Gas Exchange

Question 0

What is Charles’ law?

Answer 0

Increased temperature increases the energy and frequency of collisions, and therefore increases pressure

Pressure is directly proportional to absolute temperature (Kelvin)

Question 1

What is Boyle’s law?

Answer 1

Same amount of gas in a reduced volume increases the frequency of collisions, and therefore increases pressure

Pressure is inversely proportional to volume

Changing volume during breathing alters the pressure

Question 2

What is the relationship between pressure, volume, and temperature?

Answer 2

Pressure (Pa) x Volume = Gas constant (R) x Temperature (K)

in standard conditions (273K/0C), 101.1kPa

Question 3

Explain what a partial pressure is. How is it calculated?

Answer 3

In a mixture of gases, molecules of each type behave independently, so each gas exerts a partial pressure

Partial pressure/Total pressure = Volume of gas/Total volume

e.g. air is ~ 20.9% O2
pO2 = total pressure x 0.209 = 101.kPa x 0.209 = 21.1kPa

Question 4

What is saturated vapour pressure? How is it calculated?

Answer 4

When water molecules leave (become a vapour) and re-enter water at the same rate at a constant pressure (depends only on temperature)

Therefore, exposing gas to excess water will achieve the saturated vapour pressure

= 6.28kPa at 37C

e.g. pO2 in saturated air at 37C = (101.1 - 6.28) x 0.209 = 19.8kPa to 3s.f.

Question 5

What happens when someone is exposed to very low pressure air?

Answer 5

Reduced total pressure, but the saturated vapour pressure remains the same

No more room for air (water only)

Inhaling only water vapour -> BOIL

Question 6

What is the definition of gas tension? Why can this be useful?

Answer 6

How readily a gas will leave a liquid (NOT how much gas is in the liquid)

At equilibrium: Tension of a gas = Partial pressure of gas

e.g. pO2 in blood exposed to a gas mixture containing 14% O2 at total pressure 101.1kPa at 37C, saturated with water vapour:

pO2 = (101.1 - 6.28) x 0.14 = 13.3kPa at 3s.f.

Question 7

What is the relationship between the gas content of a mixture and the solubility and tension of a gas? Why is this important?

Answer 7

Gas content of a mixture (amount of unbound gas free to move in a mixture to establish a particular tension)

= Solubility (mmol/l/kPa) x Tension
(how readily a liquid binds to a gas)

If a gas reacts with a component of the liquid (NOT WATER), this reaction must complete before tension is established (no more gas molecules will react)

Question 8

What factors affect the diffusion of gases?

Answer 8

Surface area
Gradients of gases
Diffusion resistance (nature of barrier/nature of gas)

Question 9

What are the different components of the diffusion barrier oxygen & carbon dioxide must pass through during gas exchange?

Answer 9

1. Diffusion from inside of alveolus to the edge
2. Epithelial cell of alveolus
3. Tissue fluid
4. Endothelial cell of capillary
5. Plasma
6. Red cell membrane

Question 10

What factors affect diffusion of gases through gases? What factors affect diffusion of gases through liquid?

Answer 10

Gas: rate inversely proportional to molecular weight
Oxygen diffuses faster than carbon dioxide (therefore, diseases affecting the alveolar wall selectively affects oxygen diffusion)

Liquid: rate directly proportional to solubility
Carbon dioxide more soluble than oxygen in water and fat (therefore, by the time CO2 diffusion is affected, the oxygen deficit would have already caused death)

Oxygen exchange is ALWAYS limiting (measure colour of RBCs to determine)

Question 11

What is the definition of ventilation? How can this be measured?

Answer 11

Expansion of lungs which increases the volume of the respiratory bronchioles and alveolar ducts

note: NOT INTO THE ALVEOLI - alveolar gas remains unchanged - air flows next to the alveolar walls

Measure using a spirometer: subject breathes from closed chamber over water where the volume changes with ventilation

Question 12

Define ventilation rate. How is this calculated?

Answer 12

Amount of air moved into and out of a space per minute = volume moved per breath x respiratory rate

• pulmonary ventilation rate:
  tidal volume x respiratory rate (~8l/min at rest)
• alveolar ventilation rate:
  pulmonary ventilation rate - (dead space volume x respiratory rate)

Question 13

What is dead space? What are the different types? What proportion of inspired air is wasted at rest?

Answer 13

“Wasted” air that does not reach the alveoli (last air stays in airways and is first air our) = ~1/3 at rest

Physiological dead space = Serial/anatomical dead space + Distributive dead space

Serial/anatomical dead space: volume of the airways = ~0.15l

Distributive dead space: parts of the lung that do not support gas exchange e.g. dead/damaged alveoli or poorly perfused alveoli
= ~0.17l

Question 14

Providing gas exchange is 100% efficient, what is the difference in concentration of oxygen and carbon dioxide in the alveoli as opposed to in arterial blood?

Answer 14

Alveolar air pO2 & pCO2 = arterial pO2 & pCO2

Question 15

What is the difference in gas exchange of oxygen and carbon dioxide, and why does this matter?

Answer 15

Carbon dioxide far more soluble than oxygen, therefore diffuses faster within the blood & cytoplasm

Oxygen diffuses faster as a gas (within the alveoli)

Poor oxygen uptake in some alveoli cannot be compensated for by other alveoli as oxygen exchange is already 100% efficient

pO2 affected before pCO2 when gas exchange is affected

Question 16

What is the difference in solubility of oxygen & carbon dioxide in water? What is the significance of this in terms of oxygen supply and transport?

Answer 16

Oxygen: only ~ 0.13mmol/l (not sufficient to supply our tissues as we require ~12mmol/min, so oxygen needs to be transported chemically: combines reversibly with haemoglobin via oxygenation)

Hb saturated above 8.5kPa (normal pO2 = 13.3kPa). 1 molecule of O2 binding to Hb = 2.2mmol/l. Fully saturated Hb = 4 x 2.2 = 8.8mmol/l

Tissue pO2 ~5kPa (65% saturated) = 8.8 x 0.35 = 3mmol/l
12/3 = 4l of blood needed to adequately supply tissues with oxygen (providing adequate level of Hb)

note: tissue blood supply pO2 must be high enough to produce a gradient high enough to adequately supply cells (cannot fall below 3kPa) - this is combated by increasing capillary density

Carbon dioxide: 1.2mmol/l (therefore oxygen gas exchange will always be affected before carbon dioxide gas exchange)

Question 17

How is blood pH controlled?

Answer 17

pH depends on [CO2] in plasma (note: arterial pCO2 depends on alveolar pCO2)

RBCs buffer excess H+ in blood produced by reaction of CO2 & water (along with Cl-) (note: plasma is slightly alkaline because of this)

Bohr Shift: reduction in pH/increase in temp./BPG shifts oxygen dissociation curve to the right so oxygen is more easily lost from RBCs

pH is reduced (e.g. at respiring tissues) —> H+ & lactate are produced —> RBCs bind H+ —> oxygen released (respiring tissues supplied with oxygen) (different or the same as the Bohr effect????)

Kidney varies amount of HCO3- excreted (note: [HCO3-] is higher in venous blood: Hb has lost oxygen —> binds to H+ and releases HCO3- & increased pCO2 —> more CO2 and water reacts —> more HCO3- produced)

Question 18

What is the significance of the oxygen reserve?

Answer 18

Not all oxygen is taken up at rest

In exercise, more oxygen can be taken up

e.g. 10 fold increase in metabolism rate only requires a 5 fold increase in cardiac output (as twice as much oxygen can be extracted by tissues)

Question 19

How is carbon dioxide transported in the blood, and in what proportions?

Answer 19

80% HCO3- (CO2 + H2O —> H+ + HCO3-)
11% carbamino compounds (no function in arterial blood, removes waste CO2 in venous blood)
8% dissolved in blood