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Blood Pressure Clinical Science Applied to Nursing CopyrightCSAN2005CardiffUniversity Learning Outcomes • Define blood pressure • State the function of blood pressure • Explain how blood pressure is commonly measured • Define cardiac output and state how it is calculated • Discuss the factors that are involved in cardiac output • List the factors that regulate arterial blood pressure What is Blood Pressure? • Blood pressure is the force exerted on a blood vessel wall by the blood. • Blood must circulate through the body and organs to maintain life • The Heart is the pump that circulates the blood • Pressure difference in the vascular system ensures that blood flows around the body Effects of High Blood Pressure on Your Body • Artery Damage Effects of High Blood Pressure on Your Body • Hardening of the arteries • Stroke • Heart attack • Kidney damage • Blindness Blood pressure • Blood pressure is different in blood vessels and varies from minute to minute dependant on factors such as stress Blood Pressure • A blood pressure reading consists of two numbers: • Systolic pressure- the first, highest number – Indicates pressure when the ventricles contract to push blood out to the body • Diastolic pressure- the second, lower number – Indicates when the heart relaxes between beats Function of blood pressure • Systemic BP maintains the essential flow of substances into and out of organs • Control of BP is essential for the maintenance of homeostasis Blood flow through the vessels Blood flow through vessels is directly proportional to the difference in pressure between the ends of the tube Arterial Blood Pressure the driving force for blood in the circulatory system healthy young adults: Resting Systolic pressure = 120 mmHg Diastolic is about 80 mmHg healthy children values vary (see Whaley & Wong 2000): Rough Guide Systolic pressure • 1 –7years: age in years + 90: • 8-18 (2 x age in years) + 83 & Diastolic pressure: • 1-5 years 56mmHg, • 6-8years: age in years + 52 • How can we measure Blood Pressure? Blood Pressure can be measured in two ways: 1. Indirectly, using a sphygmomanometer and ausculating (listening) with a stethoscope, the sounds you hear are called Korotkoff sounds 2. Directly, using an arterial cannula (only used in Critical Care Areas, Operating Theatres) Measuring BLOOD PRESSURE External blood pressure measurements: We apply a cuff to the arm &apply pressure to the cuff to form a constricting band around the arm and around internal arteries. The pressure in the cuff is initially above Blood pressure in the arteries– as the cuff pressure falls, it ‘meets’ the blood pressure & 1st Korotkof sound is heard. When the cuff pressure falls below the blood pressure, the Korotkof sound disappears Making sense of what we hear Factors which influence blood pressure • Cardiac output (CO) • Total Peripheral Resistance (TPR) Or BP = CO x TPR Factors which influence BP • Cardiac output (and venous return) • Blood volume and viscosity • Peripheral resistance • Elasticity of the blood vessels Cardiac output Cardiac Output = Heart Rate x Stroke Volume l/min bpm mL In a healthy adult this is 70 x 75mL= 5L/min Factors that influence cardiac output • Stroke volume – the amount of blood ejected from each ventricle at each heartbeat. • Heart Rate – regulated by the autonomic nervous system (ANS) Stroke Volume Stroke volume is determined by three factors: Preload Afterload Contractility Preload - related to the volume of blood in the ventricle at the end of diastole: - End Diastolic Volume Factors affecting preload are: • Blood volume • Venous return • The vasomotor tone Preload: Factors which influence Venous return to the heart • Muscle pump • Respiratory Pump • Valves in the vein ‘Starling’s law of the heart’ • If, for any reason, the blood returning to the heart increases, more ventricular filling will take place during diastole; The ventricular muscle fibers will be stretched and on the next systole (contraction) it will give more powerful contraction, resulting in stroke volume. • more fibers are stretched, the greater the force of the contraction (within reason) Afterload • Described as the resistance against which the ventricle must work. • Whilst Preload is a major determinant of myocardial contractile power • Afterload is mainly mechanical factor that affects performance Total Peripheral Resistance (TPR) • Peripheral Vascular Resistance – resistance exerted by the action of the walls of the resistance vessels impeding blood flow • most resistance is provided systemically by the arterioles,and small and medium sized arteries – – exert a powerful influence in the control of blood pressure. Factors which influence TPR The length of the vessel the blood The diameter of the lumen of the vessel The viscosity of the blood The tone of blood vessels depends on the relative structure Control of Blood Pressure Short term control - mainly involves: • Baroreceptor reflex • Chemoreceptor reflex • Circulating hormones Long term control involves regulation Of blood Pressure by the kidneys. MECHANISMS OF B/P CONTROL Higher brain stimulus Proprioreceptors Baroreceptors Chemoreceptors Renin - Angiotensin - Aldosterone Mechanism Blood Volume Renal Perfusion Angiotenisogen + Reduced B/P & GFR JGA RENIN Low Na+Cl – at macula densa. Low afferent arterial Pressure. Angiotensin 1 B/P blood volume negative feedback (ACE) Angiotensin 2 Renal Na+ & H2O reabsorption Aldosterone Stimulates Adrenal Cortex arterioles & thirst Produces vaso constriction of & small arteries ADH (Anti Diuretic Hormone) Released from Posterior Pituitary in response to Low Blood Volume & B/P • EFFECTS - vasoconstriction of mainly Splanchnic circulation & fluid retention Fluid Exchange • 85% of fluid that leaves blood is returned at venous end • What about the other 15%?