Blood pressure is the force exerted by circulating blood against the inner walls of your arteries. It is measured as two numbers: systolic pressure (maximum pressure during a heartbeat) and diastolic pressure (minimum pressure between beats). When the legs are raised significantly above the heart, blood pressure generally increases. This effect is temporary and depends heavily on the person’s current physiological state, as the maneuver creates a measurable internal shift in fluid.
The Direct Answer: Impact on Central Blood Flow
Raising the legs, often called the Passive Leg Raise (PLR) maneuver, prompts an immediate shift of blood volume toward the center of the body. Gravity assists in moving venous blood from the legs and abdomen back into the chest cavity. This movement acts like a temporary, internal fluid challenge, and the volume of blood mobilized is estimated to be between 300 and 500 milliliters.
This sudden increase in central blood volume causes the heart to pump more blood per beat, temporarily increasing blood pressure. This elevation is most pronounced in individuals experiencing low blood pressure or low blood volume. For a healthy person, the body’s regulatory systems, such as baroreceptors, quickly detect this change. These systems work rapidly to maintain balance, often making the resulting blood pressure increase minimal and short-lived.
The Circulatory Mechanism
The temporary increase in blood pressure occurs because leg elevation directly increases venous return, the rate of blood flow back to the heart. When the legs are raised, gravity assists this flow, significantly increasing the volume of blood returning to the right side of the heart. This greater volume of returning blood leads to an increased preload, which is the amount of stretch placed on the heart muscle fibers at the end of the filling phase.
This stretch is the basis for the Frank-Starling mechanism, an intrinsic property of heart muscle. This mechanism states that the stroke volume (the amount of blood pumped out by the ventricle with each beat) will increase in response to a greater volume of blood filling the ventricle. The increased preload stretches the cardiac muscle fibers, causing a more forceful contraction and a larger stroke volume.
Since blood pressure is a function of how much blood the heart pumps into the arteries, a larger stroke volume directly increases the heart’s cardiac output. This greater output generates more pressure against the arterial walls, resulting in the temporary increase in blood pressure. The effect is almost instantaneous, occurring within 30 to 90 seconds of the legs being fully raised.
Clinical Context and Positional Blood Pressure
The physiological effect of leg raising is routinely utilized in medical settings through the Passive Leg Raise (PLR) Test. This test is used in emergency and intensive care units to assess a patient’s fluid responsiveness. By monitoring the change in cardiac output, clinicians determine whether a patient will respond positively to a fluid infusion.
The PLR test involves moving a patient to a supine position with the legs elevated to a 45-degree angle. A significant increase (typically 10 to 15 percent) in stroke volume or cardiac output during this brief maneuver suggests that the patient’s low blood pressure would improve with additional volume. This principle explains why raising a person’s legs can be an effective first-aid measure for temporary low blood pressure episodes, such as fainting.
It is important to distinguish this therapeutic maneuver from simple, low-level leg elevation often done for comfort or to reduce localized swelling (edema). Simple elevation, where the feet are only slightly above the heart, primarily reduces pressure within the leg veins and helps drain excess fluid from the tissues. This low-level elevation has a negligible effect on central blood pressure, unlike the rapid, high elevation used in the PLR maneuver. The maneuver is avoided in cases of head trauma or severe congestive heart failure, as the sudden increase in central blood volume could be detrimental.