15. Special Circulations

Question 1

What are the 2 circulations to the lungs?

Answer 1

Bronchial circulation - part of systemic, meets metabolic requirements of lungs
Pulmonary circulation - blood supply to alveoli, required for gas exchange

Question 2

What is the pressure in the right atrium?

Answer 2

0-8mmHg

Question 3

What is the pressure in the left atrium?

Answer 3

1-10mmHg

Question 4

What are the systolic and diastolic pressures in the right ventricle?

Answer 4

15-30mmHg

0-8mmHg

Question 5

What are systolic and diastolic pressures in the left ventricle?

Answer 5

100-140mmHg

1-10mmHg

Question 6

What are the systolic and diastolic pressures in the aorta?

Answer 6

100-140mmHg

60-90mmHg

Question 7

What are the systolic and diastolic pressures in the pulmonary artery?

Answer 7

15-30mmHg

4-12mmHg

Question 8

What are the features of the pulmonary circulation?

Answer 8

Low pressure

Low resistance - short, wide vessels, lots of capillaries, arterioles have little smooth muscle

Question 9

What are the adaptations to promote efficient gas exchange?

Answer 9

Very high density of capillaries in alveolar wall
Short diffusion distance
Large surface area and short diffusion distance produce high O2 and CO2 transport capacity

Question 10

What is the ventilation perfusion ratio?

Answer 10

For efficient oxygenation, need to match ventilation of alveoli with perfusion of alveoli
Optical V/Q ratio = 0.8
Maintaining this means diverting blood from alveoli which are not well ventilated

Question 11

What is hypoxia pulmonary vasoconstriction?

Answer 11

Ensures optimal ventilation/perfusion ratio
Most important mechanism regulating pulmonary vascular tone
Alveolar hypoxia results in vasoconstriction of pulmonary vessels
Ensures that perfusion matches ventilation
Poorly ventilated alveoli are less well perfused

Question 12

What is the downside of chronic hypoxic vasoconstriction?

Answer 12

Chronic increase in vascular resistance - chronic pulmonary hypertension
High afterload on right ventricle- can lead to right ventricular heart failure

Question 13

What can cause chronic hypoxia?

Answer 13

Can occur at altitude or as a consequence of lung disease such as emphysema

Question 14

How does gravity affect pulmonary vessels?

Answer 14

Low pressure pulmonary vessels are strongly influenced by gravity
In upright position there is a greater hydrostatic pressure on vessels in lower part of lung
Vessels at apex collapse during diastole
Vessels at level of heart continuously patent
Vessels at base distended

Question 15

What is the effect of exercise on pulmonary blood flow?

Answer 15

Increased cardiac output
Small increase in pulmonary arterial pressure
Opens apical capillaries
Increased O2 uptake by lungs
As blood flow increases, capillary transit time is reduced

Question 16

What helps minimise the formation of lung lymph?

Answer 16

Low capillary pressure, prevents pulmonary oedema

Question 17

When can pulmonary oedema occur?

Answer 17

If capillary pressure increases

If left atrial pressure rises to 20-25mmHg, or in mitral valve stenosis or left ventricular failure

Question 18

How does pulmonary oedema affect the patient?

Answer 18

Impairs gas exchange
Affected by posture due to changes in hydrostatic pressure from gravity
Forms mainly at bases when upright
Forms throughout lung when lying down

Question 19

How does the cerebral circulation meet the high demand for O2?

Answer 19

High capillary density
High basal flow rate
High O2 extraction

Question 20

How is a secure blood supply ensured to the brain?

Answer 20

Structurally - anastomoses between basilar and internal carotid arteries
Functionally - myogenic autoregulation maintains perfusion during hypotension, metabolic factors control blood flow, brain stem regulates other circulations

Question 21

What is myogenic autoregulation?

Answer 21

Blood pressure increases - vasoconstriction
Blood pressure decreases - vasodilation
Serves to maintain cerebral blood flow when BP changes
Fails below 50mmHg

Question 22

What is metabolic regulation?

Answer 22

Cerebral vessels very sensitive to changes in arterial PCO2
Hypercapnia - increased PCO2 gives vasodilation
Hypocapnia - reduced PCO2 gives vasoconstriction

Question 23

What can cause hypocapnia?

Answer 23

Panic hyperventilation

Causes cerebral vasoconstriction leading to dizziness or fainting

Question 24

What can cause vasodilation in the brain?

Answer 24

Increase PCO2
Increased [K+]
Increased adenosine
Decreased PO2

Question 25

What is Cushing’s reflex?

Answer 25

Impaired blood flow to van motor control regions of the brain stem increase sympathetic vasomotor activity
Increases arterial BP
Helps maintain cerebral blood flow

Question 26

Where do the left and right coronary arteries arise from?

Answer 26

Left and right aortic sinuses

Question 27

When does flow to the left coronary artery mainly occur?

Answer 27

Diastole

Question 28

Describe the coronary circulation

Answer 28

High capillary density facilities efficient O2 delivery
Diffusion distance short
Continuous production of NO by coronary endothelium maintains a high basal flow

Question 29

Describe how coronary blood flow changes with myocardial O2 demand

Answer 29

Coronary blood flow increases with myocardial O2 demands
Extra O2 required at high work load is supplied mainly by increased blood flow
Almost linear relationship until very high O2 demand
Small increase in amount of O2 extracted
Vasodilation (adenosine, increase [K+], decreased pH)

Question 30

What can narrowed coronary arteries lead to?

Answer 30

Angina on exercise

Question 31

What are artereovenous anastomoses?

Answer 31

Under neural control - sympathetic vasoconstrictor fibres
Decrease core temperature increases sympathetic tone in AVAs - decreases blood flow to apical skin
Increased core temperature opens AVAs