OXYGEN TRANSPORT • 98.5% oxygen in arterial blood is bound to hemoglobin and 1.5% is dissolved in plasma • Each heme group of 4 globin chains in a hemoglobin molecule may bind O2 • After binding with O2, hemoglobin changes shape to allow further uptake (positive feedback) • An oxyhemoglobin dissociation curve shows relationship between hemoglobin saturation and oxygen partial pressure - PO2.
CARBON DIOXIDE TRANSPORT • 90% as carbonic acid in plasma CO2 + H2O H2CO3 HCO3-+ H+ • 5% as carbaminohemoglobin (HbCO2)- binds to amino groups of Hb (and plasma proteins) • 5% as dissolved gas in plasma
SYSTEMIC GAS EXCHANGE • CO2 loading • carbonic anhydrase in RBC catalyzes • CO2 + H2O H2CO3 HCO3-+ H+ • chloride shift • keeps reaction proceeding, exchanges HCO3-for Cl- (H+ binds to hemoglobin) • O2 unloading • H+ binding to HbO2 causes affinity for O2 • Hb arrives 97% saturated, but leaves 75% saturated(venous reserve) • Utilization coefficient (amount of oxygen Hb has released) is 22%
ALVEOLAR GAS EXCHANGE • Reactions are the reverse of systemic gas exchange • O2 loading & CO2 unloading • as Hb loads O2 its affinity for H+ decreases, H+ dissociates from Hb and binds with HCO3- • CO2 + H2O H2CO3 HCO3-+ H+ • reverse chloride shift • HCO3-diffuses back into RBC in exchange for Cl-and free CO2 diffuses into alveoli to be exhaled
FACTORS FAVORING OXYGEN UNLOADING • Metabolic needs of tissues affect O2 unloading (HbO2 releases O2) • Low ambient PO2: tissue has PO2 • Increased temperature of tissue • Bohr effect: tissue has CO2, which raises H+ and lowers pH • bisphosphoglycerate (BPG):BPG produced by RBCs as a metabolic intermediate binds to Hb and causes HbO2 to release O2 • body temp. (fever), TH, GH, testosterone, and epinephrine raise BPG and cause O2 unloading
Oxygen Dissociation & pH Active tissue - more O2 released Bohr effect: release of O2 in response to low pH
FACTORS FAVORING CARBON DIOXIDE LOADING • Metabolic needs of tissues affect CO2 loading • Haldane effect: low level of oxyhemoglobin(HbO2) enables blood to transport more CO2 • HbO2 does not bind CO2 as well as deoxyhemoglobin (HHb) does. • HHb binds more H+ than HbO2- The CO2 + H2O HCO3-+ H+ reaction therefore shifts to the right
CHEMORECEPTORS • Monitor pH, PCO2, PO2 of body fluids • Peripheral chemoreceptors • aortic bodies - signal medulla by vagus nerves • carotid bodies - signal medulla by glossopharyngeal nerves • Central chemoreceptors on surface of medulla • primarily monitor pH of cerebrospinal fluid
DIRECT CARBONDIOXIDE EFFECT ON CHEMORECEPTORS • CO2 may directly stimulate peripheral chemoreceptors and trigger ventilation more quickly than central chemoreceptors
DIRECT EFFECT OF OXYGEN ON CHEMORECEPTORS • Usually, oxygen has little effect on chemoreceptors. • Chronic hypoxemia, PO < 60 mmHg, as is associated with emphysema & pneumonia may cause increased ventilation .