Circulation through Special Regions

Circulation through Special Regions

Circulations considered:• Coronary circulation• Cerebral circulation• Circulation through the skin

• Renal circulation• Pulmonary circulation

Coronary Circulation

Although the heart contains blood, the myocardium must be supplied by arteries, like any other organ. Two major coronary arteriesBoth arise from the root of the aorta (aortic sinuses)Major arteries run on the surface of the heart Branches go into the myocardium between muscle fibres to supply the myocardium

Coronary Circulation

Left ventricle is thicker – blood needs to go further from the major arteries

Left ventricle is more likely to suffer from ischaemia than the right ventricle

Left ventricle

Right ventricle

Coronary Circulation

Subendocardial muscle are furthest away from main arteries

Subendocardial muscle are the most likely to suffer from ischaemia

Left ventricle

Right ventricle

endocardium

Subendocardialmuscle

Coronary Circulation

Pressure in the left ventricle is greater; blood vessels going through the left ventricle are compressed more during systole

Left ventricle is more likely to suffer from ischaemia than the right ventricle

Left ventricle

Right ventricle

Coronary Circulation

Left ventricle

Right ventricle

Left ventricle

Right ventricle

SYSTOLE DIASTOLE

Coronary Circulation

During systoleLeft ventricular pressure is slightly higher than aortic

(and therefore coronary artery) pressure Very little blood flow to the left ventricular myocardium

during systole, significant flow occurs only in diastole

Right ventricular pressure is lower than aortic (and therefore coronary artery) pressure Blood flow to right ventricle continues during systole

and diastole

systole systole

Effect of heart rate on coronary blood flow

systole systole

Increased heart rate

Shorter diastole

Reduced LV blood flow

Coronary Circulation

Regulation – No significant control by sympathetic nerves to

arterioles Regulated by metabolic needs – flow increases when

the work of the heart increases (Note: sympathetic stimulation increases activity of the heart) Hypoxia - the main vasodilator Adenosine Hypercapnia, lactate, H+

Autoregulation – present but not a major regulatory mechanism

Cerebral Circulation

Cerebral circulation is part of contents enclosed within a rigid container – the skullNon-expandable spaceAny increase in any one of contents

Take space from others Increase intracranial pressure

Blood flow should not increase significantly; if it does intracranial pressure will increase

Cerebral Circulation

Brain is a critical organ Contains vital centres Regulates critical functions of the body

Function can not be compromised

Blood flow can not be allowed to decrease significantly

Cerebral Circulation

Regulation

Autoregulation is well developed

However, the total cerebral circulation is dependent on other factors too

Cerebral Circulation

RegulationAutoregulation is not the only regulatory mechanism

Increase in CO2 is the main stimulus for cerebral vasodilatation – hypercapnia is a cause of raised intracranial pressure

Hypoxia is a less potent stimulus for cerebral vasodilatation

Circulating hormones / chemicals have no effect Autonomic nervous system has little influence on the

cerebral blood flow

Cerebral Circulation

Regulation Metabolic factors are the most important regulators Parts of the brain become more active / less active

depending on what the individual is doing

These metabolic influences change the blood flow to different parts of the brain to meet the metabolic demands of the different parts, but the total blood flow to the brain remains mostly unchanged Redistribution

Cerebral Circulation

Effect of increased intracranial pressure Blood flow tends to get reduced Body attempts to compensate by increasing the arterial

blood pressure Increased pressure affects the medullary centres and

cause bradycardia

Increased intracranial pressure

Raised blood pressure, bradycardia

CUSHINGREFLEX

Skin Circulation

Little metabolic change from time to time ( except for the sweat gland activity, which is related to temperature control

Plays a major role in blood pressure and temperature control

Skin Circulation

Regulation – mainly by central nechanisms Has a rich sympathetic supply Under the control of the vasomotor centre Influenced by the temperature centre

Skin Circulation

Skin

Venule

Capillary

Small artery

Skin Circulation

Shunting blood from arterial to venous side bypassing capillaries help in temperature regulation

Skin

Venule

Capillary

Small artery

Arterio-venous shunt