Properties of Special Circulations Flashcards
What does coronary circulation consist of?
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
What are the special requirements of coronary circulation?
Special requirements:
Needs a high basal supply of O2- 20x resting skeletal muscle
Increase O2 supply in proportion to increased demand/cardiac work
What are the special structural features of coronary circulation?
Special structural features:
High capillary density large surface area for O2 transfer
Together these reduce diffusion 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
What are the special functional features of coronary circulation during different times?
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 vasodilation
High O2 extraction (75%)- average in body 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
How is oxygen unloaded to the myocardium during normal activity and how can this change?
Bohr shift-
Coronary sinus blood returning to right atrium from myocardial tissue has a 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 can we get more oxygen in times of increasing demand?
Increased O2 requirement produces increased blood flow
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)
E.g. adenosine, produced by ATP metabolism and is released from cardiac myocytes
Also increase in pCO2, H+, K+ levels
Why can so many things go wrong with the coronary arteries?
Human coronary arteries are functional end-arteries
Ischemic Heart Disease:
Coronary arteries are functional end-arteries and therefore decreased perfusion produces major problems
The heart is susceptible to sudden and slow obstruction
Sudden- acute thrombosis, produce myocardial infarction
Slow- atheroma (sub-endothelium lipid plaques) chronic narrowing of the lumen, produces angina
Systole obstructs coronary blood flow
How can we get more oxygen in times of increasing demand?
Increased O2 requirement produces increased blood flow
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)
E.g. adenosine, produced by ATP metabolism and is released from cardiac myocytes
Also increase in pCO2, H+, K+ levels
How does angina cause a patient pain?
Angina- problems during increased activity
For a normal heart, when exercising metabolic vasodilatation of arterioles reduce the total resistance of the coronary artery.
This way increased blood flow can meet increased O2 demands
When someone has angina, stenosis in large coronary artery increases resistance however metabolic hyperaemia occurs at rest so blood flow meets O2 needs
During exercise, arterioles further dilate to reduce resistance, but total resistance is still too high due to dominance of stenosis
O2 demans=d cannot be met and angina develops
What is thrombosis and what can cause it?
E.g. total occlusion of left anterior descending coronary artery
Occlusion leading to obstruction of blood flow to anterior (front) left ventricle- myocardial infarction
Ischemic tissue, acidosis, pain (stimulation of C-fibres)
Impaired contractility
Sympathetic activation
Arrhythmias
Cell death (necrosis)
How does angina cause a patient pain?
Angina- problems during increased activity
For a normal heart, when exercising metabolic vasodilatation of arterioles reduce the total resistance of the coronary artery.
This way increased blood flow can meet increased O2 demands
When someone has angina, stenosis in large coronary artery increases resistance however metabolic hyperaemia occurs at rest so blood flow meets O2 needs
What mechanical factors reduce coronary flow?
- Shortening diastole e.g. high heart rate
- Increased ventricular end-diastolic pressure e.g. heart failure (aortic stenosis, stiffening of ventricle)
- Reduced diastolic arterial pressure e.g. hypotension, aortic regurgitation
What are the special properties of cutaneous circulation?
Defence against the environment
Lewis triple response to trauma (increased blood flow)
Temperature:
Blood flow delivers heart from body core
Radiation (proportional to skin temperature)
Conduction to skin- convection from skin (skin temp.)
Sweating (latent heat of evaporation)
Skin is an organ and temperature can range from 0C to 40C (briefly) without damage (poikilothermic rather than homeothermic)
Skin temperature depends on:
Skin blood flow
Ambient temperature
What are the special structural features of 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 a1 receptors
Sudomotor vasodilator fibres- acetylcholine acting on endothelium to produce nitric oxide
Driven by temperature regulation nerve in hypothalamus
What are the special functional features of cutaneous circulation?
Responsive to ambient and core temperatures
Help heat loss- increase ambient temperature causes vaso- and venodilatation
Help to conserve heat- decrease ambient temperature causes vaso- and venoconstriction
Sever cold causes ‘paradoxical cold vasodilatation”
Core temperature receptors in hypothalamus control sympathetic activity to skin and hence skin blood flow
