Respiratory Module Lec. 1 Session 4: Blood Gas Carriage PDF

Respiratory module Lec.1 Session 4:Blood Gas Carriage Dr. Safa Amer Ali  objective state the solubility of oxygen in body fluids. draw an oxygen-hemoglobin dissociation curve, label the axes correctly and indicate the normal values of (i) alveolar pO2 (ii) ca...

Respiratory module Lec.1 Session 4:Blood Gas Carriage Dr. Safa Amer Ali  objective state the solubility of oxygen in body fluids. draw an oxygen-hemoglobin dissociation curve, label the axes correctly and indicate the normal values of (i) alveolar pO2 (ii) capillary pO2 in a typical tissue.  objective  draw the effects on the haemoglobin oxygen dissociation curve of (i) a fall in pH (ii) a rise in temperature.  state the factors influencing the diffusion of gases across the alveolar membrane.  describe in outline how the transfer factor ('diffusion capacity') of the lungs may be determined Objective 1 state the solubility of oxygen in body fluids Oxygen is not sufficiently soluble in body fluids for adequate gas transport in simple solution. The solubility coefficient of O2 = 0.01 mmol/L /kPa at 37° C. Therefore, at a partial pressure of 13.3kPa and a temperature of 37°C plasma contains 0.13 mmol/L of dissolved.oxygen. (0.01 x 13.3) Oxygen is carried in the blood in two forms, dissolved and combined with haemoglobin (Hb) which increases oxygen carrying capacity 50x (97% of O2 in blood is transported by combining.with hemoglobin and only 3% is dissolved) Partial pressure of O2 in blood leaving the lung is 95mmHg = 97% saturation and venous blood.contains 40mmHg (75% saturation) Each gram of Hb binds to 1.34ml O2, so 1.34 X 15gm = 20.1ml O2 in 100% saturation. So 95 mmHg (97%) blood carries 19.4 ml O2 , while in tissues, the saturation 40% and in veins (75%) carrying 14.4ml O2 in each 100ml of blood.  :Dissolved Oxygen amount of oxygen delivered to the tissues is only about 90 ml/min. Taking into account that the tissue requirements are about 3000 ml Oxygen/min, it is obvious that this way of transporting oxygen is.not adequate for human Objective 2 draw an oxygen-hemoglobin dissociation curve, label the axes correctly and indicate the normal values of (i) alveolar pO2 (ii) capillary pO2 in a typical tissue.  Oxyhemoglobin Dissociation Curve Graphic illustration of the % oxyhemoglobin saturation at different values of P02. –Loading and unloading of 02. –Steep portion of the sigmoidal curve, small changes in P02 produce large differences in % saturation (unload more 02). Decreased pH, increased temperature, and increased cause decrease in affinity of hemoglobin for 02 which leads to greater unloading of 02 and shift to the curve to the right. Oxyhemoglobin Dissociation Curve Difference between hemoglobin and myoglobin dissociation curve Objective 3 draw the effects on the haemoglobin oxygen dissociation curve of (i) a fall in pH (ii) a rise in temperature. Changes in the O2-Hb dissociation curve Shift to the right: Occur when there is decreased affinity of Hb for O2 and unloading of O2 in the tissues is facilitated. Factors causing right shift: Increases in PCO2 and decreases in PH Increases in temperature Increases in 2, 3 diphosphoglycerate (2,3-DPG) concentration HbO2 + 2,3 DPG → Hb-2,3 DPG + O2 Shift to the left: This occurs when the affinity of Hb for oxygen increases. When the affinity is increased unloading of O2 in tissue is more difficult Decrease in PCO2 and increases in pH Decreases in temperature Decrease in 2,3-DPG concentration Effect of temperature on the curve  Effects of pH and Temperature  The loading and unloading of O2 influenced by the affinity of hemoglobin for 02.  Affinity is decreased when pH is decreased.  Increased temperature and 2,3- DPG: Effect of CO2 on the curve  Bohr Shif' Unloading is further facilitated by the 'Bohr Shift'. The affinity of haemoglobin for oxygen is reduced by the lower pH in the tissues. Increases in temperature have.a similar effect Objective 4 estimate the rate of delivery of oxygen to the tissues at different capillary pO2's and pH's..Loading of haemoglobin with oxygen in the lungs Ventilation of the lungs maintains the composition of the alveolar gas different to that of air. The Alveolar pO2 is normally 13.3kPa. The Alveolar pCO2 is normally 5.3 kPa. Exchange of these gases across the alveolar membrane depends upon gradients of partial pressure between alveolar gas and the blood returning to the lungs from the body ('mixed venous blood') Oxygen diffuses more rapidly in the gas phase, but CO2 much more rapidly in the liquid phase, so that overall CO2 diffuses 21 times as fast.as Oxygen  Alveolar Mixed Venous Blood Air pO2 kPa 5.3 kPa 13.3 pCO2 kPa 6.6 kPa 5.3 pH2O kPa 6.3 kPa 6.3  Diffusion rate Is determined by the structure of the alveolar membrane, extra cellular fluid barrier and capillary wall, but normally the diffusion rate is sufficiently fast to allow full gas exchange in about 500 ms - half the time the blood spends in the capillaries. Under normal conditions, therefore, the blood leaving the alveolar capillaries has the same gaseous composition as alveolar air. Arterial gas tensions are therefore determined by alveolar gas composition, so respiration must be controlled to keep alveolar pCO2 at 5.3 kPa and alveolar pO2 at 13.3 kPa.  Carbon Monoxide Transfer Factor (TLCO) The 'diffusion capacity’ - or the resistance to diffusion across the alveolar membrane is estimated by the carbon monoxide transfer factor (TLCO) TLCO= Transfer Factor of the Lung for Carbon Monoxide The subject takes a single vital capacity breath of a gas mixture containing air, 14% Helium and 0.1% Carbon monoxide, and holds his or her breath for a few seconds, then exhales. After discarding the first 750 ml, the next litre is collected and the.helium and CO in exhaled air is measured.  Objective 5 describe in outline how the transfer factor ('diffusion capacity') of the lungs may be determined Inhaled CO is used because of its very high affinity for Hb. Since (almost) all the CO entering the blood binds to Hb, very little remains in plasma, and therefore the plasma pCO is taken as zero. Since the concentration gradient between alveolar pCO and capillary pCO is maintained for the entire time blood remains in contact with alveolar gas, the amount of CO transferred from alveoli to the blood is an estimate of the diffusion capacity of the lung (and is not.affected by factors such as blood flow rate) The alveolar pCO and CO uptake are easily measured, so the apparent diffusion resistance or "Transfer Factor" may be calculated. The helium in the mixture allows correction for the diluting effects of air in the lungs (residual volume) when the measurement begins Diseases affecting Diffusion (Low TLCO) If the distance that the gas has to travel from alveolus to blood is increased (i.e. thickening of blood – gas barrier) e.g. in interstitial lung diseases where the alveolar wall is thickened, or in pulmonary oedema, the TLCO is.lower than normal If there is alveolar wall destruction as in emphysema, there is less area for diffusion to occur, and again the CO.transfer factor will be low Thanks