15- Pulmonary Circulation

Question 1

“Compare the systemic and pulmonary circulations with respect to blood vessel structure, blood pressure and resistance
”

Answer 1

Systemic circulation is a large, high-pressure, high-resistance, less compliant circuit
•Pulmonary circulation is a short, low-pressure, low-resistance, more compliant circuit

Question 2

Explain the relationship between circuit pressures and cardiac ventricular morphology

Answer 2

Systemic circuit requires much greater pressure – left ventricle is much more muscular
•Pulmonary circuit requires less pressure – right ventricle is much less muscular

Question 3

Explain the differences in perfusion to the bases and apices of the lungs in a standing human
•

Answer 3

Path of least resistance, perfusion decreases from the base to the apex of the lung. 3-zone model.

Question 4

Explain, with reference to the pulmonary circulation, the meaning of the terms vascular recruitment
•

Answer 4

During increased cardiac output, a greater number of capillary beds are perfused
•Also, vessels distend more than systemic arteries to accommodate extra flow

Question 5

Explain the importance, advantages and disadvantages of hypoxic vasoconstriction in the foetus/patients with chronic lung disease

Answer 5

• In foetus, hypoxic pulonary vasoconstriction facilitates blood flow through the cardiac shunts as the ‘path of least resistance’
• Chronic lung disease patients have high pulmonary circuit resistance; this can cause the right ventricle to work extra hard, leading to LVH, pulmonary hypertension and potentially heart failure

Question 6

Explain the term ‘pulmonary embolism’, their sites of production and their potential consequences
•

Answer 6

Pulmonary embolus can consist of a clot, fatty plaque or an air bubble
•Pulmonary embolism is a serious medical event where an intravascular obstruction has occurred in a major pulmonary artery, compromising blood flow to large amounts of respiratory tissue

Question 7

Appreciate the concept of shunting, and the potential deleterious effects of pathological shunts.

Answer 7

•In foetus, shunts prevent high volume of blood entering a very high resistance circuit that could damage lungs, heart and blood vessels

Question 8

Where does the bronchial circulation enter and leave

Answer 8

Bronchial circulation comes out of the THORACIC AORTA and drains into the veins within the pulmonary circulation

Question 8

Why are pulmonary system arteries more compliant

Answer 8

greater lumen: wall thickness ratio and so they are more distensible

Question 8

What does ACE do in the walls of the pulmonary endothelium

Answer 8

Angiotensin I is then converted by ACE to ANGIOTENSIN II

ACE also degrades BRADYKININ (which works antagonistically with angiotensin II)

Question 8

How does the pulmonary system filter the blood

Answer 8

The abundant pulmonary microcirculation acts as a ‘filter’ for harmful emboli that have entered the circulation in systemic capillaries and veins.
Small air bubbles – trapped and eventually diffuse out of the circulation into the alveolar spaces
Fat emboli/thrombi – trapped in the microcirculation and degraded by the vascular endothelium
Cancerous cells – these can also be ‘filtered’ which can result in secondary metastasis (but prevent spread to the rest of the body

Question 8

What can happen if a large pulmonary embolus gets trapped

Answer 8

Sudden death

Question 8

What are 4 types of pulmonary shunt

Answer 8

“Bronchial circulation – this is a shunt only by definition, as mixed venous blood is combined with oxygenated arterial blood.

Foramen ovale – this links the atria. During foetal development a large proportion of blood bypasses the entire pulmonary circulation. At birth, this shunt closes in 90% of individuals. In the remaining 10% it can still allow interatrial blood flow to varying degrees.
Ductus arteriosus – this is a shunt linking the pulmonary artery bifurcation to the proximal descending aorta. As with the foramen ovale, this shunt normally fuses in the first days of life.

Atrial septal defect/ventricular septal defect – involve a defect in the septum separating the left and right heart. Depending on severity they allow some of the deoxygenated blood to ‘bypass’ the lungs, and often require corrective surgery.

Question 9

How is pulmonary resistance reduced during high flow- exercise
And what is the benefit of this

Answer 9

“Greater recruitment of pulmonary capillary beds
Distension of vessels due to high compliance

“Reduced risk of oedema
Reduced stress on the right ventricle
Reduced velocity for effective gas exchange

Question 10

How does hyperoxaemia effect the pulmonary and systemic system differently

Answer 10

“In the systemic circulation, blood vessels dilate in low PO2 conditions to increase distal blood flow (thereby increasing the oxygen delivery)
“In the pulmonary circulation, blood vessels constrict in the presence of low PO2 to prevent perfusion of hypoventilated lung tissue (thereby increasing the amount of blood available for perfusing better ventilated lung tissue)

Question 11

“Bronchitis and emphysema are associated with reduced alveolar ventilation, why can this trigger a global vasoconstriction in the pulmonary circuit

Answer 11

PAO2 controls this type of blood flow in the lungs.
the smooth muscle cells of the proximal arterioles (the ‘resistance’ vessels) have O2-sensitive K+ channels that close in hypoxic conditions. This impairment in K+ efflux steadily increase the membrane potential towards the threshold. Once reached, voltage-gated Ca2+ channels open and allow a massive influx, which triggers contraction.

Question 12

What can frequent pulmonary vasoconstriction lead to e.g. from COPD

Answer 12

pulmonary hypertension – consistent activation of this hypoxic vasoconstriction causes pulmonary vascular smooth muscle cells to hypertrophy and undergo hyperplasia
A greater effort is required from the right ventricle to pump into this higher-pressure circuit, so over time it will become hypertrophic through overuse and eventually fail

Question 13

What is the net force due to plasma and interstitium in the lungs

Answer 13

“1 mmHg force from the vessels to the interstitium. This steady fluid loss is small and is easily drained by the lymphatic system.

Question 14

What are 4 mechanisms of oedema and give examples

Answer 14

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