Principles of Gas Exchange

Question 1

What is the partial pressure of a gas in a mixture of gases?

Answer 1

The pressure that it would exert if it was the only gas in the container

Question 2

What is the universal gas equation?

Answer 2

PV = nRT

Question 3

What is the pressure gradient between alveolus and blood?

Answer 3

The difference in partial pressure of the respective gases in the alveolus and blood

Question 4

What is partial pressure?

Answer 4

The partial pressure of a gas in a mixture of gases is the pressure that it would exert if it was the only gas in the container

Question 5

What is Dalton’s Law?

Answer 5

Partial pressure = total pressure x fractional concentration of gas

Question 6

Why is the partial pressure of O2 lower than in room air?

Answer 6

1. Inspired air is humidified in the upper airway
2. In the alveoli, O2 is taken up while CO2 is added so CO2 dilutes air and partial pressure of O2 drops
3. Body consumes more O2 molecules than it produces CO2 molecules

Question 7

What factors determine the values of alveolar pO2 and pCO2?

Answer 7

1. The pressure of outside air
2. The partial pressures of inspired O2 and CO2
3. The rates of total body O2 consumption and CO2 production
4. The rates of alveolar ventilation and perfusion

Question 8

What is the partial pressure of water vapour?

Answer 8

6.3 kPa

Question 9

Where does gas exchange occur in the body?

Answer 9

In the lungs - O2 is picked up and CO2 is released at respiratory membrane

At the tissues - O2 is released and CO2 is picked up

Question 10

What is external respiration?

Answer 10

Exchange of gases with the external environment and occurs in the alveoli of lungs

Question 11

What is is internal respiration?

Answer 11

Exchange of gases with the internal environment and occurs in the tissues

Question 12

How does the anatomy of the lung maximise diffusion?

Answer 12

1. Respiratory membrane is highly permeable to gases
2. Respiratory and blood capillary membranes are very thin
3. Large SA throughout lung

Question 13

What does external respiration occurs as a result of?

Answer 13

Partial pressure differences in O2 and CO2 between the alveoli and the blood in the pulmonary capillaries

Question 14

Describe gas exchange during external respiration

Answer 14

1. Pulmonary artery carries deoxygenated blood from the heart to the lungs
2. This artery branches and becomes the capillary network composed of pulmonary capillaries
3. These pulmonary capillaries create the respiratory membrane with the alveoli
4. As blood is pumped through this network, gas exchange occurs

Small amount of O2 dissolves directly into plasma from alveoli, most picked up by RBCs and binds to Hb

Question 15

What happens to O2 and CO2 during gas exchange of external respiration?

Answer 15

Small amount of O2 dissolves directly into plasma from alveoli, most picked up by RBCs and binds to Hb

CO2 released in opposite direction, from blood to alveoli.

Question 16

What is the partial pressure of O2 in the alveolar air vs in the blood of the pulmonary capillaries?

What does this difference create?

Answer 16

Partial pressure of O2 in alveoli –> 13.8 kPa

Partial pressure of O2 in blood of capillary –> 5.3 kPa

A very strong pressure gradient, causing O2 to rapidly cross the respiratory membrane from the alveoli to the blood

Question 17

What is the partial pressure of CO2 in the alveolar air vs blood in of the pulmonary capillaries?

Why does it not matter that the difference in partial pressures is smaller?

Answer 17

Alveoli –> 5.3 kPa
Capillary –> 6.0 kPa

The solubility of CO2 is much greater than that of O2. As a result, the relative concentrations of O2 and CO2 that diffuse across the respiratory membrane are similar

Question 18

What does internal respiration occur as a result of?

Answer 18

Partial pressure gradient between blood and tissues

O2 diffuses out of blood and into tissues
CO2 diffuses out of tissues and into blood

Question 19

Why is partial pressure of O2 in tissues low?

Answer 19

O2 is continuously used for cellular respiration

Question 20

What is partial pressure of O2 in blood vs tissue?

What is result of this difference?

Answer 20

Blood –> 13.3 kPa
Tissues –> 5.3 kPa

Pressure gradient causes O2 to dissociate from Hb, diffuse out of blood, cross interstitial space and enter tissues

Question 21

Why is partial pressure of CO2 higher in tissues than blood? What is result of this?

Answer 21

Cellular respiration continuously produces CO2 so partial pressure is higher in tissues than blood

CO2 diffuses out of tissues into blood and carried back to lungs (via Hb, dissolved in plasma or converted form)

Question 22

What does a greater difference in partial pressure result in?

Answer 22

A more rapid movement of gas

Question 23

What does alveolar air contain in comparison to atmospheric air (water vapour, CO2, O2)

Answer 23

1. Greater amount of water vapour
2. Greater amount of CO2
3. Less O2

Question 24

How many O2 molecules are consumed for every CO2 molecule produced?

Answer 24

About 1.25x

Question 25

How does the partial pressure of a gas relate to its solubility?

Answer 25

Higher solubility = lower partial pressure (inversely proportional)

Lower solubility = higher partial pressure (hydrophobic molecules attempt to push out as much as possible)

If gas is more soluble, then more molecules are needed to achieve the same partial pressure

Question 26

Is CO2 more or less soluble than O2? How does this affect its partial pressure?

Answer 26

Approx 24x more soluble in water than O2 –> achieves less partial pressure for given number of molecules in solution than O2