Gas Exchange

Question 1

What is pulmomary ventilation ?

Answer 1

Total volume of air breathed in and out per minute
-MV = tidal volume (L/breath) x Respiratory Rate (breath/min)
-e .g. = 0.5 L X 12 breath/min = 6 L/min under resting conditions
-Max MV (MMV) could up to 180L/min

Aka minute ventilation

Question 2

What is Alveolar Ventilation ?

Answer 2

Volume of air that reaches the alveoli and participate in gas exchange per minute
-AV= (tidal volume – dead space volume) x Respiratory Rate
-E.g = (0.5 – 0.15) x 12 = 4.2 L/min under resting conditions.

Not all inhaled air gets to gas exchange

Question 3

What is anatomical dead space ?

Answer 3

Some inspired air remains in the airways where it is not available for gas exchange ( 150ml)
-Helps condition incoming air (warm, humidify, filter)

Question 4

How does Kinetic gas theory explain gas pressure ?

Answer 4

Gases are a collection of molecules moving randomly around a space
-Pressure is generated by collisions of molecules with a surface
-The more frequent and harder the collisions the higher the pressure generated by the gas

Question 5

What is the ideal gas equation ?

Answer 5

Properties of gases can be described by the ideal gas equation:
PV = nRT

Where:
P = pressure
V = volume
n = number of moles of gas
R = gas constant
T = absolute temperature (Kelvin)

Gas molecule speed depends almost all on temp

Question 6

What is Boyle’s law ?

Answer 6

Pressure (P) of a gas is inversely proportional to its volume (V) for a given temp and no. of moles in closed systen

P∝ 1/𝑉

Question 7

How is breathing an example of Boyle’s Law ?

Answer 7

Breathing occurs because of pressure gradients:
Diaphragm contracts → thoracic volume ↑ → alveolar pressure ↓ → air flows into lungs
When muscles relax → thoracic volume ↓ → alveolar pressure ↑ → air flows out

Mechanism of inspiration & expiration is an example of Boyle’s Law

Question 8

What is Dalton’s Law ?

Answer 8

In a mixture of gases, the total pressure = the sum of the partial pressures of the individual gases
-Air pressure = sum of p02 + pN2 (mostly)

Question 9

How does altitude affect pressure ?

Answer 9

At sea level: 101 kilopascals (kPa) = 1 atmosphere = 760 mmHg

At high altitudes atmospheric pressure is lower (weight of air pressing down is less)

Question 10

What are the partial pressures of the air gases at sea level ?

Answer 10

Partial pressure of O2 = 101 x 20.9% = 21.1 kPa

Partial pressure of N2 = 101 x 78 % = 78.7 kPa

Partial pressure of CO2 = 101 x 0.03%= 0.03 kPa

Atmospheric Pressure at sea level =101 kiloPascals (kPa)

Question 11

How do gases dissolve and diffuse ?

Answer 11

According to their partial pressure
-Gases diffuse down partial pressure gradients from area of higher to lower in body e.g. movement of oxygen from alveolar air ↔ blood
-Partial pressures used instead of conc to describe gases in body

Gases in mix can move independently and in different directions to other

Question 12

How is partial pressure denoted ?

Answer 12

Denoted by ‘p’ - as in pO2, pCO2, pN2

Question 13

What happens when inspired gases come into contact with the thin layer of water lining the airways and alveoli ?

Answer 13

1) Gas molecules
-Gas molecules will enter water to dissolve in liquid
-According to Henry law: PP X Solubility

2) Water molecules evaporate to enter air
-Humidify
-Reduced other gases PP (helpful for continous flow of oxygen from upper airway to lower airway; lower parts more humid than upper parts)

Gas dissolving in liquid depends on PP and solublity

Question 14

What is vapour pressure ?

Answer 14

Water molecules entering the air exert ‘vapour pressure’

At equilibrium,
-Rate of H2O evaporation = rate of H2O condensation
-The air is saturated with vapour
-Saturated Vapour Pressure (SVP) = 6.28kPa at body temp
-Inhaled air becomes saturated with water, in the upper respiratory tract

Question 15

How is equilibrium of a dissolved gas reached ?

Answer 15

Dissolved gas molecules also exert pressure in the liquid

At equilibrium:
-Rate of gas entering water = rate of gas leaving the water.
-Partial pressure of the gas in the liquid = partial pressure of the gas in the air above it

another term used for partial pressure of a gas in the liquid is ‘tension’ (e.g. oxygen tension in blood)

Question 16

How much oxygen dissolved in plasma ?

Answer 16

Solubility coefficient of O2 in plasma = 0.01 mmol L-1 kPa-1 at body temp.
pO2 of 13.3 kPa (as in alveolar air)

-0.01 x 13.3 = 0.13 mmol of O2 will dissolve (15ml/min)

(this is no where near enough, needs 250ml/min, heart would need to beat 17 times as fast for thsi to work which it very much cant do)

Question 17

What is the partial pressure of oxygen at both ends of the resp tract ?

Answer 17

Breathed in at 21.1 kPa
Alveolar air is 13.3 kPA

Question 18

How can the blood absorb enough oxygen ?

Answer 18

O2 dissolves in plasma and enters RBC to bind to Hb
-This must complete before equilibrium is reached and partial pressure is established.
-Blood contains both dissolved and Hb bound oxygen

pO2 is a measure of dissolved O2 in the blood (proportional)

Question 19

When does plasma oxygen binding to Hb stop and what does this achieve ?

Answer 19

-Process continues till Hb fully saturated (each Hb molecules binds 4 O2 molecules)
-After Hb is fully saturated, O2 continues to dissolve till equilibrium is reached
-At equilibrium, pO2 of plasma = pO2 of alveolar air

Question 20

How does Hb bound oxygen travel to tissues ?

Answer 20

-Dissolved O2 is available to diffuse into tissues down its partial pressure gradient
-As dissolved O2 leaves the blood, it will be replaced by O2 bound to Hb
-In this way, the oxygen bound to Hb will be downloaded and diffuse into tissues

The pO2 is a measure of dissolved O2 in the blood (proportional)

Question 21

What are the partial pressures of 02 and C02 in alveolar air and why ?

Answer 21

pO2 =13.3 kPa (lower than inhaled air)
pCO2 =5.3 kPa (higher than in inhaled air)

Because
-Inhaled air mixes with residual volume
-Effect of O2 diffusing across the alveolar wall
-Effect of CO2 entering the alveoli
-Alveolar air composition stays constant around this level;
-Blood equilibrates to this level

Question 22

How do partial pressures of 02 and C02 at the alveoli determine their movement ?

Answer 22

Alveolar PO2 > PO2 in mixed venous blood
Alveolar PCO2 < PCO2 in mixed venous blood

So, oxygen will diffuse into blood and carbon dioxide out

Question 23

Which are the factors affecting the rate of diffusion ?

Answer 23

Partial pressure difference (gradient) across membrane (P1 – P2 )
A - the surface area available for diffusion
T – (thickness) i.e. distance molecules must diffuse
D - diffusion coefficient of the individual gas

Rate is inversely proportional to T

Question 24

Which properities of the gas determines the rate of its diffusion ?

Answer 24

-Solubility in the liquid -Molecular weight of gas -Diffusion coefficient; Used to determine the relative rates at which different gases will diffuse across the same membrane at the same pressure

Question 25

Which factors allow 02 and C02 to diffuse at similar rates ?

Answer 25

Solubility -CO2 much more soluble than O2 (so diffuses faster than O2) -the effect of solubility is greater CO2 diffuses 20 times faster than O2 (DC02 = 20 x D02) Molecular weight -molecular weight of CO2 > O2 molecular weight (slows down CO2) (Oxygen is small and fast, but CO2 is more soluble) Partial pressire -Larger difference (ΔP) compensates for slower diffusion and lower diffusion coefficient of O2 | Also partial pressures. ## Footnote C02 chemically reacts instead of just dissolving also

Question 26

Which gas exchange is more effected by diseased lungs ?

Answer 26

In fibrosis, oedema, etc: Membrane thickness ↑ and diffusion becomes harder -O₂ has a lower diffusion coefficient (D, so when diffusion is impaired O₂ exchange falls first and CO₂ retention happens much later -O2 gas exchange is thus more impaired than CO2 because of O2 slower diffusion rate | doo doo 02 D made up for by high pp

Question 27

Rate of diffusion equation

Answer 27

Question 28

so overall, c02 and 02 have similar diffusion rates over all as c02 has a much higher diffusionn coefficient but 02 has a higher partial pressure. when lung disease makes diffusion more difficult 02 exxhcnage is first impact as due to its already lower D it is more susectible to these changes ?

Answer 28

YUP

Question 29

Which barriers must diffusion from alveolar air to RBC in capillary cross ?

Answer 29

Fluid film lining alveolus Epithelial cell of alveolus Interstitial space Endothelial cell of capillary Plasma Red cell membrane

Question 30

Why is gas exhcnage in the lung so efficient ?

Answer 30

Oxygen exchange complete in 1/3 of time blood spends capillary So, plenty of reserve – for exercise

Question 31

Which factors which affect rate of diffusion are affected in disease ?

Answer 31

Thickness of the membrane -Increase due to oedema in interstitial space and alveoli -Fibrosis increases thickness of alveolar capillary membrane Surface area of the membrane -Decreased by removal of an entire lung -Emphysema - decreased surface area Diffusion coefficient of the gas: -CO2 always diffuses much faster than O2 -So, diffusion of O2 affected → pO2 is low -Diffusion of CO2 not affected → pCO2 normal

Question 32

When is gas exchange optimal ?

Answer 32

Gas exchange optimal when: -V/Q ratio of individual alveolar units ≈ 1 -300 million alveoli - may have widely differing amounts of ventilation and of perfusion. | V/Q = ventillation perfusion ratio

Question 33

Which conditions maximise V/Q ?

Answer 33

When alveolar PO2 is low, hypoxic vasoconstriction of pulmonary arterioles occurs; diverts blood to better ventilated alveoli -Alveoli with ↑ ventilation should have ↑ perfusion -Alveoli with ↓ ventilation should have ↓ perfusion (perfusion = blood flow) ## Footnote this process is not complete, so in disease states, poorly ventilated alveoli still have significant perfusion