Ventilation:Perfusion Relationship

Question 1

What is ventilation? What is perfusion?

Answer 1

Ventilation: volume of air going in and out of the respiratory system (L/min)
Perfusion: blood flow through the pulmonary circulation L/min

Ideally ventilation and perfusion match in L/min

Question 2

How does height across the lung impact blood flow and ventilation?

Answer 2

Both blood flow and ventilation decrease with height across the lung

• At the base of the lungs blood flow is higher than ventilation as arterial pressure exceeds alveolar pressure
  
  • Blood vessels push on the alveolar and compress them
• At the apex of the lung blood flow is low because arterial pressure is less than alveolar pressure (blood flow decreases faster than ventilation does)
  
  • This compresses the arterioles in the apex of the lung

Question 3

How does the ventilation:perfusion ratio change from the base to the apex of the lung?

Answer 3

75% of the healthy lung performs well in matching blood and air
o Perfectly matched – ventilation:perfusion ratio = 1.0
o Mismatch 1 (base) – ventilation:perfusion < 1.0
o Mismatch 2 (apex) – ventilation:perfusion > 1.0

Auto regulated to keep the ratio as close to 1
(Majority of mismatch in the apex)

Question 4

What happens in a Ventilation:perfusion mismatch? How does it lead to a shunt?

Answer 4

If ventilation decreases in group of alveoli PCO2 increases and PO2 decreases

• Blood flowing past those alveoli doesn’t get oxygenated and blood going passed poorly ventilated region isn’t able to give up its CO2
• blood is taking O2 faster than it can be replenished = decrease in PO2
• leads to CO2 build up in this region
• this leads to the diluted blood going back to heart
• shunt = blood moved from right side of the left without undergoing gas exchange

Question 5

How are shunts prevented?

Answer 5

• Blood vessels around the poorly ventilated areas respond to the decrease in PO2 by constricting
  o Redirects the blood to the better ventilated areas
  o This only happens in the pulmonary vessels not in systemic circulation
  o The increase of PCO2 acts on the smooth muscle causing it to dilate
• Improves ventilation

Question 6

What is alveolar dead space?

Answer 6

Alveolar dead space = ventilation > blood flow
- (Opposite of a shunt)
o More air than blood flow = fresh air not participating in gas exchange
o Occurs at the apex
o Leads to pulmonary vasodilation + bronchial constriction.
o Common in pulmonary embolism (blood clot in vessel preventing blood flow to alveoli)

Question 7

What is anatomical dead space?

Answer 7

air in the conducting zone of the respiratory tract unable to participate in gas exchange

Question 8

What is the physiological dead space?

Answer 8

alveolar dead space + anatomical dead space

Question 9

What is a shunt?

Answer 9

Shunt = ventilation < perfusion

• alveolar PO2 falls, PCO2 rises
• pulmonary vasoconstriction
• bronchial dilation

Question 10

How is O2 carried in the blood?

Answer 10

Majority of oxygen wrapped up in haemoglobin in red blood cells (197mL/L out of 200mL)
Very little dissolved in plasma (3mL/L out of 200mL)

Question 11

How is CO2 transported?

Answer 11

Bulk (77%) of CO2 transported in solution in plasma

23% stored within haemoglobin

Question 12

What % of O2 is extracted by peripheral resting tissue?

Answer 12

25%

Question 13

What type of haemoglobin is in adult blood? What is the structure of haemoglobin?

Answer 13

Haemoglobin A

4 haem groups - each associates with on molecule of oxygen

Question 14

How does oxygen bind to haemoglobin?

Answer 14

Oxygen binds to the haem groups in a weak relationship that doesnt last very long
Cooperative binding:
- oxygen binding to haemoglobin causes conformational change making it easier for other oxygen molecules to bind
- when oxygen leaves the haemoglobin there is further conformational change making it easier for other oxygen molecules to be given up

Question 15

How does haemoglobin maintain the partial pressure gradient between alveoli and plasma?

Answer 15

Red blood cells full of haemoglobin

Haemoglobin sucks the oxygen out of the plasma maintaining the partial pressure gradient between the alveoli and plasma

Question 16

What is the haemoglobin saturation at PO2 100mmHg, 60mmHG, 40mmHg (PO2 at tissues)?

Answer 16

100mmHg: 100%
60mmHg: 90%
40mmHg: 75%

Therefore saying that venous blood is deoxygenated is misleading as still 75% oxygenated

Question 17

What is anaemia?

Answer 17

• Anaemia – any condition where the oxygen carrying capacity of the blood is compromised
  o E.g. Iron deficiency, haemorrhage, vitamin B12 deficiency

Question 18

How does anaemia affect PaO2?

Answer 18

Completely normal - possible to have normal plasma PO2, while total blood O2 is low (assuming normal lung function)
- Red blood cells can be fully saturated in anaemia assuming PO2 in plasma normal

(o Not possible to have low plasma PO2 and normal total blood O2 content
- Plasma determines how much oxygen binds to haemoglobin

Question 19

How does oxyhaemoglobin dissociation curve aid O2 loading in lungs?

Answer 19

Normal PO2 in plasma in lungs = 100mmHg —> fully saturated haemoglobin
Curve is dynamic so can slide along x-axis
- doesnt effect loading as still in top plateau phase

Question 20

How does a decrease pH, PCO2 and increase in temperature affect the oxyhaemoglobin dissociation curve?

Answer 20

Reduces saturation of the haemoglobin
- this is Bohr effect = aids oxygen unloading at peripheral tissues

Example when exercising:

• production of lactic acid lowers pH
• higher O2 metabolism = more CO2 as waste
• increase in body temperature

Question 21

How does a increase pH, PCO2 and decrease in temperature affect the oxyhaemoglobin dissociation curve?

Answer 21

Opposite effect

Hypothermia is so dangerous as haemoglobin wont give off oxygen to the peripheral tissues

Question 22

How is CO2 carried in the blood?

Answer 22

CO2 is very soluble so travels readily in solution

• CO2 diffuses into plasma (down partial pressure gradient)
• 7% of CO2 remains in solution in plasma
• 93% moves into red blood cells
  
  • 70% of this reacts with enzyme carbonic anhydrase to form carbonic acid
  • dissociates into bicarbonate and H+ ions
  • bicarbonate ions move back into plasma in exchange for Cl- ions
  • therefore 70% of CO2 travels in systemic venous blood as bicarbonate ions

Question 23

What factors favour CO2 unloading at the lungs?

Answer 23

Haemoglobin much rather associate with O2 than CO2

- PO2 increases —> haemoglobin wants to release the CO2
- haemoglobin releases H+ ions causing bicarbonate to shift back into red blood cells to neutralise H+
- carbonic anhydrase formed —> CO2 and water released from the carbonic acid formed

Question 24

What is the difference between arterial partial pressure of oxygen (PaO2) and arterial O2 concentration

Answer 24

PaO2 refers purely to O2 in solution in the plasma and is determined by O2 solubility and the partial pressure of O2 in the gaseous phase that is driving O2 into solution

Partial pressure not the same as concentration as concentration varies on the form of the molecule (e.g. 30x more O2 molecules in 1L gas than 1L plasma)

Question 25

How much of our haemoglobin is adult vs fetal?

Answer 25

92% adult

8% fetal (and others)

Question 26

Where is myoglobin found?

Answer 26

exclusively in cardiac and skeletal muscle

- Not normally found in blood
- Similar role to haemoglobin – except rather than transport oxygen myoglobin stores oxygen 
- Still has the haem group and binds the oxygen more tightly than haemoglobin does

Question 27

How does fetal haemoglobin and myoglobins affinity for oxygen differ from adult haemoglobin?

Answer 27

• faetal haemoglobin and myoglobin have higher affinities of O2 than adult haemoglobin
  o Necessary to extract O2 from maternal/arterial blood

Question 28

What is hypoxia?

Answer 28

Inadequate supply of oxygen to tissues

Question 29

Define 5 different types of hypoxia:

Answer 29

• Hypoxaemic hypoxia: most common. Reduction in O2 diffusion at lungs either due to decreased PO2 atmosphere or tissue pathology
• Anaemic hypoxia: reduction in O2 carrying capacity of blood due to anaemia (red blood cell/iron deficiency)
• Stagnant hypoxia: heart disease results in inefficient pumping of blood to lungs/around the body
• Histotoxic hypoxia: poisoning prevents cells utilising oxygen delivered to them e.g. carbon monoxide/cyanide
• Metabolic hypoxia: oxygen delivery to the tissues doesn’t meet increased oxygen demand by cells