L28 Functional Characteristics of Special Circulation PDF

 Prof Dr. Abdelaziz M. Hussein Prof and Chairman of Medical Physiology Department Contacts : Email: [email protected] Mob. #: …………………………………. Office hours : 10.0 AM- 11.0 AM Tuesday, Thursday  Lecture contents 1. Functional characteristics of Coronary Circulation 2. Functional characteri...

 Prof Dr. Abdelaziz M. Hussein Prof and Chairman of Medical Physiology Department Contacts : Email: [email protected] Mob. #: …………………………………. Office hours : 10.0 AM- 11.0 AM Tuesday, Thursday  Lecture contents 1. Functional characteristics of Coronary Circulation 2. Functional characteristics of Pulmonary Circulation 3. Functional characteristics of Cerebral Circulation 4. Functional characteristics of Skeletal ms Circulation 5. Functional characteristics of Cutaneous Circulation  Coronary Circulation ▪Functional characteristics of coronary vessels ▪O2 consumption in heart tissues ▪Coronary blood flow ▪Regulation of CBF (Intrinsic and extrinsic)  Coronary Circulation; Blood vessels Coronary circulation concerned with the blood supplying the cardiac ms Are 2 coronary arteries (Rt and Lt) arising from the aorta just above the aortic valve There are little anastomoses () the 2 coronaries, which are not sufficient to supply the cardiac ms with blood, if one of them is occluded→ functional end arteries Capillaries run parallel to the cardiac ms fibers and about one coronary capillary for each ms fiber. Venous drainage occur by coronary sinus and anterior cardiac vein drain into right atrium and deep veins drain directly into the all chambers  Coronary blood flow and O2 consumption O2 Consumption of Heart Value: At rest → is about 70% of O2 in the coronary arterial blood. On other tissues → is about 25% of O2. The coronary venous blood has low venous O2 reserve. Coronary Blood Flow Value: During rest: is about 250 ml/minute (about 5% of the COP). In severe exercise: ↑es to 3-4 fold i.e. may reach 1 L/min  Phasic changes in Coronary blood flow 1) During systole: CBF ↓ during ventricular systole (maximal during ICP) (the flow may stop completely) Such ↓ in CBF during systole is compensated by O2 delivered from myoglobin 2) During diastole: The cardiac ms relax completely and so, the blood flows rapidly into the coronary arteries. The highest CBF occurs during isometric relaxation phase. N.B.: Phasic changes in the Rt ventricle is more on Lt vent than right vent due to low pressure values So, the ischemic necrosis (myocardial infarction) affects mainly parts of the Lt ventricle rather than Rt ventricle Regulation of Coronary Bl. Flow 1) Intrinsic regulation (autoregulation) 2) Extrinsic regulation a) Nervous regulation b) Mechanical regulation c) Chemical regulation  A) Intrinsic or autoregulation of CBF Definition It is the ability of the heat to maintain its flow constant in absence of extrinsic factors Mechanism: O2 demand is the major factor in local blood flow regulation. Hypoxia or increase in O2 demand Direct relaxing effect on Release of Adenosine, K, H, CO2, smooth ms of bl vessels Bradykinin and PGI2 Stimulate endothelium to secrete Nitric oxide N.B. Active hyperaemia (marked ↑ of CBF during exercise) and reactive hyperaemia: (marked ↑ of CBF after temporary occlusion)  B) Extrinsic regulation of CBF 1) Nervous system : a) Sympathetic stimulation: Direct action → VC due to ++ of α1 receptors and mild VD due to ++ of β2 receptors. Indirect action → ↑es the metabolic activity of the heart → strong VD. ❖Net effect of sympathetic ++ is an ↑ in the coronary blood flow. b. Parasympathetic stimulation → opposite effect to sympathetic ++ c. Anrep's reflex: ↑ VR and venous pressure in the Rt atrium → reflex coronary VD and ↑es the coronary B.F. via Vagus nerve. It is important in exercise to supply the cardiac ms with more O2. d. Gastrocoronary reflex: Distension of the stomach with heavy meal → reflex coronary VC and ↓es the CBF→ anginal pain may be felt in certain persons after heavy meals B) Extrinsic regulation of CBF 2) Mechanical factors a) Phases of cardiac cycle: as above. b) Heart rate: ↑HR → ↓es the coronary blood flow by shortening of the diastolic period. ↓ HR → ↑es the coronary blood flow by prolonging the diastole. c) ABP: ↑ed ABP→ ↑es coronary flow ↓ed diastolic BP as in aortic regurgitation→ ↓es coronary flow, so anginal pain may occur in these patients. d. Cardiac output: ↑ed COP → ↑es coronary blood flow e. Mechanical occlusion of coronary artery: reactive hyperaemia occurs B) Extrinsic regulation of CBF 3) Chemical factors a. Nitroglycerin sublingually has a fast coronary dilating effect, within 2-3 minutes. b. Long acting nitrates as isosorbide dinitrate given orally also produce coronary dilatation. c. Beta-blockers, e.g. propranolol and atenolol act by reducing myocardial O2 requirements during exertion and stress. d. Calcium entry blocking agents, e.g. nifedipine and verapamil also reduce myocardial O2 requirements and induce coronary VD. Cerebral Circulation 1) Functional characteristics of cerebral vessels 2) Cerebral blood flow 4) Regulation of CBF (Intrinsic and extrinsic) Cerebral Circulation; Blood Vessels Cerebral Circulation; Blood Supply 6 cerebral arteries, no crossing between 2 sides and functional end arteries Cerebral blood flow in normal adult the brain weights 1400 gm and receives 750 ml blood /min (14% of cardiac output). In children cerebral blood flow is nearly double it's value in adult and it falls to the adult level at puberty due to sex hormones Blood flow of the grey matter is about six times that of the white matter. Blood flow increases in active area and decreases in the inactive one i.e. change in regional blood flow Very sensitive to hypoxia (fainting occurs within few seconds of cerebral ischemia) and hypoglycemia Regulation of Cerebral Bl. Flow 1) Intrinsic regulation (autoregulation) 2) Extrinsic regulation a) Chemical factors e.g. CO2, O2 & H+ b) Nervous e.g. role of sympathetic Innervation c) Mechanical i.e. Role of ICP A) Intrinsic Autoregulation of Cerebral Bl. Flow Cerebral BF remains more or less constant with a change in ABP between 60-150 mm Hg (autoregulatory range) Theories for autoregulation: - Myogenic (↑ABP→ ↑ BF → stretch of vascular smooth muscle → stretch-induced ms contraction → ↓ BF back to normal) ↓ ABP → ↓BF→ inhibition of smooth ms→ vasodilatation→ ↑BF back to normal - Metabolic theory: ↓BF→ accumulation of metabolites (adenosine, K+, lactate, ATP) →VD→↑BF B) Extrinsic regulation of Cerebral Bl. Flow 1) Chemical factors: role of CO2, O2 & H+ Changes in CO2, O2 & H+ concentrations have important effect on Cerebral BF 70%↑in PCO2 will double CBF The effect is not direct but through H+ which causes VD of cerebral Bl.V This is beneficial to get rid off excess H+ that causes depression of nervous system A decrease in cerebral tissues PO2 below 30 mm Hg stimulates VD of cerebral blood vessels  B) Extrinsic regulation of Cerebral Bl. Flow 2) Mechanical factors: intracranial pressure (ICP) The brain is enclosed within the rigid cranium (having a volume of ~ 1600-1700 ml)→ so the volume within the cranium (blood, brain and CSF) must be kept constant ICP is produced by presence of CSF Normally from 0-10 mm Hg Cerebral perfusion pressure (CPP) which is the pressure pushing blood inside the brain = MAP-ICP Thus, any factor that increases ICP will decreases CPP & consequently can reduce CBF Marked rise in ICP >33 mm Hg will compress blood vessels → marked reduction in CBF  B) Extrinsic regulation of Cerebral Bl. Flow 3) Nervous factors: Sympathetic nervous stimulation The cerebral vessels have rich sympathetic innervations However, under normal conditions they have no role in regulation of cerebral BF (overridden by autoregulatory mechanism) Their importance have been outlined as protective in cases of marked increased of ABP where it constrict the large & medium sized vessels thus protecting the smaller vessels from rupture  Pulmonary Circulation 1) Functional characteristics of pulmonary vessels 2) Pulmonary blood flow 4) Regulation of PBF(Intrinsic and extrinsic) Pulmonary Circulation; Blood Vessels It is concerned with passage of blood from the Rt ventricle, through the lungs and then to the Lt atrium. It's time is about 7 seconds at rest. It equals the COP from the Rt ventricle. The lung is supplied with blood from 2 sources; i) pulmonary arteries (venous blood) and ii) bronchial arteries (oxygenated blood) There are many anastomoses between the bronchial and pulmonary arteries Regulation of Plumonary Bl. Flow 1) Intrinsic regulation (autoregulation) 2) Extrinsic regulation a) Chemical factors e.g. CO2, O2 & H+ b) Nervous e.g. role of sympathetic Innervation c) Mechanical i.e. Role of ICP A) Autoregulation of Pulmonary Bl. Flow It is the automatic control of local pulmonary blood flow distribution. O2 concentration in the alveoli→ pulmonary V.C this effect is opposite to the response of systemic blood vessels to hypoxia which is V.D. Hypoxia leads to release of V.C substances from lung tissues. The amount of blood flow through the poorly ventilated lung is restricted and redistributed to better ventilated area. B) Extrinsic regulation of Pulmonary Bl. Flow a) Cardiac output: - Increased C O P, increases pulmonary blood flow with slight increase in pulmonary BP. The pulmonary capacity can change to buffer excessive changes in pulmonary BP b) Peripheral resistance: It equals 1/6 systemic resistance. Any  in pulmonary PR→ increase pulmonary B.P→ the Rt ventricle enlarge to  the power of contraction (starling law) If the increase in pulmonary B.P persists→→ Right ventricular hypertrophy and failure occurs c) Nervous factors Sympathetic stimulation → V.C of pulmonary blood vessels. Vagal stimulation produces dilatation of the pulmonary blood vessels. Skeletal Muscle circulation Functional characteristics Regulation of skeletal ms blood flow Skeletal ms blood vessels and blood flow The primary function of skeletal muscle is contraction & generation of mechanical force, therefore, it needs delivery of large amount of O2 & nutrients with removal of waste products This is achieved by high blood flow to skeletal muscles Value of Sk.ms BF: During resting Skeletal ms BF is about 20% of COP (1-4 ml/min/100gm tissue; low when compared with other organs). This relative high fraction (20% COP) because skeletal muscle represents ~ 40 -50% of TBW During exercise skeletal ms blood flow can increase by more than 20 folds and becomes ~ 80% of COP i.e. 20 L/min The maximal flow is 50-100 ml/min/ 100 gm.  Regulation of skeletal ms Bl. Flow 1) Nervous regulation (dominant in resting ms) 2) Local regulation (dominant in exercising ms) A) Nervous regulation of skeletal ms blood flow It is the dominant regulatory factor in resting ms. It is mediated by sympathetic NS supplying skeletal ms blood vessels a) α receptors → VC b) β2 & cholinergic receptors → VD Stimulation of symp. noradrenergic VC nerves (α adrenoreceptor-mediated) →↓ ms BF by ½ or 1/3 of normal (e.g. in Haemorrhage). During exercise or stress → vasodilatation of skeletal blood vessels occurs via blood-borne catecholamines (secreted by suprarenal medulla) acting on β2-receptor & via sympathetic cholinergic nerve fibres. B) Local regulation of skeletal ms blood flow It is the dominant regulatory factor in exercising ms. The marked increase in BF during muscle exercise is mediated by local metabolic factors that override the nervous control. The most important factor is reduction in O2 in muscle tissues (hypoxia has a direct effect & acts through releasing NO) Other VD metabolites such as adenosine, lactate, K+, ATP & CO2 Skeletal Muscle Circulation; Effect of Exercise During exercise, the increased O2 demand by the muscles will be met by an increase in: O2 extraction; increases from 7Vol% to 15Vol% Blood flow to muscles which is mediated by local metabolites, increased temperature & acidosis This increase in BF occurs in spite of increased sympathetic activity that is associated with exercise Distribution of COP in rest and exercise Skin or cutaneous circulation Functional characteristics Regulation of cutaneous blood flow  Cutaneous Circulation The major function of skin blood flow is regulating body temperature Skin represents ~ 4-5% of TBW Receives 6 % of COP The arterio-venous O2 difference is only 3 ml%→ so the flow is mostly non-nutrient flow. Blood vessels in skin are either; a) Nutritive vessels or b) Non nutritive vessels→ concerned with body temp. regulation. Regulation of cutaneous Blood Flow 1. Nervous regulation 2. Local metabolite regulation A) Nervous regulation of cutaneous blood flow The sympathetic supply to skin vessels is so powerful that cutaneous BF can range from 1% to 30% of COP e.g. a) Skin blood flow is