Thermodynamic approach to oxygen delivery in vivo by natural and artificial oxygen carriers.
Oxygen is a toxic gas, still indispensable to aerobic life. This paper explores how normal physiology uses the physico-chemical and thermodynamic characteristics of oxygen for transforming a toxic gas into a non toxic indispensable metabolite. Plasma oxygen concentration is in the range of 10(-5) M, insufficient to sustain metabolism. Oxygen carriers, present in blood, release oxygen into plasma, thereby replacing consumed oxygen and buffering PO(2) near their P(50). They are the natural cell-bound carriers, like hemoglobin inside red cells, myoglobin inside myocytes, and artificial cell-free hemoglobin-based oxygen carriers (HBOC) dissolved in plasma. Metabolic oxygen replacement can be defined as cell-bound and cell-free delivery. Cell-bound delivery is retarded by the slow diffusion of oxygen in plasma and interstitial fluids. The 40% hematocrit of normal blood compensates for the delay, coping with the fast oxygen consumption by mitochondria. Facilitated oxygen diffusion by HBOCs corrects for the slow diffusion, making cell-free delivery relatively independent from P(50). At all oxygen affinities, HBOCs produce hyperoxygenations that are compensated by vasoconstrictions. There is a strict direct correlation between the rate of oxygen replacement and hemoglobin content of blood. The free energy loss of the gradient adds a relevant regulation of tissues oxygenation. Oxygen is retained intravascularly by the limited permeability to gases of vessel walls.
Bucci
Enrico
E
Department of Biochemistry and Molecular Biology, University of Maryland Medical School, 108 North Greene St., Baltimore, MD 21201, USA.
D000818:Animals; D001802:Blood Substitutes / Q000737:chemistry / Q000378:metabolism*; D006454:Hemoglobins / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D009211:Myoglobin / Q000737:chemistry / Q000378:metabolism*; D010100:Oxygen / Q000097:blood / Q000737:chemistry / Q000378:metabolism*; D010101:Oxygen Consumption / Q000502:physiology; D013816:Thermodynamics
D001802:Blood Substitutes; D006454:Hemoglobins; D009211:Myoglobin; D010100:Oxygen