When you exercise you breathe more deeply and more rapidly. Exercising causes an increase in the amount of CO2 in the blood because muscles are oxidizing glucose more rapidly. This lowers the pH of the blood. Certain brain cells and specialized cells in the aortic arch and carotid arteries are sensitive to changes in blood pH. When they sense a lower blood pH, nerve impulses are sent more frequently to the diaphragm and intercostal muscles. These muscles contract more rapidly and more forcefully, resulting in more rapid, deeper breathing.
Gas exchange occurs in the alveoli of the lungs .
Each alveolus is about 0.25 to 0.5 mm across. However, alveoli are so numerous that the total surface area of all these sacs is about 70 m2. Associated with the alveoli are large numbers of capillaries. The walls of both the capillaries and alveoli are very thin , and the close association of alveoli and capillaries allows the easy diffusion of oxygen and carbon dioxide across these membranes. Diseases like asthma case constriction of the bronchioles reducing the flow of air into the lungs and inhibiting gas exchange.