Pulmonary circulation

Question 1

What is pulmonary circulation?

Answer 1

perfusion of the respiratory airways for gas exchange

- Low pressure circuit 
- CO same 
- Contains 10% of volume of systemic 
- Operates at 15% pressure of systemic 
- Operates at 10% gradient of systemic

Question 2

What are the functions of the pulmonary circulation?

Answer 2

1. Gas exchange
2. Metabolism of vasoactive substances:
   - luminal surface of the pulmonary epithelium expresses some specialized enzymes
   - E.g ACE, degradation of bradykinin
   - Clearance of Serotonin, noradrenaline, prostaglandins and leukotrienes
3. Filter blood from emboli and cancerous cells (may result in secondary metastasis)

Question 3

What occurs during venous thrombosis?

Answer 3

Small embolus

Eliminated in pulmonary circulation

Question 4

What occurs during ruptured fatty plaques?

Answer 4

Large embolus trapped in pulmonary microcirculation

Local perfusion obstructed

Question 5

What are the 2 circulations in the lung?

Answer 5

1. Bronchial:
   - bronchial arteries from the thoracic aorta and provide lung tissue with oxygen and nutrition, and eliminate waste products
   - bronchial veins converge and drain into the pulmonary veins
2. Pulmonary
   - Left ventricle pumps blood to lungs via pulmonary artery
   - Capillary beds converge into bronchial veins and drain into left atrium
   - Low resistance, high capacity circuit

Question 6

What is a shunt?

Answer 6

Circumstances associated with blood bypassing the respiratory exchange surface

Question 7

What are the shunts in the pulmonary system at various point in the life cylce?

Answer 7

1. Bronchial circulation: mixed venous blood is combined with oxygenated arterial blood
2. Foramen ovale: shunt linking the two atria of the heart.
   - During foetal development a large proportion of blood bypasses the entire pulmonary circulation
   - Closes at birth
3. Ductus arteriosus: shunt linking the pulmonary artery bifurcation to the proximal descending aorta
   - Closes at birth
4. Atrial/ventricular septal defect: congenital heart disease involve a defect in the septum separating the left and right heart
   - May require corrective surgery

Question 8

How does the pulmonary circulation maintain low resistance when CO increases?

Answer 8

1. Greater recruitment of capillary beds:
   - at rest, only a fraction of the respiratory exchange surface is receiving a blood supply because low GE requirement
   - When CO increases can recruit more capillary beds towards apex of lung
2. Distension of patent vessels:
   - High compliance so can stretch

Increase cardiac output considerably without compromising pressure

Question 9

What are the benefits to the pulmonary circulation to accommodate a greater volume of blood without any increase in pressure?

Answer 9

• Reduced risk of oedema (minimal fluid leakage)
• Reduced stress on the right ventricle
• No detriment to pulmonary function

Question 10

How does vascular resistance change with chaning lung volume?

Answer 10

Inspiration compresses alveolar vessels
expiration compresses extra alveolar vessels

Intraalveolar vessels exhibit high resistance near total lung capacity
Extraalveolar vessels exhibit high resistance near residual volume

see notes

Question 11

What is the effect of hypoxemia on the pulmonary vessels?

Answer 11

Vasoconstriction to prevent perfusion of hypoventilated lung tissue
Increases amount of blood available for perfusing better ventilated lung tissue
Controlled by PaO2

Question 12

How does hypoxia induced vasoconstriction in the lungs occur?

Answer 12

1. Closure of oxygen sensitive K channels
2. Decreased potassium efflux
3. Increased membrane potential
4. Membrane depolarisation and opening of voltage gated calcium channels
5. Vascular smooth muscle constriction

Question 13

What is the benefit of this mechanisms in foetuses?

Answer 13

During foetal development
○ Blood follows the path of least resistance
○ High-resistance pulmonary circuit means increased flow through shunts
○ First breath increases alveolar PO2 and dilates pulmonary vessels

Question 14

What is the consequence of this mechanisms when entire lung exposed to low PaO2?

Answer 14

1. Altitude
2. COPD
   
   • Reduced alveolar ventilation and air trapping
   • Increased resistance in pulmonary circuit
   • Pulmonary hypertension (Cor pulmonale)
   • Right ventricular hypertrophy
   • Congestive heart failure

Question 15

What are the orthostatic variations in blood flow?

Answer 15

Pulmonary blood flow to the apex (top) of the lung is lower than the base because the pulmonary circulation is a low pressure circuit

Question 16

What determines pulmonary fluid balance?

Answer 16

Pulmonary capillaries are more porous so fluid moves more easily between capillaries, interstitium and alveoli

1. Capillary hydrostatic pressure– pushes water out of the vessel (varies along capillary (13 to 6 mmHg; mean 9 mmHg)
2. Interstitial hydrostatic pressure– pushes water into the vessel (almost 0)
3. Plasma protein oncotic (colloid osmotic) pressure– draws water into the vessel (25 mmHg)
4. Interstitial protein oncotic pressure– draws water into the interstitium (17 kPa)

see notes

Question 17

What happens if lymphatic drainage is insufficient?

Answer 17

initially bepulmonary interstitial oedema, which may develop intopulmonary alveolar oedema.

Question 18

What are causes of oedema?

Answer 18

• Increasing the intravascular hydrostatic pressure
	○ Mitral valve stenosis
	○ Heart failure
• Reducing the oncotic pressure
	○ Hypoproteinaemia
	○ Protein-losing nephropathies
	○ Liver cirrhosis
	○ Protein-losing enteropathies
• Increasing interstitial oncotic pressure
	○ Pulmonary endothelial damage
	○ Infection
• Blocked lymphatic system
	○ Cancer (resulting in lymphoedema)

Question 19

What are the consequences of oedema?

Answer 19

1. Oedematous lungs are much less compliant
   
   • more effort to ventilate
   • can present in dyspnoea
2. Excessive oedema can also cause the walls of the bronchioles to become swollen
   
   • further increases resistance and breathing work
3. Excessive oedema in the interstitial space can increase the diffusion distance and impede gas exchange