0603 - Respiratory partial pressures and blood gases - EG

Question 1

What is barometric pressure?

Answer 1

force per metre squared exerted against a surface by weight of air above that surface, measured by a barometer. affected by altitude and composition of air.

Question 2

What is barometric pressure at sea level?

Answer 2

approx. to 101kPa or 760 torr.

Question 3

What is the relationship between altitude and barometric pressure? What is the comparison of barometric pressure of Everest to that at sea level?

Answer 3

barometric pressure drops with increasing altitude as the density of air drops, however this is variable with weather conditions. At 8,800 m there is ~1/3 of that at sea level.

Question 4

To what are plane cabins pressurised?

Answer 4

2,100 m ~80kPa

Question 5

State the composition of air as %volume of N2, O2, CO2, H2O and other gases.

Answer 5

N2 = 78%, 02 = 21%, CO2 = 0.05%, H2O = 0.50%, other gases 0.95%.

Question 6

what volume does 1 mol of gas take up?

Answer 6

22.4 L, irrespective of type of gas

Question 7

What gases are not metabolically relevant?

Answer 7

noble gases and nitrogen.

Question 8

What is the partial pressure (pp) exerted by O2 at sea level?

Answer 8

P(barometric) x (ppO2) = 101.3 x 0.21 = 21.3 kPa

Question 9

Is the ppH2O dependent on altitude?

Answer 9

No, it is only dependent on temperature.

Question 10

At 37 degrees Celsius, what is the ppH2O?

Answer 10

6.3 kPa

Question 11

Why does the partial pressures of all inspired gases reduce upon inhaling?

Answer 11

due to the H2O vapour mixing with the air. Pressure H2O is 6.3 kPa, therefore drops the combined pressure of other gases down (101.3-6.3=95 kPa total for remaining gases, of which O2 constitutes 21% and N2 79%, to bring to new values of 20kPa and 75 kPa respectively for O2 and N2)

Question 12

What happens to the ppO2 in the trachea?

Answer 12

reduces slightly from its atmospheric value due to body temperature of 37 degrees Celsius causing air to be H2O saturated (pp H2O = 6.3kPa). Worked out as: (101.3-6.3)*0.21 = 20.0 kPa

Question 13

What is the respiratory quotient?

Answer 13

the relationship between O2 uptake and CO2 elimination; R = V(CO2)/V(02) = ~0.8More O2 is used in the tissues than CO2 is produced, and N2 compensates for this disparity.

Question 14

In the alveoli, how much CO2 is exchanged? And how is the ppO2 in the alveoli calculated from this?

Answer 14

ppCO2 = ~5.3 kPa (or measured value from blood) exchanged from capillaries to alveoli. To calculate alveolar ppO2, the effect of CO2 (and N2) leaving the capillaries needs to be accounted for. Within tissue, CO2 produced is related to the amount of O2 used by the respiratory quotient of 0.8, therefore 5.3/0.8 = 6.6 kPa is the negative impact of alveoli gas pressure. The deficit between 6.6 and 5.3 (ie. 1.3kPa) is compensated for by N2. So as descend from tracheal gas pressure (20), the alveolar gas pressure of O2 is(20-(5.3/0.8))=13.4 kPa.

Question 15

What factors limit gas exchange via diffusion within the lung?

Answer 15

(1) membrane surface area and thickness (2) difference in partial pressure (3) diffusion capacity of the lung

Question 16

What happens to the solubility of a gas with increased temperature?

Answer 16

solubility decreases with increasing temperature.

Question 17

Are gases less or more dissolved in body fluids with fever?

Answer 17

less dissolved.

Question 18

is solubility directly or indirectly related to pressure?

Answer 18

directly

Question 19

is solubility directly or indirectly related to temperature?

Answer 19

indirectly

Question 20

Which has the highest solubility at 37 degrees Celsius: CO2, O2, or N2?

Answer 20

CO2 is 23x better than O2 which is 2x better than N2

Question 21

Under normal conditions, how is O2 exchange limited? (i.e. is it perfusion limited or diffusion limited)

Answer 21

Normally O2 exchange is perfusion limited - arterial O2 pressure can equilibrate with alveolar O2 if given enough time to equilibrate, however it can become diffusion limited in pathological situations such as inflammation and interstitial fibrosis or under strenuous exercise or at high altitude.

Question 22

What is the standard reporting for lung volumes?

Answer 22

BTPS (body temperature and pressure, saturated)

Question 23

What is the standard reporting for gas volumes?

Answer 23

STPD (standard temperature and pressure, dry)

Question 24

What measure of pressure should one never use when answering a question by Stricker?

Answer 24

mmHg

Question 25

Why is there an O2 difference between the end of the lung capillary and the aorta?

Answer 25

Due to the mixing of venous blood, called a shunt.

Question 26

What is the reference range of the pressure of O2 in arterial blood?

Answer 26

9.3-13.3 kPa

Question 27

What is the reference range of the pressure of CO2 in arterial blood?

Answer 27

4.7-6.0 kPa

Question 28

Briefly outline the oxygen cascade from inspired air to blood.

Answer 28

(1) Atmospheric air at sea level has a pressure of 101.3 kPa and of this PO2 is 21%. (2) as air is breathed in it is humidified by H2O (P=6.3 kPa), this reduces the P of all gases, and for pressureO2 reduces to become (101.3-6.3)*0.21 = 20.0 kPa. (3) in the alveoli the presence of CO2 (P=5.3 kPa and add P=1.3 kPa of N2 calculated from respiratory quotient of 0.8) exchange further diminishes PO2 (20.0-(5.3/0.8)) = 13.4 kPa. (4) the shunting of venous blood back into the arterial system causes a minor drop in P O2 (12.0), and O2 extraction from the tissues causes a further ~60% drop.