Final answer:
The correct phrase that describes the partial pressure of oxygen and carbon dioxide is that the partial pressure of oxygen in the lungs is greater than in the blood, and the partial pressure of carbon dioxide in the lungs is less than in the blood, facilitating respiratory gas exchange.
Step-by-step explanation:
The phrase that best describes the partial pressure of gases is: 'P_{O_{2}} in lungs is greater than P_{O_{2}} in blood; P_{CO_{2}} in lungs is less than P_{CO_{2}} in blood'. This is because oxygen (O_{2}) diffuses from the alveoli in the lungs where its partial pressure is higher into the blood where its pressure is lower. Conversely, carbon dioxide (CO_{2}) diffuses from the blood where its partial pressure is higher into the alveoli where its pressure is lower, facilitating the exchange of gases important for respiration.
According to Dalton's Law and Henry's Law, gases will diffuse down their pressure gradient from high to low partial pressures. This occurs across the respiratory membrane, allowing the blood to become oxygenated and release carbon dioxide back into the lungs. Hence, P_{O_{2}} is higher in the alveoli of the lungs compared to the blood in the capillaries, which facilitates the diffusion of oxygen into the blood. Similarly, P_{CO_{2}} is higher in the blood compared to the alveoli, which facilitates the diffusion of carbon dioxide into the alveoli for exhalation.