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Semester 2 - CVS > 14. Special Circulations > Flashcards

14. Special Circulations Flashcards

1
Q

What is the systemic circulation system? What is the pulmonary circulation system?

A

The systemic circulation moves blood between the heart and the rest of the body. It send oxygenated blood out to cells and returns deoxygenated blood back to the heart.

The pulmonary circulation is used to move blood between the heart and the lungs.
It transports deoxygenated blood to the lungs to absorb oxygen and release carbon dioxide.

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2
Q

What are the 2 circulations of the lungs and what is the difference between these circulations??

A 
• Bronchial circulation
– part of systemic circulation
– meets the metabolic requirements of the lungs - This means that it's used to supply parts of the lungs that wouldn't receive oxygen directly as they're not close to the alveoli.
• Pulmonary circulation
– blood supply to alveoli
– required for gas exchange
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3
Q

Compare the cardiac output in the pulmonary circulation with the cardiac output in the systemic circulation

A

The systemic circulation is demand driven whilst the pulmonary circulation is supply driven.

As a result of this the pulmonary circulation must accept the entire cardiac output, whatever the systemic circulation determines it to be.

So normal cardiac output at rest is around 5L/min but this can increase with exercise. Th pulmonary circulation must be able to accommodate this.

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4
Q

What are the pressure in different parts of the heart?

A

RA: 0-8mmHg
RV: 15-30mmHg / 0 - 8mmHg

Pulmonary artery: 15-30mmHg / 4 -12mmHg

LA: 1-10mmHg
LV: 100-140mmHg / 1-10mmHg

Aorta: 100-140mmHg / 60-90mmHg

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5
Q

Why does diastolic pressure in the ventricles have to be <= pressure in the atria?

A

To allow blood to flow into the ventricles from the atria during diastole

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6
Q

Why is the diastolic pressure in the aorta higher than LV?

A

Has lots of elastic tissue producing elastic recoil - thus the pressure does not drop as much

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7
Q

What are the heamodynamic features of pulmonary circulation?

A

Low pressures and low resistance

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8
Q

How is low resistance maintained in pulmonary circulation?

A

– short, wide vessels
– lots of capillaries (many parallel elements)
– arterioles have relatively little smooth muscle - helps keep lumen open so less resistance to blood flow

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9
Q

What are the mean arterial, capillary and venous pressures of the pulmonary circulation ?

A

– mean arterial pressure ∼ 12-15mmHg
– mean capillary pressure ∼ 9-12mmHg
– mean venous pressure ∼ 5mmHg

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10
Q

How are pulmonary arterioles different to systemic arterioles?

A

Have less smooth muscle tissue, contribute less to resistance than in systemic vasculature

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11
Q

Give the adaptations of the lungs that allows efficient gas exchange

A
  1. They have very high density of capillaries in the alveolar wall resulting in a large capillary surface area.
  2. There’s a short diffusion distance
    There’s a very thin layer of tissue separating gas phase from plasma as well as this the combined endothelium & epithelium thickness is ~ 0.3 μm

The large surface area and short diffusion distance produce high O2 and CO2 transport capacity.

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12
Q

Explain the concept of ventilation-perfusion matching in the pulmonary circulation

A

The ventilation/perfusion ratio is a ratio used to assess the efficiency and adequacy of the following 2 variables:

  1. Ventilation: this is the air that reaches the alveoli
  2. Perfusion: this is the blood that reaches the alveoli via the capillaries

So this is the ratio of air reaching the alveoli per minute to the amount of blood reaching the alveoli per minute.

For efficient oxygenation you need to match ventilation of the alveoli with perfusion of the alveoli.
The optimal V/Q ratio = 0.8

In order to maintain this ratio it’s important to divert blood from alveoli which aren’t being ventilated.

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13
Q

What mechanism used to maintain ventilation-perfusion ratio in alveoli hypoxia (what is this mechanism called)?

A

Hypoxic pulmonary vasoconstriction

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14
Q

What is hypoxic pulmonary vasoconstriction?

A

• Hypoxic pulmonary vasoconstriction is the 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
• Helps to optimise gas exchange
- blood flow to unventilated area is shut down and diverted.

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15
Q

How is the effect of hypoxia dealt with differently in the pulmonary circulation and systemic circulation?

A

In pulmonary - vasoconstriction

In systemic - vasodilation - so more o2 supplied

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16
Q

What are the side effects of chronic hypoxic vasoconstriction?

A
  • chronic increase in vascular resistance - chronic pulmonary hypertension
  • high afterload on right ventricle - can lead to right ventricular heart failure
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17
Q

Where might chronic hypoxia be seen?

A
  • high altitudes

- lung disease (e.g. emphysema)

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18
Q

Describe the influence of gravity on pulmonary vessels

A

In the upright position (orthostasis) there is greater hydrostatic pressure on vessels in the lower part of the lung
Towards the apex: capillaries are collapsed (closed) in diastole due to lower pressure

At the level of the heart: continuously patent, have similar pressure to RV

At the base: higher pressure due to gravity, so vessels are distended(increased hydrostatic pressure)

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19
Q

Describe the effect of exercise on pulmonary blood flow

A

• 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
– at rest transit time ~ 1s
– can fall to ~ 0.3s without compromising gas exchange

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20
Q

What is transit time of RBCs?

A

Time it takes for RBC to pass through a capillary

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21
Q

Describe the forces which are involved in the formation of tissue fluid in the lungs and in the systemic circulation

A

• When it comes to tissue formation starling forces are considered.
• The hydrostatic pressure of blood within the capillary is used to push fluid out of the capillary.
• The oncotic pressure (colloid osmotic pressure), this is the pressure exerted by large molecules such as plasma proteins
This is used to draw fluid into the capillary.

In the lungs the hydrostatic pressure is very close to the oncotic pressure, therefore only a small amount of fluid is formed in the lungs. It can’t be more fluid than the lympathic system is able to drain.

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22
Q

In systemic circulation pressure which pressure influences capillary hydrostatic pressure more?

A

capillary hydrostatic pressure is influenced more by venous pressure in the systemic circulation

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23
Q

What are the differences in the tissue fluid forming pressures in pulmonary capillaries compared to systemic capillaries?

A
  • Oncotic pressure of tissue fluid in lungs > than in periphery
  • Capillary hydrostatic pressure in lung < than systemic capillaries
  • Plasma oncotic pressure is the same
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24
Q

Compare the effect of low and increased capillary pressure on the formation of fluid lymph

A

Low pressure in the capillaries of the lungs minimises the formation of lung lymph, this is because the filtration ≈ reabsorption.

However increased capillary pressure causes more fluid to filter out → oedema, this is because filtration > reabsorption.

The pulmonary capillary pressure is normally low (9 - 12mmHg), this means that only a small amount of fluid leaves the capillaries (lung lymph).
However you can get pulmonary oedema if the capillary pressure increases.

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25
Q

What conditions could cause pulmonary oedema and what is the mechanism?

A
  • mitral valve stenosis
  • left ventricular failure

Lead to increase in left atrial pressure, increasing pressure in pulmonary circulation

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26
Q

What is the effect of pulmonary oedema?

A

Pulmonary oedema is the build up of fluid in the lungs. This impairs gas exchange.

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27
Q

What affects where the oedema forms in the lungs?

A

Posture:

  • Forms mainly at bases when upright
  • Forms throughout lung when lying down
28
Q

Why do patients with pulmonary oedema find it difficult to breathe when lying down but less so when upright?

A

When lying down the oedema forms through out the lung, impairing gas exchange more

When upright, oedema forms at the base so the more apical capillaries still available for gas exchange

29
Q

What drugs are used to treat symptoms of pulmonary oedema?

A

Diuretics

- necessary to treat underlying cause if possible

30
Q

What percentage of CO goes to the brain?

A

15%

31
Q

What percentage of oxygen is consumed by grey matter at rest?

A

20% of total body consumption at rest

32
Q

Describe the features of the cerebral circulation

A

The cerebral circulation refers to the movement of blood through the network of cerebral arteries and veins supplying the brain.
The cerebral arteries are used to carry oxygen and nutrients to the brain whilst the veins are used to carry deoxygenated blood back to the herat.

The brain has a very high demand for oxygen which must be met at all times.

33
Q

Explain how the cerebral circulation meets the high demand for oxygen

A

There are 3 main features of the cerebral circulation that allow it to meet the high demand for oxygen in the brain:
1. A high capillary density
This is due to a large surface area for gas exchange and a reduced diffusion distance (<10μm).
2. high basal flow rate
This is around X10 greater than the average for the whole body.
3. There’s a high O2 extraction ability
This means that the brain can extract oxygen from blood very efficiently.
It’s 35% above average

34
Q

Why does the brain require a secure O2 supply?

A

• Neurones are very sensitive to hypoxia
• Loss of consciousness after a few seconds of cerebral
ischaemia
• Begin to get irreversible damage to neurones in ~ 4 minutes
• Interruption to blood supply e.g. stroke causes neuronal death

35
Q

What is a structural feature to secure blood supply to the brain?

A

Anastomoses between basilar and internal carotid arteries (circle of Willis):
- if one path is blocked, not completely blocked supply to brain

36
Q

What is a functional feature to secure blood supply to the brain?

A
  • myogenic autoregulation maintains perfusion during hypotension
  • metabolic factors control blood flow e.g carbon dioxide levels can also be used to control blood flow, rises in the partial pressure of CO2 causes an increase in blood flow and a fall reduces it.
  • brainstem regulates other circulations (so it can prioritise the brain)
37
Q

Explain what myogenic auto regulation is

A

Myogenic autogenic regulation protects the brain from changes in cerebral perfusion pressure by adjusting the vascular resistance.
It aims to maintain cerebral blood flow when BP changes however it will fail if the blood pressure falls below 50mmHg.

For example if blood pressure increases, there’ll be an increase in the flow of blood to the brain so the vessels will vasoconstrict to reduce blood pressure and vice versa.
- vasoconstriction when BP ↑
- vasodilation when BP ↓
Maintains perfusion during hypotension

38
Q

A rise in which metabolic factors affect blood flow in the brain and how do they affect it?

A 
↑ PCO2
↑ [K+]
↑ Adenosine
(↓ PO2)    
All cause vasodilation (metabolic hyperaemia)
39
Q

How does CO2 levels affect cerebral blood flow?

A

Hypercapnia refers to when there’s too much CO2 in the bloodstream whilst hypocapnia is when there’s too little CO2 in the bloodstream.

The cerebral vessels are very sensitive to changes in arterial PCO2.
When you get an increase in PCO2 it causes vasodilatation to occur and when you get a decrease in PCO2 it causes vasoconstriction

40
Q

What is the effect of panic hyperventilation?

A

Panic hyperventilation can cause hypocapnia and cerebral vasoconstriction leading to dizziness or fainting

41
Q

What is metabolic hyperaemia?

A

Increased metabolic activity of the tissue leads to a local increase in the extracellular concentration of chemicals causing vasodilation leading to increased blood flow to tissue

42
Q

What is Cushing’s Reflex?

A

• Rigid cranium protects the brain
- but does not allow for volume expansion
• Increases in intracranial pressure impair cerebral blood flow
- Eg cerebral tumour or haemorrhage
• Impaired blood flow to vasomotor control regions of the brainstem increase sympathetic vasomotor activity.
- increases arterial BP - due to vasoconstriction
- helps maintain cerebral blood flow
- increasein arterial BP is detected by baroreceptors so there is increase in vagus activity, resulting in slowed heart rate
- It results in Cushing’s triad of increased blood pressure, irregular breathing and bradycardia.

43
Q

Where do the coronary arteries originate from?

A

Right and left aortic sinuses

44
Q

What are the major right coronary artery branches?

A

Right coronary artery

  • right marginal artery
  • posterior descending artery
45
Q

What are the major left coronary artery branches?

A

left coronary artery

  • left anterior descending (gives off diagonal branch)
  • circumflex artery (gives off left marginal artery)
46
Q

How much can the work load of the heart increase by?

A

5x

47
Q

When does flow through the left coronary artery mainly occur?

A

During diastole

  • no flow at all at the start of systole (constrict coronary heart vessels)
  • important in angina

(right coronary artery flow not affected as much by systole)

48
Q

What are the features of coronary circulation?

A
  • High capillary density facilitates efficient O2 delivery
  • Diffusion distance < 9μm
  • Continuous production of NO by coronary endothelium maintains a high basal flow
49
Q

What ensures continuous perfusion in coronary vessels?

A

Continuous production of NO by coronary endothelium maintains a high basal flow

50
Q

Compare cardiac and skeletal muscle fibres and capillaries.

A

cardiac muscle:
• Fibre diameter 18μm
• Capillary density 3000/mm2
• Capillaries continuously perfused

skeletal muscle:
• Fibre diameter 50μm
• Capillary density 400/mm2
• Not all capillaries perfused at rest

51
Q

What effect does an increase on myocardial O2 demand have on coronary blood flow? Why does this happen?

A
  • Extra O2 required at high work load is supplied mainly by increased blood flow
  • Almost linear relationship until very high O2 demand
  • When there’s extra blood flow, Small increase in amount of O2 extracted
  • also, Vasodilation due to metabolic hyperaemia
  • Vasodilators - adenosine, ↑[K+], ↓pH
52
Q

Describe the features of the coronary circulation

A

The coronary circulation is the circulation of blood in the blood vessels that supply the heart muscle (myocardium).

This circulation is based on the idea that the heart cannot stop for a rest, so blood flow through the coronary circulation must meet the metabolic demands of the myocardium.

The oxygen demand of the myocardium is determined by how much metabolic work is done. This depends on the external work done and the efficiency with which metabolic energy is converted to external work.

The external work done by the heart per beat depends upon the stroke volume and the arterial pressure.

The efficiency varies with different patterns of myocardial activity. If the ventricle is pumping stroke volume against a low pressure then efficiency is high. Pumping the same stroke volume against a higher pressure reduces efficiency. Pumping the same cardiac output into a higher arterial pressure therefore requires much more blood flow.

During systole the tension in the walls of the ventricles compresses coronary vessels and greatly reduces blood flow. Coronary blood flow is therefore almost exclusively diastolic especially in the left ventricle.

In order to attain a mean blood flow appropriate to myocardial activity, therefore, the coronary circulation must have a high blood flow in diastole to compensate for reduced blood flow in systole. At rest this problem is minimal.

As heart rate increases diastole shortens much more than systole. Consequently, the peak flow in diastole must increase very rapidly with rising heart rate in order to maintain the necessary average flow.

53
Q

Coronary arteries are said to be functional end arteries. What does this mean and why does this mean that they’re more prone to damage?

A

Functional end arteries: anastomosis exists but is incapable of providing a sufficient supply of blood
End arteries refers to an artery that is the only supply of oxygenated blood to a portion of tissue.
This means that they’re prone to atheroma and the narrowing of the lumen.

54
Q

What can cause angina?

A

• Narrowed coronary arteries leads to angina on exercise (increased O2 demand)
– blood flow mostly during diastole - diastole is reduced as heart rate increases
– Stress and cold can also cause sympathetic coronary
vasoconstriction and angina

55
Q

Describe the consequences of partial or total occlusion of coronary arteries

A

Partial occlusion can result in reduced perfusion of blood to the myocardium via the coronary arteries. This can result in stable/ unstable angina. This is chest pain due to Ischaemia.

Total occlusion by a thrombus can cause a myocardial infarction.

56
Q

Why is skeletal muscle circulation important

A

• Must increase O2 and nutrient delivery and removal of metabolites during exercise.
• Important role in helping to regulate arterial blood pressure
– 40% of adult body mass

57
Q

Discuss skeletal muscle circulation

A

• Capillary density depends on muscle type
– Postural muscles have higher capillary density
• Very high vascular tone
– Permits lots of dilatation
– Flow can increase > 20 times in active muscle
• At rest only ~ ½ of capillaries are perfused at any one time
– allows for increased recruitment
• Opening of precapillary sphincters allows more capillaries to be perfused.
– increases blood flow and reduces diffusion
distance

58
Q

What type of innervation do resistance vessels in skeletal muscles have?

A

Rich sympathetic innervation

59
Q

What maintains blood pressure in skeletal circulation?

A

Baroreceptor reflex

60
Q

What causes increase in skeletal blood flow?

A

• Various agents are thought to act as vasodilators

  • ↑[K+]
  • ↑ osmolarity
  • Inorganic phosphates
  • Adenosine
  • ↑[H+]

• Adrenaline also acts as a vasodilator at arterioles in skeletal muscle

  • Acts through β2 receptors
  • Vasoconstrictor response via NA on α1 receptors
61
Q

What is the key role of cutaneous circulation?

A

Temperature regulation
- main heat dissipating surface

  • also has a role in maintaining BP by vasoconstriction
62
Q

What is the difference between apical and nonapical skin?

A

Apical skin: found in the exposed but poorly insulated areas of the body, e.g. palms, soles, face, and ears; has artereovenous anastamoses: have large SA:Vol ratio

Non apical skin: everywhere else, lack AV anastomoses

63
Q

What are arteriovenous anastomoses?

A

Direct connections between arterioles and venules

64
Q

What controls tone of arteriovenous anastomoses?

A

• AVAs are under neural control
- sympathetic vasoconstrictor fibres

• Not regulated by local metabolites

65
Q

What causes increase/decrease in blood flow in AVAs?

A

• Decreased core temperature increases sympathetic tone in AVAs
- causes vasoconstriction: decreases blood flow to apical skin

• Increased core temperature opens AVAs

66
Q

How does increase in blood flow through AVAs help dissipate heat?

A

Reduced vasomotor drive to AVA’s allows them to dilate - diverts blood to veins near surface

Semester 2 - CVS (20 decks)

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