properties of special circulations Flashcards
what are the special requirements of coronary circulation ?
- Needs a high basal supply of O2
- Increase O2 supply in proportion to increased demand/cardiac work
what are the special structural requirements of coronary circulation ?
- High capillary density
- Large surface area for O2 transfer
- Together these reduce diffusion distance to myocytes: diffusion time is proportional to distance squared – so O2 transport is fast
what is the difference between the cardiac and skeletal muscle fibers
cardiac muscle fibers have smaller diameter so more capillaries can fit around and give rise to shorter diffusion distances
describe the special functional features at rest
- High blood flow - 10x the flow per weight of rest of body.
- Relatively sparse sympathetic innervation
- High nitric oxide released leading to vasodilatation
- High O2 extraction (75%) – average in body is 25%
describe the special functional features 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 2-adrenoceptors
what is the oxygen dissociation curve ?
a measure of uptake and removal of oxygen by hemoglobin under different partial pressure
what causes a shift to the right ?
increase carbon dioxide , acidity , temperature
describe the Bohr shift in terms of the heart
- Coronary sinus blood returning to right atrium from myocardial tissue has a greater 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 hemoglobin 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.
why does Increased O2 requirement produces increased blood flow ?
- When the body requires more oxygen (O₂), such as during exercise, the tissues signal for increased blood flow to deliver more oxygen and remove waste (like CO₂). This happens through vasodilation, where blood vessels widen to allow more oxygen-rich blood to reach the active tissues.
- Myocardium metabolism generates metabolites to produce vasodilatation, increase blood flow
(metabolic hyperaemia).
what are functional end arteries ?
- arteries that have anastomoses (connections with other arteries) but are insufficient to provide adequate blood supply if the main artery is blocked.
- This means that while there is some collateral circulation, it is not enough to fully compensate for a blockage, potentially leading to tissue damage or ischemia.
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what are ischemic diseases ?
refers to conditions caused by reduced blood supply (ischemia) to tissues, leading to oxygen deprivation and potential damage. It usually results from narrowed or blocked blood vessels due to atherosclerosis (plaque buildup), thrombosis (clot formation), or embolism (blockage by a foreign particle).
what is thrombosis ?
Thrombosis is the formation of a blood clot (thrombus) inside a blood vessel, which can partially or completely block blood flow. It can occur in arteries or veins and may lead to serious complications if the clot obstructs circulation or breaks loose (embolism).
what is angina ?
- when a stenosis in a coronary artery causes an increase in resistance.
- during exercise arterioles dilate to reduce resistance but total resistance is still to high so o2 demand cannot be met
describe the mechanical factors that reduce coronary blood flow
- Shortening diastole, eg. high heart rate.
- Increased ventricular end-diastolic pressure, eg. Heart failure (aortic stenosis, stiffening of ventricle).
- Reduced diastolic arterial pressure, eg. hypotension, aortic regurgitation
note - ventricles only fill up during diastole
describe the special properties of the cutaneous system
- Defence against the environment
- Lewis triple response to trauma (increased blood flow)
Temperature regulation
* Blood flow delivers heat from body core by conduction
* Radiation (proportional to skin temperature) in the infra-red
Convection from skin as heat carried away by the air
* Sweating (latent heat of evaporation)
what does skin temperature depend on ?
- Skin blood flow
- Ambient temperature
Skin is an organ and temperature can range from 0oC to 40oC (briefly) without damage: poikilothermic (wide range of temps) rather than homeothermic (constant temp).
what are the special structural features of the cutaneous system ?
Arterio-venous anastomoses (AVAs)
* are direct connections between arteries and veins, bypassing capillaries. They help regulate blood flow and temperature, especially in the skin. When AVAs open, more blood flows to the skin to release heat. When they close, less blood reaches the skin, conserving heat.
sympathetic vasoconstrictor fibres
* nerves that control blood vessel narrowing (vasoconstriction). They release noradrenaline, which makes blood vessels contract, reducing blood flow. This helps regulate blood pressure, temperature, and blood distribution during stress or exercise.
Sudomotor vasodilator fibres
* nerves that help with sweating and blood vessel widening (vasodilation). They are part of the sympathetic nervous system and release chemicals like acetylcholine to increase blood flow to the skin. This helps with heat loss and temperature regulation, especially in hot conditions.
describe the affect of ambient temperature on skin blood flow
what does increasing core temperature do ?
Stimulates temperature receptors in anterior hypothalamus.
Causing:
Sweating
Increased sympathetic activity to sweat glands
mediated by acetylcholine.
Vasodilatation
Increase sympathetic sudomotor activity such that acetylcholine act on endothelium to produce NO which dilates arterioles in extremities.
