Some Physiological Control Systems
It is typical of living organisms that their different parts interact functionally. Much of modern physiology—systems physiology—is preoccupied with both the formal and quantitative aspects of such interactions, which may be of several different kinds. The very simplest are trivial in the sense that they are direct results of mass transfer or reaction stoichiometry. For example, the increase in arterial pO2observed after an increase in alveolar pO2 is required by the permeability of the alveolar-capillary membrane to oxygen, and the decrease in red cell pH upon oxygenation is a direct consequence of the simultaneous binding of O2/release of protons by hemoglobin. Much more complex interactions stem from feedback mechanisms, and these are invariably interpreted as serving some specific function, e. g., the maintenance of blood pH or tissue pO2 within certain ranges. In describing such cases, the variables kept constant are often said to be controlled, and the same word is also used to describe situations in which a complete set of variables is changed in concert so as to maintain a certain function.
KeywordsPentose Phosphate Pathway Pyruvate Kinase Polycythemia Vera Glycolytic Rate Phosphoglycerate Mutase
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