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CVS > Session 8: Special Circulations > Flashcards

Session 8: Special Circulations Flashcards

1
Q

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

A

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

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2
Q

What is the normal cardiac output at rest?

A

5 l/min

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3
Q

What is the maximum cardiac output as a non athlete?

A

20-25 l/min

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4
Q

What is the normal pressure of the left atrium?

A

1-10 mmHg

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5
Q

What is the normal pressure of the right atrium?

A

0-8 mmHg

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6
Q

What is the normal pressure of the left ventricle?

A

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

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7
Q

What is the normal pressure of the right ventricle?

A

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

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8
Q

What is the normal pressure of the aorta?

A

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

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9
Q

What is the normal pressure of the pulmonary artery?

A

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

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10
Q

Give some important features of the pulmonary circulation.

A

It has a low pressure and also a low resistance.

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11
Q

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

A

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

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12
Q

How does the pulmonary system keep a low resistance?

A

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

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13
Q

Give some important adaptions to promote efficient gas exchange.

A

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.

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14
Q

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

A

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.

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15
Q

Give an example of a high V/Q ratio.

A

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.

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16
Q

Give an example of a low V/Q ratio.

A

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.

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17
Q

What is the optimal V/Q ratio?

A

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

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18
Q

What is hypoxic pulmonary vasoconstriction and why is it important?

A

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.

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19
Q

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

A

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.

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20
Q

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

A

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.

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21
Q

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

A

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.

22
Q

Give effects of exercise on pulmonary blood flow.

A

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.

23
Q

What is capillary transit time?

A

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.

24
Q

What forces determine fluid formation?

A

Starling forces.

25
Q

What are the starling forces?

A

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.

26
Q

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

A

The venous pressure.

27
Q

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

A

Mitral valve stenosis causing increased hydrostatic pressure and pulmonary oedema can follow.

Also left ventricular failure will cause this increase.

28
Q

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?

A

Larger in the lungs.

Lower in lungs.

29
Q

What prevents pulmonary oedema to form in lungs?

A

A low capillary pressure around 9-12 mmHg

30
Q

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

How do you treat it?

A

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.

31
Q

What are main feature of cerebral circulation?

A

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.

32
Q

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

A

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.

33
Q

Why is the O2 supply vital to the brain?

A

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

34
Q

How is a secure blood supply ensured?

A

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.

35
Q

Explain myogenic autoregulation.

Does it ever fail?

A

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.

36
Q

Explain metabolic regulation in cerebral vessels.

A

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.

37
Q

Why would vasodilation happen in hypercapnia?

A

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.

38
Q

What can happen during panic hyperventilation?

A

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

39
Q

Give metabolites which will lead to vasodilation.

A

Increase in:

PCO2

[K+]

Adenosine

Decrease in:

PO2

40
Q

Explain Cushing’s reflex (A triad)

A

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.

41
Q

Where do the left and right coronary arteries arise from?

A

The right and left aortic sinuses.

42
Q

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

A

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.

43
Q

Label

A
44
Q

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

A

Production of NO by coronary endothelium

45
Q

What makes coronary blood flow increase?

A

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.

46
Q

What features of coronary arteries make MIs common?

A

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.

47
Q

What is a major role of skeletal muscle circulation?

A

Helping to regulate arterial blood pressure.

48
Q

How are resistance vessels innervated in the skeletal muscle circulation?

A

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

49
Q

Give features of the skeletal muscle circulation.

A

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.

50
Q

Give metabolites which will increase flow in metabolic hyperaemia.

A

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.

51
Q

Give features of cutaneous circulation.

A

Special role in temp reg.

Role in maintaining blood pressure

Vasonconstriction in cutaneous circulation to maintain BP

52
Q

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

A

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.

CVS (18 decks)

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