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Lecture 13

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Stephen Reid

1    Lecture 13: Blood Gas Transport, Ventilation-Perfusion Matching and the Control of Breathing 1. Oxygen – Haemoglobin Binding 1A. The Oxygen Equilibrium Curve (OEC) We can quantify the binding of oxygen to haemoglobin using an oxygen equilibrium (or disassociation) curve, which plots the partial pressure of oxygen in mmHg onthe x-axis. The Y-axis can be the level of oxygen-Hb saturation (as a percentage value) or it can be a measure of oxygen content (units of oxygen or units of oxygen per units of Hb such as volume %, or molO /molH2, or gO /gHb). 2 The curve issigmoidal in shape. It is relatively flat at the lowest O 2 levels and then becomes quite steep before reaching a plateau at higher P O 2levels. The sigmoidal nature of this curve results from positive cooperativity, in which the binding of one O mo2ecule to a Hb subunit facilitates the binding of a second, and so on. When we have very low partial pressures, the hemoglobin molecule does not a have a particular high affinity for oxygen - but as one molecule becomes bound, it causes the other heme groups to increase their affinity for and as these subsequent heme groups bind oxygen, the rate of O -H2 saturation increases. 1B. The p50 Value: Quantifying Oxygen – Haemoglobin Binding Affinity The variable that is used toquantify the affinity of haemoglobin for oxygen is called thep50. It is defined as the partial pressure of oxygen at which 50% of haemoglobin is bound with oxygen. If we plot concentration on the y-axis, then the p50 value is defined as the point when the oxygen content of the blood if 50% of its maximum value. The OEC curve can shift to the left or to the right depending on regulatory factors such as temperature, pH, CO 2 and the presence of small organic ions (i.e., 2, 3 DG). If the curve shifts to the left, then there is a decrease in the p50 value which reflects an increase in the affinity for oxygen of haemoglobin. If the curve shifts to the right, this leads to an increase in p50 and indicates a decrease in haemoglobin-oxygen affinity. 1C. The p50 Value and Oxygen Loading and Unloading When blood flows through the lungs, with relatively high partial pressures of oxygen, then O2-Hb saturation will be high. When blood flows to the tissues, then we come to the steep part of the oxygen equilibrium curve: the low partial pressures will facilitate the removal of oxygen from Hb and therefore its diffusion out of the blood and into the tissues. 1D. Resting Oxygen Consumption In the systemic arteries the partial pressure of oxygenis 100 Torr while in the systemic veins it is 40 Torr. If we look at where these values lie on an oxygen equilibrium curve, we see that at 100mmHg, the O2*Hb saturation of the arteries is 100% whereas at 40mmH saturation is still quite high, 75%. Thus, even as the blood travels through thesystemic circulation and oxygenatesthe tissues, it is only giving up about a quarter of the oxygen that is has bound to it. When the deoxygenated blood enters the lungs, its partial pressure is low, but the oxygen content of the 2    blood is still quite high. Why do we have thisexcess capacity? It is for whenour metabolic rate increases, such as during exercise - essentially, the body has a 'reserve' of saturated oxygen in case it requires more to maintain metabolic function. When we remember that we only use about a third of our alveolar capillary length for gas exchange, we can see just how tremendous our capacity is for increasing metabolic function. 1E. Oxygen-Hb Saturation versus Oxygen Content Anaemia is a condition where there area lower number of red cells.Polycythaemia is a condition where there is an overproduction of red cells. More red cells mean more Hb which means a greater capacity to carry oxygen. Less red cells mean leas Hb and therefore a reduced capacity to carry oxygen. In all cases (normal, anaemia and polycythaemia), all of the Hb that is available to bind oxygen can, in theory, bind oxygen. Therefore in all cases it is possible to achieve 100% oxygen-Hb binding. However, this doesn’t mean that the oxygen content is the same in all three cases. Oxygen content will be higher in the case of polycythaemia because there is more Hb availableto carry oxygen. Oxygen content will be lowest during anaemia because there is less Hb available to carry oxygen. 1F. Modification of Oxygen-Hemoglobin (Hb) Binding Oxygen-haemoglobin binding can be affected by several factors the two most important, from a physiological-regulation point-of-view are temperature and pH while the partial pressure of CO can 2 also affect it Hb-O 2inding but to a lesser extent. In humans, a byproduct of glycolysis called 2,3-diphospoglycerate can also affect binding as can chloride ions (but, in humans, to a much lesser degree). These factors alter the oxygen-haemoglobin binding reaction primarily to promote oxygen uptake by the blood in the lungs and tohelp deliver oxygen from Hb to the tissues when blood is flowing through the tissues. 1F-1. Modification of O -Hb2Binding: Temperature Body temperature is relatively constant in humans; approximately 37 degrees Celsius. However, if we are breathing air cooler than this, then the lung gaswill be cooler than the average body temperature. This reduced temperature will lower the p50, increasing oxygen-haemoglobin binding affinity and promote O 2loading onto Hb. In contrast, metabolically active tissues (such as muscles) will have an elevated temperature as heat is a by-product of metabolism. As blood reaches the tissues, this elevated temperature will increase the p50, and thereby lower oxygen-haemoglobin binding affinity. This facilitates oxygen delivery to the tissues. 1F-2. Modification of O -Hb2Binding: pH The pH of the blood can also influence oxygen-Hb binding to encourage oxygen uptake in the lungs and oxygen delivery to the tissues. In metabolically active tissues pH is reduced causing an increase in p50 and a decrease in O 2-Hb affinity. This
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