Central Venous PressureWe have just examined the arterial blood pressure, which is a measure of the pressure of the blood leaving the heart. Another pressure of great interest is the pressure of the blood entering the heart. All the venous blood (from the veins) of the body drains into the right atrium. The pressure in the large vein (vena cava) just outside the right atrium is called the central venous pressure (CVP). This pressure is of special interest because it provides an indication of the volume of blood that is flowing through the veins and into the right atrium. If the heart is beating weakly, blood tends to "back up"in the veins. The CVP, as a measure of this back-up, will increase. Also, if the blood begins to back up (measured by increased CVP), the heart can respond in two ways: (1) it can beat faster, and (2) it can pump more blood with each beat. When the heart responds in this way, then the back-up is relieved. As the back-up is relieved, the CVP decreases back to normal.Besides a weakly beating heart, another factor that increases the flow of blood into the right atrium, and thus causes the CVP to become elevated, is an increase in blood volume. This can happen when we go to bed at night, because when we lie down, gravity is no longer pulling the blood "down" in the same way it does when we stand up. Again, the elevated CVP is alleviated by the heart's response to move the blood faster. Now, let us add to our picture of the heart and blood vessels to understand how oxygen from our atmosphere enters the blood system to "feed" and provide energy for all the activities we carry out every second of our lives. We will examine the gateway through which we take in oxygen and breathe out carbon dioxide: the respiratory system.
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