blood to lower partial pressure in the alveoli. This effective gas exchange is crucial for maintaining proper physiological function and ensuring that cells receive adequate oxygen for energy production while removing harmful carbon dioxide.