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CVS > Characteristics of Circulation in Different Organs (flipped) > Flashcards

Characteristics of Circulation in Different Organs (flipped) Flashcards

1
Q

What is the theoretical maximum volume of oxygen that can be extracted from the blood?

What is the actual value?

A
  • Theoretically, 20ml oxygen / 100ml blood.

- In reality, 15ml oxygen / 100ml blood.

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2
Q

What can be said about a tissue which is supplied by a vessel whose a-v O2 difference is high?

A

It has a high demand for oxygen.

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3
Q

What proportion of oxygen does the heart extract from the blood supplying it?

A

2/3 of the maximum volume that could be extracted from the blood, i.e. 10ml oxygen / 100ml blood.

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4
Q

What proportion of oxygen does the skin extract from the blood supplying it?

A

1/10 of the maximum volume that could be extracted from the blood, i.e. 1.5ml oxygen / 100ml blood.

*The skin has the lowest metabolic rate of all circulations.

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5
Q

What proportion of the cardiac output does the coronary circulation receive?

A

5%.

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6
Q

List and explain 2 adaptations of the coronary circulation that allows it to maintain a sufficient supply of oxygen to the myocardium.

A

1 - A high capillary density of 3000-5000 capillaries per square mm, or 1 capillary per myocyte:

This increases surface area and decreases diffusion distance.

2 - The presence of myoglobin in cardiac myocytes:

Myoglobin can only bind to one molecule of oxygen, but its affinity for oxygen is much higher than haemoglobin. Adjacent myoglobin molecules between myocytes can exchange oxygen, facilitating rapid diffusion of oxygen across the heart.

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7
Q

What is the autoregulatory range of coronary flow?

A

50-150mmHg.

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8
Q

Define coronary flow reserve.

A

The difference between the resting level of flow and the maximum level of flow that can be obtained by vasodilation of the coronary vessels.

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9
Q

List 2 mechanisms of control of coronary flow.

Which mechanism dominates?

A

1 - Metabolic / functional hyperaemia (dominates).

2 - Sympathetic control.

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10
Q

List 6 molecules which mediate the functional / metabolic regulation of coronary flow.

A

1 - Adenosine.

2 - K+.

3 - Nitric oxide.

4 - Prostacyclin.

5 - H+ (indirectly via acidosis or alkalosis caused by low O2 or high CO2).

6 - Prostaglandins.

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11
Q

What proportion of a coronary vessel diameter must a stenosis block in order to have a significant effect on flow?

A

60-70%.

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12
Q

How might the molecular mechanisms of functional / metabolic autoregulation of coronary flow be affected by a stenosis?

A

A stenosis can cause endothelial damage through hypoxia, impairing nitric oxide and prostacyclin release.

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13
Q

What causes angina?

A

Cardiac tissue hypoxia.

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14
Q

List the two types of angina.

How do they differ?

A

1 - Stable angina.

2 - Unstable angina.

Stable angina occurs predictably and upon exertion, and doesn’t indicate a high risk of complete occlusion of a coronary vessel.

Unstable angina is chest pain that occurs both at rest and with exertion, and indicates a high risk of complete occlusion of a coronary vessel.

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15
Q

Describe the treatment of both forms of angina.

A
  • Stable angina can be treated with drugs that decrease cardiac work.
  • Unstable angina must be treated with surgery, such as a balloon angioplasty, insertion of a stent or coronary bypass graft surgery.
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16
Q

What proportion of the body’s energy is taken by the brain at rest?

A

20%.

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17
Q

Which neural tissue receives most of the blood supply to the brain?

A

The grey matter.

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18
Q

How long must a loss of consciousness be sustained to cause neuronal damage?

A

4 minutes.

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19
Q

Which arteries comprise the blood flow to the brain?

A
  • 2 internal carotid arteries.
  • 2 vertebral arteries.
  • These anastomose to form the circle of Willis.
  • From the circle of Willis, the distributing arteries arise.
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20
Q

Give an example of an adaptation of the cerebral circulation that allows it to maintain a sufficient supply of oxygen to the brain.

A
  • A high capillary density of 3000-4000 capillaries per square mm.
  • This increases surface area and decreases diffusion distance to less than 10 microns.
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21
Q

How do the vessels that supply the brain differ from those that supply the peripheral circulation?

Why is this necessary?

A
  • In the peripheral circulation, the vessels are fenestrated.
  • In the brain (at the blood-brain barrier), the vessels are continuous and form tight junctions with the tissues that they supply.
  • This is necessary in order to ensure that bulk flow and diffusion of water and ions does not occur in neural tissue. Only lipophilic molecules can pass through the bilayer.
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22
Q

List 5 lipophilic solutes that cross the blood-brain barrier.

A

1 - Oxygen.

2 - Carbon dioxide.

3 - Alcohol.

4 - Nicotine.

5 - Caffeine.

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23
Q

How do glucose and plasma proteins pass through the blood-brain barrier?

A

Via carrier proteins.

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24
Q

How does the concentration of mitochondria in the endothelium of neural tissue differ from that of muscle endothelium?

A

The endothelium of neural tissue has a mitochondrial concentration that is 5-6x greater than that of the muscle endothelium.

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25
Q

How is blood flow to the brain regulated outside of cerebral vasodilation / vasoconstriction?

A

By reducing perfusion to other organs (except to the heart) via peripheral vasoconstriction.

26
Q

What is the autoregulatory range of cerebral flow?

A

60-150mmHg.

27
Q

List 2 risks of cerebral hypotensions below 60mmHg.

A

1 - Confusion.

2 - Syncope.

28
Q

List 3 molecules which mediate the functional / metabolic regulation of cerebral flow.

Which molecule is the primary mediator?

A

1 - Primary mediator: pCO2 (greater sensitivity to pCO2 than pO2).

2 - Nitric oxide.

3 - H+ (indirectly via acidosis or alkalosis caused by low O2 or high CO2).

29
Q

What is the response of cerebral and coronary vessels to hypercapnia and hypocapnia?

A
  • Hypocapnia causes vasoconstriction.

- Hypercapnia causes vasodilation.

30
Q

List the values for high, normal and low pCO2 in the cerebral circulation.

A
  • High = >7kPa.
  • Normal = 5kPa.
  • Low = <2kPa.
31
Q

Why does hyperventilation cause dizziness?

A

Hyperventilation will cause hypocapnia in the cerebral circulation, which leads to vasoconstriction of cerebral vessels.

32
Q

What is the effect of hypoxia on cerebral flow?

Why is this effect often undetectable?

A
  • Hypoxia causes vasodilation.
  • Hypoxia also causes hyperventilation, which eventually causes hypocapnia.
  • The cerebral response to hypocapnia is vasoconstriction, which masks the vasodilation caused by hypoxia as the cerebral circulation is more sensitive to changes in pCO2.
33
Q

What is neuronal activity-evoked functional hyperaemia?

A

The selective distribution of cerebral flow to areas of the brain that are particularly metabolically active.

34
Q

Describe the mechanisms that produce neuronal activity-evoked functional hyperaemia.

A
  • Neuronal activity increases:

1 - Increased interstitial K+.

2 - Adenosine production.

3 - Nitric oxide production.

4 - Metabolite release from astrocytes.

5 - pCO2.

  • These products of neuronal activity act as local vasodilators to increase cerebral flow to active areas of the brain.
35
Q

What is the effect of maximal sympathetic stimulation on the resistance of vessels supplying neural tissue?

A

An increase in resistance by 20-30%.

36
Q

What is the effect of maximal sympathetic stimulation on the resistance of vessels supplying skeletal muscle?

A

An increase in resistance by 500%.

37
Q

Why is it important that baroreceptors evoke small changes in cerebral flow?

A

To ensure that standing up and exercise doesn’t compromise cerebral flow by increasing sympathetic stimulation.

38
Q

What is the effect of sympathetic stimulation on the autoregulatory curve for cerebral circulation?

Why is this necessary?

A
  • Sympathetic stimulation shifts the autoregulatory curve to the right.
  • This is necessary in order to protect the brain from the damaging effects of elevated pressure.
39
Q

What is the greatest threat to normal cerebral flow?

A

Raised intracranial pressure.

40
Q

List 3 volumes that contribute to intracranial pressure.

A

1 - Volume of neural tissue.

2 - Volume of blood.

3 - Volume of cerebrospinal fluid.

41
Q

List 3 events that can increase intracranial pressure.

A

1 - Cerebrovascular haemorrhage.

2 - Cerebral oedema (caused by trauma).

3 - Tumour growth.

42
Q

Why does high intracranial pressure reduce cerebral flow?

A

Because it causes vascular compression.

43
Q

What is the equation for cerebral perfusion pressure?

A

Cerebral perfusion pressure = mean arterial blood pressure - intracranial pressure.

44
Q

What is the average range of intracranial pressure?

A

0-10mmHg.

45
Q

At which intracranial pressure is cerebral blood flow significantly reduced?

A

20mmhg.

46
Q

Define postural hypotension.

A

The natural fall in blood pressure that occurs during standing.

47
Q

List 2 circumstances under which the baroreceptor reflex produces syncope in response to postural hypotension.

A

1 - Diabetic neuropathy.

2 - Ageing.

48
Q

What is a transient ischaemic attack?

A

A temporary reduction in cerebral flow that may last between a few minutes to a few hours.

49
Q

What is a cerebrovascular accident (stroke)?

List 2 causes of stroke.

A

Total interruptions of cerebral flow.

1 - Obstruction due to atherosclerosis or migrated clots.

2 - Haemorrhage.

50
Q

What is the primary purpose of the cutaneous circulation?

A

Thermoregulation.

51
Q

Give an example of an adaptation of the cutaneous circulation.

A

It can operate over a very wide range of blood flow, from 1 - 200ml / min / 100g.

52
Q

How is the skin able to produce such a low flow?

A
  • Arterial blood from the dermis that normally enters the epidermis via capillary loops is able to bypass the capillaries by entering arteriovenous anastomoses, which connect directly with the venous plexuses.
  • This is made possible by the thick muscular walls of the arteriovenous anastomoses, which are richly supplied by noradrenergic sympathetic nerves which act on alpha adrenoceptors.
  • These nerves respond to temperature receptors in the CNS.
53
Q

Do arteriovenous anastomoses show functional hyperaemia?

What about autoregulation?

A

They show neither of these properties.

54
Q

List the responses of the cutaneous circulation to increased temperature.

A

A fall in sympathetic stimulation of alpha adrenoceptors causes:

1 - Cutaneous arteriolar vasodilation, increasing total flow to the skin.

2 - Vasodilation of arteriovenous anastomoses, increasing flow to, and therefore heat loss from, the venous plexuses (where most of the cutaneous blood is stored).

55
Q

What type of innervation do sweat glands receive?

A

Sympathetic cholinergic innervation (this is an exception as it is normally noradrenergic).

56
Q

How does sweat affect the cutaneous circulation?

A

Sweat contains bradykinin, a vasodilator which is able to act in a paracrine fashion on the cutaneous arterioles.

57
Q

Why might an increase in systemic temperature lead to an increase in heart rate?

A

Primary mechanism:

  • Vasodilation of arterioles and arteriovenous anastomoses leads to a decrease in total peripheral resistance.
  • This causes arterial blood pressure to decrease.
  • The baroreceptor reflex causes the heart to increase cardiac output in order to maintain blood pressure.
  • This is done by increasing heart rate.

Secondary mechanism:

  • Temperature has a direct effect on the cells of the SA node, causing an increased heart rate by 10 beats / min / degree celsius.
58
Q

List the acute responses of the cutaneous circulation to decreased temperature.

A

An increase in sympathetic stimulation of alpha adrenoceptors causes:

1 - Cutaneous arteriolar constriction, decreasing total flow to the skin.

2 - Constriction of arteriovenous anastomoses, decreasing flow to, and therefore heat loss from, the venous plexuses (where most of the cutaneous blood is stored).

59
Q

Describe the response of the cutaneous circulation to prolonged exposure to decreased temperature.

Describe the physiological mechanism behind this response.

A
  • The vasoconstriction in the cutaneous circulation changes to a paradoxical cold vasodilation.
  • Due to the paralysis of noradrenergic neurotransmission, and the release of vasodilators such as prostacyclins.
60
Q

What is the advantage of the response of the cutaneous circulation to prolonged exposure to low temperature?

A

The paradoxical vasodilation in response to prolonged exposure to low temperature protects against cold temperature damage to the tissues.

61
Q

Why does blood appear more red at colder temperatures?

A

Because the affinity of blood for oxygen increases as temperature decreases.

62
Q

What mechanism keeps cold temperature away from major vessels and towards the periphery (in hands, feet etc.)?

A

Countercurrent exchange which is set up by the close proximity of arteries and veins.

  • The same mechanism can keep the major arteries at body temperature when peripheral temperature increases.

CVS (18 decks)

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