Session 8: Special Circulations

Question 1

What are the two circulations of the lung? What’s the main purpose of them?

Answer 1

Bronchial circulation which helps the lungs meeting the metabolic requirement. Pulmonary circulation which supplies blood to the alveoli in order for required gas exchange.

Question 2

What is the normal cardiac output at rest?

Answer 2

5 l/min

Question 3

What is the maximum cardiac output as a non athlete?

Answer 3

20-25 l/min

Question 4

What is the normal pressure of the left atrium?

Answer 4

1-10 mmHg

Question 5

What is the normal pressure of the right atrium?

Answer 5

0-8 mmHg

Question 6

What is the normal pressure of the left ventricle?

Answer 6

100-140 mmHg (systolic) 1-10 mmHg (diastolic)

Question 7

What is the normal pressure of the right ventricle?

Answer 7

15-30 mmHg (systolic) 0-8 mmHg (diastolic)

Question 8

What is the normal pressure of the aorta?

Answer 8

100-140 mmHg (systolic) 60-90 mmHg (diastolic)

Question 9

What is the normal pressure of the pulmonary artery?

Answer 9

15-30 mmHg (systolic) 4-12 mmHg (diastolic)

Question 10

Give some important features of the pulmonary circulation.

Answer 10

It has a low pressure and also a low resistance.

Question 11

What’s the normal pressure of the following: Mean arterial pressure Mean capillary pressure Mean venous pressure

Answer 11

MAP: 12-15 mmHg MCP: 9-12 mmHg MVP: 5 mmHg

Question 12

How does the pulmonary system keep a low resistance?

Answer 12

Due to short and wide vessels with a lot of capillaries causing many parallel ‘circuits’. Also the arterioles have relatively little smooth muscle

Question 13

Give some important adaptions to promote efficient gas exchange.

Answer 13

A very high density of capillaries in alveolar wall making sure there is a large capillaries surface area. A short diffusion distance meaning there is a very thin layer of tissue separating gas phase from plasma. Combined the endothelium and epithelium thickness is around 0.3 micrometer. Both of these produce a high O2 and CO2 transport capacity.

Question 14

What is the ventilation/perfusion ratio also called V/Q ratio?

Answer 14

It’s the ratio between ventilation (the air that reaches the alveoli) and perfusion (blood that reaches the alveoli via the capillaries). It shows the efficiency and adequacy of the lungs. For efficient oxygenation the ventilation of alveoli must meet the perfusion of the alveoli.

Question 15

Give an example of a high V/Q ratio.

Answer 15

Where there is a failure in perfusion would give a high V/Q ratio. This is when the blood can’t reach the alveoli so ventilation goes ‘wasted’. This is a common consequence of a pulmonary embolism.

Question 16

Give an example of a low V/Q ratio.

Answer 16

When ventilation isn’t working adequately but perfusion is. This means that there is no problem with blood getting to alveoli but sufficient oxygen is not getting there. This is common in asthma, chronic bronchitis, acute pulmonary oedema and hepatopulmonary syndrome.

Question 17

What is the optimal V/Q ratio?

Answer 17

0.8 This means that you can divert blood from alveoli that are not well ventilated.

Question 18

What is hypoxic pulmonary vasoconstriction and why is it important?

Answer 18

It means that alveoli that are not well ventilated will have their capillaries constricted. This is to not ‘waste’ blood going to less ventilated alveoli so it can go to the well ventilated instead. Alveolar hypoxia results in vasoconstriction. It ensures that perfusion matches ventilation and optimise gas exchange.

Question 19

How can chronic hypoxic vasoconstriction occur and what are its consequences?

Answer 19

It can occur due to altitude or because of lung disease such as emphysema. The consequences are as follows: Chronic increase in vascular resistance leading to chronic pulmonary hypertension. This causes high afterload on right ventricle which can lead to RV hypertrophy and right ventricular heart failure.

Question 20

Why would right ventricle failure and RV hypertrophy happen in the absence of a pulmonary defect?

Answer 20

In case of a mitral stenosis for example causing atrial enlargement and backing up into pulmonary system. This can cause pulmonary oedema and RV hypertrophy.

Question 21

What is the low pressure pulmonary vessels most strongly influenced by? Explain it.

Answer 21

Gravity. In an upright position called orthostasis there is a hydrostatic pressure on vessels in the lower part of the lung.

This means that vessels above the heart will have lower pressure and vessels below the heart will have higher pressure.

Vessels on level with the heart will have the same pressure.

Question 22

Give effects of exercise on pulmonary blood flow.

Answer 22

Increased cardiac output.

A small increase in pulmonary arterial pressure.

It will open the apical capillaries (ones that cant open at rest)

Increased O2 uptake by lungs

Also as the blood flow increases the capillary transit time is reduced.

Question 23

What is capillary transit time?

Answer 23

The amount of time RBCs spend at a particular place in the capillary.

At rest the transit time is around 1s, it can fall to 0.3s without comprising gas exchange.

Question 24

What forces determine fluid formation?

Answer 24

Starling forces.

Question 25

What are the starling forces?

Answer 25

Hydrostatic pressure of blood within the capillary which pushes fluid of the capillary. Hydrostatic pressure is also found in interstitium. Oncotic pressure which is exerted by large molecules such as plasma proteins. Draws fluid into the capillary. Oncotic pressure is also found in the interstitium.

Question 26

What is capillary hydrostatic pressure influenced most by in the systemic circulation. (Rather than starling forces)

Answer 26

The venous pressure.

Question 27

Give an example of why hydrostatic pressure would increase in pulmonary system?

Answer 27

Mitral valve stenosis causing increased hydrostatic pressure and pulmonary oedema can follow. Also left ventricular failure will cause this increase.

Question 28

How does the oncotic pressure of tissue fluid in lungs relate to the periphery? How does the capillary hydrostatic pressure in lungs related to the systemic capillaries?

Answer 28

Larger in the lungs. Lower in lungs.

Question 29

What prevents pulmonary oedema to form in lungs?

Answer 29

A low capillary pressure around 9-12 mmHg

Question 30

Where does pulmonary oedema form standing up, compared to lying down? In what instance will symptoms be more severe? How do you treat it?

Answer 30

The fluid collects at bases when upright. It forms throughout the lungs when lying down. It will be more severe lying down. Treat with diuretics to relieve symptoms.

Question 31

What are main feature of cerebral circulation?

Answer 31

High O2 demand Receives about 15% of CO O2 consumption of grey matter accounts for 20% of total body consumption Must provide a secure O2 supply.

Question 32

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

Answer 32

By a high capillary density with a large surface area and reduced diffusion distance. A high basal flow rate which is 10x the rest of the body. A high O2 extraction 35% above average.

Question 33

Why is the O2 supply vital to the brain?

Answer 33

Neurones are very sensitive to hypoxia and will result in a loss of consciousness only seconds after cerebral ischaemia/hypoxia. At around 4 minutes the neurones will get irreversible damage. E.g. stroke causes neuronal death

Question 34

How is a secure blood supply ensured?

Answer 34

Structurally it is supplied via anastomosess between the basilar and internal carotid arteries. This is called the circle of willis. Functionally: There is myogenic autoregulation maintaining perfusion during hypotension. Metabolic factors which control blood flow Brainstem regulates other circulations as well.

Question 35

Explain myogenic autoregulation. Does it ever fail?

Answer 35

When blood pressure decreases and hypotension ensues vasodilation will occur by cerebral resistance vessels (arterioles in brain). When blood pressure increases vasoconstriction will follow. It fails below 50 mmHg.

Question 36

Explain metabolic regulation in cerebral vessels.

Answer 36

Cerebral vessels are very sensitive to changes in arterial PCO2 In case of hypercapnia (high PCO2) vasodilation will occur. In case of hypocapnia (low PCO2) vasoconstriction will occur.

Question 37

Why would vasodilation happen in hypercapnia?

Answer 37

Because in the metabolically active parts of the brain PCO2 will be high. This causes vasodilation to occur in order to ensure blood will get there.

Question 38

What can happen during panic hyperventilation?

Answer 38

It can cause hypocapnia and cerebral vasoconstriction which will lead to dizziness or fainting.

Question 39

Give metabolites which will lead to vasodilation.

Answer 39

Increase in: PCO2 [K+] Adenosine Decrease in: PO2

Question 40

Explain Cushing's reflex (A triad)

Answer 40

The rigid cranium protects the brain but does not allow for volume expansion. This means that increases in intracranial pressure will lead to impaired cerebral blood flow. Impaired blood flow to vasomotor control regions of the brainstem increase sympathetic vasomotor activity. This increases arterial BP and helps maintain cerebral blood flow.

Question 41

Where do the left and right coronary arteries arise from?

Answer 41

The right and left aortic sinuses.

Question 42

Why might angina occur in atherosclerosis of coronary arteries at exercise but no angina at rest?

Answer 42

Because as you exercise the heart will work harder and need more blood. The coronary arteries and especially the left coronary artery fills most during diastole. This means that as you exercise -\> diastole will decrease -\> less time to fill coronary arteries to adequate demand -\> inadequate blood supply and angina.

Question 43

Label

Answer 43

Question 44

Production of what metabolite ensures a high basal flow in the coronary circulation?

Answer 44

Production of NO by coronary endothelium

Question 45

What makes coronary blood flow increase?

Answer 45

Myocardial O2 demand increase Small inrease in amount of O2 extracted. Vasodilation occur due to metabolic hyperaemia, vasodilators such as: Adenosine, increase in K+ conc. and decrease in pH.

Question 46

What features of coronary arteries make MIs common?

Answer 46

There are few arterio-arterial anastomoses. They are prone to atheromas and narrow coronary arteries can lead to anginas. Sudden obstruction by thrombus causes myocardial infarction.

Question 47

What is a major role of skeletal muscle circulation?

Answer 47

Helping to regulate arterial blood pressure.

Question 48

How are resistance vessels innervated in the skeletal muscle circulation?

Answer 48

A rich innervation by sympathetic vasoconstrictor fibres. Baroreceptor reflex maintains blood pressure.

Question 49

Give features of the skeletal muscle circulation.

Answer 49

Capillary density which depends on the muscle type. (Postural muscles have higher capillary density) Very high vascular tone which permits lots of dilation and flow can increase 20 times in active muscles. At rest only half of capillaries are perfused which allows for increased recruitment. Opening of precapillary sphincters allows more capillaries to be perfused which increased blood flow and reduces diffusion distance.

Question 50

Give metabolites which will increase flow in metabolic hyperaemia.

Answer 50

Increased: [K+] Osmolarity Inorganic phosphates Adenosine [H+] Also adrenaline acts as a vasodilator at arterioles in skeletal muscle via beta2 receptors. Vasoconstriction is via NA on alpha receptors.

Question 51

Give features of cutaneous circulation.

Answer 51

Special role in temp reg. Role in maintaining blood pressure Vasonconstriction in cutaneous circulation to maintain BP

Question 52

What are the special acral skin structures that are responsible for heat dissipation?

Answer 52

Artereovenous anastomoses which regulate heat loss from the apical skin. They are under neural control and are not regulated by local metabolites. A decrease in core temperature increases sympathetic tone in AVAs which will reduce blood flow to apical skin. A reduced vasomotor drive to AVAs allows them to dilate and diverts blood to veins near surface.