Pulmonary blood flow and gas exchange Flashcards
Describe the blood supply to the lungs
Pulmonary circulation:
- Left and right pulmonary arteries from right ventricle of heart
- Branches into capillaries which are involved in gas exchange
Bronchial circulation:
- Bronchial circulation supplies the smooth muscle, nerves and lung tissue
- Part of the systemic circulation
- These are not involved with gas exchange
How does the pressure inside the pulmonary circulation compare to that of the systemic circulation?
Low pressure
High flow
In a perfect scenario, the pO2 and pCO2 in arterial blood should be equal to…
Alveolar pO2 and pCO2
O2 and CO2 diffuse down PP gradients, until the pp is the same on both sides
In a perfect scenario, the pO2 and pCO2 in venous blood should be equal to….
The pO2 and pCO2 in the tissues
same reason as the other question
What is the normal value for alveolar pO2
100 mmHg
13.3 kPa
What is the normal value for arterial pCO2?
40 mmHg
5.3 kPa
What is a normal value for venous pCO2?
46 mmHg
6.2 kPa
What 4 factors determine the rate of diffusion across a membrane?
Partial pressure gradient - proportional
Gas solubility - proportional
Surface area - proportional
Thickness of membrane - inversely proportional
Why is CO2 much better at diffusing across membranes than O2?
CO2 is much more soluble
Describe a pathology which affects surface area, for exchange, in the lungs
Emphysema
Type of COPD which causes destruction of alveoli
Describe 2 pathologies which affect the thickness of the membranes over which gas is exchanged
Fibrotic lung disease (fibrosis) - thickening of alveolar membranes due to scarring
Pulmonary oedema - fluid build up in interstitial space
Pulmonary oedema often has a larger effect on oxygen exchange than carbon dioxide exchange
Why is this?
Fluid build up largely consists of water
CO2 relatively soluble in water so can still diffuse(ish) whereas O2 can not
Explain where blood flow highest in the lungs
At the base
- Arterial pressure exceeds alveolar pressure. This compresses the alveoli
Where is ventilation highest in the lungs?
At the base
however blood flow is greater
Explain why there is an imbalance in blood flow - ventilation at the apex of the lungs
Arterial pressure is greater than alveolar pressure
Alveoli are therefore compressed
At what rib number is ventilation and perfusion matched?
3
(although when youre upright, gravity and shit means about 75% of your lung has pretty much matched ventilation - perfusion)
What is shunt?
The dilution of oxygenated blood from better ventilated areas, with poorly oxygenated blood from poorly ventilated areas
How does the body react to shunt?
Decreased tissue pO2 around under-ventilated alveoli causes vasoconstriction
Blood is redirected to better ventilated areas
How is the bodies reaction to shunt, different from other hypoxia induced responses?
In the systemic circulation, hypoxia causes vasodilation
In pulmonary circulation, hypoxia (shunt) causes constriction
What is ‘alveolar dead space’?
When blood flow is too low
ventilation > blood flow
How does the pulmonary circulation respond to alveolar dead space?
Increased alveolar pO2 causes vasodilation to increase blood flow
Decreased alveolar pCO2 causes mild bronchial constriction
What 2 components make up physiological dead space?
Anatomical dead space
+
Alveolar dead space
Describe how oxygen and carbon dioxide are transported in the blood
Oxygen:
- Haemoglobin
- Small amount dissolved in plasma
Carbon dioxide:
- dissolved in plasma
- carbonic acid (in RBCs)
- carbamino compounds
What is the carrying capacity of Haemoglobin for oxygen?
200 ml/L
