There is a steep concentration gradient because the lungs have a good supply and are well ventilated.

The statement touches upon the physiological principle of gas exchange in the lungs. In the lungs, oxygen from inhaled air diffuses into the blood due to a steep concentration gradient, which is made possible by several factors:

1. Good Blood Supply: The lungs are richly supplied with blood vessels, particularly the pulmonary capillaries. This allows for a large surface area for gas exchange, facilitating quick movement of oxygen into the blood and carbon dioxide out of it.

2. Ventilation: Proper ventilation ensures that fresh air, which has a higher concentration of oxygen, constantly replaces the air in the alveoli. This maintains a high concentration of oxygen in the alveoli compared to the oxygen concentration in the deoxygenated blood flowing through pulmonary capillaries.

3. Alveolar Structure: The alveoli, tiny air sacs in the lungs, provide an extensive surface area (about 70 square meters in total for an adult) for gas exchange. Their thin walls minimize the distance that gases must travel to diffuse.

4. Steep Concentration Gradient: The difference in concentration of oxygen between the alveolar air and the blood creates a driving force for diffusion, allowing oxygen to move from the alveoli into the blood where its concentration is lower, and carbon dioxide to move from the blood into the alveoli to be exhaled.

In summary, the combination of effective ventilation, a rich blood supply, and optimal lung structure creates a steep concentration gradient that is essential for efficient gas exchange, which is crucial for maintaining adequate oxygen levels in the body and removing carbon dioxide.