L45-regulation Of Coronary Blood Flow Flashcards
What happens to coronary circulation during exercise
Increased oxygen delivery to contracting skeletal muscle which can be provided by combination of increased blood flow and increased extraction of oxygen from hemoglobin
In the heart there is a limited scope for increasing oxygen delivery by this route it is provided by increased coronary blood flow
Why is the heart extracting 60 to 70% of 02
Because heart muscle has more mitochondria up to 40% of cell is occupied by mitochondria which generates energy for contraction by aerobic metabolism therefore the heart needs oxygen
What does the coronary blood flow particularly to the left ventricle affected by
Contraction of the myocardium which crushes coronary vessels
When is blood flow maximum and why
During the diastole since the muscle fibers are relaxed
What in systole the contracting fibers collapse the supply vessels and flow falls to zero
Which blood vessels are the most affected
In the subendocardial layer therefore they are most prone to ischemic damage and most common site of MI
Which blood vessels are relatively unaffected
The blood vessels on or close to the surface of the heart
Why is coronary blood flow to the right side not affected during systole
In the right side of the heart the interventricular pressures are lower and so the effect of ventricular systole on coronary blood flow is less marked
What happens during exercise to diastole and how does it affect
It is shortened more Markedly then the duration of the systole
This imposes a limitation on increase in coronary blood flow and is probably the limiting factor on maximum exercise ability in normal individuals
What is autoregulation
Currently refers to the ability of the myocardium to maintain stable blood flow despite variations in coronary pressure
What does it mean that the coronary blood flow is auto regulated
▪️It means over range of mean arterial pressures from about 50 to 120 mm hg coronary blood flow is relatively independent of arteria pressure
▪️This is thought to result especially from responses of arterioles which are less than 150 µm in diameter
▪️Thus as the arterial pressure increases through the auto regulatory range the smooth muscle in the wall of these arterioles contracts to maintain flow constant
What are the control mechanisms
▪️Neurohormonal factors such as adrenaline, noradrenaline and acetylcholine
▪️Physical factors such as arterial pressure ,coronary pressure
▪️Endothelial factors such as nitric oxide
▪️Metabolic factors such adenosine,PO2
What does the lack of O2 do
Increase local concentration of CO2, H+ K+ , lactate,adenine nucleotide adenosine
All these vasodilator metabolites Act in an integrated fashion to increase coronary blood flow by opening of ATP sensitive K+ channels leading to hyper polarization
What is the role of adenosine
Source of vasodilator
Under hypoxic conditions
Complete de phosphorylation of ATP
Non polar and can leave myocardial cell once formed
What is the role of nitric oxide
When there is increased shear stress on the endothelium the production of nitric oxide increases leading to vasodilation particularly in large Coronary arteries but it does not appear to contribute to the mechanism of auto-regulation
What is shear stress
Tangential force of the flowing blood on the endothelial surface of the blood vessel
