Transport of Gases PDF - Gulf Medical University

Diffusion and transport of gases Sovan Bagchi Professor of Physiology www.gmu.ac.ae COLLEGE OF HEALTH SCIENCES  Learning Objectives List the mechanisms of oxygen and carbon dioxide transport De...

Diffusion and transport of gases Sovan Bagchi Professor of Physiology www.gmu.ac.ae COLLEGE OF HEALTH SCIENCES  Learning Objectives List the mechanisms of oxygen and carbon dioxide transport Describe the reaction between Hemoglobin and oxygen Explain oxygen dissociation curve Describe Bohr effect Describe chloride shift Explain Carbon dioxide dissociation curve Describe Haldane effect  Gas Exchange in Lungs & Tissues Exchange of gases between lungs and blood and gas movement at the tissue level progress passively by diffusion, depending on their pressure gradients. Area of 70sq.m and thinness of 0.5µ make alveolar membrane as an ideal one for diffusion Pressure gradient for O2 = 100-40 mmHg Pressure gradient for CO2=46mmmHg-40  Gas Exchange in the Lungs PO2 in alveoli :100 mm Hg PO2 in pulmonary capillaries : 40 mm Hg Result: O2 moves into pulmonary capillaries PCO2 in pulmonary capillaries : 46 mm Hg Average Arterial blood gases ✓PO2 100 mm Hg ✓PCO2 40 mm Hg Gas transfer capacity may be impaired by Thickening of membrane Reduction in surface area  Transport of O2 in the Blood Occurs in 1.Dissolved form in plasma 0.3 ml/100ml of blood Dissolved O2 establishes the PO2 of the blood. Regulates breathing Determines loading of hemoglobin 2. Oxyhemoglobin - Each Hb carries 4 molecules of O2 as there are 4 heme in each Hb molecule. Oxygen carrying capacity Each gram of Hb can carry 1.34 ml of O2 and in 100 ml of blood the oxygen carrying capacity would be 1.34 ml X 15g = 20 ml oxygen  Bohr Effect in the tissues for unloading of O2 Conditions creating the Bohr effect Increased PCO2 Increased temperature Increased 2,3-DPG Decreased pH Helps Unloading of oxygen from Hb With shift to the right P50 increased Therefore in actively contracting muscles as in exercise, O2 availability is enhanced. Myoglobin present in skeletal muscle binds to O2 at a low pO2 and releases it at very low Po2 in the tissue.ie shift to left  Oxygen hemoglobin dissociation curve It is a Sigmoid shaped curve. In the pulmonary capillaries the Hb saturation is 98% for PO2 of 100 mmHg The plateau in the curve indicates that increasing PO2 from 90 mmHg, the % saturation of Hb does not rise. The steep of the curve lies between PO2 of 60 mmHg and 20 mmHg P50 is the pressure at which Hb saturation of oxygen is 50%. Normal value 24- 28 mmHg In the arterial blood the PO2 is 98 mmHg and Hb saturation is 95% In the venous blood the PO2 is 40 mmHg and Hb saturation is 75% P50 P50 : it is the pressure at which O2 is 50% saturated. Normal value is 28mmHg P50 is increased when the curve is shifted to right as in Bohr effect (With more of CO2 in blood more of O2 is released from blood. ie its O2 carrying capacity comes down. Similarly with high temp., higher H+ ions, more of DPG (with chronic hypoxia, severe anemia,lung diseases etc associated with high DPG) P50 is decreased when the curve is shifted to left as in CO poisoning. O2 dissociation becomes difficult here. Fetal Hb also shows the same nature and it transports more of O2.  CO2 Transport Three mechanisms 1. CO2 is transported as dissolved CO2 ( 5%) The dissolved CO2 establishes the pCO2 of the blood 2. The main form of CO2 transport in the blood occurs as BICARBONATE (70%). Plays important role in Acid Base Balance and pH maintenance. 3. CO2 is also transported in the blood by combining with Hb as Carbaminohemoglobin (25%) In normal resting conditions, 4 ml of CO2 per 100 ml of blood is transported from tissues to the lungs Chloride shift in Tissue capillaries CO2 enters into RBC and with the help of carbonic anhydrase enzyme bicarbonate is formed. This bicarbonate enters into plasma. For electrical neutrality This binds to Na that is formed out of NaCl. So the negatively charged Cl enters inside RBC in place of Bicarbonate. This is called chloride shift or Hamburger’s shift. O2 dissociation curve in anemia and CO poisoning In anemia In anaemia the PO2 , % saturation of Hb are normal. Only the O2 content is reduced In CO poisoning: HbCO (Carboxy hemoglobin) CO has 200 times more affinity to Hb than O2. CarboxyHb cannot bind to O2. Hb saturation of O2 is reduced and also the oxygen content. Moreover, shift to left occurs and dissociation of O2 becomes difficult. So O2 delivery is better in anemic condition than in carbon monoxide poisoning.  CO2 transport - Summary In lungs reversal of chloride shift occurs.ie chloride comes out of RBC and bicarbonate gets into RBC. From this bicarbonate , CO2 is produced that is eliminated into the alveoli from there to atmosphere. This is the reversal of chloride shift. CO2 dissociation curve The curve is linear In the venous blood, the PCO2 is 46 mmHg and volume is 52% In the arterial blood, the PCO2 is 40 mmHg and volume is 48% Haldane effect for the release of CO2 in lungs In lungs, the PO2 is 100 mmHg and O2 diffuses into pulmonary capillaries with PO2 of 40mm Hg. According to the pressure gradient. The entry of O2 displaces H+ ion from the Hb , freeing it to combine with O2. The reverse chloride shift occurs to form H2CO3 which dissociates into CO2 and H20. CO2 is released. If Hb is oxygenated, CO2 dissociation curve shifts right. That blood looses CO2 as it gets oxygenated. This is called Haldane effect Haldane effect for the release of CO2 in lungs At the lungs, the PO2 is 100 mmHg and when pulmonary capillaries carry the deoxygenated Hb, the O2 diffuses into the pulmonary capillaries from alveoli along the pressure gradient. The entry of O2 displaces H+ ion from the Hb , freeing it to combine with O2. The reverse chloride shift occurs to form H2 CO3 which dissociates into CO2 and H20. The release of CO2 by the Oxygen in the lungs is called Haldane effect ie When blood get oxygenated CO2 dissociation curve shifts to right.  Learning Resources Text Book: Marieb EN. Human Anatomy and Physiology, 9th Edition, Pearson International Edition; 2014. ISBN-13: 978-1-2920-2649-7. Chapter 22, pp. 892-897. Power-point presentation in the moodle www.gmu.ac.ae COLLEGE OF HEALTH SCIENCES