Among mammals, humans are known for having exceptional endurance. For example, persistence hunters can run for 2-5 hours to wear out their prey, while marathoners routinely run ~4.5 hours on average (Schulte et al., 2015). Sure, a cheetah can run fast, but it can’t run as long as a human.
How did humans develop such exceptional endurance?
When vertebrates moved to dry land, they needed to develop lungs to extract oxygen from the surrounding air. Mammals have smaller red blood cells because they are missing a nucleus (more on the oddity of mammalian red blood cells). Because mammalian red blood cells are larger than capillaries, they are forced to squeeze through them, which results in higher contact between the red blood cells and vascular wall to help exchange oxygen and carbon dioxide. Mammals also have more capillaries in their muscles than amphibians, which requires higher blood pressure to push the blood through the capillary networks. These adaptations allow humans to engage in endurance exercise and also explains why animals with lower blood pressure and capillarization are not able to engage in endurance exercise. Humans, in particular, also have an advantage over other mammals as we are relatively hairless and covered in sweat glands, allowing us to regulate body temperature pretty well (Schulte et al., 2015).
An analysis of animals that died from natural causes in the London Zoological Garden revealed that high blood pressure comes with a cost, however. Among vertebrates, birds have rather high blood pressure. Falconiformes (e.g. eagles, hawks) have among the highest resting blood pressures (~200 mmHg) but ~54% of specimens showed evidence of arteriosclerosis. Penguins, on the other hand, have resting blood pressures of ~94 mmHg and do not show any signs of arteriosclerosis. Although pet parrots can develop high blood pressure, regular veterinary care allows them to live long lives even though 70-90% end up developing arteriosclerosis, a major cause of death in these birds (Schulte et al., 2015).
There are limits, of course, to how high our blood pressure can go before it leads to disease and increased risk of death. Although exercise is often recommended to improve cardiovascular health, tolerance for participating in exercise activities may be impaired in people with hypertension. A recent study published in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology examined how hypertension impacts men performing cycling exercises. Heart rate and mean arterial blood pressure were higher during exercise in men with hypertension compared to healthy participants whereas blood flow to their legs was decreased. Such a decrease in leg blood flow may explain why men with hypertension develop neuromuscular fatigue faster than those without hypertension and why they may not feel very good when participating in exercises.
Sources
K Schulte, U Kunter, MJ Moeller. The evolution of blood pressure and the rise of mankind. Nephrology Dialysis Transplantation. 30(5): 713-723, 2015.
TS Thurston, JC Weavil, H-Y Wan, MA Supiano, PA Kithas, M Amann. Hypertension restricts leg blood flow and aggravates neuromuscular fatigue during human locomotion in males. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. 327: R517-R524, 2024.
Categories: Comparative Physiology, Diet and Exercise, Exercise, Extreme Animals, Nature's Solutions
Tags: American Journal of Physiology, American Physiological Society, blood pressure, evolution, fitness, health, hypertension, wellness
Life Lines by Dr. Dolittle 