Ventilation and Diffusion

Question 1

What is the volume of dead space?

Answer 1

150mls

Question 2

What is partial pressure?

Answer 2

the pressure that would be exerted by one of the gases in a mixture if it occupied the same volume on its own at the same temperature

Question 3

How would you calculate partial pressure?

Answer 3

percentage of the gas (proportion) x atmospheric pressure

Question 4

What determines how much oxygen will dissolve in water?

Answer 4

• solubility of O2 in water

- PO2

Question 5

What is the normal partial pressure of PN2, PO2, PCO2 and water vapour?

Answer 5

• PN2 = 593.48 mmHg
• PO2 = 159.22 mmHg
• PCO2 = 0.23 mmHg
• water vapour 47mmHg
  (divide by 7.5 to get to KPa)

Question 6

What is the alveolar gas composition of PN2, PO2, PCO2?

Answer 6

• PN2 = 556.78 mmHg
• PO2 = 149.37 mmHg
• PCO2 = 0.21 mmHg
  (divide by 7.5 to get KPa)

Question 7

What is Henry’s Law?`

Answer 7

• the concentration of O2 dissolved in water is proportional to the partial pressure in the gas phase
• [O2]dis = s x PO2
  (s = solubility of O2 in water)

Question 8

What are the requirements for oxygen for gas exchange to the bloodstream?

Answer 8

• dissolve in the aqueous layer
• diffuse across the membranes
• enter the blood

Question 9

What is rate of diffusion proportional to?

Answer 9

• partial pressure difference (∆P)
• surface area
• solubility (D, diffusion coefficient)
• molecular mass (D, diffusion coefficient)

Question 10

What is rate of diffusion inversely proportional to?

Answer 10

• tissue thickness (T)

Question 11

Describe the normal conditions for gas exchange between the alveoli and blood to take place

Answer 11

• surface area of lungs is large
• large number of alveoli
• thickness is small
• concentration gradient is large so diffusion is rapid (PO2 alveolar air is 100 mmHg and PO2 venous blood is 40 mmHg)
• molecular mass insignificant
• CO2 20 times more soluble than O2

Question 12

Describe the conditions and process of the movement of CO2 in gas exchange

Answer 12

• CO2 moves in other direction from capillary to alveoli
• smaller concentration gradient (alveoli PCO2 is 40mmHg, venous PCO2 is 45mmhg)
• however greater solubility so CO2 diffuses faster than O2
• same amount of gas moves

Question 13

Describe how different disease states can limit gas exchange

Answer 13

• oedema: thickness of barrier increases so transit time through capillar may not be sufficient to complete full gas exchange, gas exchange reduced
• emphysema: surface area reduced, reducing gas exchange
• pulmonary fibrosis: tissue thickness increases, reducing gas exchange
• mucus, inflammation of airways, tumours which reduce gas entry reducing gas exchange

Question 14

How does altitude affect atmospheric pressure?

Answer 14

decreases PO2

Question 15

What are the physiological adaptations to acute altitude?

Answer 15

• hypoxia sensed by peripheral chemoreceptors
• ventilatory drive increases initially but blunted by central chemoreceptors that respond to decreased PaCO2 due to increased ventilation
• CO increases due to suppression of cardioinhibitory centre

Question 16

What are the physiological adaptations to adaptive altitude?

Answer 16

• central chemoreceptors adapt so ventilation rate continues to increase
• PaCO2 drops leading to respiratory alkalosis, kidneys compensate by reducing acid secretion blood pH normalises
• alkalosis stimulates 2,3 DPG production leading to rightward shift of O2 dissociation curve

Question 17

What are the changes to blood in altitude?

Answer 17

• erythropoietin release stimulated

- Hb concentration increases

Question 18

What are the changes to vasculature in altitude?

Answer 18

• hypoxia stimulates angiogenesis

- capillary density increases throughout body

Question 19

What are the changes to the cardiopulmonary system in altitude?

Answer 19

• vascular and ventricular remodelling
• smooth muscle growth increase vascular wall thickness
• right ventricle hypertrophies

Question 20

How does diving affect atmospheric pressure?

Answer 20

atmospheric pressure increases by 760mmHg every 10m depth

Question 21

What effect does depth have on the blood?

Answer 21

• increase in partial pressure
• N2 and O2 dissolve into blood at lethal excess
• volume decreases

Question 22

Describe the gas toxicity associated with diving?

Answer 22

• N2 narcosis: partial pressure of N2 rises and starts to dissolve in body tissues
• O2 poisoning: high pressure O2 dissolves in blood in excess of buffering capacity of Hb
• TREATMENT - Heliox: N2 replaced by helium and percentage of O2 tailored to reduce harm
• less readily dissolves in body tissues and less narcotic