Physiology Lectures 21 - Gas Exchange Ch. 12

Front 1

Define the respiratory quotient; what is its normal value?

Back 1

RQ is a ratio of the amount of CO2 produced per O2 consumed, and is normally 200ml/250ml = 0.8.

Front 2

What are normal values of PACO2, PaCO2, and PvCO2?

Back 2

PACO2 equilibriates with PaCO2 so that they are roughly equal at 40mmHg. PvCO2 is higher at 46mmHg. If someone breathes faster and blows off more CO2, both alveolar and arterial CO2 will drop.

Front 3

What defines a person hyperventilating or hypoventilating?

Back 3

A blood sample of arterial blood: if the PaCO2 is less than 40mmHg, the person is hyperventilating. If the PaCO2 is more than 40mmHg, the person is hypoventilating. You cannot tell if a person if hyper/hypoventilating just by looking at them.

Front 4

What is the diffusion capacity of the lung?

Back 4

DLCO measures how easily blood takes up a diffusible gas, in this case CO.
DLCO = V(CO)/PA(CO) = uptake of CO by blood/driving force for CO.
CO diffuses at a similar rate to O2 and is more readily measured.

Front 5

When is diffusion capacity increased decreased?

Back 5

Decreased in interstitial fibrosis (alveolar-capillary block) and emphysema (loss of surface area)

Increased in exercise (recruitment of previously closed blood vessels)

Normal in chronic bronchitis and asthma

Front 6

What is the most important cause of hypoxemia in patients with oxygen-diffusion barrier?

Back 6

Ventilation/Perfusion imbalance is more a cause of hypoxemia than, for example, the actual alveolar-capillary block in interstitial fibrosis

Front 7

PaO2 = 60mmHg

SaO2 = ?

Back 7

SaO2 = 90

Front 8

PO2 = 40mmHg

SO2 = ?

Back 8

SO2 = 75%

This is the saturation of mixed venous blood

Front 9

Describe what each part of the Hb O2 binding curve represent

Back 9

The graph is curvilinear due to the cooperative effect: binding affinity of Hb for O2 increases with each O2 bound. The steep part at low PaO2 represent the unloading phase, and the flat upper part of the curve at high PaO2 represents the loading phase.

Front 10

PaO2 = 27mmHg

SaO2 =?

Back 10

SaO2 - 50%
This is a crisis-- this person, if not unconscious, will lose consciousness shortly and will die unless you administer O2 immediately.

Front 11

Who do you not give O2 to?

Back 11

Do not give O2 to someone with a PaO2 above 60mmHg (or SaO2 above 90%). It will not help significantly, and it may be harmful. A major exception is a patient with suspected myocardial infarct.

Front 12

What is the concentration of O2 bound to plasma or hemoglobin in arterial and venous blood?

Back 12

In arterial blood:
0.3ml O2/dL in plasma, 19.7ml O2/dL bound to Hgb
In venous blood:
0.12ml O2/dL in plasma, 15 ml O2/dL bound to Hgb

Total amount of O2 in the blood decreases from 20ml/dl to 15 ml/dl

Front 13

What factors influence oxyhemoglobin dissociation?

Back 13

The curve will shift to the right with an increase in PaCO2, 2,3-DPG, Temperature, or [H+] (decrease in pH). The opposite will result in a leftward shift.

Front 14

What is the total gas pressure in aterial and mixed venous blood?

Back 14

Total gas pressure in arterial blood: 760mmHg
Total gas pressure in venous blood: 706mmHg.
The decreased total blood gas is important for reabsorbing gas pockets such as swallowed air in the intestines or air outside the lungs in the chest.

Front 15

How does CO poisoning affect PAO2, PaO2, SaO2, CaO2, CvO2, PvO2?

Back 15

CO binds at 210 times the affinity to Hb than O2, so that 0.1% CO competes 1:1 with O2. This would reduce the oxygen carrying capacity of blood (CaO2: 20ml/dl -> 10ml/dl), as well as the Hb-O2 saturation (SaO2: 100% -> 50%). The Hb-O2 curve is reduced and shifted to the left so that unloading is affected (Hb O2 affinity increases at lower PaO2). PAO2 and PaO2 are normal, however in mixed venous blood PO2 will drop to 15mmHg, CvO2 will drop to 5ml/dl, and SvO2 will drop to 25% due to O2 consumption by the tissues.

Front 16

How much O2 will one gram of hemoglobin combine with?

Back 16

One gram of Hb will combine with 1.34ml of O2.

Front 17

How does anemia affect the CaO2, PaO2, SaO2, as well as the CvO2, PvO2, and SvO2?

Back 17

The CaO2 is reduced, but the PaO2 is normal because this value depends on the small amount of O2 in physical combination with the plasma, and the SaO2 is the same because the fewer Hb molecules are saturated.

On the veinous side, all the values are reduced due to O2 consumption from the Hb reservoir and the plasma O2.

Front 18

How to chemoreceptors get affected by CO poisoning?

Back 18

The receptors in the carotid and aortic bodies respond to low O2 tensions (PaO2 < 55mmHg) by increasing ventilation. However, they do not respond to a low CaO2 (O2 content of blood). Because the PaO2 is normal in CO poisoning, a patients' breathing will not increase to serve as a warning of severe hypoxemia.