If a patient is lying flat, venous pressure is typically:

• A. Highest
• B. Lowest
• C. Moderate
• D. Unchanged

Answer: (B)

Posture changes venous pressure because gravity creates a hydrostatic column in the veins. When a person is lying flat, there is no vertical height between the heart and the rest of the body, so the hydrostatic pressure in peripheral veins is minimized and central venous pressure sits near baseline. This means venous pressure in the legs and other peripheral veins is at its lowest in the supine position. In contrast, standing or sitting upright adds a gravitational column below the heart, increasing venous pressure in the legs and promoting venous pooling, which raises venous pressure there and reduces venous return to the heart. So, lying flat typically results in the lowest venous pressure. (Central venous pressure is often around a few mmHg when supine.)