Elevating an injured limb is a fundamental step in first aid for controlling external bleeding, often recommended alongside direct pressure. This technique manipulates the forces governing the cardiovascular system. By raising the wound, it taps into physical principles to mechanically decrease the pressure that pushes blood out of severed vessels, slowing the escape of blood.
The Physics of Blood Flow: Hydrostatic Pressure
Blood pressure within the body is a combination of the force generated by the heart’s pumping action and hydrostatic pressure. Hydrostatic pressure is the pressure exerted by a fluid due to gravity, increasing in proportion to the depth of the fluid column. In the human body, this means blood pressure is not uniform, varying based on a body part’s vertical position relative to the heart. The force of gravity creates a pressure gradient of approximately 75 millimeters of mercury (mmHg) over a meter.
When a person is standing, blood vessels in the feet experience significantly higher pressure due to the added weight of the column of blood extending from the heart. Conversely, vessels in the head experience lower pressure than those at heart level. This principle is similar to the plumbing in a tall building. For example, the arterial pressure in the lower legs of a standing adult can be elevated by about 100 mmHg compared to the pressure measured at the heart’s level.
How Elevation Reduces Pressure at the Wound Site
When a bleeding wound is elevated above the level of the heart, the hydrostatic pressure component begins to work against the heart’s pumping force. The elevation effectively creates a negative pressure gradient relative to the heart. The heart continues to beat with the same intensity, but the blood must now travel “uphill,” overcoming the downward pull of gravity.
This mechanical resistance reduces the force with which blood is pushed into the injured vessels. By reducing the overall pressure, the volume of blood escaping the wound is minimized. Raising an arm 30 centimeters (12 inches) above the heart, for instance, can decrease local blood pressure by roughly 22 mmHg. This reduction in transmural pressure slows the leak, providing a favorable environment for platelets and coagulation factors to aggregate and form a clot.
When and How to Safely Elevate a Bleeding Wound
The correct technique for using elevation involves positioning the injured limb above the level of the heart. This is typically achieved by raising the limb 6 to 12 inches higher than the heart, using supports like pillows or folded clothing. It is important to maintain this position consistently while simultaneously applying firm, direct pressure to the wound with a clean cloth or dressing.
Elevation is always a supplementary measure; direct, sustained pressure remains the primary method for controlling most external bleeding. There are circumstances where elevation should be avoided to prevent further harm. The technique should not be used if there is a suspected fracture or dislocation of the limb, as movement could worsen the injury. Likewise, if a foreign object, such as a piece of glass or a knife, is embedded in the wound, elevation should be skipped to avoid causing internal damage.