Properties of Special Circulations Flashcards
Describe coronary circulation
- 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
State the special requirements of coronary circulation
- Needs a high basal supply of O2 – 20x resting skeletal muscle
- Increase O2 supply in proportion to increased demand/cardiac work
State the special structural features of coronary circulation
- 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
Describe coronary circulation during normal activity
- 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%
Describe coronary circulation during increased demand
- 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
Describe the Bohr shift
- 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
How does the heart produce higher oxygen supply during increased demand without needing to rely on extraction?
- By increasing blood flow
- Myocardium metabolism generates metabolites to produce vasodilatation, increase blood flow - metabolic hyparaemia
- Extraction is already near-maximal during normal activity.
Describe what functional end arteries are and why they are problematic
- 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
State what can cause a sudden blockage of the arteries
Acute thrombosis, produce myocardial infarction
State what can cause a slow blockage of the arteries
Atheroma, chronic narrowing of lumen, produces angina
Describe what thrombosis is and where it occurs most often
Total occlusion - usually of the left anterior descending coronary artery
What can thrombosis lead to?
- 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
What is angina?
Chest pain during exercise
What occurs in a normal heart during exercise?
- Arterioles dilate
- Decreases TPR
- Increases blood flow to meet the increased oxygen demands
What occurs in someone with stable angina?
- 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
State some mechanical factors that can reduce coronary flow
- Shortening diastole
- Increased ventricular end-diastolic pressure, eg. heart failure (aortic stenosis)
- Reduced diastolic arterial pressure, eg. hypotension
State the special properties of cutaneous circulation
INVOLVED IN:
- Defence against the environment
- Lewis triple response to trauma and skin injuries (increased blood flow)
- Temperature regulation
State the special structural features of the cutaneous circulation
- Arterio-venous anastomoses (AVAs)
- Sympathetic vasoconstrictor fibres
- Sudomotor vasodilator fibres
What occurs at the AVAs?
Direct connections of arterioles and venules expose blood to regions of high surface area
- Involved in conduction, radiation and evaporation
What occurs at the sympathetic vasoconstrictor fibres?
Release noradrenaline acting on α1 receptors
What occurs at the vasodilator fibres?
Acetylcholine acting on endothelium to produce nitric oxide
State the special functional features of cutaneous circulation
- 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’
Describe cold induced vasoconstriction
- 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
Describe paradoxical cold vasodilation
- Protects against skin damage
- Caused by paralysis of sympathetic transmission.
- Long term exposure causes oscillations of contraction/relaxation
Describe how cutaneous perfusion changes with increased core temperature (eg. exercise)
- Increased cutaneous perfusion with increased core temperature (eg. exercise)
- Increased core temperature stimulates temperature receptors in hypothalamus
What is the effect of stimulation of temperature receptors?
- 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
Describe other specialised functions of cutaenous circulations
- 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
Describe the Lewis triple response
- 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
Describe what arises when there is prolonged obstruction of flow by compression.
Severe tissue necrosis
Bed sores
How can prolonged obstruction of flow be avoided?
- Shifting position / turning causing reactive hyperaemia (on removal of compression)
- High skin tolerance to ischemia
Describe what arises during postural hypotension/oedema due to gravity
- Decrease in central venous pressure (hypotension) and increased capillary permeability (oedema)
- Causes person to feel faint
Describe the resistance in a normal artery/ arteriole at rest.
At rest, resistance is low in a large coronary artery. Then resistance is high in the arterioles
Describe the resistance in a normal artery/ arteriole during exercise
During exercise, low resistance to flow in arteries.
- Vasodilation of arterioles to reduce resistance, so increased blood flow to meet oxygen demands
Describe the resistance in a stenosis artery/ arteriole at rest
In artery, stenosis increases resistance.
- Metabolic hyperaemia occurs at rest, so blood flow meets needs.
- Resistance in arterioles is reduced.
When does the heart get its blood supply?
Diastole
What does it mean to say that the skin is poikilothermic?
Temperature can vary over a large range without damage
RECAP: What do the x and y axes on oxygen dissociation curves represent?
X-AXIS: Amount of oxygen dissolved in plasma
Y-AXIS: Percentage saturation of haemoglobin
RECAP: What does it mean when the oxygen dissociation curve shifts to the right?
- Reduced affinity for oxygen due to high metabolic demands/pH changes etc.
Why is there no coronary perfusion following systole?
- Ventricular pressure > arterial pressure.
- Aortic pressure is not enough to force blood through the arteries in the contracted
myocardium
Why does coronary perfusion occur following diastole?
- Aortic pressure remains high because of compliance of the aorta
- Ventricular pressure decreases dramatically, allowing blood to flow around the arteries of the heart
How is coronary perfusion influenced by heart failure?
- 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
What three factors may cause diastolic pressure to be lower?
- Hypovolemia
- Haemorrhage
- Aortic valve incompetence
What is the initial response to skin exposure to the cold?
Cold-induced vasoconstriction
