Properties of Special Circulation Flashcards
Name examples of special circulation systems within the body
- cerebral
- pulmonary
- skeletal muscle
- renal
- gastrointestinal
> all have unique requirements
What 3 characteristics define a special circulation system
- special requirements met by the special circulation
- special structural / functional features of the circulation
- specific problems relating to that circulation
Explain the route of special circulation of the heart
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
What are the special requirements of the heart’s special circulation system?
Needs high basal O₂ supply (20 x that of resting skeletal muscle)
Increase O₂ supply in proportion to demand / cardiac work
Describe the dynamics of skeletal muscle
Fibre diameter - 50 µm
Capillaries - 400 / mm²
Outline the dynamics of Cardiac Muscle
Fibre Diameter - 18 µm
Capillaries - 3000 / mm²
What are the special structural requirements of cardiac circulation?
- high capillary density
- large SA for O₂ transfer
> together these reduce diffusion distance to myocytes
(t ∝d²) so O₂ transport is fast
Why is the diffusion distance so short in cardiac muscles?
Cardiac muscle contains high numbers of fibres and capillaries giving rise to shorter diffusion distances
What are the special functional features of cardiac circulation during normal activity?
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)
During increased demand, how do the special functional features of cardiac circulation differ?
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
Explain why the coronary sinus has greater CO₂ content compared to the rest of circulation?
Coronary sinus (blood returning to right atrium from myocardial tissue) has greater CO₂ content due to high capillary density, SA and small diffusion distances
Why does high CO₂ levels in coronary sinus mean for oxygen loading?
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
What is the consequence of an increase in O₂ demand to blood flow?
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
How does myocardium metabolism contribute to increased blood flow?
Myocardium metabolism generates metabolites to produce vasodilation, increasing blood flow (metabolic hyperaemia)
Give an example of metabolic hyperaemia
Adenosine produced by ATP metabolism and released via cardiac myocytes
- also increases PCO₂ , H⁺, K⁺ levels
Describe the pathologies of Ischaemic Heart Disease
Coronary arteries (functional end-arteries) have decreased perfusion causing major problems systole obstructs coronary blood flow Heart is very susceptible to sudden & slow obstruction
What is sudden obstruction leading to ischaemic heart disease?
Acute thrombosis producing MI
What is slow obstruction ischaemic heart disease?
Atheroma (sub-endothelial lipid plaques), causing chronic narrowing of lumen -> producing angina
What is thrombosis?
Total occlusion of left anterior descending coronary artery
- occlusion leading to the obstruction of blood flow to
anterior left ventricle -> MI
Outline the consequences of ischaemic hart disease
- Ischaemic tissues, acidosis, pain (stimulation of C fibres)
- Impaired contractility
- Sympathetic activation
- Arrhythmias
- Cell death (necrosis)
Explain the effects of metabolic vasodilation during exercise?
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
Explain the effects of Angina on resistance during normal activity
Stenosis in large coronary artery increases resistance
Metabolic hyperaemia occurs at rest, so blood flow meets needs
What are the effects of angina during exercise
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
Outline the stages of contraction & relaxation in a heart with angina during exercise
coronary blood flow is restricted during diastole
- Shortening diastole e.g. high HR
- Increased ventricular end diastolic Pressure
e. g. heart failure (aortic stenosis, ventricles stiffening) - Reduced Diastolic arterial pressure
e. g. hypotension, aortic regurgitation
Explain the significance of diastolic blood pressure
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
Why do skin ulcers occur?
Skin ulcer due to Impaired micro vascular flow
What are the specialised properties of cutaneous circulation?
- lewis triple response to trauma (increased blood flow)
- temperature regulation
Explain how cutaneous circulation contributes to temperature regulation
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)
Describe skin and it’s temperature range
Skin is an organ
It’s temperature can range from 0 - 40 ℃ (briefly) without damage (poikilothermic rather than homeothermic)
What is skin temperature dependent upon?
Skin temperature depends on skin blood flow and ambient temperature
What are the special structural features of cutaneous circulation?
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
What are the special functional features of of cutaneous circulation?
- responsive to ambient & core temperatures
- helps heat loss
How does cutaneous circulation help heat loss?
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
Explain what occurs during cold induced vasoconstriction
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
Explain how paradoxical cold vasodilation helps regulate cutaneous blood flow
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
Why does the skin go red during cold?
Skin turns red due to paradoxical cold vasodilation
Why can cold cause pain?
Prolonged cold can cause pain as cold stimulates Nociceptive C fibres
Describe the relationship of cutaneous perfusion & core temperature
Increased cutaneous perfusion = increased core temp
- e.g. exercise
How does increased circulation cause an increase in core temperature?
Increased core temperature stimulates warmth receptors in the anterior hypothalamus
What does the stimulation of the anterior hypothalamus cause?
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
What other functional specialities cause stimulation of vasoconstriction of skin blood vessels?
- Baroreflex
- RAAS
- ADH
> cause stimulation of skin blood vessel vasoconstriction
What is the advantage of RAAS/ADH/Baroreflex stimulation of skin vessels vasoconstriction?
Blood is directed to more important organs/tissues during loss of BP following sepsis, haemorrhage or acute cardiac failure etc.
What regulates the RAAS/ADH/Baroreflex stimulated vasoconstriction of skin blood vessels?
Mediated by sympathetic vasoconstrictor fibres, vasopressin (ADH), adrenaline, and angiotensin II
- responsible for pale cold skin of shock patients
What is the consequence of warming up the body too quickly following haemorrhage?
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
What is the significance of cutaneous circulation?
Cutaneous circulation affects
- core temperature
- emotional communication
- response to skin injury
How does cutaneous circulation contribute to emotional communication?
e.g. blushing via sympathetic sudomotor nerves
Explain the role of cutaneous circulation in the response to skin injury
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
How is the flare to trauma regulated?
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
What special problems can occur within cutaneous circulation?
- prolonged obstruction of flow by compression
- postural hypotension / oedema due to gravity
Explain what happens during prolonged obstruction of flow by compression
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
Describe how postural oedema can occur
Due to prolonged orthostasis - especially in hot weather
- decreased central venous pressure -> hypotension
- causes feeling faint, rings on fingers get tighter due to
swelling