RESPIRATORY GAS TRANSPORT

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.

Oxyhemoglobin Dissociation Curve

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

Alveolar Gas Exchange

Systemic Gas Exchange

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

Bohr effect: release of O2 in response to low pH

Active tissue - more O2 released

• 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

FACTORS FAVORING CARBON DIOXIDE LOADING

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

Peripheral Chemoreceptor Pathways

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 .