In scuba diving, why do you tend to rise when you inhale and sink when you exhale?

In scuba diving, the tendency to rise when you inhale and sink when you exhale is primarily due to a change in displacement. As you inhale, your lung volume increases, which effectively makes you a larger object in the water. This increase in volume leads to an increase in buoyancy because you are displacing more water. The principle of buoyancy states that an object will float in a fluid if it displaces a volume of water that weighs more than the object itself. Therefore, when your lungs fill with air, you are displacing a greater volume of water, resulting in positive buoyancy and causing you to rise.

Conversely, when you exhale, your lung volume decreases, leading to less displacement of water. As you expel air, your overall body density increases, which can make you less buoyant, allowing you to sink. This dynamic movement is crucial for maintaining control of your ascent and descent in the water column while diving. While changes in weight and ambient pressure can influence buoyancy and diving dynamics, in this particular scenario, the most pertinent factor is the change in displacement related to your lung volume.