Cremo 2: Factors that control CO2 and O2 in alveoli and blood

Question 1

Sensors of blood CO2 levels

Answer 1

1. Central chemoreceptors in medulla

2. Peripheral chemoreceptors in carotid and aortic bodies

Question 2

Arterial CO2 is controlled by two rates. What are they?

Answer 2

1. Metabolic production of CO2 (VCO2)

2. Alveolar ventilation (VA)

Question 3

PCO2 ventilation equation

Answer 3

PaCO2 = VCO2 * 0.863 / VA

Question 4

H-H equation (acid-base)

Answer 4

pH = 6.1 + log [HCO3-]/0.03(PaCO2)

Question 5

Alveolar gas equation (oxygenation)

Answer 5

PAO2 = PIO2 - 1.2(PaCO2)

Question 6

What is a buildup of CO2 in the blood due to?

Answer 6

A failure in some component of the respiratory system (not an increase in metabolic CO2 production)

Question 7

What is VCO2?

Answer 7

Rate of CO2 production by metabolism

Question 8

What two rates control PaCO2?

Answer 8

VA: rate of alveolar ventilation by lung
VCO2: rate of CO2 production by metabolism

Question 9

Alveolar ventilation and CO2 production double during moderate exercise in a 24-year old man. What is the effect on PaCO2?

Answer 9

No effect! Both variables change together!

Question 10

Clinical definition of hypercapnia or hypoventilation.

Answer 10

PaCO2 of >45mmHg

Question 11

Clinical definition of hypocapnia or hyperventilation.

Answer 11

PaCO2 of <35mmHg

Question 12

Normal PaCO2 levels

Answer 12

35-45mmHg

Question 13

A 32 year old male has just run into the classroom after being late for class. He can be considered to be hyperventilating if his PaCO2 is at what level?

Answer 13

<35mmHg

Question 14

The only physiologic reason for elevated PaCO2

Answer 14

A level of alveolar ventilation inadequate for the amount of CO2 produced and delivered to the lungs.

Question 15

What does VA refer to in the PCO2 equation?

Answer 15

VA = VE - VD = total volume of lung - dead space

Question 16

What is the equation for VE, or total ventilation rate?

Answer 16

VE = Total ventilation rate (L/min) = respiratory rate · tidal volume

Question 17

Is dead space ventilated? Is it perfused?

Answer 17

Yes; no

Question 18

Anatomic dead space + all other dead space

Answer 18

Physiologic dead space

Question 19

3 processes required for gas exchange

Answer 19

1. Gas entering and leaving
2. Blood flow = perfusion
3. Diffusion of gases across capillary membrane

Question 20

What is another way to think of VA?

Answer 20

VE - VD

Question 21

Level of VE needed to keep PaCO2 constant

Answer 21

Minute ventilation requirement

Question 22

In a normal lung, what is PACO2? What is PaCO2?

Answer 22

40mmHg

Question 23

In a normal lung, what is PvCO2?

Answer 23

45mmHg

Question 24

What is a typical PAO2 for a normal lung?

Answer 24

100mmHg

Question 25

Is the PAO2 equal to the PaO2 in a normal lung?

Answer 25

No, PaO2 is slightly lower

Question 26

PO2 at sea level

Answer 26

159mmHg = 0.21 * 760mmHg

Question 27

PO2 at the trachea

Answer 27

150mmHg

Question 28

PaO2 at the terminal respiratory units

Answer 28

100mmHg

Question 29

Anatomic unit used by pathologists | 10 to 12 terminal respiratory units

Answer 29

Acinus

Question 30

At the terminal respiratory unit, is PAO2 = PaO2?

Answer 30

No

Question 31

Mass conservation equation for O2

Answer 31

PAO2 = PIO2 - (PaCO2 x 1.2)

Question 32

Recall: PACO2~PaCO2 | But does PAO2 = measured PaO2?

Answer 32

At the level of a single alveolus, yes | At the level of the lung, terminal respiratory unit, or patient, no

Question 33

PaO2 will always be less than PAO2 | But how much less?

Answer 33

ranges between 5 and 20 mm Hg

Question 34

Calculating age appropriate normal PaO2

Answer 34

Normal PaO2 = 100-(0.4 x age)

Question 35

Normal P(A-a)O2 difference

Answer 35

[Patient age / 4] + 4

Question 36

Hypoxemia vs hypoxia

Answer 36

Hypoxemia: Partial pressure of oxygen (PaO2) in the blood < normal for the subject’s age Hypoxia: Decreased oxygen supply to organs and tissues

Question 37

What happens to the P(A-a)O2 difference with age?

Answer 37

It increases

Question 38

Things that make diffusion faster

Answer 38

Large diffusing area High solubility of gas in barrier and blood Large difference in gas partial pressures

Question 39

Things that make diffusion slower

Answer 39

Thicker barrier membrane | Gases with higher mass

Question 40

Which is more soluble in liquid? CO2 or O2?

Answer 40

CO2 is 20 times more soluble than O2

Question 41

About how long does it take for the capillary to reach the alveolar PO2?

Answer 41

1/4 of the transit time. Total transit time = .75 sec

Question 42

Capillary reserve time

Answer 42

Time left over to reach appropriate levels of PAO2 or PACO2

Question 43

Is the amount of O2 in the end capillary blood diffusion or perfusion limited?

Answer 43

Normally it is perfusion-limited – the amount of gas taken up by the blood is limited by the amount of blood flow (not the rate of diffusion)

Question 44

The time in the pulmonary capillary after which the PO2 has reached PAO2

Answer 44

Capillary reserve time

Question 45

The typical time it takes for an RBC to traverse the length of a pulmonary capillary

Answer 45

Capillary transit time

Question 46

The anatomic dead space can be altered by…

Answer 46

Brochodilation