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For further investigation into the progress of the blood pressure, we will measure a patient's blood pressure in the superior vena cava measured during inhalation and during an exhalation. His heart function remains the same during the measurements

Why is this subject's blood pressure in the superior vena cava during inhalation less than during the exhalation?

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The diaphragm moves down during inspiration. This leads to air being sucked in, due to the pressure difference created. The converse is true during expiration. As the thoracic space is where the SVC sits, it's pressure decreases during inspiration as it is subjected to the surrounding decrease in pressure.

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