Explain Change of posture - Decreased Cardiac Output

When a person stands up from lying down posture, there is an immediate but transient fall in systolic blood pressure. This is because, upon standing, there is a pooling of venous blood in the lower limbs due to gravity. This leads to a decreased venous return, which in turn decreases the stroke volume and cardiac output causing a fall in the systolic pressure.

However, within fifteen seconds, the baroreceptor reflexes bring about a cardio acceleration and vasoconstriction. As a result, the blood pressure soon returns to normal or may rise above value due to overcompensation. You may work out the changes on standing.