General Pulmonary Diffusion and Blood Flow Questions

Question 1

What is Dalton’s Law?

Answer 1

The total pressure exerted by a mixture of gases is equal to the sum of the partial pressure of each gas in the mixture

The partial pressure exerted by each gas is directly proportional to the percentage of the gas in the mixture

Question 2

What is Atm air and its constituents?

Answer 2

Total pressure = 760mmHg at sea level

O2 = 21% = 159mmHg
N = 79% = 600mmHg
CO2 = .04 % = 0.03mmHg
H2O = .46% = 3.7 % 

The Atm pressure may change with altitude but (as far as we’re concerned for respiratory pathology) the percentages of each gas in the mixture do not change

Question 3

Water vapour pressure at 37 degrees?

Answer 3

47mmHg

Question 4

What effect does humidification have on inspired air?

Answer 4

Decreases O2 partial pressure to 149mmHg (from 159)
Decreases nitrogen to 563 from 600
Increases H2O to 47mmHg
Increases CO2 to 0.29

Question 5

Describe alveolar air gas partial pressures:

Answer 5

O2 = 104
CO2 = 40

H2O = 47
N = 569

Question 6

What is the significance of taking 16 breaths to fully recycle alveolar air (with tidal vol of 500ml and FRC of 2.3 L)

Answer 6

Helps to prevent sudden alveolar gas concentration changes

Thus stabilises respiratory control mechanisms

Question 7

What is the composition of expired air?

Answer 7

Mixture of dead space and alveolar air

First 100ml = Dead Space Air

Next ~250mls = Dead Space/Alveolar Air Mixture

The remainder = Alveolar Air

Question 8

What is the Fick Principle?

Answer 8

Expresses the gas diffusion rate

Vgas = (pressure difference x diffusion surface area x diffusion constant) / distance through which the gas must diffuse

Vgas = (Pressure difference x A x D) / d

Question 9

What is Henry’s Law?

Answer 9

At a constant temperature, the amount of gas absorbed by a liquid is proportional to the solubility of the gas in the liquid, and the partial pressure of that gas upon the lquid

Means that high solubility gases diffuse faster

And the greater the pressure gradient, the faster the diffusion will be

Question 10

CO2 solubility?

Answer 10

23 x that of O2

Question 11

What factors affect diffusion?

Answer 11

The solubility of the gas

The pressure gradient of the gas across the diffusion membrane

The area available for diffusion (e.g. reduced in emphysema)

Diffusion Distance (e.g. increased in pulmonary oedema or fibrotic lung disease)
(normal respiratory membrane = 0.2-0.6um)

Question 12

Which parts of Fick’s Principle relate to the respiratory gases?

Answer 12

Pressure difference: difference in alv PO2/PCO2 and blood PO2/PCO2

D - diffusion constant (related to solubility and molecular weight)

Question 13

Which parts of Fick’s Principle relate to Anatomy only?

Answer 13

A - Area over which diffusion can occur

d - distance through which diffusion must occur

Question 14

What is meant by ‘perfusion limitations to gas exchange’?

Answer 14

O2 and CO2 are generally considered ‘perfusion limited’.

Low solubility molecules tend to create a higher partial pressure (e.g. because they don’t interact well with water, and thus tend to move around more). By creating this higher partial pressure, their ability to diffuse across a membrane is increased. These fast-diffusing molecules (such as CO2 and O2) tend to be ‘perfusion limited’ in that their diffusion is limited by the perfusion of blood (faster perfusion = more new blood to equilibrate with)

By comparison, molecules which are more soluble tend to create lower partial pressures, and thus don’t tend to diffuse as quickly. These molecules/gases are considered ‘diffusion limited’

Question 15

What factors mainly affect diffusion of CO2 and O2, considering that they are ‘perfusion limited’

Answer 15

How quickly the RBC passes by in the capillary - e.g. very fast in heavy exercise

Higher altitudes (lower partial pressure leads to slower diffusion, thus exercise and high altitude in those not adjusted to the high altitude is bad news!)

Ventilation-perfusion mismatch - e.g. PE

Rate of reaction with Hb (think left and right shifts in O2-Hb dissociation curve)

Question 16

What is venous admixture?

What constitutes ‘normal venous admixture’

Answer 16

Any blood that passes through pulmonary circulation without undergoing gas exchange remains only partly oxygenated. This constitutes venous admixture.

Result = dilution of oxygenated blood entering LA

Normal Venous Admixture:
Thebesian Veins (part of coronary circulation, most drain into RA, but some drain into the ventricles)

Bronchial Veins (may drain into pulmonary veins)

Question 17

Why is the pulmonary circulation generally a low pressure system?

Answer 17

Low pressure, low resistance, high flow

Facilitated by:
Thin-walled vessels (compliant, distensible)

Less smooth muscle in vasculature

Higher Distensibility

Can engage in Recruitment & Distension

Recruitment = Previously closed capillaries can open in response to increases in pressure (results in more vessels in parallel)

Question 18

How does inspiration and expiration affect blood flow through pulmonary circulation?

Answer 18

Lung volume.

Extra-alveolar vessels (in the parenchyma) are exposed to Pip. With higher lung volumes, they tend to become more distended as they are connected to the surrounding parenchyma (which stretches during inspiration). Thus, Extra-alveolar vessels have higher flow with higher lung volumes

Intra-alveolar vessels run through septae between adjacent alveoli. They tend to become compressed with increasing lung volumes because the alveoli fill with air and compress them. Thus, intra-alvolar vessels have decreased flow with higher lung volumes

Question 19

Regional differences in blood flow are attributable to which factors?

Answer 19

Gravity

Alveolar Pressure

Hypoxia (local hypoxic vasoconstriction)

Neural control (minor)

Question 20

Explain the effect of gravity on regional differences in perfusion?

Answer 20

Because the pulmonary circulation is a relatively low-pressure circulatory system (the RV does not create a large amount of pressure), perfusion is limited because the pressure cannot cope with the gravity. The lung is ~30cm high, and the pressure of blood being ejected from the RV results in decreased perfusion to the apical regions.

This is position-dependent. If lung were up-side-down, the situation would reverse.

Question 21

What are the ‘zones’ of the lung in terms of perfusion.

Answer 21

Zone 1 = Palv exceeds arterial pressure.
Creates increased resistance to flow - little or no flow. Does not normally occur in the normal lung

Zone 2 = Palv is intermediate to arterial pressure.
There is moderate resistance to flow, thus only systolic flow occurs.

Zone 3 = Arterial & venous pressure both exceed Palv.
Low resistance to flow, thus perfusion rate is high and continuous.

In the normal lung, Zone 2 occurs and the apex, and Zone 3 is everywhere else.

Question 22

Describe the mechanism of hypoxic vasoconstriction

Answer 22

Local decreases in PO2 of ALVEOLAR gas will lead to local vasoconstriction, to divert blood away from under-perfused regions.

*Increased alveolar PCO2 elicits a similar response