Alveolar perfusion Flashcards
Describe the two systems of blood circulation in the lungs
Bronchial arteries give high pressure low flow circulation that provides systemic arterial blood to the trachea and bronchial tree, which is then drained by the bronchial veins and added to the pulmonary vein returning oxygenated blood to the left side of the heart. Therefore approx 2% deoxygenated blood returns to the heart and gives 2% extra volume to blood in left atrium compared to right ventricular output.
Pulmonary circulation provides a low pressure high flow circulation that supplies venous blood from the body via the right side of the heart and the pulmonary artery to the alveolar. The pulmonary artery and arterioles follow the bronchi branches and provide blood to the pulmonary capillaries for gas exchange. The oxygenated blood then returns to the left atrium via the pulmonary veins (4).
What are the important features and differences of pulmonary circulation compared to systemic circulation?
The pulmonary artery trunk is short and divides quickly into left and right branches
It has a wall thickness less than a 3rd of the aorta
The arteries and arterioles have larger diameters than their systemic counterparts
The thin distensible nature of the vessels gives a large compliance (7ml/mmHg) allowing the pulmonary arteries to accomodate the stroke volume of the right atrium whilst maintaining a low blood pressure.
Describe the pressure distribution in the pulmonary circulation.
Pressure in the pulmonary arteries are driven by right ventricle pressure which at systole is about 25 mmHg. When the pulmonary valve closes the right ventricular pressure falls much quicker than that in the pulmonary arteries as the blood flows slowly to the pulmonary capillaries. Diastolic pulmonary pressure reaches around 8mmHg, whilst right ventricular pressure reaches about 1mmHg giving a mean pulmonary arterial presure of 15mmHg. The pulmonary capillary pressure is about 7mmHg (compared to an average of about 17mmHg in systemic capillaries). Pulmonary veins and the left atrium have pressures of betwen 1 and 5mmHg (averaging about 2mmHg).
How does the control of pressure distribution differ between systemic and pulmonary circulation?
The pressure drop in systemic circulation is notable at the point of arterioles where the smooth muscle in their walls enables the control of regional blood flow and here there is an increased resistance. However in pulmonary vessels the pressure drops gradually and is distributed evenly as pulmonary arterioles lack smooth muscle and pulmonary resistance is determined by blood pressure changes and passive distention.
How is left atrial pressure measured?
Left atrial pressure is estimated from pulmonary wedge pressure in which a catheter is passed from the right atrium into a small pulmonary artery branch stopping the flow of blood through this artery. The other side of this connects directly to the pulmonary veins and left atrium and therefore a pressure reading can be made of these structures. The reading is usually 2-3mmHg greater than left arterial pressure which is between 8-10mmHg, but it will rise and fall as pressures in these structures rise and fall giving an insight into changes particularly in congestive heart failure.
What is the blood volume of the lung and what factors change this?
Blood volume is approximately 450ml or 9% of total blood volume. 70ml is in the pulmonary capilaries and remainder is divided between the arteries and veins.
The distensibility of the lungs allow them to act as a blood reservoir and the volume can alter significantly. When pressure builds in the lungs (e.g. blow our hard - blow a trumpet) as much as 250ml of blood can be displaced into the systemic circulation. Likewise in the event of a major haemorrhage blood can move from pulmonary to systemic circulation.
Cardiac pathology can shift blood from the left side of the heart to the pulmonary circulation. e.g. mitral stenosis or regurge causing increases in pulmonary vascular pressure (but mismatch in volume effects pulmonary circulation more than systemic)
What is the blood flow in the lungs? What changes the pulmonary vascular resistance?
The blood flow in the lung is essentially equal to cardiac output.
The pulmonary vessels act as passive distensible tubes therefore an increase in pressure results in greater distention of the tubes and therefore reduced vascular resistance. This means an increase in cardiac output is accomodated with a small rise in pulmonary pressure as happens in exercise.
However decreased alveolar oxygen levels (below 70% normal) results in constriction of the adjacent blood vessels (the opposite to that seen in systemic circulation) and an increase in pulmonary vascular resistance. This constriction of small arteries and arterioles ensures blood is distributed away from poorly ventilated alveoli.
What is the effect of hydrostatic pressure on pulmonary blood flow?
In the normal lung the lowest point is 30cm below the highest point. This represents a 23mmHg pressure difference, 15mmHg of which is above the heart and 8mmHg is below. This means the blood flow per unit of tissue in the top, middle and bottom of the lung are significantly different.
Describe what is meant by zone 1, 2 and 3 of pulmonary blood flow
Zone 1: No blood flow during all portions of the cardiac cycle as capillary pressure is never higher than alveolar air pressure.
Zone 2: Intermittant blood flow only during the peaks of pulmonary arterial pressure as systolic pressure is greater than alveolar air pressure.
Zone 3: Continuous blood flow because alveioar capillary pressure remains greater than alveolar air pressure during the entire cardiac cycle.
In healthy lung there is normally zone 2 flow in the apices and zone 3 in the rest.
At the apices the blood pressure is 15mmHg less than that at the heart so 25-15 in systole and 8-15 in diastole. It is therefore less than the 1cmHg pressure being exerted by the alveolar air pressure.
When might Zone 1 blood flow happen?
If breathing against positive air pressure so that intra alveolar pressure is greater than 10mmHg.
Alternatively if systolic pulmonary pressure drops following severe blood loss.
What is the effect of lung volume on pulmonary vascular resistance?
Pulmonary vascular resistance is lowest and Functional residual capacity and rises at volumes greater or less than this.
Within the lung there are alveolar blood vessels that get flattened as the alveolar wall stretches such that their pulmonary resistance increases with increased lung volume.
There are also extra alveolar blood vessels that expand as the lung expands and so their pulmonary resistance falls with increased lung volume.
The total resistance is the sum of the alveolar and extra alveolar resistances and both have their lowest resistance at FRC.
How is the increased cardiac output of exercise accomodated by the pulmonary circulation?
In exercise…
the pulmonary vascular pressure rises this means that the apices of the lungs are more consistently perfused and they move from zone 2 intermittant blood flow to zone 3 continuous.
the number of capillaries open increases by as much as 3 fold
the capillaries are distended reducing resistance and increasing flow by as much as two fold
These factors limit the rise in pulmonary arterial pressure therefore conserving the energy of the right side of the heart and also limiting the risk of a raised pulmonary capillary pressure which could lead to pulmonary oedema.
What is the effect of a rise in left atrial pressure in left sided heart failure?
An increase to around 7mmHg (from 1-2mmHg) can be accomodated by distention of the venules and opening up of more capillaries, but above this a back pressure will result in increased pulmonary vascular pressure and a concomitant increase in load on the right side of the heart. Above 30mmHg pulmonary oedema is likely to develop.
Describe the factors that determine the capillary exchange of fluid in the lungs.
Pulmonary capillary hydrostatic pressure is lower than systemic (7mmHg vs 17mmHg)
The interstitial fluid pressure is slightly more negative than in peripheral subcutaneous tissue (creating fluid outward force of 8mmHg)
The interstitial colloid osmotic pressure is higher than peripheral tissue again increasing the outward force to 14mmHg.
The plasma colloid osmotic pressure is 28mmHg represents the total inward force.
This gives totals: outward force 29
inward force 28
mean filtration pressure +1
What factors can cause pulmonary oedema?
What factors cause Pleural effusion?
Left sided heart failure will increase hydrostatic pressure therefore increasing outward forces
Damage to pulmonary capillary membranes e.g. in infection such as pneumonia result in leakage of plasma proteins into the interstitial space and fluid will follow.
Blockage of lymphatics, cardiac failure, reduced plasma colloid pressure allowing excessive transudation of fluid and infection
