Special Circulations

Question 1

Why do the lungs have two blood supplies, and what are they called?

Answer 1

There is both the bronchial and the pulmonary circulation. The bronchial supplies the metabolic needs of the lungs whilst the pulmonary carries deoxygenated blood through the lungs to be reoxygenated.

Question 2

How does the pressure vary between the pulmonary and systemic circulation? And why?

Answer 2

There is much lower resistance in the pulmonary circulation and so it works at much lower pressures.

Question 3

What major feature of the pulmonary system means that it needs to operate at low pressure?

Answer 3

The vessel walls are very thin to allow gas exchange, and so it is necessary that the pressure is low to prevent rupture and haemorrhage through these.

Question 4

What two features of the pulmonary circulation assist with efficient gas exchange?

Answer 4

The capillary walls are very thin, and there is a high density of capillaries in the alveolar wall.

Question 5

What is controlled by pulmonary vascular tone?

Answer 5

This means that blood is redirected to areas where there is high oxygen levels and away from areas of hypoxia to maintain the ventilation perfusion ratio.

Question 6

What leads to pulmonary hypertension?

Answer 6

Chronic Hypoxic vasoconstriction. This can occur at altitude or in conditions such as emphysema.

Question 7

What often occurs with pulmonary hypertension?

Answer 7

Right heart hypertrophy due to the increased afterload.

Question 8

There are relatively small amounts of smooth muscle in the pulmonary circulation, what consequence does this have?

Answer 8

This means that blood vessels are much more affected by gravity and so there is a greater hydrostatic pressure in the base of the lungs.

Question 9

On exercise, what is the effect on the pulmonary circulation?

Answer 9

When exercising there is an increase in cardiac output. This causes an increase in arterial BP and leads to the opening of apical capillaries.

Question 10

What do apical capillaries in the lungs do?

Answer 10

These allow for increased O2 uptake despite capillary transit time being reduced.

Question 11

What is the outward force in tissue fluid formation?

Answer 11

Hydrostatic pressure of the blood within the capillaries.

Question 12

What is the inward force in formation of tissue fluid?

Answer 12

Oncotic pressure of the plasma proteins within the blood.

Question 13

What can cause an increase in hydrostatic pressure?

Answer 13

Increased venous pressure.

Question 14

Explain the link between peripheral oedema and heart failure.

Answer 14

When there is increased hydrostatic pressure, this leads to oedema occurring. Increased hydrostatic pressure is caused by increased venous pressure which can be due to heart failure.

Question 15

Name two factors which prevent lung tissue fluid formation.

Answer 15

In the lungs, there is a lower hydrostatic pressure due to the lower blood pressure. Tissue fluid in the lungs also has a higher oncotic pressure due to less fluid being pushed out due to hydrostatic pressure.

Question 16

What can cause pulmonary oedema?

Answer 16

Pulmonary oedema can be caused by increased capillary pressure due to mitral valve stenosis or left ventricular failure.

Question 17

Why do patients with left sided heart failure often struggle to sleep lying down?

Answer 17

The increased preload means that there is pulmonary oedema formation, and this is at the base of the lungs when upright but when lying down affects a greater area of the lungs.

Question 18

How does the brain secure 02 supply?

Answer 18

It has a high capillary density, there is a high flow rate through these capillaries and it has a high extraction rate for oxygen.

Question 19

What happens if blood flow to the brain is lost?

Answer 19

Neurones are very sensitive to hypoxia, and so this leads to a loss of consciousness.

Question 20

How long is it after blood supply loss before neuronal death begins?

Answer 20

4 minutes.

Question 21

How does the brain control the blood supply to it?

Answer 21

In the brain stem are the circulation controllers so this can prioritise its own needs. There is also myogenic autoregulation and metabolic factors which control blood flow.

Question 22

What is myogenic autoregulation?

Answer 22

This is where cerebral blood vessels can respond to changes in transmural pressure and either vasoconstriction or vasodilatory in order to maintain blood pressure.

Question 23

To what metabolite, are cerebral vessel particularly sensitive to?

Answer 23

CO2. High levels cause vasodilation and low levels cause vasoconstriction.

Question 24

Explain why syncope can result from hyperventilation.

Answer 24

When hyperventilating, CO2 levels in the blood can fall and this leads to vasoconstriction of cerebral blood vessels which leads to inadequate perfusion.

Question 25

What is the clinical term for low CO2 levels?

Answer 25

Hypocapnia

Question 26

Name a powerful vasodilator of cerebral arteries

Answer 26

Adenosine and Co2.

Question 27

What is cushings reflex?

Answer 27

This is where if there is impaired blood supply to the brain stem then a sympathetic nervous response occurs and there is increased blood pressure to maintain blood flow.

Question 28

What molecules can diffuse across the blood brain barrier?

Answer 28

CO2 and O2

Question 29

What is the consequence of an increase in intracranial pressure?

Answer 29

This can impair blood flow, leading to haemorrhage or cerebral tumour because the skull is rigid and cannot expand.

Question 30

What is the role of the coronary circulation?

Answer 30

This supplies the heart with oxygen and must be maintained constantly. It also needs to be able to increase when work rate increases such as during exercise.

Question 31

What is produced by the coronary epithelium which maintains a high basal flow rate?

Answer 31

NO

Question 32

What can be said about the difference in perfusion between skeletal and coronary muscle capillaries?

Answer 32

Skeletal muscle capillaries are not all perfused at rest, whilst coronary capillaries are all perfused at rest.

Question 33

How do the coronary capillaries react to increased myocardial demand?

Answer 33

There is a small increase in the amount of O2 extracted but the main change is vasodilation by adenosine, K+ and a fall in pH.

Question 34

What does it mean that coronary arteries are functional end arteries?

Answer 34

There are very few anastomoses.

Question 35

Why does angina only occur on exercise?

Answer 35

When exercising, the heart is under higher demand. This means that obstruction of the blood vessel is more likely to lead to ischaemia which is felt as pain - angina.

Question 36

What is recruitment in skeletal muscle?

Answer 36

This links to the fact that only half of the capillaries are perfused at rest. Recruitment of more capillaries leads to increased blood flow and reduced diffusion distance.

Question 37

What is a major cause of increased blood supply to muscle on exercise?

Answer 37

Increased metabolites such as adenosine and K+ which lead to vasodilation. Also activation of the sympathetic nervous system.

Question 38

What is the purpose of the cutaneous circulation?

Answer 38

This can maintain body temperature, and also has a role in maintenance of BP.

Question 39

What are arteriovenous anastomoses? Which nervous system controls them?

Answer 39

These are found in apical skin and decrease heat loss as less blood passes closer to the surface. These are controlled by sympathetic fibres.

Question 40

When do arteriovenous anastomoses open?

Answer 40

These open when there is a decrease in core body temperature, thus reducing blood flow to the skin.

Question 41

How does heat dissipation occur in non-apical skin?

Answer 41

The sympathetic fibres activate sweat glands and these also release bradykinin which causes vasodilation.