Pulmonary Circulation

Question 1

What is the bronchial circulation?

Answer 1

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

Question 2

What is the pulmonary circulation?

Answer 2

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

Question 3

Describe pulmonary capillaries.

Answer 3

Very thin walls with low vascular resistance

Much more distensible and compressible than systemic capillaries

Question 4

What is the effect of anastomoses between the bronchial and pulmonary capillary?

Answer 4

Little function in healthy adults

May open in pathological conditions, such as occlusion of either bronchial or pulmonary blood flow

Question 5

What are the vascular pressures in the pulmonary circulation compared to that of they systemic circulation?

Answer 5

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

Question 6

How does gravity affect pressures in pulmonary arteries?

Answer 6

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

Question 7

How does gravity affect perfusion?

Answer 7

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)

Question 8

How does hemorrhage or general anesthesia affect perfusion?

Answer 8

Increased portions of Zone 1 (unperfused) due to low pulmonary systolic pressure

Question 9

How does exercise or high cardiac output affect perfusion?

Answer 9

Increase in Zones 2 and 3

1 is recruited to 2, 2 is recruited to 3

Question 10

What will perfusion look like on a patient that is on a pressure ventilator with PEEP (Positive end-expiratory pressure)?

Answer 10

High amounts of Zone 1

Question 11

How does alveolar hypoxia affect Pulmonary Vascular Resistance (PVR) and what is its significance?

Answer 11

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

Question 12

How does pulmonary vasoconstriction affect those with obstructive or restrictive diseases?

Answer 12

Alveolar PO2 is low in these patients, causing pulmonary vasoconstriction throughout the lung

This causes increases PVR, and increased workload on the right heart.

Question 13

What is cor pulmonale?

Answer 13

Right heart failure do to increased PVR

Question 14

What occurs to PVR during exercise?

Answer 14

Cardiac Output rises, however there is only a modest increase in pulmonary arterial pressure due to a decrease in PVR

Question 15

What two processes account of the decrease in PVR during increased cardiac output?

Answer 15

Recruitment - opening of previously collapsed arteries

Distension - Increase in the radius of the existing vessels to allow blood flow

Question 16

What occurs to alveolar vessel resistance during normal inspiration?

Answer 16

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

Question 17

What occurs extralveolar vessel resistance during normal inspiration?

Answer 17

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

Question 18

What occurs to PVR during mechanical ventilation?

Answer 18

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

Question 19

What is the effect of mild/moderate hypoxia on perfusion?

Answer 19

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

Question 20

What is pulmonary edema?

Answer 20

Accumulation of fluid in the lung, results in impaired gas transfer

Depending on capillary and interstitial hydrostatic and colloid pressure

Question 21

What is cardiogenic/hydrostatic edema?

Answer 21

Caused by increases in capillary pressure, usually occurs in left heart failure

Mitral stenosis or Left ventricular infarction

Question 22

What is permeability edema?

Answer 22

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

Question 23

What is Adult Respiratory Distress Syndrome (ARDS)?

Answer 23

Severe form of lung injury marked by persistent lung inflammation and increased capillary permeability

Causes diffuse alveolar damage and pulmonary edema