Matching ventillation to perfusion requirements

Question 1

What is Henry’s law?

Answer 1

The law that states that gasis in contact with liquid will dissolve in the liquid in proportion to its partial pressure.

At equilibrium partial pressures in alveoli and capillaries will be equal.

Alveoli - Capillary (A - a gradient) gradient determines the direction of diffusion.

Question 2

What makes O2 and CO2 good for gas exchange?

Answer 2

They are easily able to cross cell membranes and are soluble in aqueous solution

Question 3

What is partial pressure?

Answer 3

Percentage pressure caused by a certain gas

Question 4

What is the total pressure of air at sea level?

Answer 4

760mmHg (partial pressures of different gases are a proportion of this number)

Question 5

What do gases depend on in addition to partial pressure to determine how much they dissolve in a liquid?

Answer 5

Solubility (which is why very little N2 dissolves in water)

Question 6

What is more soluble in water? CO2 or O2?

Answer 6

CO2 is 20 times more soluble in water than O2. Very little N2 dissolves in water.

Question 7

What happens to oxygen partial pressure when air enters the alveoli?

Answer 7

It drops due to presence of residual air in the alveoli that is rich in CO2. (drops from 150 - 100 mmHg)

Question 8

What determines the time it takes for O2 and CO2 to reach equilibrium in the alveoli?

Answer 8

Fick’s law of diffusion describes the parameters that are important for the lungs:

Distance

Solubility (CO2>O2)

Surface Area

Concentration gradient

Question 9

How long do RBCs spend in pulmonary capillaries?

Answer 9

0.8 seconds at rest

Less time if exercising

Question 10

How long do RBCs need to spend in the lungs to reach equilibrium with pressure in normal functioning lungs?

Answer 10

0.25 to reach maximal loading

Question 11

What happens if distance is increased between air and capillaries?

Answer 11

Equilibration time increases dramatically

Question 12

Which conditions cause an increase in diffusion distance?

Answer 12

Pneumonia

Question 13

How can partial pressure of oxygen be decreased?

Answer 13

Going to higher elevations

Question 14

What does exercise do to equilibration time?

Answer 14

It decreases time due to faster movement of blood through the pulmonary capillaries

Question 15

What can limit pulmonary gas exchange?

Answer 15

Low inspired oxygen (PiO2)

Hypoventilation (high ventilation rate and low lung volume can cause this)

Diffusion limitations (Can be caused by anything disturbing the alveolar - arterial PO2 difference)

Ventilation - Perfusion mismatching (Right to left shunts can cause this)

Question 16

What is the Va/Q ratio?

Answer 16

Ratio of air reaching alveoli to blood reaching alveoli

Question 17

What is normal Va/Q ratio?

Answer 17

Va= 4.2L/min of air 
Q = 5L/min

Va/Q=0.84

Question 18

What does a low Va/Q ratio tell us?

Answer 18

Impaired ventilation and so there is a shunt of blood from left to right past the alveoli without sufficient oxygenation

Question 19

What does a high Va/Q ratio tell us?

Answer 19

Impaired perfusion which indicates dead space

Question 20

How does the body solve the issue of ventilation-perfusion mismatch?

Answer 20

Autoregulation:

Alveolar change in PO2 causes change in arteriole diameter (vasodilation)

Alveolar PCO2 causes change in bronchiole diameter (Bronchodilation)

Question 21

What happens when alveolar oxygen increases?

Answer 21

Arterioles dilate to take oxygen in

Question 22

What happens when alveolar oxygen decreases?

Answer 22

Arterioles are constricted

Question 23

What happens when alveolar CO2 is increased?

Answer 23

Bronchodilation to get rid of excess CO2

Question 24

What happens to ventillation in a shunted alveolar supply?

Answer 24

No alveolar ventilation and no oxygenation of blood. Alveolar air instead equilibrates with venous blood O2 and CO2 resulting in no exchange.

Question 25

What happens to ventilation in dead space?

Answer 25

No capillary blood flow = wasted ventilation. This results in alveoli equilibrating to atmospheric air because there is no CO2 being released.

Question 26

How does the heart’s hydrostatic pressure affect Va and Q? Why does this matter?

Answer 26

Q = hydrostatic P is greater lower, so stronger flow. Lung is centered vertically around the heart. Part of the lung is above and part is below. Top part is going to be underperfused and lower part of the lung will be overperfused.. This matters because it means that parts of the heart would be problematic to remove by surgery.

Question 27

What common pathology would result in a high Va/Q ratio?

Answer 27

Pulmonary embolism

Question 28

What common pathology would result in low Va/Q?

Answer 28

Chronic bronchitis, asthma, pulmonary oedema

Question 29

What affects haemoglobin release of oxygen?

Answer 29

pH (lower pH results in more O2 release)

Temperature (higher temperature allow more release of oxygen)

Increase in CO2 results in more O2 release.

Question 30

What causes more O2 loading?

Answer 30

Alkaline pH

Low temperature

Low PCO2 (Bohr effect)

Question 31

Where is CO2 located on in the blood?

Answer 31

7% is dissolved

20% binds with proteins and is known as carbamino binding (most is bound to terminal groups of Hb. R-NH groups can always bind to CO2)

70% is bicarbonate (CO2 + H2O H2CO3 H+ + HCO3)-

Question 32

What does carbonic anhydrase do?

Answer 32

It speeds up the conversion of CO2 + H2O H2CO3 reaction

Question 33

How are the 3 methods of CO2 different to each other?

Answer 33

They are more or less labile than each other. I.e Bicarb takes time to produce so it takes a long time to release CO2 from it in the lungs. Carbonic anhydrase makes this method 5000x faster in RBCs.

Question 34

Where is most of the CO2 unloaded at the lungs?

Answer 34

In the carbamino and dissolved CO2 groups

Question 35

What is the relationship between CO2 loading and O2 bound? What is the effect called?

Answer 35

More O2 bound = less affinity for CO2 and vice versa. This is called the haldane effect (same as bohr effect but for CO2).