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Medicine 2 - 06 Haemodynamics > 3 Control of Blood vessels: Blood flow regulation > Flashcards

3 Control of Blood vessels: Blood flow regulation Flashcards

1
Q

Describe the regulation of blood flow to the Brain

A

Autoregulation - receive constant blood supply

  • receives ~14% of Cardiac Output (at rest)
  • around 50ml/100g/min
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2
Q

Describe the neural control of blood flow to the brain (cerebral)

A

Relatively minor neural control (vasoconstriction)

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3
Q

Describe the hormonal control of blood flow to the brain (cerebral)

A

Minor importance

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4
Q

Describe the local control of blood flow to the brain (cerebral)

A

There is good autoregulation of flow across a wide pressure range (that resets in hypertension) 60-160mmHg, abolished by hypercapnia

  • important metabolic control during metal activity (regional)
  • H+, K+, adenosine, hypercapnia (Too much CO2), hypoxia ALL cause vasodilation
  • Endothelin may be an important (potent) vasoconstrictor in pathological states e.g. subarachnoid haemorrhage
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5
Q

Describe the mechanical control of blood flow to the brain (cerebral)

A

As the brain is constrained in the rigid cranium
- this is influenced by Cerebrospinal Fluid pressure (CSF)

e.g. space-occupying lesions can increase the ICP (intracranial pressure) + reduce Cerebral Blood flow (CBF)

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6
Q

Describe some special features of blood flow control of the brain (cerebral)

A

Medullary ischaemic reflex (Cushing)

  • e.g. tumour-induced reduction in Cerebral Blood flow (CBF) causes medullary ischaemia
  • which stimulates an increase in BP in an attempt to restore CBF
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7
Q

How much blood flow is directed to the heart (coronary)?

A

The heart receives around 4% of Cardiac Output (CO)

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8
Q

Describe the neural control of blood flow to the heart (coronary)

A

Minor direct influence (vasoconstriction), but the secondary effect on blood flow due to changes in cardiac function and hence metabolism

  • Sympathetic stimulation causes a B-mediated increase in Heart Rate
  • and supraventricular Tachycardia, which increases O2 consumption
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9
Q

Describe the local control of blood flow to the heart (coronary)

A

Metabolites have a major influence:

- Hypoxia, hypercapnia, adenosine ALL cause vasodilation

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10
Q

Describe the hormonal control of blood flow to the heart (coronary)

A

Adrenaline - vasodilator

- and also stimulates metabolism

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11
Q

Describe the mechanical control of blood flow to the heart (coronary)

A

A major influence on flow during the cardiac cycle
- peak flow in early diastole
- 0 or -ve flow on the onset of systole
(heart contracts - blood vessels are compressed against myocardium)

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12
Q

Describe some special features of blood flow control of the heart (coronary)

A

There is a parallelism between metabolism and flow

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13
Q

How much blood flow is directed to the skin?

A

The skin receives around 4% of cardiac output

- at rest in a thermoneutral environment

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14
Q

Describe the neural control of blood flow to the skin

A

Arteries can have a relatively weak innervation (a vasoconstriction)

AV anastomoses have a dense innervation (a vasoconstriction)
- increase in core temperature causes AVAs to dilate, increasing skin blood flow and hence heat loss

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15
Q

Describe the local control of blood flow to the skin

A

Arterioles show some degree of myogenic autoregulation

AV anastomoses show no autoregulation and no reactive hyperaemia
- Endothelin may be involved in pathological states (Raynaud’s)

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16
Q

Describe the mechanical control of blood flow to the skin

A

Minimal influence

17
Q

Describe some special features of blood flow control of the skin

A

The primary function is thermoregulation

  • sweat glands have cholinergic innervation (sudomotor)
  • which can cause vasodilation via the release of Bradykinin
18
Q

How much blood flow is directed to Skeletal Muscles?

A

Skeletal muscles receive around 15% of Cardiac output

  • at rest
  • it can vary between 3 and 60ml/100g/min
19
Q

Describe the neural control of blood flow to skeletal muscles

A

At rest
- important a vasoconstriction, some B vasodilation, maybe cholinergic vasodilation
- involved in systemic BP regulation.
> Skeletal muscle - 40% of the mass, hence vasoconstriction has a large influence on total peripheral resistance

Excercise

  • very little neural influence
  • some B vasodilation
20
Q

Describe the hormonal control of blood flow to skeletal muscles

A

Adrenaline at low concentrations

- will Vasodilate (B receptors)

21
Q

Describe the mechanical control of blood flow to skeletal muscles

A

Muscle pumping

22
Q

Describe some special features of blood flow control of skeletal muscles

A

Capacity to increase flow in exercise (20-fold) - Active hyperaemia

A large increase in blood flow post-occlusion - Reactive hyperaemia (increased blood flow)
- Reactive hyperaemia - transient increase in organ blood flow - that occurs following a brief period of ischaemia (arterial occlusion)

23
Q

How much blood flow is directed to the splanchnic circulation (GI tract, pancreas, liver etc)

A

Superior mesenteric: 10% of cardiac output

Hepatic: around 25% of cardiac output

  • 70-75% is via Hepatic portal vein (low [O2], low pressure
  • 25-30% is via the hepatic artery (high [O2], high pressure)
24
Q

Describe the neural control of blood flow to the splanchnic circulation (GI tract, pancreas, liver etc)

A

Intestinal: moderate (a vasocontriction)

Hepatic: important (a vasoconstriction)

The liver stores around 15% of total blood volume
- and hepatic vasoconstriction can expel around 50% of hepatic blood volume into the systemic circulation

25
Q

Describe the local control of blood flow to the splanchnic circulation (GI tract, pancreas, liver etc)

A

Intestinal: poor autoregulation, but importantly influenced by local peptides
- (Ang II - vasoconstrictor)

Hepatic:

  • portal vein - no autoregulation
  • hepatic artery - good autoregulation
26
Q

Describe the hormonal control of blood flow to the splanchnic circulation (GI tract, pancreas, liver etc)

A
  • GI hormones (gastrin, cholecystokinin) vasodilate

Vasopressin + Angiotensin vasoconstrict potently

27
Q

Describe the mechanical control of blood flow to the splanchnic circulation (GI tract, pancreas, liver etc)

A

Minimal influence

28
Q

Describe some special features of blood flow control of the splanchnic circulation (GI tract, pancreas, liver etc)

A

Intestinal circulation exhibits functional hyperaemia following feeding

Intense vasoconstriction can lead to damage + release of toxins
- Vasoconstriction (neurohormonal) beneficial in baroreflex, but can be detrimental in haemorrhage/septic shock

29
Q

How much blood flow is directed to the Kidneys (renal)?

A

The kidneys receive around 25% of the Cardiac Output

30
Q

Describe the neural control of blood flow to the Kidneys (renal)

A

Important a vasoconstriction
Some B vasodilation

Renin secreting cells have sympathetic innervation (B adrenoceptors)

31
Q

Describe the local control of blood flow to the Kidneys (renal)

A

Good autoregulation of flow over a wide pressure range

32
Q

Describe the hormonal control of blood flow to the Kidneys (renal)

A

Noradrenalin, adrenaline, angiotensin
- ALL cause Vasoconstriction

Vasopressin may cause vasodilation
- via prostaglandin/NO release

Dopamine - vasodilation

33
Q

Describe the mechanical control of blood flow to the Kidneys (renal)

A

The renal capsule may restrict flow in pathological states

34
Q

Describe some special features of blood flow control of the Kidneys (renal)

A
  • Excretory function of the kidney depends on well-maintained flow (autoregulation)
  • Vascular connection provides for the capacity to regulate afferent/efferent resistances
35
Q

How much blood flow is directed to the Lungs (pulmonary)?

A

The lungs receive 100% of cardiac output

36
Q

Describe the neural control of blood flow to the Lungs (pulmonary)

A

Relatively minor neural influence

- a vasoconstriction

37
Q

Describe the local control of blood flow to the Lungs (pulmonary)

A

Unlike elsewhere, hypoxia causes vasoconstriction
- which is augmented by hypercapnia
(possibly mediated by endothelin)

NO causes vasodilation
(therapeutic uses)

  • Pulmonary hypertension - possible therapeutic strategies include endothelin receptor antagonism and NO inhalation
38
Q

Describe the mechanical control of blood flow to the Lungs (pulmonary)

A

Flow is affected by changes in:

  • Alveolar pressure
  • Lung volume

An increase in blood flow (cardiac output) is associated with recruitment and distension of microvessels, and a decrease in vascular resistance
- If alveolar pressure > intravascular pressure, blood flow is reduced

Lung inflation

  • reduces resistance in extra/alveolar vessels (traction)
  • and increases resistance in intra-alveolar vessels (compression)

Medicine 2 - 06 Haemodynamics (6 decks)

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