You open a “valve” thousands of times per day. It’s called your “mouth.”
When you open your mouth and drink water, for example, you’re adding fluid into the system that is circulated throughout your blood stream. Blood is not 100 percent blood cells. Much of it is water.
Your kidneys act as another “valve,” by removing excess water from your system in the form of urine. Similarly, many blood pressure medications act to lower blood pressure by increasing the rate at which water is removed from your system.
The pipe in your analogy doesn’t really work because a pipe is constant in size, whereas the vessels in your circulatory system can constrict or dilate, making the blood flow through a larger or smaller circuit depending on the circumstances (consequently altering resistance to flow and pressure). In addition, your heart is a pump with variable output, increasing or decreasing the output by changing how fast it is beating and the amount of force (and therefore volume) of each beat. So although the circulatory system is essentially a closed system it can vary significantly in volume, and the pump can vary significantly in output.
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