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.
Department of Biochemistry and Molecular Biology, University of Maryland Medical School, 108 North Greene St., Baltimore, MD 21201, USA.
Enrico
E
Bucci
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