Gas Laws & Gas Transfer

Question 1

Explain the difference in movement of gas by mechanical force or diffusion.

Answer 1

Mechanical force (convective transport): bulk flow of matter down a pressure gradient via muscular work in + elastic recoil out (in large airways + circulation)

Diffusion: Gas movement down concentration gradient of each individual gas i.e. no movement of matter (from terminal bronchioles to mitochondria of metabolising tissues)

Question 2

What are the 4 main functions of the cardio-respiratory system?

Answer 2

1. Transport O2 + nutrients to tissues
2. Remove CO2 + other waste products from tissues
3. Mobility of cell-mediated + humoral immunity
4. Transport of hormones, cytokines + other mediators

Question 3

What is the kinetic theory of gases?

Answer 3

Posits that gases are composed of a collection of submicroscopic molecules in constant random motion proportional to their energy - these molecules only form a tiny proportion of total gas volume, the majority is empty space

Aids understanding of macroscopic properties of gas

Question 4

How is pressure generated in terms of the kinetic theory of gases?

Answer 4

By force of the impacts of the molecules in the gas with the container walls/other molecules so more frequent/harder collision generates higher pressure

Question 5

How is pressure (P) defined?

Answer 5

As force (F) per unit area (A) - P = F/A

Unit is in Pascal (Pa)

Question 6

What is Boyle’s law?

Answer 6

Pressure (P) inversely proportional to volume (V) if temperature + amount of gas constant (c) -> P = c/V meaning rearranged:

PV = c so P1V1 = P2V2 = constant

Question 7

What is Charles’s law?

Answer 7

Volume (V) is directly proportional to temperate (T) on absolute temperate scale if pressure + amount of gas are constant (c) -> T = cV meaning rearranged:

T/V = c so T1/V1 = T2/V2 = constant

Question 8

How does the universal gas law come about?

Answer 8

Rearranging the equations of Boyle’s + Charles’s law gives you:

B: P = 1/V and V = 1/P
C: V = T
So V = T/P -> T = V x P thus, pressure and volume are both proportional to temperature

Can be rearranged to PV/T = constant (R) where R = gas constant

Final equation: PV = nRT where n is no. of moles of gas

Question 9

What does the universal gas law allow for?

Answer 9

Calculation of one change in a parameter when the others are changed but are know i.e. need to know all of the values of the equation except one

Question 10

What are partial pressures in terms of Daltons law?

Answer 10

When non-reactive gases (e.g. O2, N + CO2) mix, molecules of each type till behave independently + exert their own partial pressure which is a fraction of the total pressure

Total pressure = sum of partial pressures of individual gases

Same fraction of total pressure = fraction of total volume of gas in mixture so can work out partial pressure by multiplying fraction of volume by total pressure

Question 11

If there is 2 gases with the same pressure + amount in 2 separate boxes, and then the partition is removed allowing the gases to mix, what will be the partial pressure of the gases?

Answer 11

The volume of the box has doubled and each gas will take up half of the box therefore, the partial pressure of both gases will be 50%

Question 12

Using Daltons law, what would be the equation for atmospheric pressure?

Answer 12

= Sum of partial pressures of nitrogen, oxygen, water vapour + any other gases in the air

Question 13

In Dalton’s law, in a mixture of gases, each gas exerts the same pressure that it would exert if it ___ __ __ ___ __.

Answer 13

Occupied the same volume alone

Question 14

In biological systems gas mixtures are always in contact with water (major constituent of tissues + blood), what happens at this contact point?

Answer 14

Water molecules evaporate + gas molecules dissolve but there is no chemical reaction between them

Equilibrium is reached when partial pressures of each substance on either side of interface are equal

Question 15

How can equilibrium between water and gas be attained? When is it impossible to reach equilibrium?

Answer 15

Water molecules enter gas exerting a vapour pressure

If they enter a closed volume of gas (e.g. in alveolus) equilibrium is reached typical of biological systems -> equilibrium pressure for water vapour = saturated vapour pressure (SVP)

If gas volume is unlimited, there will be fluid evaporation to dryness as no equilibrium can be reached

Question 16

What is Saturated Vapour Pressure (SVP) and what does it depend on?

Answer 16

SVP = equilibrium pressure for water vapour = 6.28 kPa at 37 degrees (good approximation of temperature in biological systems)

SVP depends only on temperature of system as SVP increases with temperature because increased kinetic energy allows more vapour to escape liquid surface

Question 17

The temperature at which the Saturated Vapour Pressure (SVP) is equal to the atmospheric pressure is called the ___ __.

Answer 17

Boiling point

Question 18

What is Henry’s law?

Answer 18

The physical amount of gas (θ) dissolving in a solvent is directly proportional to the pressure of the gas (δ) at the solvent’s surface (if temperature is constant)

Constant of proportionality (c) of this relationship is an expression of solubility of gas in the liquid

So θ = c x δ

Question 19

What gas movement is occurring at the alveolar-blood interface? Why?

Answer 19

O2 is stored in the blood in the lungs and will come in from bronchi and diffuse down its concentration gradient into the blood

CO2 is higher in the blood as it is stored in the blood of tissues for transport so it will diffuse down its concentration gradient into the alveoli

Question 20

Why is CO2 higher in the tissues/blood leaving the tissues?

Answer 20

Because it is a metabolism waste product produced at the tissues

Question 21

What is gas solubility in a liquid? What factors can alter it?

Answer 21

Represents the gas’ ability to dissolve in that liquid and is constant for the specific gas + liquid depending on their physical properties - it also depends on temperature, pressure + pH of solution

Question 22

The amount of gas which enters the liquid to establish a particular tension is determined by its ____.

Answer 22

Solubility

Question 23

How is gas solubility in a liquid expressed?

Answer 23

As amount of solute per amount of solvent

Measured at saturation concentration where adding further solute results in its precipitation e.g. adding sugar to water until the sugar starts to precipitate = solubility

Question 24

What happens when a gas dissolved in water?

Answer 24

Gas molecules exert ‘tension’ i.e. a pressure within the liquid

At equilibrium, the tension of each constituent gas is proportional to its partial pressure in the adjacent gas mixture (Henry’s law)

Question 25

What does gas tension in liquids indicate?

Answer 25

How readily the gas will leave the liquid

Measure of partial pressure of solute in solvent (kilopascals) e.g. oxygen in blood so does not indicate physical amount of gas in liquid

Question 26

What is the gas content in a liquid? What does it depend upon?

Answer 26

Physical amount of given gas in a liquid e.g. mls O2 per 100ml blood

Changes with temperature + gas pressure above it

Proportional to its solubility in that liquid + gas tension at equilibrium

Question 27

How is gas exchanged facilitated in the lungs?

Answer 27

At rest, 5L of blood must pick up 12 mmol of O2 per minute so exchange needs a very large SA ~ 70 square metres to expose enough blood to gas coming from the atmosphere

Requires large no. of small compartments = 300 million alveoli (thin hexagonal structure surrounded by capillary plexus + fed through tiny bronchiole called alveolar duct)

Question 28

What is molecular diffusion?

Answer 28

Principal mechanism of gas mixing in lung periphery

Requires presence of molecular concentration gradient

Due to random motion of individual gas moelcules -> net transfer of a given gas from a region of high to low concentration

Question 29

What is the role of blood?

Answer 29

Picks up O2 in lungs + deposits it in tissues & picks up CO2 in tissues + deposits it in lungs due to properties of Hb carried in RBCs

Gas transfer occurs because blood is exposed to appropriate gaseous environments by the pulmonary system + tissues

Question 30

What will happen if a gas enters a liquid and then reacts with?

Answer 30

Gas may react with component of liquid (e.g. RBC) - reaction must be complete + reached equilibrium itself before tension within fluid reaches its equilibrium value

Total content of gas in liquid = reacted gas (with for e.g. Hb) + dissolved gas in liquid (e.g. plasma)

Question 31

What 3 factors is the final gas content at equilibrium in a liquid dependent on?

Answer 31

1. Gas tension
2. Solubility of gas in liquid
3. Affinity of Hb for O2

This occurs in order as equilibria for each reaction must occur in sequence

Question 32

How does gas exchange occur between the air and the blood?

Answer 32

Via diffusion across the alveolar membrane

Alveolar gas has different composition to atmosphere - less O2 + more CO2 because exchange occurs between alveolar gas + adjacent gases carried in blood

Question 33

What are the normal partial pressures of atmospheric air + alveolar air?

Answer 33

Atmospheric: pO2 = 21.1 kPa, pCO2 = minimal + vapour pressure depends on humidity

Alveolar: pO2 = 13.3 kPa, pCO2 = 5.3 kPa + saturated with water vapour

Question 34

What is mixed venous blood? What are the typical partial pressures within it?

Answer 34

Returns to lungs from systemic circulation

pO2 = 6.0 kPa + pCO2 = 6.5 kPa but varies with metabolism

Question 35

What are the gradients of partial pressure of O2 and CO2 at the alveolar membrane + how does this influence diffusion of gases?

Answer 35

pO2 in alveolar gas > pO2 in returning mixed venous blood

pCO2 in alveolar gas < pCO2 in returning mixed venous blood

= O2 will diffuse into blood + CO2 will diffuse out

Question 36

What is proportional + inversely proportional to diffusion?

Answer 36

P: SA of alveoli + gradient down which individual gas diffuses caused by partial pressure differences

IP: Resistance to diffusion of alveolar capillary membrane to gas (affected in disease states)

Question 37

What are the 3 factors that affect the diffusion rate of a gas?

Answer 37

1. Nature of barrier
2. Nature of gas
3. Interaction between the gas + barrier i.e. solubility of gas in tissue/fluid

Question 38

What 5 components make up the diffusion barrier of the alveoli that gases must get through?

Answer 38

1. Epithelial cell of alveolus
2. Tissue fluid + connective tissue
3. Endothelial cell of capillary
4. Plasma
5. RBC membrane + cytoplasm

(a lot can therefore go wrong here to resist/limit diffusion rate)

Question 39

In terms of the nature of the gas, how does this affect the diffusion rate?

Answer 39

Gases diffuse at rate inversely proportional to square root of their MW - big MW = slower diffusion so CO2 diffuses a little slower than O2

Question 40

In terms of gas solubility, how does this affect diffusion rate?

Answer 40

Gases diffuse at rate proportional to solubility - CO2 has higher tissue solubility than O2 so diffuses 21x faster overall even though O2 is smaller

Question 41

Define solubility.

Answer 41

The amount of solvent that can dissolve in unit volume of solute e.g. ml of O2/L of water

Question 42

Why is O2 transfer to tissues perfusion limited under normal circumstances? What can happen in disease states?

Answer 42

As O2 has fully saturated Hb by the time it is 25% along the capillary length

However, in disease states, transfer may become DIFFUSION LIMITED with full equilibration not occurring by the end of the capillary

Question 43

What is the equation for the diffusion rate (DR)?

Answer 43

DR = A/TD X (P1-P2)

A = Area
T = Thickness
D = diffusion constant = Solubility (Sol)/√MW
P - Partial pressure