5.4 Physiology of Gas Exchange

Question 1

What is partial pressure? How can we calculate it?

Answer 1

• Partial pressure is the proportion of the pressure of a gas created by a single component of that gas
• It can be calculated as follow:

Part. Pressure = Total Pressure * Proportion of Component

Question 2

State Fick’s law in words

Answer 2

The molar flux due to diffusion is proportional to the concentration gradient

Question 3

State the equation of fick’s law

Answer 3

Rate of transfer = diffusing capacity * partial pressure gradient

Question 4

How do we calculate diffusing capacity?

Answer 4

Rate of transfer / partial pressure gradient

(i.e. what volume of gas will diffuse per unit pressure?)

Question 5

What are the four categories of factors that can modify the diffusing capacity of a the blood-air barrier?

Answer 5

• Diffusion coefficient of gases
• Changes in effective surface area
• Changes in physical properties of membrane
• Changes in O2 uptake by RBCs

Question 6

List some factors that influence a person’s blood-air barrier membrane surface area

Answer 6

• Height
• Lung voluime
• V/Q Mismatch
• Pathology (e.g. emphysema)

Question 7

List some pathologies/factors that decrease the efficiency of diffusion across the blood-air barrier

Answer 7

• Pulmonary congestion
• Interstitial oedema
• Membrane thickening

Question 8

List some factors that influence blood-air barrier gas uptake

Answer 8

• Hb concentration
• Capillary transit time

Question 9

What is capillary transit time?

Answer 9

The time available for diffusion exchange before blood returns to the heart

Question 10

Normal haemoglobin values in males/females?

Answer 10

Males: 13-18
Females: 12-16

(g/dL)

Question 11

How much O2 can be bound to 1g of haemoglobin?

Answer 11

~1.34mL

Question 12

What percentage of O2 in blood is bound to haemoglobin?

Answer 12

~97%

Question 13

External vs internal respiration

Answer 13

External: exchange of substances with external environment, between air and alveoli

Internal: exchange of substances within organism, between blood and tissue

Question 14

List three factors that increase the tendency of haemoglobin to offload oxygen

Answer 14

1. Increased H+
2. Increased CO2
3. Increased temperature

(all three of these indicate increased metabolic demand)

Question 15

Explain the Bohr effect

Answer 15

CO2 and H+ increase the tendency of Hb to release its oxygen

Low CO2: oxygen binding promoted
High CO2: oxygen offloading promoted

Question 16

Explain the three mechanisms of CO2 transport

Answer 16

1. Dissolved (10%)
2. Bicarbonate ion (70%)
3. Haemoglobin (20%)

Question 17

Is the operating concentration range of CO2 wider or thinner than the operating concentration range of O2?

Answer 17

Much thinner (40-46mmHg)

Question 18

Explain the Haldane effect

Answer 18

Greater binding of O2 with Hb increases release of CO2 into the blood

Question 19

Do gases exert a partial pressure when bound to haemoglobin?

Answer 19

No

Question 20

Average minute ventilation human

Answer 20

5-8L/min

Question 21

Typical cardiac output

Answer 21

5L/min

Question 22

How do ventilation and blood flow increase from the apices to the bases of the lungs?

Answer 22

Ventilation and perfusion are highest at the base of the lungs, and lowest at the apices.

Question 23

List four factors that cause regional differences in ventilation throughout the lungs

Answer 23

1. Gravity
2. Anatomical expansion (bases of lungs have more room to expand)
3. Compliance (bases are more compliant)
4. Breathing pattern (alveolar recruitment can differ at slow v fast filling rates)

Question 24

List three factors that influence regional differences in lung perfusion

Answer 24

1. Gravity - creates a hydrostatic gradient
2. Hypoxic pulmonary vasoconstriction (redirects flow away from poorly ventilated areas)
3. Lung volume

Question 25

Describe West’s four zones of the lungs. Which are most common in a healthy human?

Answer 25

Zone 1: PA > Pa > Pv (no flow)
Zone 2: Pa > PA > Pv (flow during systole, not diastole)
Zone 3: Pa > Pv > PA (constant flow)
Zone 4: Interstitial pressure > PA (no flow)

Most common: Zone 2 and Zone 3