Lecture 17 - Respiratory Physiology II Transport and exchange of gases Flashcards Preview

1060 Human form and function > Lecture 17 - Respiratory Physiology II Transport and exchange of gases > Flashcards

Flashcards in Lecture 17 - Respiratory Physiology II Transport and exchange of gases Deck (18):

What is Henry's Law?


Amount of a gas dissolved in a liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid



Gas diffuses from region of high partial pressure to region of low partial pressure down its partial pressure gradient (P1-P2)

What does the direction of diffusion depend on?


Direction of diffusion depends on direction of P1-P2 gradient NOT

on direction of concentration gradient.



Why use Partial Pressures rather than concentrations of gases in a liquid?


1). Partial pressure is a better indication of how a gas will diffuse from one compartment to another




2). Content of gas dissolved in liquid depends on solubility as well as partial pressure




Content (ml/dl) = solubility (ml/dl/kPa) x partial pressure (kPa)





in Gaseous exchange between air and blood

alveolar epithelium + capillary endothelium = ??


diffusion barrier



Rate of transfer  (uptake) through a sheet of tissue of area A and thickness T = ???


A x (P1-P2) x solubility x (1/ √Mol. Wt.)




Systemic blood PO2 is determined by alveolar PO2 and alveolar PCO2 is determined by capillary blood PCO2

But wait! systemic arterial PO2 is 12.5 kPa, slightly less

than alveolar PO2 (13.3 kPa). Why?


Because of:

1). The anatomical right-to-left shunt (next lecture) of deoxygenated bronchial vein blood mixing with oxygenated pulmonary vein blood, lowers PO2 entering left atria

2). Drainage of part of the coronary venous blood directly into the left ventricle, further lowers PO2 of blood flowing from left ventricle into aorta







In gaseous exchange between air and blood

what is the rate of gaseous transfer determined by?



Partial Pressure gradient (P1-P2) 



In gaseous exchange between blood and tissues, what is the gaseous exchange driven by?


by P1-P2

but in opposite direction to that in lung





What are the 4 important measures of O2 transport?


1). Percent saturation = % of O2 binding sites on haemoglobin

(Hb) that are bound to O2

2). O2 content (ml/dl) = amount of gas present in blood

= sum of amount of O2 bound to Hb

+ amount dissolved in plasma

3). Maximum O2 carrying capacity (ml/dl) = amount of O2 blood is capable of carrying when Hb is 100% saturated.

4). Partial pressure (kPa) = pressure exerted by the gas in solution

P1-P2 = driving force for diffusion and uptake by Hb

= Main determinant of amount of gas dissolved in plasma

AND of % saturation




O2 transport to tissues per min = ??


O2 transport to tissues per min = 5L/min x 200 mlO2/L = 1000 mlO2/min

CO= 5L/min

O2 in systemic arterial blood at 12.5 kPa:

O2 dissolved in plasma = 0.3ml/dl (1.5% of total)


O2 bound to Hb = 19.7ml/dl   (78.5% of total)

= Total O2 content of 20ml/dl (100% of total) =200 mlO2/L





What are the factors hat influence binding Oto Hb


PCO2, pH


Structure of Hb

[2,3 diphosphoglycerate]


Key Point:

The way O2-Hb binding is influenced by these factors facilitates uptake in lungs and unloading of O2 in tissues






deficiency in the amount of oxygen reaching the tissues.



What happens during carbon monoxide (CO) poisoning?






What happens when you have anaemia?


you have a reduced red blood count --> reduced Hb content --> reduced O2 carrying capacity --> mild tissue hypoxia



What is thee haldane effect and what's it due to ?



At any given PCO2, the quantity of CO2 carried is greater in partially deoxygenated (venous) blood than in oxygenated (arterial) blood

Due to:

A. Better buffering of H+ by deoxy-Hb promotes formation of


B. Hb forms carbamino compounds with CO2 more readily when





The fick equation for VO2 and VCO2 ?


VO2    =   cardiac output  x  (arterial - mixed venous O2 content)

5000 ml/min  x   (20 - 15) ml/dl =  250 ml/min


VCO2  = cardiac output  x  (mixed venous – arterial CO2 content)

5000 ml/min  x   (52 - 48) ml/dl =  200 ml/min