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 arteries Both 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 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 Right ventricle

Coronary Circulation Left ventricle Subendocardial muscle are furthest away from main arteries Subendocardial muscle are the most likely to suffer from ischaemia Right ventricle Subendocardial muscle endocardium

Coronary Circulation Left ventricle 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 Right ventricle

Coronary Circulation SYSTOLE DIASTOLE Left ventricle Left ventricle Right ventricle Right ventricle

Coronary Circulation During systole Left 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 Effect of heart rate on coronary blood flow 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 skull Non-expandable space Any 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 Regulation Autoregulation 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 CUSHING REFLEX Raised blood pressure, bradycardia

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 Capillary Venule Small artery

Skin Circulation Shunting blood from arterial to venous side bypassing capillaries help in temperature regulation Skin Capillary Arterio-venous shunt Venule Small artery