What is the primary function of the bladder?

The primary function of the bladder is to store urine until it is expelled from the body. This organ is a muscular sac that expands as it fills with urine produced by the kidneys. It plays a crucial role in the urinary system by allowing for the temporary storage of urine, providing the body with the ability to manage the timing of urination. The bladder's walls are elastic, allowing it to stretch and hold varying volumes of urine, which is important for maintaining bodily fluid balance and preventing continuous urination. When the bladder reaches a certain level of fullness, nerve signals inform the brain that it is time to empty the bladder, allowing for voluntary control over the process.

In contrast, the other options are related to different functions and organs within the body. Filtering waste from the blood is the primary role of the kidneys, which remove waste products and excess substances to form urine. Regulating blood pressure is primarily handled by the heart and blood vessels, with numerous mechanisms in place involving both these organs and the kidneys. Lastly, the absorption of nutrients is a function of the digestive system rather than the urinary system, as nutrients are absorbed from food in the intestines, not in the urine.