Properties of Special Circulations Flashcards
Describe coronary circulation
- Two coronary arteries originate from the left side of the heart at the beginning (root) of the aorta, just after it exits the left ventricle
- 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
- 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
Compare coronary circulation during normal activity and during increased demand
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%
During increased Demand
- 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 b2-adrenoceptors
Describe the bohr shift in coronary circulation
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 has shifted the curve to the right meaning that haemoglobin has less affinity for oxygen and more O2 is given up to the myocardial tissues
- The myocardium is able to extract 75% of the oxygen as opposed to typically 25% in other tissues
How does the heart produce higher oxygen supply during increased demand despite a limit to oxygen efficiency
- Extraction of oxygen is near max during normal activity
- Therefore to provide more O2 during demand, we must increase blood flow
- Myocardium metabolism generates metabolites to produce vasodilatation, increase blood flow (metabolic hyperaemia)
- eg. Adenosine, produced by ATP metabolism and is released from cardiac myocytes. Also, increases in pCO2, H+, K+ levels
Describe what functional end arteries are and why they are problematic
- These are low numbers of cross-branching collateral vessels
- Ischaemic Heart Disease - many coronary arteries are functional end-arteries and therefore decreased perfusion in one of them can cause major problems
- Heart is susceptible to both sudden and slow obstruction
State what can cause a sudden and slow blockage of the arteries
- Sudden - acute thrombosis, produce myocardial infarction
- Slow - atheroma (sub-endothelium lipid plaques) chronic narrowing of lumen, produces angina
- Systole obstructs coronary blood flow.
Describe what thrombosis is, where it occurs most often and what it leads to
- Total occlusion - usually of the left anterior descending coronary artery
- Occlusion leads to obstruction of blood flow to the anterior (left) ventricle - leads to a myocardial infarction
- This leads to ischemic tissue (tissue with a lack of perfusion) which causes acidosis and pain (stimulation of c fibres)
- Then causes impaired contractility due to necrosis leading to sympathetic activation, arrhythmias and cell death (necrosis) if perfusion is lost for a period of time
State what angina is, what happens in a normal heart in exercise and what happens in someone with stable angina
Angina -
- Symptoms (chest pain) arise during exercise (stable angina)
Normal heart -
- The arteriole dilates which decreases the total peripheral resistance and so increases blood flow to meet the increased oxygen demands
In angina -
- Stenosis in large coronary artery - increases resistance. Metabolic hyperaemia occurs at rest, so blood flow meets needs
- During exercise the arterioles further dilate to reduce resistance and allow more blood flow to meet oxygen demand but the total resistance is still too high due to dominance of the stenosis - oxygen demand cannot be met and angina develops
State some mechanical factors that can reduce coronary flow
- When the pressure in ventricles is ≥ aorta, there is no coronary perfusion during systole
- Shortening diastole, eg. high heart rate
- Increased ventricular end-diastolic pressure, eg. Heart failure (aortic stenosis, stiffening of ventricle)
- Reduced diastolic arterial pressure, eg. hypotension, aortic regurgitation
State the special properties of cutaneous circulation
- Defence against the environment
- Lewis triple response to trauma (increased blood flow)
- Temperature regulation
- Blood flow delivers heat from body core by conduction
- Radiation (proportional to skin temperature) in the infra-red
- Convection from skin as heat carried away by the air
- Sweating (latent heat of evaporation)
- Skin is a organ and temperature can range from 0oC to 40oC (briefly) without damage (poikilothermic rather than homeothermic)
- Skin temperature depends on:
- Skin blood flow
- Ambient temperature
State the special structural features of the cutaneous circulation
- Arterio-venous anastomoses (AVAs) -
Direct connections of arterioles and venules expose blood to regions of high surface area
Convection, conduction, radiation, evaporation - Sympathetic vasoconstrictor fibres -
Release noradrenaline acting on α1 receptors - Sudomotor vasodilator fibres -
Acetylcholine acting on endothelium to produce nitric oxide - Driven by temperature regulation nerves in hypothalamus
State the special functional features
Responsive to ambient & core temperatures:
- 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’
- Core temperature receptors in hypothalamus control sympathetic activity to skin & hence skin blood flow
Describe cold induced vasoconstriction and paradoxical cold vasodilation
- Cold-induced vasoconstriction - Conserves heat;
Sympathetic nerves react to local cold by releasing noradrenaline which binds to a2 receptors on vascular smooth muscle in skin. a2 receptors bind NA at lower temperatures than a1 receptors - Paradoxical cold vasodilatation - Protects against skin damage
Caused by paralysis of sympathetic transmission. Long-term exposure leads to oscillations of contract/relax
