Final answer:
The partial pressure of carbon dioxide is generally higher in water-breathing animals than in air-breathing animals due to differences in the respiratory systems and the efficiency of gas exchange in water vs. air.
Step-by-step explanation:
The partial pressure of carbon dioxide is generally higher in water-breathing animals compared to air-breathing animals. When considering the gas exchange in alveoli, the partial pressure of carbon dioxide in the alveolar air is around 40 mm Hg, and for the blood in the capillaries, it is approximately 45 mm Hg. This difference promotes the diffusion of carbon dioxide from the blood into the alveoli to be expelled from the body. The greater solubility of carbon dioxide compared to oxygen facilitates this process, allowing for comparable concentrations to diffuse across the respiratory membrane despite the differences in partial pressures.
During internal respiration, the carbon dioxide produced by cellular respiration diffuses out of the tissues, crosses the interstitial fluid, and enters the bloodstream due to its lower partial pressure in the blood (compared to the tissues). This transferred carbon dioxide is then carried back to the lungs and released during external respiration. The efficient removal of carbon dioxide in water-breathing animals requires a higher partial pressure gradient to overcome the water's capacity to hold CO2.