When you're swimming, buoyancy determines whether you float or sink. According to Archimedes' principle, any object in water experiences an upward buoyant force equal to the weight of water it displaces. If this buoyant force is greater than your body weight, you float. If it's less, you sink. The key factor is density - your body's density compared to water's density.
The human body is composed of different tissues with varying densities. Muscle tissue has a density of about 1060 kilograms per cubic meter, bone is around 1900, while fat is only 900. Most importantly, our lungs contain about 5 to 6 liters of air, which has a very low density. When we combine all these components, the average human body density is approximately 985 kilograms per cubic meter, which is less than water's density of 1000. This is why humans can float naturally - our air-filled lungs act as built-in flotation devices.
The key to understanding why exhaling causes sinking lies in lung volume changes. During full inhalation, your lungs can hold up to 6 liters of air, expanding to maximum capacity. However, when you exhale completely, only about 1.2 liters of residual air remains in your lungs. This represents an 80 percent reduction in air volume. Since air is much less dense than water, this dramatic decrease in lung volume significantly reduces your body's total volume while your mass stays the same, causing your overall density to increase.
Let's calculate the exact density changes with a concrete example. Consider a 70-kilogram person. When their lungs are full, their total body volume is about 72.5 liters, giving a density of 965 kilograms per cubic meter - less than water, so they float. However, after exhaling, their body volume decreases to 68.7 liters while their mass remains 70 kilograms. This gives a new density of 1020 kilograms per cubic meter, which is greater than water's density. This 55-unit increase in density is enough to make the person sink. The key insight is that exhaling reduces your volume while keeping your mass constant, inevitably increasing your density.
Understanding buoyancy control has many practical swimming applications. For controlled diving, swimmers and pearl divers exhale to increase their density, allowing them to sink faster and reach greater depths. When treading water, keeping lungs partially filled helps maintain neutral buoyancy, making it easier to stay at the surface with minimal effort. During swimming, athletes inhale during stroke recovery for buoyancy and exhale underwater for better streamlining. In emergency situations, taking a deep breath maximizes buoyancy, which is a crucial survival technique for conserving energy while floating. By controlling your breathing, you can effectively control your position in the water.