Cremo 2: Factors that control CO2 and O2 in alveoli and blood Flashcards
Sensors of blood CO2 levels
- Central chemoreceptors in medulla
2. Peripheral chemoreceptors in carotid and aortic bodies
Arterial CO2 is controlled by two rates. What are they?
- Metabolic production of CO2 (VCO2)
2. Alveolar ventilation (VA)
PCO2 ventilation equation
PaCO2 = VCO2 * 0.863 / VA
H-H equation (acid-base)
pH = 6.1 + log [HCO3-]/0.03(PaCO2)
Alveolar gas equation (oxygenation)
PAO2 = PIO2 - 1.2(PaCO2)
What is a buildup of CO2 in the blood due to?
A failure in some component of the respiratory system (not an increase in metabolic CO2 production)
What is VCO2?
Rate of CO2 production by metabolism
What two rates control PaCO2?
VA: rate of alveolar ventilation by lung
VCO2: rate of CO2 production by metabolism
Alveolar ventilation and CO2 production double during moderate exercise in a 24-year old man. What is the effect on PaCO2?
No effect! Both variables change together!
Clinical definition of hypercapnia or hypoventilation.
PaCO2 of >45mmHg
Clinical definition of hypocapnia or hyperventilation.
PaCO2 of <35mmHg
Normal PaCO2 levels
35-45mmHg
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?
<35mmHg
The only physiologic reason for elevated PaCO2
A level of alveolar ventilation inadequate for the amount of CO2 produced and delivered to the lungs.
What does VA refer to in the PCO2 equation?
VA = VE - VD = total volume of lung - dead space
What is the equation for VE, or total ventilation rate?
VE = Total ventilation rate (L/min) = respiratory rate · tidal volume
Is dead space ventilated? Is it perfused?
Yes; no
Anatomic dead space + all other dead space
Physiologic dead space
3 processes required for gas exchange
- Gas entering and leaving
- Blood flow = perfusion
- Diffusion of gases across capillary membrane
What is another way to think of VA?
VE - VD
Level of VE needed to keep PaCO2 constant
Minute ventilation requirement
In a normal lung, what is PACO2? What is PaCO2?
40mmHg
In a normal lung, what is PvCO2?
45mmHg
What is a typical PAO2 for a normal lung?
100mmHg
Is the PAO2 equal to the PaO2 in a normal lung?
No, PaO2 is slightly lower
PO2 at sea level
159mmHg = 0.21 * 760mmHg
PO2 at the trachea
150mmHg
PaO2 at the terminal respiratory units
100mmHg
Anatomic unit used by pathologists
10 to 12 terminal respiratory units
Acinus
At the terminal respiratory unit, is PAO2 = PaO2?
No
Mass conservation equation for O2
PAO2 = PIO2 - (PaCO2 x 1.2)
Recall: PACO2~PaCO2
But does PAO2 = measured PaO2?
At the level of a single alveolus, yes
At the level of the lung, terminal respiratory unit, or patient, no
PaO2 will always be less than PAO2
But how much less?
ranges between 5 and 20 mm Hg
Calculating age appropriate normal PaO2
Normal PaO2 = 100-(0.4 x age)
Normal P(A-a)O2 difference
[Patient age / 4] + 4
Hypoxemia vs hypoxia
Hypoxemia: Partial pressure of oxygen (PaO2) in the blood < normal for the subject’s age
Hypoxia: Decreased oxygen supply to organs and tissues
What happens to the P(A-a)O2 difference with age?
It increases
Things that make diffusion faster
Large diffusing area
High solubility of gas in barrier and blood
Large difference in gas partial pressures
Things that make diffusion slower
Thicker barrier membrane
Gases with higher mass
Which is more soluble in liquid? CO2 or O2?
CO2 is 20 times more soluble than O2
About how long does it take for the capillary to reach the alveolar PO2?
1/4 of the transit time. Total transit time = .75 sec
Capillary reserve time
Time left over to reach appropriate levels of PAO2 or PACO2
Is the amount of O2 in the end capillary blood diffusion or perfusion limited?
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)
The time in the pulmonary capillary after which the PO2 has reached PAO2
Capillary reserve time
The typical time it takes for an RBC to traverse the length of a pulmonary capillary
Capillary transit time
The anatomic dead space can be altered by…
Brochodilation
