Lecture 8 - Special Circulation Flashcards
How much of the coronary venous blood returns to the coronary sinus? And where does the rest drain?
And why is there slight deoxygenation of arterial blood?
95% of the coronary venous blood returns to the coronary sinus.
The rest drains through anterior coronary and thebesian veins.
The thebesian vein drains into the left ventricle - This is why their is slight deoxygenation of arterial blood in the left ventricle (saturation ~97%)
What is the resting and active coroanry blood flow?
Resting - 80-70ml/min/100g of tissue
it’s 300-400ml/min/100g of tissue during heavy exercise
The coronary circulation is the _______ in the human body, with a mean transit time of 6-8 secs
Shortest
What are the special tasks of the coronary circulation?
- Needs to maintain a high basal supply of O2
- 20x more O2 needed than skeletal muscle
- Increase in O2 delivery is proportional to demand (cardiac work)
In regards to capillaries, what are the special structures found in the heart?
In cardiac muscle each muscle fibre has at least one capillary.
Whereas in skeletal muscle not all capillaries are open - so the travelling distance is longer, whereas in cardiac muscle the travelling distance is shorter.
What are the benefits of the high capillary density in the heart??
- Gives a big surface area for O2 transfer - since there is a larger surface area for O2 transport.
- Short diffusion distance into cardiomyocye - so it can reach the cell in a much shorter time.
What percentage of the O2 does the myocardium extract from the blood to meet it’s high demand?
65-75%, whereas most organs only extract only ~25% of O2 (like skeletal muscle) in resting conditions, so for most organs 75% of O2 remains in veins.
What is the main controller of the coronary circulation?
And what is one of the vasodilator metabolites that contributes to this?
And what state causes vasodialtion for the coronary arteries?
Via metabolic hyperaemia to increase blood flow, so there is increase in coronary blood flow in proportion to demand (like skeletal muscle).
One of the key metabolites is adenosine, which is produced as a break down product of ATP (so more the heart works the more adenosine produced), this acts on the beta adrenergic receptor to relax the coronary arteries.
Hypoxia causes vasodilation, since O2 is a potent vasoconstrictor - so if low O2 = vasodialtion in order to increase O2 delivery.
What are the benefits of hypoxic vasodilation, and what does it prevent to the arteries?
And how does it help with a heart attack?
Hypoxic/ ischaemic vasodilation helps preserve myocardial perfusion downstream of the narrowed artery (reactive hyperaemia). This helps prevent ischaemic damage by coronary artery disease.
During a MI hypoxic vasodilation helps maintain perfusion at the margins of the myocardal infarct, thereby limiting the extent of the damage.
Why does coronary stenosis usually cause angina during exercise?
hypothetically, say that to meet O2 demand the overall vessel needs 20 units of resistance.
When stenosis occurs due to artherogenesis and dilatation suppose the arterial resistance raises to 10 uints, and in response to maintain the 20 units the downstream resistance decreases from 19 to 10.
During exercise the reserve dilatation of resistance vessels reduces the downstream resistance further, e.g. from 10 to to 3 units. The total resistance now equals 13 units. The total resistance is now dominated by the stenosis (the vessel now has much less reserve dilatation since it’s already dilated). As a result the myocardial blood flow only increases by 1.5x fold (due to increased resistance when compared to normal vessel in exercise, which has a total resistance of 5 and allows for a 4 fold increase in exercise). This 1.5x fold increase cannot meet O2 demand, thus causing angina.
Look at slide for pic
What are the special problems of the coronary arteries due to them being functional end-arteries?
There are NO aterio-arterial anastomoses (in the large coronary arteries) - therefore if a vessel becomes occluded, the area of heart that that vessel supplies will become ischaemic and die (MI)
THERE are NO anastomoses in the coronary circulation.
What happens to the coronary blood flow when systole occurs?
2/3rd of coronary arterial circulation is intramural, therefore systole obstructs coronary blood flow. As it compresses the vessels during isovolumetric contraction.
When does most of myocardial perfusion occur?
80% of myocardial blood flow occurs during diastole
Beta-blockers in heart failure or after MI increases the diastolic length and hence increase cardiac perfusion perfusion.
What are the special tasks of the cutaneous circulation?
- Defence against the environment
- it increases the blood flow in response to local damage.
- Regulation of body core temperature, which is done by:
- Radiation
- conduction and convection
- Sweating
All three mechanisms depend on increased blood flow to deliver heat from the core.
What special strucural features of the cutaneous circulation allow it to increase heat loss efficiently?
It has aterio-venous anastomoses in the extremeties.
When the skin temp increases (due to core temp increasing), this heat needs to be removed. the sympathic vasoconstrictive fibres are inhibited, and the ateriovenous anastomoses dilate for increased heat loss.
The advantage of having arteriovenous-anastomoses is because since arteries have a thicker wall compared to the veins, if we send more blood to veins it’s easier for heat to be removed compared to the arterioles
What is cutaneous blood flow affected by?
It’s affected by both ambient and core temperature.
After a period of time with cooling by ice, what happens to the vessels after vasoconstricting for a period of time?
There is paradoxical cold vasodilation. We can’t continually vasoconstrict blood flow to the hands, so dilation occurs.
This dilation is caused by paralysis of noradrenergic neutrotransmission by the cold, and by the release of vasodilators such as prostacyclin
In regards to extrinsic control, what allows skin blood flow to change in response to increased core temp? (e.g. for exercise or fever)
Vasodilation and sweating is caused by:
- Caused by increased sympathetic cholinergic fibre activity to the skin resistance vessels
- And a decrease in sympathetic vasoconstrictor drive to AVAs in extremities
What is the special structural feature of teh brain that protects O2 supply from by blockage of an artery? (an occlusion)
And how does this assist in high O2 delivery to neurons?
The major cerebral arteries anastomose, forming the circle of willis
This allows there to be a high O2 delivery to neurons due to there being a high capillary density
How does the brain maintain perfusion during hypotension?
Cerebral autoregulation maintains perfusion during hypotension
When arterial blood pressure falls cerebral resistance blood vessels undergo vasodilatation
What happens to cerebral blood flow as age increases?
Cerebral blood flow decreases
