Special circulation

Question 1

Special circulations

Answer 1

There are many special circulations, including cerebral, pulmonary, skeletal muscle renal gastrointestinal and all have unique requirements

Question 2

Special circulations have 3 characteristics to consider

Answer 2

1) Special requirements met by the circulation
2) Special structural or functional features of the circulation
3) Specific problems relating to that circulation

Question 3

Cardiac veins function

Answer 3

• 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

Question 4

Coronary circulation special requirements

Answer 4

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

Question 5

Coronary special structural features

Answer 5

• High capillary density
• Large surface area for O2 transfer
• Together these reduce diffusion distance to myocytes.
• Diffusion time is proportional to distance squared – so O2 transport is fast
• Cardiac muscle contains high numbers of fibres and capillaries giving rise to shorter diffusion distances

Question 6

Special functional features of coronary circulation during normal activity

Answer 6

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

Question 7

Special functional features of coronary circulation during increased demand

Answer 7

• Coronary blood flow increases in proportion to demands
• Production of vasodilators (adenosine, K+, acidosis) out-compete relatively low sympathetic vasoconstriction
• Circulating adrenaline dilates coronary vessels due to abundance of β2-adrenoceptors

Question 8

Oxygen unloading to myocardium during normal activity

Answer 8

BOHR SHIFT

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

Question 9

Sudden obstruction

Answer 9

Acute thrombosis - produce myocardial infarction

Question 10

Slow obstruction

Answer 10

Atheroma (sub-endothelium lipid plaques) chronic narrowing of lumen - produces angina

Question 11

Thrombosis can lead to

Answer 11

Ischaemic tissue, acidosis, pain (stimulation of C fibres)
Impaired contractility
Sympathetic activation
Arryhthmias
Cell death (necrosis)

Question 12

Coronary blood flow during diastole

Answer 12

1) Shortening diastole, e.g. high heart rate
2) Increased ventricular end – diastolic pressure, e.g. heart failure (aortic stenosis, stiffening of ventricle)
3) Reduced disatolic arterial pressure, e.g. hypotension, aortic regurgitation

Question 13

Special properties of cutaneous circulation

Answer 13

Defence against environment
Lewis triple response to trauma
Temperature regulation

Question 14

Skin temperature depends on

Answer 14

Skin blood flow

Ambient temperature

Question 15

How is skin temperature regulated

Answer 15

Blood flow delivers heat from body core
Radiation
Conduction to skin - convection from skin
Sweating

Question 16

Special structural features of cutaneous circulation

Answer 16

Arterio-venous anastomoses (AVAs)
Convection, conduction, radiation, evaporation
Sympathetic vasoconstrictor fibres
Sudometer vasodilator fibres
Driven by temperature regulation nerves in hypothalamus

Question 17

Arterio-venous anastomoses (AVAs)

Answer 17

Direct connections of arterioles and venules, expose blood to regions of high surface area

Question 18

Sympathetic vasoconstrictor fibres

Answer 18

Release noradrenaline acting on α 1 receptors

Question 19

Sudometer vasodilator fibres

Answer 19

ACh acting on endothelium to produce nitric oxide

Question 20

Special functional features for cutaneous features

Answer 20

• Responsive to ambient core temperatures
• Help heat loss - increase ambient temperature causes vaso- and venodilation
• Help to conserve heat - decrease ambient temperature causes vaso and venoconstriction
• Severe cold causes ‘parodixcal cold vasodilation’
• core temperature receptors in hypothalamus control sympathetic activity to skin and hence skin blood flow

Question 21

Cold induced vasoconstriction

Answer 21

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

Question 22

Paradoxical cold vasodilation

Answer 22

Protects against skin damage
Caused by paralysis of sympathetic transmission
Long-term exposure leads to oscillations of contract/relax

Question 23

Sweating

Answer 23

Increased sympathetic activity to sweat glands meditated by ACh

Question 24

Vasodilation

Answer 24

Increased sympathetic sudomotor activity such that ACh act on endothelium to produce NO which dilates arterioles in extremities

Question 25

Lewis triple response

Answer 25

Local redness - site of trauma
Local swelling - inflammatory oedema
Spreading flare - vasodilation spreading out from site of trauma

Question 26

Other functional specialisations of cutaneous circulations

Answer 26

Baroreflex/RAAS/ADH stimulated vasoconstriction of skin blood vessels

Question 27

Special problems

Answer 27

Prolonged obstruction of flow by compression

Postural hypotension/oedema due to gravity

Question 28

Prolonged obstruction of flow by compression

Answer 28

Severe tissue necrosis

Bed sores - heels, buttocks, weight bearing areas

Question 29

Postural hypotension/oedema due to gravity

Answer 29

Often standing for long period in hot weather will decrease CVP and increase capillary permeability