Properties of special circulations Flashcards
Coronary circulation
- Special requirements
* Needs a high basal supply of O2 - 20x resting skeletal muscle
* Increase O2 supply in proportion to increased demand/cardiac work - Special structure features
* High capillary density
* Large SA for O2 transfer
* This reduces diffusion distance to myocytes; diffusion time is proportional to distance ^2 - so transport of O2 is fast
Special structural features
Cardiac muscle containes a lot of fibres and capillaries, giving rise to shorter diffusion distances.
Special functional features
- During normal activity:
* High blood flow
* High nitric oxide - vasodilatation
* High O2 extraction - During increased demand:
* Coronary blood flow increases in proportion to demands
* Production of vasodilators out compete vasoconstriction
* Adrenaline dilates coronary vessels due to abundance of β2-adrenoreceptors
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Haemoglobin oxygen binding
Oxygen haemoglobin dissociation curve
- Measyre of the uptake and removal of oxygen by haemoglobin under different partial pressure.
- How much oxygen pressure leads to what % of oxygen saturation of blood saturation.
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Oxygen haemoglobin dissocation curve - affinity shifts
LEFT SHIFT - Increased affinity.
RIGHT SHIFT - Decreased affinity.
Oxygen unloading to myocardium - During normal activity
Bohr shift:
* The blood coming back to the right atrium from the heart muscle through the coronary sinus has a lot of carbon dioxide because of the high number of tiny blood vessels, large surface area, and a small difference in the diffusion of gases.
* Because of the high carbon dioxide and low pH, the oxygen-carrying capacity of hemoglobin is reduced, so it releases more oxygen to the heart muscles. In this case, about 75% of the oxygen in the blood is taken up by the heart muscle, which is much higher compared to the usual 25% in other tissues.
Increased O2 requirement produces increased blood flow
- Extraction of oxygen is near max during normal activity - To provide mroe O2 during demand - we must increase blood flow.
- Myocardium metabolism generates metabolites which produce vasodilatation - this increases blood flow - metabolic heperaemia.
- EXAMPLE: Adenosine produced by ATP metabolism is released from cardiac myocytes - also increases in pCO2, H+, K+.
Functional end-arteries
Ischaemic Heart Disease:
- End-arteries: Decreased perfusion in one of them can cause major problems.
Sudden and slow obstruction:
* Sudden - accute thrombosis, produce myocardial infraction.
* Slow - atheroma chronic narrowing of lumen produces angina.
Systole obstructs coronary blood flow.
Thrombosis
Occlusion leading to obstruction of blood flow to anterior (front) left ventricle - myocardial infraction.
Symptoms of thrombosis:
* Ischaemic tissue, acidosis, pain - stimulation of C-fibres.
* Impaired contractility.
* Sympathetic activation,
* Arrhythmias.
* Cell death (necrosis).
Angiography
Angina - problem during increased activity
Coronary blood flow during diastole
Mechanical factors reducing coronary flow:
1. Shortening diastole - high heart rate.
2. Increased ventricular end-diastolic pressure. Eg: heart failure - aortic stenosis, stiffening of ventricle.
3. Reduced diastolic arterial pressure. Eg: hypotension, aortic regurgitation.
Special properties of the cutaneous circulation
- Defence against the environment.
- Lewis triple response to trauma (increased blood flow).
- Temperature regulation:
1. Blood flow delivers heat from the body core by conduction,
2. Radiation - proportional to skin temperature - In the infra-red.
3. Convection from skin as heat carried away by the air.
4. Sweating - latent heat of evaporation. - Skin is an organ and can range from 0 degrees to 40 degrees celsius - It’s poikilothermic (wide range of temperatures, rather than homeothermic.
Skin temperature depends on skin blood flow and ambient temperature.
Special structural features
Arterio-venous anastomoses (AVA): direct connections of arterioles and venules expose blood to regions of high surface area.
Convection, conduction, radiation, evaporation.
Sympathetic vasoconstrictor fibres - release noradrenaline acting on α1 receptors.
Sudomotor vasodilator fibres: acetylcholine acting on endothelium to produce nitric oxide.
Effect of ambient temperature on skin blood flow
- Cold induced vasoconstriction: conserves heat
* Sympathetic nerves react to local cold by releasing noradrenaline which binds to α2 receptors on vascular smooth muscle in skin.
* α2 receptors bind NA at lower temperatures than α1 receptors. - Paradoxical cold vasodilatation: protects against skin damage
* Caused by paralysis of sympathetic transmission.
* Long-term exposure leads to oscillations of contract/relax
Cutaneous perfusion and core temperature
Increased cutaneous perfusion with increased core temperature Eg exercise.
- Increased core temperature - stimulate temp 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.
Other functional specialisations
Special problems
