Why is it important for the pressure in the pulmonary circulation to be low?
Low pulmonary pressure reduces the amount of tissue fluid formation in the lung. This helps to increase compliance of the lung, reduce the work of breathing and aid exchange of respiratory gases across the alveolar epithelium.
How is reabsorption of fluid in the lung acheived?
In the pulmonary circulation, hydrostatic pressure is less than oncotic pressure from the arteriorlar and venular end. Therefore reabsorption occurs across the pulmonary circulation.
Why does left heart failure result in pulmonary oedema?
Pulmonary oedema occurs if hydrostatic pressure in the pulmonary circulation is increased.
Normally, hydrostatic pressure in the pulmonary circulation is lower than oncotic pressure, so there is reabsorption.
In heart failure, the heart is unable to maintain cardiac output despite normal venous pressure. Blood accumulates in the circulation, raising central venous pressure.
Pressure rises in the pulmonary veins Fluid accumulates in the interstita and alvoelar spaces of the lung.
Blood flow to a capillary bed
Movement of air in and out of the lungs
The volume of the respiratory tract not involved in gas exchange.
Occcurs clinically where parts of the lung are poorly perfused or over ventilated.
What determines the rate of gas exchange in the lungs?
Perfusion (blood flow through the alveolar tissue)
What determines the efficiency of gas exhange?
Ventilation and perfusion.
Describe the distribution of perfusion pressure in the lung
Blood flow through different parts of the lungs in uneven.
In the upright position, blood flow at the apex is lowes and highest at the bases. There is a 7-10fold increase in perfusion at the base of the lungs compared to the apex.
There are therefore differences in gaseous exchange down the normal lung. Lowest at the apex, highest at the bases.
The ratio of the volume of gas delivered to the alveoli compared with the volume of blood delivered.
Mean ratio 0.8 (4litres of gas, 5litres of blood)
For efficient gas exchange, the ratio should be the same in all parts of the lung
Effect of gravity on pulmonary circulation
Lower parts perfused better
Effect of gravity on lung tissue
Upper parts ventilated better
Lower parts denser and less ventilated
How would failute of ventilation affect the V/Q ratio?
Failure of ventilation can be caused by blockage of the airways e.g. asthma, COPD, pneumothorax
Would result in a venous admixture. Lack of airflow to a part of the lung means blood from that area returning to the pulmonary vein is still deoxygenated.
Blood less well oxygenated compared to normal
How would reduced blood flow affect the V/Q ratio?
Uneven blood flow can arise from anatomical shunts due to airway obstruction, embolism, destruction of vascular beds or increased pulmonary vascular resistance (heart failure)
If there is an area of lung with no perfusion a dead space results (there is no contribution to gas exchange)
Leads to hypoxia (low pO2) and hypercarbia (high pCO2)
How is pulmonary flow regulated?
1. pO2 influences pulmonary vascular tone.
Low pO2 is caused when ventilation is low. This results in vasoconstriction (reducing blood flow) High pO2 is caused when ventilation is high results in vasodilation (increases flow).
This means that the perfusion of the alveoli matches the ventilation of the lung
2. Rise in cardiac output increases venous return. Pulmonary vessels are distensible and accomodate the change without increasing pulmonary arterial pressure
Hypoxic pulmonary vasoconstriction
Hypoxia is sensed in the alveoli by endothelium
Smooth muscle of the pulmonary arterioles constrict
(Affected by cardiac output, anaesthetics)
Why does coronary blood flow occur in diastole?
Contraction of the myocardium compresses the vessels and constricts blood flow
The coronary arteries originate from the aorta, just above the aortic valve, and fill during diastole when backflow of blood closes the aortic valve.
What is the effect of exercise on coronary blood flow?
During exercise cardiac output increases. Heart rate and stroke work increase. As heart rate rises, diastole shortens, meaning there is less time for coronary blood flow.
The heart is more vulnerable to the effects of arterial occlusion during exercise.
What is the main factor controlling coronary blood flow?
Oxygen consumption/demand from the myocardium.
Increased metabolic activity increases O2 consumption and concentration of metabolic vasodilators e.g. adenosine
Blood flow increases proportionally to oxygen demand (metabolic hyperaemia)
Name 4 things that affect coronary blood flow
Adenosine (produced by metabolic activity)
O2 demand (low pO2, high pCO2)
K+ ions (released during electrical activity)
How could you determine if low pO2 was due to poor perfusion or poor ventilation?
In patients with ineffective ventilation leading to a shunt (venous admixture) mechanical ventilation will restore normal pO2.
If there is a lack of perfusion, the area for gas exchange is reduced because there is increased dead space. Therefore increasing O2 supply will have a minimal effect.
What is the volume of blood in coronary flow?
What is the volume of blood in cerebral circulation?
750ml/min. Remains constant. Cannot be changed by changes of fluid content.
Cerebral circulation shows strong autoregulation. Flow constant at mean systolic pressures of 60-160mmHg
What is the volume of blood in pulmonary flow?
What happens if the blood flow to the brain is reduced?
What local factors affect cerebral blood flow?
pCO2 is the main factor affecting cerebral flow.
If pCO2 rises, vasodilation occurs. This helps to improve blood flow in asphyxia.
In low pCO2 there is vasoconstriction so flow is reduced e.g. in hyperventilation.
Adenosine and K+ also act as vasodilator metabolites. Causes reactive hyperaema so blood flow to different regions of the brain is increased with metabolic activity.
What changes occur in the cerebral circulation when intracranial pressure increases?
Rises in intracranial pressure occurs in pathalogical conditions e.g. tumour, brain haemorrhage, inflammation.
Rise in intracranial pressure causes the cerebral arteries to become compressed. Blood flow to the brain is reduced, causing cerebral ischemia.
Systolic blood pressure increases to overcome intracranial pressure and maintain blood flow to the brain. The rise in pCO2 from metabolic activity of the brain causes vasodilation, which also helps increase flow.
Pulse pressure drops because the raised systolic is detected by the carotid sinus which sends impulses to the medulla oblongata to increase vagal tone.
How is the microcirculation controlled?
Small arterioles constrict and dilate which controls blood flow to capillary beds (responsible for autoregulation and rective hyperaemia)
Precapillary sphincters open and close
Capillaries bypassed by ateriovenous anastomoses (under sympathetic control)
What is the main role of the cutaneous circulation?
To regulate temperature (allow heat loss)
How does the cutaneous circulation respond to changes in temperature?
Changes in core temperature are monitored by receptors in the hypothalamus.
In the cold, arteriovenous anastomoses are closed. Arterioles are constricted and only small amounts of blood can pass through.
When the core temperature rises arterioles dilate and arteriovenous anastamoses open up. This allows a high flow rate in the venous plexi which have a large surface area to allow heat loss across the skin.
Bradykinin, produced by sweat secretion, also acts as a vasodilator
How is blood flow to skeletal muscle controlled?
Sympathetic nervous system
Adenosine and local metabolites.
Result in :
Opening and closing of precapillary sphincters
Change of arteriolar diameter
How is vasodilation produced in skeletal muscle?
Adrenaline acting on ß2-receptors
Reduction in the firing frequency of vasoconstrictor nerve fibres, reducing NorA action on a1-receptors
Local metabolites (adenosine, K+, CO2)
What are the pressures in the systemic and pulmonary circulations?
RV: 15-50mmHg systolic, 0mmHg diastolic
LV: 100-140mmHg systolic, 1-10mmHg diastolic
Aorta: 100-140mmmHg systolic, 60-90mmHg diastolic
Pulmonary artery: 15-30mmHg systolic, 4-12mmHg diastolic
What is normal cerebral perfusion pressure
Mean arterial pressure - intracranial pressure