Alveolar Gas Exchange

Question 1

Diffusion

Answer 1

the movement of molecules from high to low concentration area (of o2 and co2).

In the lungs, this happens across the alveolar-capillary membrane.

Question 2

The vascular system that forms the gas exchange network surrounding the alveoli

Answer 2

the pulmonary circulation

Question 3

How does blood flow in the pulmonary (lung) circulation?

What is the primary function of the pulmonary circulation?

Answer 3

The pulmonary circulation is the portion of the circulatory system which carries deoxygenated blood from the right ventricle, to the lungs, and returns oxygenated blood to the left atrium and left ventricle of the heart.

The pulmonary circulatory system begins at the pulmonary artery which recieves venous blood from the right side of the heart. This artery then divides into left and right branches and continues to branch until it forms the capillaries that surround the alveoli. After gas exchange happens between the capillaries and the alveoli, the blood is returned to the left side of the heart through the pulmonary veins.

Its primary function involves the exchange of gases across the alveolar membrane which ultimately supplies oxygenated blood to the rest of the body and eliminates carbon dioxide from the circulation.

Question 4

Which circulation is the only one that receives the entire cardiac output?

Answer 4

Pulmonary Circulation

Question 5

What is the normal range for Pulmonary artery systolic pressure? (PAS)

Answer 5

15-30mm Hg (milimeter)

the force of the blood against the artery walls as your heart beats

Question 6

What is the normal range for Pulmonary artery diastolic pressure? (PAD)

Answer 6

4-12mm Hg

the blood pressure between heartbeats.

Question 7

What is the Diffusion - coefficient and what does it describe?

Answer 7

It defines the velocity of a substance moving from one area to another.

• CO2 diffuses 20 times more rapidly than 02 across the Alveolar-Capillary (AC) Membrane. Even if the thickness of the AC membrane changes it will continue to cross into the alveoli. As it crosses into the alveoli it’s waiting to be removed by exhalation. If ventilation is not good enough, the concentration of co2 in the alveoli will increase

Question 8

What is Alveolar ventilation?

Answer 8

The portion of total ventilation that takes part in gas exchange . The portion that doesn’t is called “wasted ventilation”

Question 9

What is Physiologic Dead Space?

Answer 9

Areas in the lungs that are ventilated but in which no gas exchange occurs are known as” dead space regions”

Ex: the conducting airways are called anatomical dead space because they are ventilated but not perfused, and therefore can’t take part in GE. Also the trachea etc.

Without perfusion, GE can’t take place, and the ventilation is wasted.

Dead Space ventilation refers to the rest of the gas taken in during a breath that stays in spaces not capable of gas exchange (conducting airways, trachea..)

Question 10

What is the purpose/function of Alveolar Ventilation?

Answer 10

To remove co2 from the blood and to add more O2 the the blood so it can bind to Hgb and be delivered to the body.
–> this is why the lungs stay inflated!

Question 11

What is driving pressure?

Answer 11

The difference in the concentrations of the gases (O2 and co2). The greater the driving pressure of the gas through the membrane the greater the rate of diffusion!

• if o2 is in higher concentration in the alveoli, it will diffuse at a greater rate into the capillaries
• if co2 is higher in concentration in the capillaries, it will diffuse at a greater rate into the alveoli, allowing for more co2 expulsion…. But if ventilation is not good enough, co2 will build up and will not be removed (hypercapnia)

Question 12

How can driving pressure be increased?

Answer 12

With the administration of supplemental o2 via nasal prongs, face mask, ventilator, BiPAP.
The HIGHER the concentration of o2, the HIGHER the driving pressure.

Question 13

___ is in high concentration within the alveoli, and ___ is in high concentration within the pulmonary capillaries.

Answer 13

O2, CO2

Question 14

When ventilation is DECREASED, less ___ is expelled and ___ levels increase in the alveoli. This increase results in a decreased driving pressure for ___ into the alveoli. So the movement of ___ from the blood pulmonary capillaries to the alveoli will decrease, allowing alveoli ___ levels in arterial blood (____) to rise.

Answer 14

CO2, PACO2

Question 15

What is a V/Q (P) Relationship and why is it so important?

Answer 15

Ventilation (V) and perfusion (Q) in the lungs should be equally matched at the alveolar capillary membrane level for optimal gas exchange, but normal variations do occur.

Question 16

What is normal alveolar ventilation rate? Pulmonary capillary perfusion rate?
What is the normal v/q ratio?

Answer 16

Normally alveolar ventilation is 4 L/min
Normally, pulmonary capillary perfusion is 5L/min
The normal ventilation/perfusion ratio (v/q) is 4:5, or 0.8

Question 17

What is PaCO2? and its normal value

Answer 17

The partial pressure of CO2 in the arterial blood. (35-45mm Hg range.)

Question 18

What is PaO2? and its normal value

Answer 18

The partial pressure of O2 in the blood. (75-100mm Hg)

Peripheral chemoreceptors increase when PaO2<60 mmHg.

Question 19

What is SaO2? and its normal value

Answer 19

O2 saturation. The % of Hgb sites bound to O2. (97-100%).

Question 20

What is a Ventilation/Perfusion mismatch?

Answer 20

When ventilation is happening but perfusion is not, and vice versa (unequal balance of perfusion and ventilation)

Question 21

What is alveolar dead space?

Answer 21

Decreased alveolar perfusion.
- alveoli are ventilated but not perfused

When at one end during gas exchange, the alveolus is receiving ventilation but is not receiving any perfusion (or decreased) and blood is unable to participate in gas exchange.

alveolar dead space = the volume of air in alveoli that are ventilated but not perfused … they have little or not blood flowing through their pulmonary capillaries
–> blood cannot flow properly

Question 22

What are some causes of alveolar dead space?

Answer 22

Anything that decreases perfusion to the lungs. Decreased CO, PE, Hypovolemia (blood loss), MI, HF…

Question 23

What is Intrapulmonary shunting?

Answer 23

Decreased Alveoli Ventilation. (o2 can’t get in)

On the other end, the alveolus is receiving perfusion but is not receiving any ventilation (or decreased)and is unable to participate in GE (O2/CO2 exchange between the lungs and bloodstream)

you have blood coming through but it’s not being oxygenated

In a right to left shunt, a portion of the pulmonary blood flow is shunted away from the alveoli, resulting in ventilation without perfusion and a higher V/Q ratio.

Question 24

What are some causes of intrapulmonary shunting?

Answer 24

Physiological conditions like atelectasis (alveoli/lung collapse – o2 cannot get into alveoli = decreased ventilation, bit good blood flow), airway obstruction, bronchoconstriction, decreased RR, inflammation, secretions, pneumonia, COPD, pulmonary edema, acute respiratory distress syndrome (fluid leaks into alveolus) can cause extreme V/Q fluctuations.

Question 25

What is the V/Q ratio for alveolar dead spacing?

Answer 25

V/Q >0.8 (if ventilation exceeds perfusion)

Question 26

What is the V/Q ratio for pulmonary shunting?

Answer 26

V/Q < 0.8 (if perfusion exceeds ventilation)

Question 27

What is hypoxic vasoconstriction?

Answer 27

When the pulmonary vessels constrict when the Pao2 is less than 60 mm Hg.
- Usually occurs when a portion of the pulmonary capillaries perfuses unventilated or under ventilated alveoli.
A compensatory response used to limit the return of unoxygenated blood to the left side of the heart.

Question 28

Why does hypoxemia (decreased o2 in blood) occur when there is a V/Q mismatch?

Answer 28

Less oxygen to diffuse across the alveolar-capillary membrane to the rest of the body. If there isn’t enough ventilation happening, less o2 is coming into the alveoli to be diffused into the capillaries.
If there isn’t enough perfusion happening, there is less hgb for o2 to bind to.

Question 29

What 4 factors influence diffusion? How so?

Answer 29

1. ) Alveolar - Capillary Membrane Thickness
   - can increase with secretions of fluid shifting in lungs, inflammation, which impairs diffusion
2. ) Anatomical - Surface Area
   - increased SA is better for GE as there are more opportunities for gases to diffuse across.
3. ) Diffusion - Co-efficient
   - remember CO2 diffuses 20 times more rapidly than 02 across the Alveolar-Capillary (AC) Membrane. Proper ventilation is needed to ensure there’s a balance between o2 and co2 levels.
4. ) Driving Pressure of gas
   - the greater the difference of the concentration gradients, the steeper the gradient and the increase in driving pressure.
   - adding more o2 increases the difference in concentration between the alveoli and capillaries to promote diffusion.
   - ex: supplemental o2 increases the driving pressure of o2 into the blood capillaries as more o2 is inhaled into the alveoli of the lungs.

Question 30

What does it mean by “dead-space producing unit?”

Answer 30

alveoli are ventilated; perfusion is present but diminished

Question 31

What does it mean by “shunt-producing unit?”

Answer 31

alveoli are perfused; ventilation is present but diminished.

Question 32

What is hypercapnia?

Answer 32

When you have too much carbon dioxide (CO2) in your bloodstream.
- It usually happens as a result of hypoventilation, or not being able to breathe properly and get oxygen into your lungs

Question 33

Despite a thickened Alveolar Capillary membrane, PaCO2 levels will remain ____ as long as ventilation is adequate.

Answer 33

normal

- due to fast PaCO2 rate or diffusion into alveoli – hyperventilation will decrease PaCO2