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Lecture 10

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PHGY 210
Ann Wechsler

Lecture Ten- Monday, January 25 , 2010h H. Measuring Pulmonary Blood Flow (Q) Using Fick's principle O2 consumption per minute (V(dot)O2) is equal to the O2 taken up by the blood in the lungs in one minute. The [O2] in the blood entering the lungs is Cv(venous)O2 and that leaving is Ca(arterial)O2. It then follows that: V(dot)O2= Q(dot)(CaO2 - CvO2) or Q(dot)= V(dot)O2/ (CaO2 – CvO2) where: V(dot)O2: measured by comparing [O2] in the expired gas collected in a large spirometer and [O2] in inspired gas; CaO2: measured from an artery; CvO2: measured via a catheter from the pulmonary artery. V TRANSPORT OF O2 and CO2 A. O2 Physically Dissolved in Plasma The amount of dissolved gas carried by the blood is directly proportional to the partial pressure of the gas, according to Henry’s Law (Henry's Law states that the number of gas molecules dissolved in a liquid is proportional to the partial pressure of the gas above the liquid). Because O2 is relatively insoluble in H2O, the amount of O2 dissolved in blood is very small, and linearly proportional to PO2. In 100 ml of plasma, there is 0.3 ml of O2 (i.e. 0.3 volume %) when equilibrated with PO2 of 100 mm Hg. O2 consumption (VO2) by the body cells, even at rest, is much greater than what can be supplied from the amount dissolved in blood. At rest, O2 is about 300 ml O2/min. Therefore, if O2 were only found in plasma, the tissue demand for O2 would never be met. B. O2 Bound to Hemoglobin Hemoglobin, Hb, is found in red blood cells, and permits whole blood to take up to 65 times as much O2 as plasma, at a PO2 of 100 mm Hg. Hb constitutes 1/3 of the total weight of red blood cells; there are 147g of Hb in each liter of blood. Each molecule of Hb consists of a heme (iron porphyrin) joined to a protein globin which ++ consists of 4 polypeptide chains, each containing an Fe ion that can bind to 1 molecule of O2 (Figure 21). Hb is essential for the transport of O2 by blood because it combines rapidly and reversibly with O2: Hb + O2 ↔ HbO2 At PO2 of 100 mm Hg, the total amount of O2 physically dissolved in the blood is 0.3 vol.%, and the total amount of O2 bound to Hb is 19.5 vol. %. This means that the total amount of O2 in arterial blood is about 20 vol. %. One can see from this that Hb plays an important role in determining the total amount of O2 carried by the blood. Note that the O2 that is bound to Hb does not contribute to the PO2 of the blood. Only molecules physically dissolved in the blood plasma are responsible for PO2. However, the PO2 of the plasma does determine the amount of O2 that combines with Hb. see picture in slides C. The O2 dissociation curve The HbO2 dissociation curve (Figure 22) determines the amount of O2 carried by Hb for a given partial pressure of O2.
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