Lwcture 16 Respiration 2 Flashcards
Gas exchange
Pulmonary circulation
Behaviour of gases
Exchange of CO2 and O2
Matching ventilation- perfusion
Pulmonary circulation
Conc. Of CO2 and O2 in arterial blood relatively constant
- O2 moves from alveoli to blood same rate consumed by tissues
- CO2 moves from blood to alveoli same rate that tissues produce it
Ratio amount CO2 produced to amount of O2 consumed is the respiratory quotient (RQ)
Respiratory quotient (RQ)
At rest
250ml O2 min-¹ consumed
200ml CO2 min-¹ produced
RQ = 200/250 = 0.8
Gas exchange
Gas exchange involves simple diffusion of O2 & CO2 down partial pressure gradients
Partial pressure exerted by each gas in a mixture = total pressure X fractional composition of gas in mixture
Diffusion gradients at level of lungs and tissue affected by size of conc. gradient, s.a. and permeability
Alveoli: large SA thin membrane so effective diffusion
Dalton’s law: -P total = P1+P2+P3…
E.g.
total atmospheric pressure =760mmHg
79%N2 so N2 = 600mmHg
PN2= 760mmHgx0.79= 600mmHg
Partial pressure gradients
Alveolar PO2 high PCO2 low because a portion of alveolar air is exchanged for fresh air each breath
Venous blood entering lungs has low O2 and high CO2 from giving out O2 and picking up CO2 at systemic capillary level
This establishes partial pressure gradient between alveoli air and pulmonary capillary blood inducing O2 diffusion to blood and CO2 out until partial pressures equal
Blood leaving lungs high in O2 low in CO2 delivered to tissues
Partial pressure gradients for gas exchange at tissue level favour O2 into tissue and CO2 out to blood
Having equilibrated with tissue cells blood leaves tissue low in O2 and high in CO2
Blood returns to lungs to fill up on O2 and dump CO2
Establishment of gradients
Air: N2=79% O2=21% (CO2 = 0.003%)
P air= PN2+PO2+PH2O+PCO2
- depending on humidity water can become critical, decreasing contribution of N2 and O2 to total pressure
Seal level air pressure = 760mm Hg
PH2O assuming humidity 0 = 0
PN2= 0.79x760= 600mmHg
PO2=0.21x 760= 160mm Hg
PCO2= 0.003x760=0.23mm Hg
Air moves through conducting zone becomes humidified to saturation (100%)
Water contributing considerably to partial pressure air entering lungs
E.g. pN2=575mmHg pO2=152mm Hg and pCO2= 0.21mm Hg
Solubility of gases
Gas molecules exist as gases or dissolved in liquids
Important for CO2 and O2 exchange from air in alveoli to blood - primarily water
At any partial pressure concentrations of dissolved gas diff. As some gases are more soluble than others e.g. CO2 30x more soluble in blood than O2
Henry’s law C= KP
C= molar conc. Of dissolved gas
K= Henry’s law constant that varies depending on gas
P= pressure in atmospheres
Gases in solution
When temp remains constant amount of gas that dissolves in liquid depends on both solubility of gas and partial pressure of gas
E.g. initial state no O2 in solution
PO2 atmosphere= 100mm Hg
PO2 solution = 0mm Hg
Oxygen dissolves
At equilibrium
PO2 atmospheric= 100mm Hg
(O2) = 5.20mmol/L
PO2 solution = 100mm Hg
(O2)= 0.15mmol/L
(gas) diss = alpha (Pgas)
Gas exchange in alveoli
Air: PO2 = 160mm Hg PCO2=0.23mmHg
Alveoli: PO2 =100mmHg PCO2=40mmHg
Because:
Gas exchange between alveoli and capillaries is continuous
Air mixes with alveolar air
Alveoli air is saturated water vapour
O2 AND CO2 at alveoli
Diffusion rapid, 0.25s blood to equilibrate with with alveolar air
Because:
Thin respiratory membrane
Blood vessels close to resp membrane
In exercise blood flow further increased and equilibrium still achieved
Exchange of O2 and CO2 in tissue
At tissue exchange occurs diffusion gradients
PCO2 varies depending on metabolic activity and blood flow to tissue
Intense exercise PO2 low and PCO2 high in tissue
Large pressure gradients more gas exchanged
Venous blood from active tissue low in PO2 high in PCO2
Venous blood mixed in right atrium until PO2 and PCO2 average
Determination of alveolar PO2 and PCO2
Alveolar PO2 and PCO2
3 factors determine alveolar partial pressure
1) PO2 and PCO2 inspired air
2) minute ventilation (vol of fresh air reaching Alveoli/ min)
3) rate respiring tissue consume O2 and produce CO2
When alveolar vent exceeds demands of tissue then PO2 up and PCO2 down
When alveolar vent does not keep up PO2 down PCO2 up
Under normal conditions ventilation matches tissue metabolism
Ventilation perfusion relationship
VA/Q
Ratio of alveolar ventilation to pulmonary blood flow
Need to match ventilation(V) with perfusion (Q)
In an upright lung gravity has an effect.
- increases pulmonary arterial hydrostatic pressure more at base than apex
- alveolar ventilation varies in same direction as blood flow
(Though not as great as blood flow variations)
Matching pulmonary blood flow to alveolar ventilation
Blood flow decreases 3x faster than ventilation
Normal ventilation: perfusion matched average V/Q= 0.8
Ventilated alveoli close to perfused capillaries- ideal for gas exchange
Effect of obstructing ventilation or perfusion
Normal V/Q =0.8
Infinity: ventilation no perfusion
Zero: perfusion no ventilation
Airway obstruction Va/Q = 0 blood gas content remains unchanged during passage through capillary e.g. mucus plug
Normal Va/Q =0.8 gas exchange is complete initial 1/3 of capillary
Vascular obstruction Va/Q = infinite
Alveolar gas remains at atmospheric levels e.g. pulmonary embolism
