Ventilation - perfusion and gas transport (physiology) Flashcards
Define ventilation
The amount of air getting into the lungs (alveolar ventilation)
Define perfusion
The blood flow through the pulmonary circulation
What is the relationship between ventilation and perfusion?
Ideal relationship is where ventilation = perfusion
(all air getting into alveoli is passed on to circulation)
How does ventilation vary throughout the lungs?
Greatest at base and least at apex
(ventilation»_space;> perfusion at apex due to lower blood pressure)
How does perfusion vary throughout the lung?
Greatest at base and least at apex
(perfusion»_space;> ventilation at base due to gravity and high blood pressure)
Why is ventilation»_space;> perfusion at apex of lung?
Arterial pressure is less than alveoli pressure which then compresses the arterioles.
Why is perfusion»_space;> ventilation at base of lung?
Arterial pressure exceeds alveolar pressure so alveolar are compressed.
What happens when blood flow is greater than ventilation?
Poor ventilation leads to a build up of PCO2 and a fall in PO2 in alveoli.
Blood flow continues but there is little gas exchange.
A shunt occurs = where there is dilution of oxygenated blood from better ventilated areas. Blood moves from left to right side of the heart without being oxygenated.
Results in pulmonary vasoconstriction (to redirect blood flow to better ventilated areas) and bronchial dilation (to increase ventilation)
What happens when ventilation is greater than blood flow?
Results in alveolar dead space. Where the alveolar are well ventilated but due to poor perfusion little gas exchange occurs.
Alveolar PO2 increases and PCO2 falls
Results in pulmonary vasodilation (to increase blood flow ) and bronchial constriction (to reduce ventilation.)
Define alveolar dead space
Air just sat in alveoli due to poor perfusion (little gas exchange occurring)
Define anatomical dead space
Air sat in the respiratory tract unable to participate in gas exchange as wall of region are too thick.
Define physiological dead space
Alveolar DS + anatomical DS
What happens to O2 movement between alveoli and blood?
(in terms of partial pressure)
O2 moves down its partial pressure gradient.
Diffusion of O2 stops when PO2 alveolar = PO2 plasma.
What happens to CO2 movement between alveoli and blood?
(in terms of partial pressure)
CO2 moves down its partial pressure gradient.
(out of tissue into blood, out of blood into lungs)
What is the role of haemoglobin in the transport of O2 in the blood?
O2 diffuses into the blood and binds to Hb which transports it round the body to tissues
There are 4 binding sites per HB
Hb is cooperative
- When one O2 binds in increases affinity for other O2 to bind
What is the saturation of Hb in oxygenated and deoxygenated blood?
Oxygenated blood = 98%
Deoxygenated blood = 75%
(Therefore 25% O2 is used in tissues)
How quickly does saturation of Hb occur?
Within 0.25 seconds
Occurs quickly as a safety mechanism
What is the relationship between O2 and Hb and when is it reversable?
Hb + O2 <> Hb)2
(reversable when PO2 falls)
Define cooperativity
Once one O2 binds to Hb the affinity of Hb for O2 increases and so more O2 binds (same process for O2 unloading)
Describe the oxygen-haemoglobin dissociation curve
As partial pressure increases percentage of O2 saturation of haemoglobin increases.
Rapid increase from 0 to 40 PO2 then tails of at about 60 PO2 (90% saturation)
Between 60 PO2 and 100 PO2 (alveolar) saturation levels rise to 98%
What does this mean for changes in PO2/O2?
You can have a big decrease in PO2 without having a big affect on O2 saturation of Hb
There is little change in O2 saturation until PO2 falls below 60 mmHg
A decrease of PO2 below 60 mmHg results in a large decrease in O2 saturation.
What factors affect the oxygen-haemoglobin dissociation curve? (4)
pH
PCO2
Temperature
DPG
How does pH affect oxygen-haemoglobin dissociation curve?
An increase in pH
- Increases the affinity of Hb for O2.
- Reduces O2 delivery to peripheral tissues as Hb hangs onto it
A decrease in pH
- Bohre effect
- aids O2 unloading in peripheral tissues by reducing affinity of Hb for O2
How does temperature affect oxygen-haemoglobin dissociation curve?
Increase in temperature
- Bohre effect
- Aids O2 unloading in peripheral tissues by reducing affinity of Hb for O2
Decrease in temperature
- Increase Hb affinity for O2
- Reduces O2 delivery to peripheral tissues as Hb onto it.
How does PCO2 affect oxygen-haemoglobin dissociation curve?
Increase in PCO2
- Bohre effect
- Aids O2 unloading in peripheral tissues by reducing affinity of Hb for O2
Decrease in PCO2
- Increase Hb affinity for O2
- Reduces O2 delivery to peripheral tissues as Hb onto it.
How does DPG affect oxygen-haemoglobin dissociation curve?
Reduces affinity of Hb for oxygen
Increases O2 delivery to periphery tissues
Regions of hypoxia produce more DPG
What is anaemia?
When oxygen carrying capacity of the blood is compromised
What causes anaemia?
Iron deficiency
Loss of RBCs
Vit B deficiency (treated with B12)
How does anaemia show PaO2 determines but is independent of total blood O2 content?
PO2 is NORMAL despite the low O2 content in anaemic patients
The saturation of Hb is normal but don’t have as much Hb so hence the low O2 content.
In what forms is CO2 carried in the blood? (and in what proportions)
7% - remains dissolved in plasma
23% - carbamino compounds (CO2 combines with deoxyhaemoglobin in erythrocytes).
70% - bicarbonate ions and H+ ions (CO2 and water combine in erythrocytes to form carbonic acid which then dissociates)
What is the action of carbonic anhydrase in CO2 transport?
Aids in the conversion of carbon dioxide to carbonic acid and bicarbonate ions.
(catalyses the conversion of CO2 into carbonic acid which then hydrolyses into H+ ions and bicarbonate ions)
What factors favour CO2 unloading to the alveoli at the lungs?
Increased acidity
What is the difference between partial pressure and gas content?
Partial pressure is the force driving gas into/out of solution.
Gases do not travel in gaseous phase in blood, if so bubbles = fatal air embolism
How is foetal Hb different to HbA and why is this helpful?
It has a higher affinity for O2 than HbA
Helpful so that foetus can get O2 from the mother.
How is myoglobin Hb different to HbA and why is this helpful?
It has a higher affinity for O2 than HbA
Allows myoglobin to extract O2 from HbA and so muscles can get more O2 out of the blood.
What are the 5 types of hypoxia?
Hypoxaemia hypoxia
Anaemic hypoxia
Stagnant hypoxia
Histotoxic hypoxia
Metabolic Hypoxia
What happens in hypoxaemia hypoxia?
Reduction in O2 diffusion at lungs either due to decreased PO2 (atmos) or tissue pathology
What happens in anaemic hypoxia?
Reduction in O2 carrying capacity of blood due to anaemia (RBC loss, iron/vit B deficiency)
What happens in stagnant hypoxia?
Heart disease results in inefficient pumping of blood to lungs and around the body
What happens in histotoxic hypoxia?
Poisoning prevents cells utilising O2 delivered to them e.g. carbon monoxide/cyanide poisoning.
What happens in metabolic hypoxia?
Oxygen delivery to the tissues does not meet the increased O2 demand by cells.
Occurs during exercise, can be acute and short and is reversable
If a pathological problem then it is chronic
