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The Effect of Gravity and Upright Posture on Circulation

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The Heart and Circulation

Abstract

Adaptation to gravity is an essential requirement for life on earth and plays a fundamental role in structuring musculoskeletal support and in the organizing of bodily fluids. About 60% of muscle mass is dedicated to opposing gravity. Moving blood possesses a significant momentum, and a change in posture requires a separate set of adaptive responses for its optimal distribution. Unlike the quadrupeds whose long bodily axis is parallel with the ground, humans spend over two thirds of their lives in the upright or sitting position in line with the gravitational pull. To counteract the effect of gravity, humans and tall vertebrates have developed a host of short-term neuronal and long-term humoral adjustments which correct and maintain a steady mean arterial pressure during change in posture or sudden acceleration. The aim of this chapter is first to examine the physiologic circulatory response to upright posture and the levels of organization responsible for its maintenance. The controversial issue of the siphon effect on perfusion of the brain is discussed in the context of environmental adaptations. Next, the salient features of cerebral blood supply and the dependence of brain function on buoyancy are examined. Human exposure to microgravity removes hydrostatic gradients and causes a headward shift of blood and extracellular body fluids. Microgravity experiments suggest the existence of body’s own “buoyancy field” (upthrust) generated by total body water (60% of total body weight) which bathes the water-free elements consisting of connective tissue, fat, tendons, and bone matrix. This buoyancy vector is parallel with the long axis of the body and works in opposition to gravitational loading. The heart is positioned at the fulcrum between the forces of gravity and levity (buoyancy).

The human upright posture is fundamentally different from the body postures of higher animals in that it is freed from the constraints of gravity.

Jos Verhulst (2003)

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Notes

  1. 1.

    Orthostatic fainting is not known to occur in animals even under extreme environmental conditions. Several bird species can cross the Himalayas in a single flight over 9000 m above the sea level, at atmospheric pressure of 0.3 atm, 220 mm Hg (30 kPa), and ambient oxygen tension of 30 Torr (3 kPa) [1]. At the other end of the spectrum are deep-diving mammals. Sperm whales typically dive to 600–1200 m for 30–50 min duration and elephant seals sink to depths of 1500 m (ambient pressures of 150 atm) for as long as 120 min [2].

  2. 2.

    See Refs. [4, 5] for in-depth, comparative morphological discussion on human uprightness.

  3. 3.

    When standing upright the body’s center of gravity is located in front of the second sacral vertebra. Cumulative centers of gravity of individual organs and limbs are located on the frontal plane that runs through the heels, knees, hips, and ascends through the atlanto–occipital joint where it bisects the line connecting the organs of balance. Thus, the location of the organs of balance in humans at the top of the body, close to the center of gravity, is optimal. In contrast, the organs of balance are positioned in front of the frontal plane in anthropoid apes and at extreme end of the body in quadrupeds [4].

  4. 4.

    It is of interest that William Harvey ascribed the flow of blood through the arteries to the siphon effect [17] (see also Sect. 14.1).

  5. 5.

    Lowering of the head in a large giraffe from 5 m above ground to the ground level would add hydrostatic gradient to arterial pressure resulting in values in the range of 350 mm Hg. “Carotid rete mirabilis,” a meshwork of arteries immersed in venous blood at the base of giraffe’s skull protects the brain from sudden surges in pressure. An S bend of carotid artery at its entry through the carotid foramen, the “carotid siphon,” may serve a similar function [27].

  6. 6.

    The theory concerning the effects of atmospheric pressure and vacuum pumps dates to Galileo and Pascal (see footnote 4, Chap. 14)

  7. 7.

    The evolution of human cognitive faculties has traditionally been inferred from anthropological estimates of brain size. However, the number of neurons in primates increases linearly with the brain mass, with volume of individual neurons remaining constant. This implies that, from a neurological perspective, the human brain could be a scaled-up version of the primate brain. It has been proposed recently that high cerebral metabolic rate (which is proportional to cerebral perfusion) may be a more reliable indicator of evolutionary drive of hominin evolution. Estimation of brain perfusion from the size of carotid foramina from skulls of 11 species of human ancestors indicate a sixfold increase in cerebral blood flow during 3 million years of hominin evolution [33].

  8. 8.

    Archimedes principle states that the buoyant force or upthrust of an immersed object is equal to the weight of displaced fluid.

  9. 9.

    To prevent ingress of air, surgeons apply bone wax to exposed edges of craniectomy wounds. A practical rule of thumb is: if blood does not come out, air is sucked in.

  10. 10.

    Lower body negative pressure is used to reverse the cephalad fluid shifts. Rubber pants with adjustable suction, known as the “chibis suit,” are worn by astronauts for a period before the descent to Earth. Simulated gravity produced by a short-arm centrifuge is a more effective way of reducing the adverse effects of microgravity [48].

  11. 11.

    Experiments on the International Space Station showed that astronauts experience distortions in the perception of their visual environment; distances are underestimated and objects appear higher and shallower when compared to standing on Earth [53].

  12. 12.

    Recumbent position with a head-down tilt is an accepted model for the simulation of microgravity. Similar results are achieved in body-water immersion experiments [48].

  13. 13.

    It appears that the broader question of the siphon principle is related to the nature of circular motion as the archetype of natural motions originally proposed by Aristotle (cf. footnote 1, Chap. 14). Before a siphon will operate automatically, it must be primed. Such priming is not needed in an organism where physical laws are superseded and emerge as higher-level lawfulness in plant and/or animal organization (cf. open system, Sect. 23.3.1). Importantly, pressure (or suction) measured in a blood vessel or a tissue space is ontogenetically different from pressure/suction generated by a mechanical pump. Notwithstanding the fact that both can be quantified by the same instrument, the two belong to different categories.

  14. 14.

    The debate about the circulation in giraffe spurred a related discussion about blood supply to the head in long-necked sauropod dinosaurs. Diplodocus Barosaurus and Argentinosaurus with necks towering 7 and 12 m above the heart, respectively, would require blood pressure in the range of 700 mmHg and a massive heart making up to 5% of body weight, an amount far exceeding the typical mammal (0.6%). Computer models predict that full neck extension in these species was not possible due to inadequate blood pressure. Assumptions based on such models would lead to the conclusion that giraffes could not exist [27].

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  1. Professor of Anesthesiology, Albany Medical College, Albany, NY, USA

    Branko Furst MD, FFARCSI

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Furst, B. (2020). The Effect of Gravity and Upright Posture on Circulation. In: The Heart and Circulation. Springer, Cham. https://doi.org/10.1007/978-3-030-25062-1_24

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