4. Properties of gases and Gas exchange in the lung

Question 1

what is boyle’s law?

Answer 1

The pressure of a fixed quantity of gas at a constant temperature (e.g. body temperature) is inversely proportional to its volume.

Question 2

What is Gas pressure caused by?

Answer 2

the collisions of gas particles with the walls of the container

Question 3

What is partial pressure?

Answer 3

The pressure exerted by a particular gas in a mixture of gases (proportional to its volume percentage in the mixture)

Question 4

What is the atmospheric pressure At sea level?

Answer 4

101.1 kPa = 1 atmosphere = 760 mmHg

Question 5

What percentage of air is made up of oxygen and therefore what is its partial pressure?

Answer 5

20.9%. pO2 = 101.1 x 0.209 = 21.1 kPa

Question 6

Describe how to calculate Partial Pressure of a Gas in a Gas Mixture

Answer 6

Multiply the fractional percentage (percent converted to a decimal) of the individual gas by the total pressure exerted by the mix of gases
(% gas A)(total pressure) = partial pressure gas A

Question 7

What should be assumed when calculating partial pressure of a gas in a gas micture?

Answer 7

Assume there are no chemical reactions between the gases

Question 8

What is dalton’s law?

Answer 8

In a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.

Question 9

What is atmospheric pressure?

Answer 9

• pressure exerted by the weight of the air above the earth in the atmosphere

Question 10

How does the atmospheric pressure change at higher altitudes?

Answer 10

goes lower

Question 11

Which gas makes up 78% of air?

Answer 11

nitrogen

Question 12

What happens to inhaled air in the nostril?

Answer 12

When we breathe in air it is moistened – water molecules in vapour form are added to air

Question 13

What is the result of moistening air in the airways?

Answer 13

Water vapour pressure – called saturated vapour pressure- displaces a proportion of the total pressure of the mixture of gases – in this case, total atmospheric pressure

Question 14

What is the saturated water pressure dependant upon?

Answer 14

temperature - does not change with altitude

Question 15

What is the saturated water pressure at body temp?

Answer 15

6.28 kPa

Question 16

How does the saturated water pressure affect the calculation of Partial Pressure of a Gas in a Gas Mixture?

Answer 16

• need to subtract the water vapour pressure from the total pressure of the dry gas mixture to arrive at the total pressure of gas mixture in a moist environment
• therefore 101(kPa) – 6.28 (kPa) = 94.7 kPa =total pressure of gas mixture in a moist environment
• The other gases are still in the same ratios as in dry air

• pO2 = 94.7 kPa x .209 = 19.8 kPa= partial pressure oxygen in URT

Question 17

What is alveolar partial pressure of oxygen?

Answer 17

13.3 kPa

Question 18

Why is the alveolar partial pressure of oxygen different in URT and alveoli?

Answer 18

• Oxygen from alveolar air constantly diffusing into pulmonary circulation
• Alveolar air only partially replaced with each breath - certain amount of air remain in alveoli and not fully emptied so the Fresh air diluted by residual already in the lungs which is constantly having its oxygen removed

Question 19

What determines alveolar pO2?

Answer 19

Rate at which O2 is taken up by the blood and the rate at which it is replenished by alveolar ventilation

Question 20

What determines alveolar pCO2?

Answer 20

Alveolar pCO2 is determined by the rate at which CO2 enters the alveoli from blood and the rate at which it is removed from alveolar gas by ventilation

Question 21

is the alveolar partial pressure of carbon dioxide different in URT and alveoli?

Answer 21

yes

Question 22

What is the typical value for tidal volume, how much of this reaches the alveoli and what happens to the rest?

Answer 22

450ml, 300ml reaches alveoli, 150ml (30%) remains in anatomical dead space

Question 23

What is the anatomical dead space?

Answer 23

Volume of non-gas exchange surface in the respiratory system (trachea, bronchi, bronchioles)

Question 24

Compare Pulmonary ventilation and alveolar ventilation

Answer 24

• Pulmonary ventilation - amount of air we bring into our lungs
• alveolar ventilation - volume of air that reaches respiratory portion of lung after minusing the 30% that remain in the anatomical dead space

Question 25

Compare Pulmonary Minute Ventilation and alveolar Minute Ventilation

Answer 25

• Pulmonary Minute Ventilation – We breathe ≈ 12 times a minute - the total volume of air moved is 450 ml × 12 = 5400 ml/min

• Alveolar Minute Ventilation (ml/min) is the rate of air flow reaching the alveoli
– 12 breathes/minute × 300 ml = 3600 ml/min

Question 26

What happens when inspired gases come in contact with body fluids?

Answer 26

gas molecules will enter fluid and dissolve in the liquid

Question 27

What determines amt of gas that dissolves in liquid?

Answer 27

directly proportional to the partial pressure of that gas - higher the partial pressure,

Question 28

When is equilibrium reached in a gas-liquid interface?

Answer 28

Partial pressure of the gas in the liquid is equal to the partial pressure of that gas in the gas phase in contact with the liquid.
(rate gas entering liquid = rate gas leaving liquid)

Question 29

Down what gradient will A gas in air dissolve into a fluid/

Answer 29

along its partial pressure gradient – from high partial pressure to low

Question 30

What two factors determine the rate out of liquid to gas?

Answer 30

1. conc of dissolved gas

2. solubility of gas for liquid medium

Question 31

When equilibrium is reached in a gas-liquid interface, will the concentration of the gas be the same in the ags phase and liquid phase?

Answer 31

No

• if a molecule has a high solubility to the liquid, a higher concentration gradient is needed for the molecule to move out of the liquid and for equilibrium to be reached - liquid has higher conc
• if a molecule has a low solubility to the liquid, only a low conc gradient is needed for it to move out of the liquid to gas, and to reach equilibrium - gas has higher conc

Question 32

explains why ALVEOLAR gas partial pressures determine ARTERIAL gas partial pressures

Answer 32

• Alveolar partial pressure oxygen > partial pressure of oxygen in the venous blood arriving at the alveoli
• Oxygen gas diffuses and dissolves down its partial pressure gradient into the blood
• Equilibrates with blood
• Partial pressure oxygen in blood *leaving alveoli (which will become arterial blood) is now equal to partial pressure oxygen in alveoli
• The partial pressure of oxygen in arterial blood is the equilibrium derived partial pressure established at the air-fluid interface – i.e. at the alveolar-blood interface

Question 33

What factors determine Dissolved Gas Content?

Answer 33

• Depends on the partial pressure of the gas
• And
• The solubility coefficient of that gas

Question 34

What is the solubility coefficient?

Answer 34

Amount (in mmol) of a gas that will dissolve in a

litre of plasma at 370C (body temperature), when exposed to a given partial pressure