Pulmonary Circulation Flashcards
What is the bronchial circulation?
Arterial blood that perfuses the bronchi, part of the systemic circulation
About 2% of the CO, considered a shunt
About 2/3 drains into the pulmonary veins
What is the pulmonary circulation?
Circulation that is delivered to the lungs via the right ventricle
Represents the whole cardiac output
Pulmonary blood flow is equal to the total blood flow of the systemic circulation
Describe pulmonary capillaries.
Very thin walls with low vascular resistance
Much more distensible and compressible than systemic capillaries
What is the effect of anastomoses between the bronchial and pulmonary capillary?
Little function in healthy adults
May open in pathological conditions, such as occlusion of either bronchial or pulmonary blood flow
What are the vascular pressures in the pulmonary circulation compared to that of they systemic circulation?
Pulmonary arterial pressure averages about 15mmHg compared to 100mmHg of systemic arterial pressure
This is largely due to the lack of high resistance arterioles that are characteristic of systemic arteries
How does gravity affect pressures in pulmonary arteries?
Gravity creates a pressure gradient because blood has to be pumped against a column of water
The pressure at the top of the lung is about 16cmH2O less than that of the hilum, which is 16cmH2O less than the base
How does gravity affect perfusion?
The capillaries at the top of the lung have a lower pressure than the alveolus, therefore they are under high resistance and generally not perfused
Resistance decreases as you move towards the bottom of the lung and the capillary pressure increases
Perfusion is smallest in the top (Zone 1) and greatest in the lowest part of the lung (Zone 2)
How does hemorrhage or general anesthesia affect perfusion?
Increased portions of Zone 1 (unperfused) due to low pulmonary systolic pressure
How does exercise or high cardiac output affect perfusion?
Increase in Zones 2 and 3
1 is recruited to 2, 2 is recruited to 3
What will perfusion look like on a patient that is on a pressure ventilator with PEEP (Positive end-expiratory pressure)?
High amounts of Zone 1
How does alveolar hypoxia affect Pulmonary Vascular Resistance (PVR) and what is its significance?
Pulmonary vascular smooth muscle contracts in low PO2
(This is the opposite of the effect of low PO2 in systemic circulation)
This allows mixed venous blood to be redistriubted to other, well-aerated areas of the lung
How does pulmonary vasoconstriction affect those with obstructive or restrictive diseases?
Alveolar PO2 is low in these patients, causing pulmonary vasoconstriction throughout the lung
This causes increases PVR, and increased workload on the right heart.
What is cor pulmonale?
Right heart failure do to increased PVR
What occurs to PVR during exercise?
Cardiac Output rises, however there is only a modest increase in pulmonary arterial pressure due to a decrease in PVR
What two processes account of the decrease in PVR during increased cardiac output?
Recruitment - opening of previously collapsed arteries
Distension - Increase in the radius of the existing vessels to allow blood flow
What occurs to alveolar vessel resistance during normal inspiration?
Alveolar volume increases, causing the vessels to stretch, i.e. decrease in diameter and increase in length
According to Poiseuille’s equation, this causes an increase in resistance in these vessels
The opposite (decrease in resistance) occurs at low lung volumes
What occurs extralveolar vessel resistance during normal inspiration?
The larger arteries and veins are exposed to the intrapleural pressure, which becomes more negative, causing these vessels to distend (transmural gradient increases)
As lung volumes increase, extralveolar vessel resistance decreases
Radial traction also contributes to this decrease in resistance
What occurs to PVR during mechanical ventilation?
During mechanical positive-pressure ventilation, alveolar pressure and intrapleural pressure are positive during inspiration
Both alveolar and extra-alveolar are compressed as lung volume increases, therefore increasing resistance to blood flow
What is the effect of mild/moderate hypoxia on perfusion?
Causes a decrease in Zone 1, increasing perfusion
The hypoxia will cause some vasoconstriction, increasing the mean pulmonary pressure and allowing more blood to reach the top of the lung
What is pulmonary edema?
Accumulation of fluid in the lung, results in impaired gas transfer
Depending on capillary and interstitial hydrostatic and colloid pressure
What is cardiogenic/hydrostatic edema?
Caused by increases in capillary pressure, usually occurs in left heart failure
Mitral stenosis or Left ventricular infarction
What is permeability edema?
Edema caused by an increase in the permeability of the vessel wall that separates blood from the tissue
Principle disorder in Adult Respiratory Distress Syndrome (ARDS)
Other causes include bacterial toxins and oxygen toxicity
What is Adult Respiratory Distress Syndrome (ARDS)?
Severe form of lung injury marked by persistent lung inflammation and increased capillary permeability
Causes diffuse alveolar damage and pulmonary edema
