Session 8

Question 1

What are the two circulations of the lungs?

Answer 1

Pulmonary circulation - supply driven rather than demand led. Works with a low pressure (arterioles with little smooth muscle) and a low resistance (capillaries in parallel).
Bronchial circulation - part of the systemic circulation. Meets the metabolic requirements of the lungs.

Question 2

State the mean arterial, capillary and venous pressures of the pulmonary and systemic circulations.

Answer 2

Pulmonary: arterial - 12 to 15. Capillary - 9 to 12. Venous - 5.
Systemic: arterial ~90. Capillary - 25 to 30. Venous - 0 to 8.

Question 3

What allows for efficient oxygenation in the lungs?

Answer 3

A matched perfusion (blood flow) and vertilation (air flow) in each part of the lung.
The optimal ventilation to perfusion ratio is 0.8.

Question 4

What is hypoxic pulmonary vasoconstriction?

Answer 4

Alveolar hypoxia results in vasoconstriction of pulmonary vessels. Blood is diverted from alveoli that are not well ventilated to maintain the ventilation to perfusion ratio.

Question 5

What can cause chronic hypoxia and what are the complications?

Answer 5

Altitude of lung disease (e.g. Emphysema).
Leads to chronic increase in vascular resistance (by hypoxic pulmonary vasoconstriction) and chronic pulmonary hypertension that can result in right ventricular heart failure.

Question 6

Describe the changes in the pulmonary circulation during exercise

Answer 6

There is a small increase in pulmonary arterial pressure, which opens apical capillaries (usually collapse during diastole at rest due to gravity) and increases O2 uptake by the lungs. Capillary transit time decreases without compromising gas exchange.

Question 7

What forces determine tissue fluid formation?

Answer 7

Starling forces. Hydrostatic pressure within the capillaries pushes fluid out and colloid osmotic pressure exerted by plasma proteins draws fluid back in.

Question 8

What normally limits the formation of lung lymph and pulmonary oedema compared to the periphery?

Answer 8

Capillary hydrostatic pressure in lungs

Question 9

What conditions can lead to pulmonary oedema?

Answer 9

Mitral valve stenosis and left heart failure. It impairs gas exchange. Diuretics can relieve symptoms.

Question 10

How does cerebral circulation meet the high demand for blood flow?

Answer 10

There is a high capillary density (large SA and small diffusion distance), high basal flow rate and high O2 extraction.

Question 11

How is cerebral blood flow secured?

Answer 11

Structurally
-anastomoses between basilar and internal carotid arteries.

Functionally

• myogenic autoregulation that maintains perfusion during hypotension
• metabolic factors that control blood flow (CO2, K+, adenosine)
• brainstem regulates other circulations

Question 12

Describe the myogenic autoregulation of the cerebral circulation

Answer 12

Cerebral resistance vessels respond to changes in blood pressure to dilate or constrict accordingly. Serves to maintain cerebral blood flow. Fails below 50mmHg.

Question 13

Why does panic ventilation lead to dizziness/fainting?

Answer 13

Leads to hypocapnia in the cerebral vessels which causes vasoconstriction.

Question 14

What is Cushing’s reflex?

Answer 14

Increases in intracranial pressure (haemorrhage or tumour) leads to impaired cerebral blood flow.
Impaired blood flow to the vasomotor control region of the brainstem increases sympathetic activity, hence increasing blood pressure to maintain cerebral blood flow.

Question 15

What forms the tight blood brain barrier?

Answer 15

Cerebral capillaries - only allows lipid soluble molecules through (including O2 & CO2)

Question 16

Describe the features of the cutaneous circulation

Answer 16

Most blood flow is not nutritive and most flows through sympathetically controlled arteriovenous anastomoses rather than capillaries - they are not influenced by metabolites.
Increased core temperature decreases vasomotor drive which opens AVAs to dissipate heat.