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
What is the carrying capacity of PLASMA for oxygen?
3 ml/L
At rest, what is the max amount of oxygen that can be pumped around the body per minute?
Cardiac output = 5 L/min
Max capacity = 200 ml/L
5 x 200 = 1000 ml/min
What is the normal demand for O2, of resting tissues?
250 ml/min
Describe the different types of haemoglobin found in red blood cells
HbA - 92% abundant
HbA2
HbF
Glycosylated Hb
Haemoglobin binds to oxygen that has diffused into the pulmonary circulation
This maintains a high rate of diffusion across the membrane, how?
Haemoglobin takes up the oxygen, so the partial pressure gradient remains high
High PP gradient means rate of diffusion remains high
Aside from the effect created by haemoglobin binding to oxygen, what else maintains a high partial pressure gradient between the alveoli and blood?
High blood flow
Oxygenated blood is quickly replaced with new de-oxygenated blood
How saturated is haemoglobin at normal venous partial pressures of oxygen?
How is this helpful for reducing hypoxia?
At pO2 of 40 mmHg, Haemoglobin is still 75% saturated
75% reserves of oxygen can be unloaded to tissues if needed - stops hypoxia
Explain why foetal haemoglobin (HbF) differs from HbA?
They have a higher affinity for O2 than HbA
This is necessary so a foetus can extract oxygen from maternal blood at the placenta
What is anaemia?
any condition where the oxygen carrying capacity of the blood is compromised
Aside from the partial pressure of oxygen, other factors affect the affinity of Haemoglobin for oxygen
What would cause a decrease in affinity?
Decrease in pH
Increase in pCO2 (this would cause a decrease in pH)
Increase in temperature
2,3-disphosphoglycerate production
What is the purpose of 2,3-diphosphoglycerate?
2,3-DPG is synthesised by erythrocytes
If inadequate supply of O2 (hypoxia)
By decreasing affinity, stimulates unloading of O2 to hypoxic tissues
Why is Carbon monoxide shit for you
CO binds to haemoglobin to form carboxyhaemoglobin
Higher affinity than O2
Causes a bunch of problems
What is hypoxia?
Inadequate supply of oxygen to tissues
What are the 5 causes of hypoxia?
Hypoxaemic hypoxia
Anaemic hypoxia
Stagnant hypoxia
Histotoxic hypoxia
Metabolic hypoxia
What is hypoxaemic hypoxia?
Reduction in O2 diffusion at lungs either due to decreased PO2atmos or tissue pathology
What is anaemic hypoxia?
Reduction in O2 carrying capacity of blood due to anaemia (red blood cell loss/iron deficiency).
What is stagnant hypoxia?
Heart disease results in inefficient pumping of blood to lungs/around the body
What is histotoxic hypoxia?
poisoning prevents cells utilising oxygen delivered to them e.g. carbon monoxide/cyanide
What is metabollic hypoxia?
when oxygen delivery to the tissues does not meet increased oxygen demand by cells.
How would hypoventilation affect the pH of the blood?
CO2
Hypoventilation = CO2 retention
CO2 forms carbonic acid
Carbonic acid = higher [H+]
Decrease in pH = respiratory acidosis
