Blood pressure is the force exerted by circulating blood against the walls of blood vessels, measured in millimeters of mercury (mmHg). This hydraulic pressure drives oxygen and nutrients to every tissue in the body, a process called tissue perfusion. Without sufficient pressure, blood flow would cease. The required force varies dramatically across the animal kingdom, reflecting diverse physiological demands and environmental conditions. This variation is most apparent when comparing the high pressures needed by tall terrestrial animals against the extremely low pressures found in certain aquatic species.
Identifying the Animal with the Lowest Blood Pressure
The animal possessing the lowest recorded systemic arterial blood pressure among all vertebrates is the hagfish, a jawless, eel-like marine scavenger. Measurements from the Atlantic hagfish (Myxine glutinosa) reveal an extremely low arterial pressure, typically ranging from 3 to 8 mmHg. This maximum reading is lower than the minimum diastolic pressure considered healthy for a resting human. This pressure, measured in the dorsal aorta, is far below that of other low-pressure animals like chelonians (turtles and tortoises), which maintain resting arterial pressures closer to 15 to 30 mmHg. The hagfish’s exceptionally low pressure results from its primitive, specialized circulatory design, which operates under minimal resistance due to its simple aquatic existence and low activity level.
Biological Adaptations Enabling Low Circulation
The hagfish sustains its bodily functions despite its nearly negligible blood pressure through physiological and behavioral adaptations. A primary adaptation is its extremely low metabolic rate, which reduces the overall demand for oxygen delivery to the tissues. Consequently, the circulatory system does not need to pump blood with high force to achieve adequate tissue perfusion.
Its circulatory system is highly unusual among vertebrates, featuring multiple accessory hearts distributed throughout its body. While the primary branchial heart drives the main systemic circulation, auxiliary pumps (such as the portal and caudal hearts) assist in boosting venous return and local circulation. This decentralized pumping action compensates for the main heart’s very low myocardial power output and lack of strong pressure.
The hagfish also benefits from the physical properties of its aquatic environment. Since water’s density is similar to that of blood, the effects of gravity on blood flow are significantly minimized. This effectively makes the animal “weightless,” eliminating the need to pump blood vertically against gravity, a challenge faced by terrestrial animals. Furthermore, the hagfish possesses a high blood volume relative to its body mass, allowing for sufficient oxygen transport even with slow, low-pressure flow.
Comparing Resting Blood Pressure Across Vertebrates
The hagfish’s pressure of 3 to 8 mmHg sits at one extreme of a vast physiological spectrum. For comparison, a healthy resting human typically maintains a systemic arterial pressure of approximately 120/80 mmHg. Birds, which have high metabolic rates for flight, often exhibit even higher mean arterial pressures than mammals. At the opposite extreme is the giraffe, which requires an extraordinary systemic pressure of around 250 mmHg at the heart level. This immense force is necessary to overcome gravity and perfuse the brain, which is two meters above the heart. These dramatic differences highlight how factors like body size, posture, and metabolic demands dictate the necessary blood pressure for survival.