Special Circulation Qiang XIA (夏强), PhD Department of Physiology Room C518, Block C, Research Building, School of Medicine Tel: 88208252 Email: [email protected] System Overview The blood flow to organs depends on ⒈ The difference between aortic pressure and central venous pressure ⒉ The diastolic and systolic state of blood vessel in this organ The blood flow to individual organs must vary to meet the needs of the particular organ, as well as of the whole body System Overview Neural, myogenic, metabolic, and endothelial mechanisms control regional blood flow Neural mechanism: Autonomic nervous system (sympathetic division) Blood pressure Cardiac output Local blood flow Myogenic mechanism: Autoregulation in the vessels of heart, brain, skeletal muscle, and kidneys Metabolic mechanism: Local control in the vessels of heart, brain, skeletal muscle during exercise PO2, pH, etc. Endothelial mechanism: NO, EDHF, PGI2, ET, EDCF, etc. Relaxing or contracting VSMCs System Overview Neural mechanism: Autonomic nervous system (sympathetic division) Myogenic mechanism: Metabolic mechanism: Sophisticated feedback, Mechanical forces, etc. Local circulation Resting vasomotor tone Vasomotor control Electrical and chemical signalling PO2, pH, etc. Endothelial mechanism: NO, EDHF, PGI2, ET, EDCF, etc. VSMCs ECs Gap junction Coronary circulation 冠脉循环 Coronary circulation Heart: view from front Coronary circulation Heart: view from diaphragm Coronary circulation • Coronary circulation receives 5%of the resting cardiac output form the left heart, and mostly returns it to the right heart • Heart muscle consumes as much O2 as does equal mass of SM during vigorous exercise • Heart tissue extracts maximal amount of O2 at rest • The only way to increase of energy is by increasing blood flow • Autoregulation: relative stable flow between 70 and more than 150mmHg Diagram of the epicardial, intramuscular, and subendocardial coronary vasculature • The branches of left and right coronary artery often penetrate myocardium in direction perpendicular to cardiac surface • Myocardial capillary distribution is extremely abundance • Collateral coincidence between coronary is less Extravascular compression impairs coronary blood flow during systole Isovolumic contraction phase ↓↓→ rapid ejection phase ↑→reduced ejection phase ↓→ diastolic phase ↑ (isovolumic relaxation phase↑↑) Myocardial blood flow parallels myocardial metabolism • Metabolic signals are the principal determinants of O2 delivery to myocardium – Resting: 60-80 ml/100g/min – Exercise: 300-400 ml/100g/min – O2 consumption 7-9 ml/100g/min is about 65-70% of O2 extraction • Adenosine activates purinoceptors to induce vasodilation by lowing [Ca2+]i Local metabolic changes that cause vasodilation in the systemic circulation CHANGE MECHANISM ↓ PO2 ↓ [ATP]i, adenosine release ↑ PCO2 ↓ pHo ↓ pH ↓ pHo ↑ [K+]o Depolarization → opens voltage-gated Ca2+ channels ↑ [lactic acid]o Probably ↓ pHo ↓ [ATP]i Opens KATP channels ↑ [ATP]o Activates purinergic receptors ↑ [ADP]o Activates purinergic receptors ↑ [Adenosine]o Activates purinergic receptors Notes • Although sympathetic stimulation directly constricts coronary vessels, accompanying metabolic effects predominate, producing an overall vasodilation • Collateral vessel growth can provide blood flow to ischemic regions • Vasodilator drugs may comprise myocardial flow through “coronary steal” Collateral vessel Coronary steal • Which of the following is the most common cause of an increased coronary blood flow? A A decreased coronary perfusion pressure B An increased ventricular diastolic pressure C An increased stimulation of α-adrenergic receptors in the heart D An increased stimulation of β-1 adrenergic receptors in the heart E An increased stimulation of β-2 adrenergic receptors in the heart • Coronary blood flow A Is greatest during diastole in the left ventricle B May increase twelve-fold at maximal myocardial work levels C Is dependent upon the difference between aortic pressure and coronary sinus pressure D Is not affected by heart rate or myocardial contractile state E Is increased by incomplete ventricular relaxation Cerebral circulation circle of Willis The major arteries of the brain. (A) Ventral view, Lateral (B) and (C) midsagittal views, (D) Idealized frontal section Cerebral circulation Blood supply of the three subdivisions of the brainstem. (A) Diagram of major supply. (B) Sections through different levels of the brainstem indicating the territory supplied by each of the major brainstem arteries Cerebral circulation • Brain weight: 2% of body weight • Blood flow: 15% of cardiac output at rest • Brain is the least tolerant of ischemia • Arteries: internal carotid arteries, vertebral arteries • Brain lacks lymphatic vessels 1. 脑循环特点 Changes in regional blood flow SENS 1: Lowintensity electrical stimulation of hand SENS 2:Highintensity electrical stimulation of hand Cerebral Blood Flow • Neural control: – Sympathetic nerve – Parasympathetic nerve – Sensory nerve: “axon reflex” • Metabolic control: – PO2 – PCO2 – pH • Myogenic control Autoregulation • Nearly constant blood flow: perfusion pressure from 70 to 150 mmHg Cushing Reflex • Cushing reflex is a physiological nervous system response to increased intracranial pressure (ICP) • Cushing's triad: – Hypertension – Bradycardia – irregular respiration • It was first described in detail by American neurosurgeon Harvey Cushing in 1902. • Which of the following would be expected to DECREASE cerebral blood flow? A Hyperventilation B Hypoventilation C Activity of sympathetic adrenergic nerves D Activity of parasympathetic cholinergic nerves E Moderate exercise • The circulation through all of the following tissues is almost exclusively locally controlled, EXCEPT A Skin B Brain C Heart D Skeletal Muscle • A 16-year-old male presents to your emergency room with a gun shot wound to his abdomen. The bullet entered the upper left quadrant, perforating the spleen and removing the splenic flexure of the large intestine. His heart rate is rapid, and he is bleeding profusely. He is unconscious, and his blood pressure is low, but his pupils still respond to light. Under normal circumstances what percent of cardiac output goes to the brain? A 6% B 10% C 14% D 18% E 22% The End.