Asthma narrows the airways by causing an allergy-induced spasms of surrounding muscles or by clogging the airways with mucus . Bronchitis is an inflammatory response that reduces airflow and is caused by long-term exposure to irritants such as cigarette smoke, air pollutants, or allergens . Cystic fibrosis is a genetic defect that causes excessive mucus production that clogs the airways. The differences between oxygen and carbon dioxide concentrations are measured by partial pressures. The greater the difference in partial pressure the greater the rate of diffusion.
Respiratory pigments increase the oxygen-carrying capacity of the blood.
Humans have the red-colored pigment hemoglobin as their respiratory pigment. Hemoglobin increases the oxygen-carrying capacity of the blood between 65 and 70 times. Image from W.H. Freeman and Sinauer Associates, used by permission. Carbon dioxide concentration in metabolically active cells is much greater than in capillaries, so carbon dioxide diffuses from the cells into the capillaries. Water in the blood combines with carbon dioxide to form bicarbonate .