How do gases in the body equalize with outside pressure?

The correct choice is that gases in the body equalize with outside pressure primarily by diffusion through membranes. This process involves the movement of gas molecules from an area of higher concentration to an area of lower concentration, typically across the alveoli in the lungs. When external pressures change, gases such as oxygen and carbon dioxide move in and out of the body via the respiratory system and through cellular membranes according to partial pressures.

The equalization process is crucial at different altitudes and is affected by various physiological factors. For instance, as a person ascends to higher altitudes, the external pressure decreases, leading to a change in the partial pressures of gases within the body. In response, gases will diffuse from the bloodstream into the lungs and then into the atmosphere to maintain equilibrium with the outside environment.

The other aspects touch on processes that are not the primary means by which gases equalize with pressure. Dissolving in the blood plays a role in gas transport but is not the main mechanism for pressure equalization. Expansion and contraction relate to volume changes in the lungs but are secondary to diffusion. The movement of fluids pertains more to the circulatory system and not directly to the gas exchange process itself.