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SGUL - Physiology 1 > Properties of Special Circulations > Flashcards

Properties of Special Circulations Flashcards

1
Q

Describe coronary circulation

A
  • Two coronary arteries originate from the left side of the heart at the beginning of the aorta
  • Cardiac veins carry blood with a poor level of oxygen, from the myocardium to the right atrium.
  • Most of the blood of the coronary veins returns through the coronary sinus
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2
Q

State the special requirements of coronary circulation

A
  • Needs a high basal supply of O2 – 20x resting skeletal muscle
  • Increase O2 supply in proportion to increased demand/cardiac work
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3
Q

State the special structural features of coronary circulation

A
  • High capillary density and high proportion of capillaries
  • Large surface area for O2 transfer
    Reduced diffusion distance to myocytes - diffusion time is proportional to distance squared so O2 transport is fast
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4
Q

Describe coronary circulation during normal activity

A
  • High blood flow - 10x the flow per weight of rest of body
  • Relatively sparse sympathetic innervation
  • High nitric oxide released leading to vasodilatation
  • High O2 extraction (75%) – average in body is 25%
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5
Q

Describe coronary circulation during increased demand

A
  • Coronary blood flow increases in proportion to demands
  • Production of vasodilators out-compete relatively low sympathetic vasoconstriction
  • Circulating adrenaline dilates coronary vessels due to β2-adrenoceptors
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6
Q

Describe the Bohr shift

A
  • Coronary sinus blood returning to right atrium from myocardial tissue has a high carbon dioxide content due to high capillary density, surface area and small diffusion difference
  • The high CO2 and low pH has shifted the curve to the right
  • Haemoglobin has less affinity for oxygen and more O2 is given up to the myocardial tissues
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7
Q

How does the heart produce higher oxygen supply during increased demand without needing to rely on extraction?

A
  • By increasing blood flow
  • Myocardium metabolism generates metabolites to produce vasodilatation, increase blood flow - metabolic hyparaemia
  • Extraction is already near-maximal during normal activity.
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8
Q

Describe what functional end arteries are and why they are problematic

A
  • Arteries where only a single artery supplies a specific tissue
  • Ischaemic Heart Disease - many coronary arteries are functional end-arteries and therefore decreased perfusion can cause major problems
  • Heart is susceptible to both sudden and slow obstruction
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9
Q

State what can cause a sudden blockage of the arteries

A

Acute thrombosis, produce myocardial infarction

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10
Q

State what can cause a slow blockage of the arteries

A

Atheroma, chronic narrowing of lumen, produces angina

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11
Q

Describe what thrombosis is and where it occurs most often

A

Total occlusion - usually of the left anterior descending coronary artery

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12
Q

What can thrombosis lead to?

A
  • Obstruction of blood flow to the left ventricle - leads to a myocardial infarction
  • Ischaemic tissue (tissue with a lack of perfusion) which causes acidosis and pain (stimulation of c fibres)
  • Impaired contractility due to necrosis leading to sympathetic activation, arrhythmias and cell death (necrosis) if perfusion is lost for a period of time
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13
Q

What is angina?

A

Chest pain during exercise

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14
Q

What occurs in a normal heart during exercise?

A
  • Arterioles dilate
  • Decreases TPR
  • Increases blood flow to meet the increased oxygen demands
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15
Q

What occurs in someone with stable angina?

A
  • Stenosis in large coronary artery - increases resistance.
  • During exercise the arterioles further dilate to reduce resistance and allow more blood flow to meet oxygen demand
  • Resistance is still too high due to stenosis
  • Oxygen demand cannot be met and angina develops
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16
Q

State some mechanical factors that can reduce coronary flow

A
  • Shortening diastole
  • Increased ventricular end-diastolic pressure, eg. heart failure (aortic stenosis)
  • Reduced diastolic arterial pressure, eg. hypotension
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17
Q

State the special properties of cutaneous circulation

A

INVOLVED IN:
- Defence against the environment
- Lewis triple response to trauma and skin injuries (increased blood flow)
- Temperature regulation

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18
Q

State the special structural features of the cutaneous circulation

A
  • Arterio-venous anastomoses (AVAs)
  • Sympathetic vasoconstrictor fibres
  • Sudomotor vasodilator fibres
19
Q

What occurs at the AVAs?

A

Direct connections of arterioles and venules expose blood to regions of high surface area
- Involved in conduction, radiation and evaporation

20
Q

What occurs at the sympathetic vasoconstrictor fibres?

A

Release noradrenaline acting on α1 receptors

21
Q

What occurs at the vasodilator fibres?

A

Acetylcholine acting on endothelium to produce nitric oxide

22
Q

State the special functional features of cutaneous circulation

A
  • Help heat loss - increase ambient temperature causes vaso- and venodilatation
  • Help to conserve heat - decrease ambient temperature causes vaso- and venoconstriction
  • Severe cold causes ‘paradoxical cold vasodilatation’
23
Q

Describe cold induced vasoconstriction

A
  • Conserves heat
  • Sympathetic nerves react to local cold by releasing noradrenaline which binds to α2 receptors on vascular smooth muscle in skin.
    -α2 receptors bind NA at lower temperatures than α1 receptors
  • Decreased rate of heat loss
24
Q

Describe paradoxical cold vasodilation

A
  • Protects against skin damage
  • Caused by paralysis of sympathetic transmission.
  • Long term exposure causes oscillations of contraction/relaxation
25
Q

Describe how cutaneous perfusion changes with increased core temperature (eg. exercise)

A
  • Increased cutaneous perfusion with increased core temperature (eg. exercise)
  • Increased core temperature stimulates temperature receptors in hypothalamus
26
Q

What is the effect of stimulation of temperature receptors?

A
  • Sweating:
    Increased sympathetic activity to sweat glands mediated by acetylcholine
  • Vasodilatation:
    Increase sympathetic sudomotor activity such that acetylcholine acts on endothelium to produce NO which dilates arterioles in extremities
  • Increase rate of heat loss
27
Q

Describe other specialised functions of cutaenous circulations

A
  • RAAS / ADH stimulated vasoconstriction of skin blood vessels - blood is directed to the more important organs / tissues during loss of blood pressure following haemorrhage and acute cardiac failure
  • Mediated by sympathetic vasoconstrictor fibres + adrenaline + vasopressin + angiotensin II - responsible for pale cold skin in shock
  • During haemorrhage warming the body too quickly may reduce cutaneous vasoconstriction and be dangerous
28
Q

Describe the Lewis triple response

A
  • Redness, caused by capillary vasodilation
  • Flare, a redness in the surrounding area due to arteriolar dilation mediated by axon reflex
  • Wheal, exudation of extracellular fluid from capillaries and venules
  • Increased delivery of immune cells & antibodies to site of damage to deal with invading pathogens
29
Q

Describe what arises when there is prolonged obstruction of flow by compression.

A

Severe tissue necrosis
Bed sores

30
Q

How can prolonged obstruction of flow be avoided?

A
  • Shifting position / turning causing reactive hyperaemia (on removal of compression)
  • High skin tolerance to ischemia
31
Q

Describe what arises during postural hypotension/oedema due to gravity

A
  • Decrease in central venous pressure (hypotension) and increased capillary permeability (oedema)
  • Causes person to feel faint
32
Q

Describe the resistance in a normal artery/ arteriole at rest.

A

At rest, resistance is low in a large coronary artery. Then resistance is high in the arterioles

33
Q

Describe the resistance in a normal artery/ arteriole during exercise

A

During exercise, low resistance to flow in arteries.
- Vasodilation of arterioles to reduce resistance, so increased blood flow to meet oxygen demands

34
Q

Describe the resistance in a stenosis artery/ arteriole at rest

A

In artery, stenosis increases resistance.
- Metabolic hyperaemia occurs at rest, so blood flow meets needs.
- Resistance in arterioles is reduced.

35
Q

When does the heart get its blood supply?

A

Diastole

36
Q

What does it mean to say that the skin is poikilothermic?

A

Temperature can vary over a large range without damage

37
Q

RECAP: What do the x and y axes on oxygen dissociation curves represent?

A

X-AXIS: Amount of oxygen dissolved in plasma
Y-AXIS: Percentage saturation of haemoglobin

38
Q

RECAP: What does it mean when the oxygen dissociation curve shifts to the right?

A
  • Reduced affinity for oxygen due to high metabolic demands/pH changes etc.
39
Q

Why is there no coronary perfusion following systole?

A
  • Ventricular pressure > arterial pressure.
  • Aortic pressure is not enough to force blood through the arteries in the contracted
    myocardium
40
Q

Why does coronary perfusion occur following diastole?

A
  • Aortic pressure remains high because of compliance of the aorta
  • Ventricular pressure decreases dramatically, allowing blood to flow around the arteries of the heart
41
Q

How is coronary perfusion influenced by heart failure?

A
  • Incomplete expulsion of the blood during systole, which means end systolic volume, (ESV) will still exert a pressure in the ventricle during diastole.
  • Pressure would oppose cardiac circulation, so over time inefficient systole can lead to
    poor cardiac circulation, which might decrease inefficiency of systole
42
Q

What three factors may cause diastolic pressure to be lower?

A
  • Hypovolemia
  • Haemorrhage
  • Aortic valve incompetence
43
Q

What is the initial response to skin exposure to the cold?

A

Cold-induced vasoconstriction

SGUL - Physiology 1 (68 decks)

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