Blood Supply & Control of Blood Flow PDF

Blood Supply & Control of Blood Flow to the Heart, Brain, Kidneys and Lower Limbs Dr Aaron Murray Spring Semester – MGEM2019 Learning objectives u To describe the major arterial supply and venous drainage of the heart, brain, kidneys and lower limbs u To relate deficiency of supply of oxygenate...

Blood Supply & Control of Blood Flow to the Heart, Brain, Kidneys and Lower Limbs Dr Aaron Murray Spring Semester – MGEM2019 Learning objectives u To describe the major arterial supply and venous drainage of the heart, brain, kidneys and lower limbs u To relate deficiency of supply of oxygenated blood to the heart, brain, kidneys and lower limbs to symptoms and signs in patients u To describe the mechanisms of how blood flow is controlled Arteries Supplying Blood to the Brain Internal Carotid Arteries • Branch of common carotid • Enters skull via carotid canal •Main supply for the Anterior Cerebral Arteries (ACA) and the Middle Cerebral Arteries (MCA) Vertebral Arteries • Branches of subclavian • Enter skull via foramen magnum So what is the Circle of Willis? Why might this be Advantageous? •Re-join to form the basilar artery •Main supply for the Posterior Cerebral Arteries (PCA) Arterial of the Brain ti d o r Ca V te r e a br l ACA ACA MCA PCA PCA PCA Venous Drainage of Brain Inferior sagittal sinus Superior petrosal sinus Superior sagittal sinus Transverse sinus Sphenoparietal sinus Sigmoid sinus Straight sinus Cavernous sinus Inferior petrosal sinus Anatomy of the Jugular Veins IJV sits under sternocleidomastoid (SCM) and the EJV sits above it External jugular vein (EJV) • Runs from angle of mandible to where SCM meets clavicle • Sits under platysma • Injury can result in venous air embolus Subclavian vein Internal jugular vein (IJV) • Runs from jugular foramen to jugular notch • Sits lateral to common carotid artery • Accessed via lesser supraclavicular fossa Symptoms of Reduced Blood Flow to the Brain Loss of motor function Syncope Sensory loss Visual field loss Confusion Temporary loss of consciousness Blindness Loss of consciousness Amnesia Ataxia Behavioural change General term for the loss of coordinated functions Problems with balance Drowsiness Speech problems Jugular venous pulse (JVP) Internal jugular vein • This is a vein without valves • Thus, pressure changes on right side of circulation are transmitted directly to it • In a normal individual, the JVP in the IJV will be just visible above the clavicle when the person is lying at 45°. • Jugular venous pulse can be elevated in heart failure • This is because the heart is finding it difficult to pump out sufficient amounts of blood and venous return is impaired • It can also be elevated in other conditions External jugular vein • Often has valves so less useful when estimating jugular venous pressure Arterial supply to kidneys • Usually, each kidney is supplied by a single renal artery • therefore these are end-arteries with no collateral supply • These renal arteries are direct branches off the abdominal aorta • They sit at vertebral level L1 • Turbulent flow in these arteries may be heard as an epigastric bruit on auscultation Venous drainage of kidneys – less important, but interesting • Renal veins drain directly into the inferior vena cava (IVC) • The left renal vein passes anterior to the abdominal aorta (not shown above) • Blood from the left adrenal gland and the left gonad drains into the left renal vein • Conversely, Blood from the right adrenal gland and the right gonad drains directly into the IVC Symptoms of Reduced Renal Blood Flow Kidneys are the organs that filter waste products from the blood and help regulating blood pressure, electrolyte balance, and red blood cell production in the body. Reduce Blood flow = Reduced renal function Malignant hypertension Depending on how long this goes on, may cause: High blood pressure (hypertension) that's difficult to control Nausea, Vomiting, Loss of appetite, Fatigue and weakness, Sleep problems, Changes in how much you urinate, Decreased mental sharpness, Muscle twitches and cramps, Swelling of feet and ankles, Persistent itching, Think about the role of the RAAS (renin–angiotensin– aldosterone system) in regulating fluid volume as discussed in supply and demand modules last year NOTE: The diagnosis of kidney failure usually is made by blood tests measuring Blood Urea Nitrogen, Creatinine, and Glomerular filtration rate (GFR). Coronary Arteries – Important Right coronary artery • Arises above the anterior cusp of the aortic valve (therefore flow greatest during diastole) • Gives off various branches (incl. marginal branch) • Curves around to posterior surface of heart as well, gives off posterior interventricular artery • Supplies: • both atria • most of right ventricle • posterior left ventricle • SA node (usually) Coronary Arteries – Important Left coronary artery • Arises above the left posterior cusp of the aortic valve (therefore flow greatest during diastole) • Within 2 cm, splits into: • the anterior interventricular branch and • the circumflex branch • Supplies: • both atria • anterior left ventricle • some right ventricle Cardiac Veins – Not so Important • There are four main cardiac veins: • Great cardiac vein • Small cardiac vein • Anterior cardiac vein • Middle cardiac vein • They drain blood from local areas of cardiac muscle • Eventually all of them come together at the coronary sinus, which eventually drains directly into the right atrium • There are also many so-called Thebesian veins that drain directly into the other cardiac chambers Symptoms when myocardial oxygen demand outstrips oxygen supply Referred pain Chest ‘tightness’ Chest pain Chest ‘heaviness’ Retrosternal pressure Dyspnoea (breathing difficulties) Diaphoresis Nausea Fatigue (sweating) Weakness None Arterial supply to the lower limbs • All lower limb blood supply originally flows down the external iliac artery and then into the femoral artery • The various branches can be palpated at the points marked X • This is one of the key steps in examination of the peripheral vasculature • In someone with peripheral vascular disease, some of these pulses may be weak or absent • More complex analyses (such as ankle:brachial pressure index) may be performed Venous drainage of the lower limbs • Most of the time, gravity is working against venous return from the lower limbs • Venous drainage is critically-dependent on movement, the presence of valves in leg veins and the function of the ‘muscle pump’ • Any degree of stasis of blood in the lower limbs predisposes the person to thrombosis (development of clots) • Lack of drainage may also cause peripheral oedema • Varicose veins may also develop Reduced blood flow to lower limbs - symptoms Leg Intermittent claudication Buttock / thigh Foot (rare) Cold feet / toes Hair loss on legs Gangrene Ischaemic rest pain Paraesthesia Peripheral Nerve Damage Sensory loss Ulcers Autoregulation of Blood Flow: Principle • In all tissues of the body, blood flow is controlled by neural and hormonal influences • In most tissues these neural and hormonal influences are essential for adequate regulation of blood flow • In the case of the brain, the heart and the kidneys, blood flow can be regulated in the absence of neural and hormonal influence (see supply and demand lectures) • Blood flow is maintained at a constant level over a wide range of perfusion pressures • This is termed autoregulation of blood flow Autoregulation of Blood Flow: How? • Signals of the tissue’s metabolic status (pO2, H+, K+, lactic acid, adenosine, inorganic phosphate) • Intrinsic myogenic mechanisms of the arteriolar smooth muscle (e.g. stretch-induced vasoconstriction) • Nitric oxide (NO, EDRF). Increased shearing forces on endothelial cells caused by increased perfusion pressure lead to increased synthesis of NO by NO synthase (NOS) • Other endothelial factors such as endothelin-1 (vasoconstrictor) and prostacyclin (vasodilator)… there are many and more are being found each year! • The ‘local hormones’ histamine and bradykinin Autoregulation: the Metabolic Hypothesis Nitric oxide (NO) • A number of different autacoids, neurotransmitters and blood constituents can trigger release of NO from endothelial cells • This can also be elicited by various types of blood flow • NO synthase is ‘switched on’ in the endothelial cell, resulting in the production of NO from L-arginine • The NO can then diffuse out of the endothelial cells and stimulate guanylyl cyclase in smooth muscle cells to elicit vasodilation • Pharmacologically, nitrate vasodilators can target this system Important Regulators of Coronary Blood Flow • The myogenic mechanism • Metabolic signals • Endothelial substances • Sympathetic input (+ circulating adrenaline) o Coronary arterioles have a significant population of b2-adrenoceptors, which mediate vasodilation Summary u Covered the major arterial supply and venous drainage of the heart, brain, kidneys and lower limbs (don’t get too caught up on the minor secondary and tertiary arteries, focus on the ones talked about today) u Related deficiency of supply of oxygenated blood to the heart, brain, kidneys and lower limbs to symptoms and signs in patients u Describe the mechanisms of how blood flow is controlled, stimulation and autoregulation Reading • Color Atlas and Text of Clinical Medicine, 3rd Ed. pp 212-215, 239-242, 473-476. UNaD WB141 FOR • Rhoades & Tanner Medical Physiology, 2nd Ed. pp 276-281, 300-303, 384-386. UNaD QT104 MED