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Six facts about physiology

The Mayo Clinic Scientific Press suite of publications is now available on Oxford Medicine Online. To highlight some of the great resources, we’ve pulled together some interesting facts about physiology from James R. Munis’s Just Enough Physiology.

(1)     In physiologic terms, we are exposed to three main sources of pressure: (1) the weight of the atmosphere; (2) hydrostatic forces exerted by the weight of body fluids; and (3) mechanical pressure generated by the heart or other muscles that contract around those fluids.

(2)     In 1978, two climbers (Reinhold Messner and Peter Habeler) reached the summit of Mount Everest without using supplemental oxygen. Physiologists originally thought such a climb was impossible because a calculation of lung oxygen levels at that altitude indicated that nothing beyond basal metabolism could be supported, even with hyperventilation. Thus, a climber theoretically would not have sufficient oxygen available for the exercise that was needed to reach the summit in the first place. Messner and Habeler proved the physiologists wrong.

(3)     Transmural pressure is the pressure inside a vessel minus the pressure outside. Perfusion pressure is inlet minus outlet pressure.

(4)     Stroke volume is determined by three factors — preload, afterload, and inotropy — and these determinants are in turn dependent on how the left ventricle handles pressure.

(5)     A little more than a hundred years ago, the physiologist Ernest Starling told a story and asked a riddle: why does heart failure cause capillary edema? His audience, sheltered from a London winter inside the halls of the Royal College of Surgeons, had come to hear about heart failure. When Ernest Starling lectured, however, there was usually something remarkable to hear. The experiment that Starling highlighted was very simple. He placed a catheter into the pericardium of an anesthetized dog and slowly infused oil into it. This produced an incremental cardiac tamponade, i.e. a form of acute heart failure.

(6)     Like mammals, birds are warm-blooded animals (homeotherms). This means that they are metabolically active, even in the coldest or highest environments, and they have to maintain a constant flow of oxygen to the brain if they want to remain neurologically intact and still enjoy the view. To keep oxygen flowing at high altitudes, the bird must massively hyperventilate. One consequence of hyperventilation, though, is severe hypocapnia and respiratory alkalosis. Regardless of competing needs, though, it is still remarkable that these birds can achieve high enough oxygenation to support life (and exercise) at such extraordinary altitudes. That said, birds do have an unfair advantage over us — their lungs use a system of ventilation/perfusion matching that is more efficient than that of the mammalian to-and-fro alveolar lungs.

James R. Munis is a member of the College of Medicine at the Mayo Clinic and the author of Just Enough Physiology. Physiology is the science that is applied at the boundary between life and death; this is why it’s so important to those of us who tread that same boundary every day in the practice of anesthesiology and critical care.

The Mayo Clinic Scientific Press suite of publications is now available on Oxford Medicine Online. With full-text titles from Mayo Clinic clinicians and a bank of 3,000 multiple-choice questions, Mayo Clinic Toolkit provides a single location for residents, fellows, and practicing clinicians to undertake the self-testing necessary to prepare for, and pass, the Boards and remain up-to-date. Oxford Medicine Online is an interconnected collection of over 250 online medical resources which cover every stage in a medical career, for medical students and junior doctors, to resources for senior doctors and consultants.

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