2.11 - Special Circulations Flashcards
Describe why coronary flow is largely during diastole
There is mechanical compression of the left coronary artery depending on the position relative to the heart wall
Subepicardial vessels are affected very little –> better perfusion
Subendocardial vessels are compressed during systole –> flow stops temporarily during systole. Therefore vessels are prone to hypoxia.
However perfusion of the RCA is possible due to smaller cavity pressure –> limited “squeeze” on vessels
Describe autoregulation
The process of keeping blood pressure in a vessel within a normal range to maintain perfusion of a tissue/organ. Can maintain a constant blood flow over a wide pressure range (60-180mmHg)
It is an immediate response
Most likely a myogenic response that change arteriolar resistance in pre-capillary sphincters
Describe the key metabolic factors responsible for vasodilation as a result of increased cardiac work
ATP: decreased ATP production activate KATP channels. KATP hyperpolarises VSMC –> decreased calcium influx –> vasodilation
Adenosine: transiently released from myocardial cells. Activates adenosine receptors and also activates KATP channels –> vasodilation
NO: relaxes smooth muscle cells
Describe the changes in substrate utilisation in exercise
At rest, glucose, fatty acids and lactate serve equally to provide energy. However during exercise, lactate from muscle is icnreasingly imporant in energy production
Most increase via increased perfusion. But O2 consumption increases slightly more than perfusion increases. Therefore there is increased o2 extraction during exercise
Describe haemodynamic steal in coronay vessels
Atherosclerotic plaques prevent blood flow.
When this and surrounding vessels dilate, there is decreased resistance in each vascular arm. This results in increased flow. However as resistance if constant around sclerotic plaques, the relative resistnace increases locally and there is decreased blood flow over the plaque. Thus the healthy blood vessels “steal” blood from the compromised vessel.
This can be exacerbated by exercise and can lead to a heart attack
Describe the local control of coronary blood flow
Autoregulaiton is a major player in maintaining constant blood flow (myogenic)
Regional neural activity results in increases in blood flow due to increased metabolism leading to local hyperaemia. E.g. shining a light in the eye causes a massive increase in the visual cortex of the brain
Coronary blood flow is also very sensitive to changes in PCO2. Increased PCO2 –> vasodilation. Mechanism: perivascular pH changes with CO2. however the acidosis itself does not affect CBF because of the BBB
What degree of stenosis affects coronary blood flow
Only large stenosis (80%) causes a significant flow limitation at rest. However this is accentuated under exrecise –> then we start to see effects much earlier
What makes the cerebral vascular bed different to others, in terms of its innervation?
It has parasympathetic innervation. Dilation via greater superficial petrosal nerve (VII)
However neural control of CBF is quite small under normal conditions
Helmut Fringer, a 28 year-old medical student, has an assessment of his coronary circulation under strenuous exercise. Which of the following statements best describes the coronary circulation during exercise?
A: Increased O2 supply is mainly achieved via increased O2 extraction than an increase in blood flow.
B: Increased cardiac sympathetic fibre activity raises coronary perfusion.
C: Increased ATP availability contributes to vasodilation.
D: Blood flow is increased by release of acetylcholine.
E: Autoregulation is suppressed.
B: Increased cardiac sympathetic fibre activity raises coronary perfusion.
