Properties of Special Circulation

Question 1

Name examples of special circulation systems within the body

Answer 1

• cerebral
• pulmonary
• skeletal muscle
• renal
• gastrointestinal

> all have unique requirements

Question 2

What 3 characteristics define a special circulation system

Answer 2

1. special requirements met by the special circulation
2. special structural / functional features of the circulation
3. specific problems relating to that circulation

Question 3

Explain the route of special circulation of the heart

Answer 3

2 coronary arteries originating from the left side of the heart at the root of the aorta, just after it exits the left ventricle provide oxygenated blood

Cardiac veins carry blood with low O₂ levels, from the myocardium to the right atrium

most blood from coronary veins returns via the coronary sinus

Question 4

What are the special requirements of the heart’s special circulation system?

Answer 4

Needs high basal O₂ supply (20 x that of resting skeletal muscle)

Increase O₂ supply in proportion to demand / cardiac work

Question 5

Describe the dynamics of skeletal muscle

Answer 5

Fibre diameter - 50 µm

Capillaries - 400 / mm²

Question 6

Outline the dynamics of Cardiac Muscle

Answer 6

Fibre Diameter - 18 µm

Capillaries - 3000 / mm²

Question 7

What are the special structural requirements of cardiac circulation?

Answer 7

• high capillary density
• large SA for O₂ transfer

> together these reduce diffusion distance to myocytes
(t ∝d²) so O₂ transport is fast

Question 8

Why is the diffusion distance so short in cardiac muscles?

Answer 8

Cardiac muscle contains high numbers of fibres and capillaries giving rise to shorter diffusion distances

Question 9

What are the special functional features of cardiac circulation during normal activity?

Answer 9

High blood flow (10 x flow per weight of rest of body)
relatively sparse sympathetic innervation
High NO released -> vasodilation
High O₂ extraction (75% compared to 25% in rest of body)

Question 10

During increased demand, how do the special functional features of cardiac circulation differ?

Answer 10

Coronary blood flow increases in proportion to demands
Production of vasodilators (adenosine, K+, acidosis)
out-competes relatively low sympathetic vasoconstriction
Circulating adrenaline dilates coronary vessels due to ꞵ₂ adrenoceptor abundance

Question 11

Explain why the coronary sinus has greater CO₂ content compared to the rest of circulation?

Answer 11

Coronary sinus (blood returning to right atrium from myocardial tissue) has greater CO₂ content due to high capillary density, SA and small diffusion distances

Question 12

Why does high CO₂ levels in coronary sinus mean for oxygen loading?

Answer 12

High CO₂ and low pH shifts the oxyhaemoglobin curve to the right => lower O₂ affinity for Hb, so is unloaded to myocardial tissues more readily

Question 13

What is the consequence of an increase in O₂ demand to blood flow?

Answer 13

Increased O₂ demand = increased blood flow
Extraction of O₂ is near max. during normal activity ∴ to provide more O₂ during increased demand, increase the blood flow

Question 14

How does myocardium metabolism contribute to increased blood flow?

Answer 14

Myocardium metabolism generates metabolites to produce vasodilation, increasing blood flow (metabolic hyperaemia)

Question 15

Give an example of metabolic hyperaemia

Answer 15

Adenosine produced by ATP metabolism and released via cardiac myocytes
- also increases PCO₂ , H⁺, K⁺ levels

Question 16

Describe the pathologies of Ischaemic Heart Disease

Answer 16

Coronary arteries (functional end-arteries) have decreased perfusion causing major problems 
systole obstructs coronary blood flow
Heart is very susceptible to sudden & slow obstruction

Question 17

What is sudden obstruction leading to ischaemic heart disease?

Answer 17

Acute thrombosis producing MI

Question 18

What is slow obstruction ischaemic heart disease?

Answer 18

Atheroma (sub-endothelial lipid plaques), causing chronic narrowing of lumen -> producing angina

Question 19

What is thrombosis?

Answer 19

Total occlusion of left anterior descending coronary artery
- occlusion leading to the obstruction of blood flow to
anterior left ventricle -> MI

Question 20

Outline the consequences of ischaemic hart disease

Answer 20

• Ischaemic tissues, acidosis, pain (stimulation of C fibres)
• Impaired contractility
• Sympathetic activation
• Arrhythmias
• Cell death (necrosis)

Question 21

Explain the effects of metabolic vasodilation during exercise?

Answer 21

In exercise metabolic vasodilation of arterioles reduces total Resistance
In arterioles resistance is in series so all adds together
Increased blood flow to meet increased O₂ demands (vasodilation) -> total R reduced

Question 22

Explain the effects of Angina on resistance during normal activity

Answer 22

Stenosis in large coronary artery increases resistance

Metabolic hyperaemia occurs at rest, so blood flow meets needs

Question 23

What are the effects of angina during exercise

Answer 23

During exercise, arterioles further dilate to reduce resistance, but total resistance is still too high due to dominance of stenosis
> O₂ demand can’t be met, angina develops
> Total R not reduced enough

Question 24

Outline the stages of contraction & relaxation in a heart with angina during exercise

Answer 24

coronary blood flow is restricted during diastole

1. Shortening diastole e.g. high HR
2. Increased ventricular end diastolic Pressure
   e. g. heart failure (aortic stenosis, ventricles stiffening)
3. Reduced Diastolic arterial pressure
   e. g. hypotension, aortic regurgitation

Question 25

Explain the significance of diastolic blood pressure

Answer 25

Diastolic blood pressure drives coronary circulation

If BP becomes really low / incompetent aortic valve, blood flows back into the heart during diastole rather than staying in the aorta and maintaining pressure

Question 26

Why do skin ulcers occur?

Answer 26

Skin ulcer due to Impaired micro vascular flow

Question 27

What are the specialised properties of cutaneous circulation?

Answer 27

• lewis triple response to trauma (increased blood flow)

- temperature regulation

Question 28

Explain how cutaneous circulation contributes to temperature regulation

Answer 28

Blood flow delivers heat from body to core
Radiation (proportional to skin temperature)
Conduction to skin - convection from skin (skin temp)
Sweating (latent heat of evaporation)

Question 29

Describe skin and it’s temperature range

Answer 29

Skin is an organ

It’s temperature can range from 0 - 40 ℃ (briefly) without damage (poikilothermic rather than homeothermic)

Question 30

What is skin temperature dependent upon?

Answer 30

Skin temperature depends on skin blood flow and ambient temperature

Question 31

What are the special structural features of cutaneous circulation?

Answer 31

Arteriovenous anastomoses
- direct connections of arteriole s& venules exposing
blood to regions of high SA
- Convection, conduction, radiation & evaporation

Sympathetic vasoconstrictor fibres
- release NA acting on ⍺₁ receptors

Sudomotor Vasodilator Fibres
- Ach act on endothelium, producing NO
- driven by temperature regulation nerves in the
hypothalamus

Question 32

What are the special functional features of of cutaneous circulation?

Answer 32

• responsive to ambient & core temperatures

- helps heat loss

Question 33

How does cutaneous circulation help heat loss?

Answer 33

Decreased ambient temperature causes veno- and vasoconstriction
Severe cold causes ‘paradoxical cold vasodilation
Core temperature receptors in hypothalamus control sympathetic activity to skin and ∴ skin blood flow

Question 34

Explain what occurs during cold induced vasoconstriction

Answer 34

Vasoconstriction conserves heat
sympathetic nerves react to local cold by releasing NA which binds to ⍺₂ receptors on vascular smooth muscle in skin

⍺₂ bind NA at lower temperatures than ⍺₁ receptors

Question 35

Explain how paradoxical cold vasodilation helps regulate cutaneous blood flow

Answer 35

Protects against skin damage

Caused by paralysis of sympathetic transmission
long term exposure leads to oscillations of relaxation and contraction
- as skin warms up sympathetic transmission returns for
another round of vasoconstriction

Question 36

Why does the skin go red during cold?

Answer 36

Skin turns red due to paradoxical cold vasodilation

Question 37

Why can cold cause pain?

Answer 37

Prolonged cold can cause pain as cold stimulates Nociceptive C fibres

Question 38

Describe the relationship of cutaneous perfusion & core temperature

Answer 38

Increased cutaneous perfusion = increased core temp

- e.g. exercise

Question 39

How does increased circulation cause an increase in core temperature?

Answer 39

Increased core temperature stimulates warmth receptors in the anterior hypothalamus

Question 40

What does the stimulation of the anterior hypothalamus cause?

Answer 40

Sweating:
- increased sympathetic activity to sweat glands
mediated by Ach

Vasodilation
- increased sympathetic sudomotor activity so Ach acts
on endothelium to produce NO to dilate arterioles in
extremities

Question 41

What other functional specialities cause stimulation of vasoconstriction of skin blood vessels?

Answer 41

• Baroreflex
• RAAS
• ADH
  > cause stimulation of skin blood vessel vasoconstriction

Question 42

What is the advantage of RAAS/ADH/Baroreflex stimulation of skin vessels vasoconstriction?

Answer 42

Blood is directed to more important organs/tissues during loss of BP following sepsis, haemorrhage or acute cardiac failure etc.

Question 43

What regulates the RAAS/ADH/Baroreflex stimulated vasoconstriction of skin blood vessels?

Answer 43

Mediated by sympathetic vasoconstrictor fibres, vasopressin (ADH), adrenaline, and angiotensin II
- responsible for pale cold skin of shock patients

Question 44

What is the consequence of warming up the body too quickly following haemorrhage?

Answer 44

During haemorrhage, warming up the body too quickly can reduce cutaneous vasoconstriction and be potentially dangerous as blood flow to the skin is not as vital as other visceral organs

Question 45

What is the significance of cutaneous circulation?

Answer 45

Cutaneous circulation affects

• core temperature
• emotional communication
• response to skin injury

Question 46

How does cutaneous circulation contribute to emotional communication?

Answer 46

e.g. blushing via sympathetic sudomotor nerves

Question 47

Explain the role of cutaneous circulation in the response to skin injury

Answer 47

The Lewis triple response occurs causing
- local redness at site of trauma
- local swelling due to inflammatory oedema (wheal)
- spreading flare due to vasodilation spreading out from
the trauma site

Question 48

How is the flare to trauma regulated?

Answer 48

The C fibre axon reflex mediates the flare to trauma

Increased delivery of immune cells and antibodies to site of damage to deal with invading pathogens

Question 49

What special problems can occur within cutaneous circulation?

Answer 49

• prolonged obstruction of flow by compression

- postural hypotension / oedema due to gravity

Question 50

Explain what happens during prolonged obstruction of flow by compression

Answer 50

Severe tissue necrosis
- bed sores on heels, buttocks, weight bearing areas

can be avoided by shifting positions / turning causing reactive hyperaemia on removal of compression
High skin tolerance to ischaemia

Question 51

Describe how postural oedema can occur

Answer 51

Due to prolonged orthostasis - especially in hot weather
- decreased central venous pressure -> hypotension
- causes feeling faint, rings on fingers get tighter due to
swelling