Specialised Circulation Flashcards
What is a special structural feature of the coronary circulation?
Supplies O2 even in high demand because there is high capillary density (no. of capillaries per cardiac muscle cell). This is important because with a high capillary density it gives a large surface area for O2 transfer and reduces the diffusion distance to the myocytes. Both together increase the speed of passive diffusion of CO2 removal and O2 uptake. This is because time taken for diffusion (t) is proportional to distance squared (X2).
What are some of the special functional features of the coronary circulation?
Coronary circulation has a very high blood flow. It does this by basal nitric oxide release (releasing of NO) causing vasodilatation. This much wider radius of vessels increases blood flow. Also at rest the heart is also able to extract much higher amounts of oxygen (75%) than the rest of the body (25%).
During metabolic hyperemia [(e.g. in exercise), increase in demand], coronary blood flow increases in proportion to the demand, there is production of vasodilators (e.g. adenosine) and these outcompete sympathetic vasoconstriction.
Circulating adrenaline also dilates coronary vessels due to abundance of β2 adrenoceptors (PKA pathway).
Explain how we know that the myocardium extracts more oxygen from arterial blood. Why does this happen?
Arterial blood has an O2 content of 200. Mixed venous blood has O2 content of around 150, this means that 25% of oxygen has been extracted from tissues in the circulation.
If we look at the coronary sinus flow we can see that the O2 content is 50, so this means that 75% of the oxygen has been removed from the arterial blood.
The greater CO2 in the venous blood is what shifts the curve to the right (bohr shift) which means that it lets go off more O2, because pCO2 is higher in coronary circulation the curve is shifted even more to the right and hence even more O2 is extracted by tissues.
Why does blood flow increase when there is an increased demand for oxygen by the heart?
Myocardium metabolism generates metabolites to increase blood flow.
Metabolites such as adenosine which is produced by ATP metabolism and released from cardiac myocytes, it acts a major vasodilator which will increase blood flow.
Also, things such as pCO2, H+, K+ levels which are all products of metabolism will increase blood flow and contribute to the metabolic hyperaemia.
What are the 2 special problems that occur in the coronary circulation?
Ischemic heart disease and systole, which obstructs coronary blood flow.
Why are human coronary arteries susceptible to ischaemia?
They are functional end-arteries. One coronary artery will be supplying a certain area of tissue and if you have a blockage to that artery then that area of cardiac tissue is now very susceptible to ischemia as it won’t be receiving any blood.
Give an example of a sudden obstruction in the heart and a slow obstruction.
Sudden - Acute thrombosis in a coronary artery which completely blocks it off so there is no O2 delivery producing a myocardial infarction.
Slow - Buildup of atheroma (sub-endothelial lipid plaques) which causes chronic narrowing of the lumen producing angina due to a reduction in O2 supply.
What happens if patient has occlusion of the coronary artery due to thrombosis?
Occlusion will lead to a myocardial infarction. This is because the blockage will prevent blood flow to that area of cardiac tissue leading to ischemic tissues. This will produce metabolic problems, there will be a buildup of acid (acidosis) and this will cause pain (C-fibres).
The contractility of the heart will become impaired, there will be sympathetic activation and arrhythmias and cell death (necrosis).
Why is coronary blood flow restricted to the diastole window?
When the ventricular pressure is higher than or equal to the aorta the coronary vessels are occluded and there is no coronary blood flow.
During relaxation, because this diastolic BP in the aorta is greater than in the ventricles (there is a large pressure difference) it allows coronary circulation.
List some of the factors which effect coronary flow?
- Shortening diastole, when heart rate increases
- Increased ventricular end diastolic pressure (if not ejecting enough blood from heart and filling heart lot), blue line will be further up.
- Reduced diastolic arterial pressure -> aorta line will drop down lower so window will be less.
What type of organ is the skin and what does the skins temperature depend on?
The skin is a poikilo-thermic (not homeo-) organ – Meaning its temperature can range from 0oC to 40oC briefly without damage for short periods of time (e.g. hand in cooker, ice)
Skin temperature depends on:
• Skin blood flow
• Ambient temperature (outside temp)
Name and describe a special structural feature of the skin.
Blood vessels in our skin have arterio-venous anastomoses (AVAs), which are connections direct from arterioles to veins (shunt blood straight from arteriole to vein). The blood supply is mainly to the dermis and oxygen etc. will diffuse up to the epidermis.
A lot of blood goes through these AVAs allowing blood to enter easily into the veins which have little resistance and this allows temperature to move out of the veins and into the surrounding area ultimately to be dissipated into the ambient temperature (environment).
Arterioles going into the AVAs are under control of sympathetic vasoconstrictor and sudomotor vasodilator fibres. So, these fibres can control blood flow to the skin and are linked back to the hypothalamus which controls temperature.
Describe what happens to skin blood flow after being placed in cold temperature for 20 minutes.
Initially your hand would go pale due to cold-induced vasoconstriction to conserve heat. This is caused by the abundance of alpha2 receptors in the skin which bind NA at lower temperatures. This can be seen in the drop in blood flow initially.
There is then an increase in blood flow after a while, this is paradoxical cold vasodilatation. This is to protect the skin against damage. As it is now severely cold there is paralysis (shut down) of sympathetic transmission – stops vasoconstriction so there is just local vasodilatation through NO etc. so you get redness of the hand.
What happens to cutaneous perfusion when there is an increase in core temperature?
Higher cutaneous perfusion. Increased core temperature will stimulate warmth receptors in the anterior hypothalamus. This leads to increased sweating – through sympathetic activity (Ach) on sweat glands and you will get more vasodilatation from increased sympathetic sudomotor activity (Ach) to the arterioles in extremities. This will increase heat loss
Explain the baroreflex/RAAS/ADH-stimulated
vasoconstriction of skin blood vessels.
This is where blood pressure drops due to haemorrhage, shock, sepsis etc. and blood is directed to more important tissues/organs.
This is mediated by baroreceptors being stimulated which switches on sympathetic vasoconstrictor fibres, adrenaline, vasopressin and angiotensin II.
This all causes vasoconstriction of arterioles to skin and results in pale cold skin of a patient that is in shock.
