Lecture 29: Gases And Respiration

Question 1

External respiration involves 4 processes, what are they?

Answer 1

1. Pulmonary ventilation (bulk flow)
2. Gas exchange at lungs (diffusion)
3. O2 and CO2 transport
4. Gas exchange at tissues

Question 2

Adaptations for gas exchange:

Answer 2

• gas exchange in all organisms relies on diffusion
• diffusion: spread of particles from regions of high concentration to regions of low concentration
• in external respiration, O2 and C2 move by simple diffusion

Question 3

Factors affecting gas diffusion:

Answer 3

Rate of diffusion depends on: 
-pressure gradient 
-surface area
-diffusion distance 
-diffusion coefficient 
Diffusion is greatest when pressure gradient, surface area and diffusion coefficient are large BUT diffusion distance is small

Question 4

1. Pressure gradient:

Answer 4

Pressure represents the number if collisions occurring between the gas molecules themselves and their container walls.
-the greater the number of gas molecules the more collisions that occur, therefore the greater the pressure exerted
Net diffusion of gas will always be from regions of high to regions of low pressure

Question 5

2. Surface area:

Answer 5

Increased surface area means more oxygen can enter the body over a given time.
Eg lungs have a branching network of airways that gives them 70m 2 surface area

Question 6

3. Diffusion distance

Answer 6

The thinner the gas exchange membrane, the faster the rate of diffusion.
-gas exchange membranes must be very thin

Question 7

4. Diffusion coefficient

Answer 7

Diffusion coefficient is dependant on 2 factors:
1. Gas solubility
2. Molecular weight
Those gases which are highly soluble will have a larger diffusion coefficient and diffuse more quickly
The diffusion capacity of CO2 is 20x greater than O2

Question 8

Factors effecting gas diffusion

1. Pulmonary oedema
2. Emphysema

Answer 8

1. Pulmonary oedema:
   - fluid collected in the interstitial and eventually the alveoli
   - fluid on lungs increases diffusion distance
   - obstructs normal exchange of gases across respiratory membrane
2. Emphysema:
   - progressive destruction of walls of alveoli
   - leads to decreased surface area for gas exchange

Question 9

Name the 6 gas laws

Answer 9

1. Atmospheric pressure and altitude
2. Ideal gas law
3. Dalton’s law of partial pressures
4. Henry’s law
5. Boyle’s law
6. Bulk Air Flow

Question 10

1. Describe Law 1: atmospheric pressure

Ie why does air pressure decrease with increasing altitude?

Answer 10

Pressure is measured in mm Hg
-the pressure exerted by atmospheric air can push a column of mercury (Hg) to a height of 760 mm.
Patm= 760 mm Hg
Why?
Air above us compresses the air around us
-as we ascend, less compression occurs and air I’d less dense.
-at higher altitudes, there are less gas (oxygen) molecules in a given volume ie gas partial pressure decreases

Question 11

Describe law 2: the ideal gas law

Answer 11

We just need to understand that gas pressure is proportional to the number of gas molecules

Question 12

Describe law 3: Dalton’s law of partial pressures

Answer 12

The total pressure of a mixture of gases is a sum of the partial pressures exerted by each gas
Ptotal= P1 + P2 + P3 etc
Note: increased water vapour pressure (ie humidity) decreases the contribution of the other gases

Question 13

Describe law 4: Henry’s law

Answer 13

The amount of gas that will dissolve in a liquid is determined by its partial pressure and solubility.
-in order to diffuse into a cell, gas molecules in air must first dissolve in liquid.
-the higher the partial pressure and solubility coefficient (dissolving capability), the more soluble gas is.
If a substance is more soluble it means it is attracted to water. More can build up before the pressure is raised. Therefore, if a gas is very soluble, large numbers of molecules will go into solution at low pressures. If gas is less soluble, high partial pressures may cause.

When gas is in contact with a liquid, nett movement will occur between states IF a pressure gradient exists.
This will occur until equilibrium is reached ie PO2 in air=PO2 in liquid
WARNING: equilibrium doesn’t mean that numbers of gas molecules in a liquid equals number of gas molecule in air. The final concentration of gas in a liquid will depend on its solubility at equilibrium.

Question 14

Describe law 5: Boyle’s Law

Answer 14

Pressure is inversely related to volume given temp remains constant
-when volume decreases, gas pressure increases

Question 15

Describe Law 6: bulk air flow

Answer 15

Air flow = pressure gradient/ resistance
-bulk flow of air increases as the pressure gradient increases and resistance decreases.
-bulk air flow decreases as the pressure gradient decreases and resistance increases.
Resistance is strongly determined by radius