Blood pressure Blood pressure Dr Badri Paudel www.badripaudel.com Ø Blood pressure means the force exerted by the blood against the vessel wall Ø ( or the force exerted by the blood against any unit area of the vessel wall) Ø Blood Pressure is stored energy (potential energy) Formation of the blood pressure Mean circulatory filing pressure (MCFP): Total peripheral resistance Cardiac pumping Elasticity of Windkessel vessel Formation of the blood pressure: Ø (1) Mean circulatory filing pressure (MCFP): Ø when heart beat is stopped, the pressure in any point of cardiovascular system is equal. This pressure is called MCFP Ø systemic circulation, 7 mmhg; Ø pulmonary circulation, 10 mmhg. Ø (2) Total peripheral resistance. Formation of the blood pressure: Ø (3) Cardiac pumping Ø Energy released from heart contraction is transferred into parts, Ø 1) kinetic energy (1% of the total), Ø 2) potential energy (pressure) (99% of the total). Ø That means most part of energy used to create the blood pressure 1
Blood Pressure: Generated by Ventricular Contraction Formation of the blood pressure: Ø (4)Elasticity of Windkessel vessel Ø 1 diastolic blood pressure Ø 2 continuous blood flow in diastole Ø 3 buffering blood pressure Physical Characteristics of the Systemic Circulation Ø (1)The velocity of blood flow in each segment of the circulation is inversely proportional to its cross-sectional area. Physical Characteristics of the Systemic Circulation Ø (2) Pressure and resistance in the various portion of the systemic circulations. Ø The decrease in pressure in each part of the systemic circulation is directly proportional to the vascular resistance. Concept of Arterial Pressure Ø Blood pressure in the aorta and other big arterials. 2
2. Normal Range of Arterial Pressure Mean arterial pressure Ø Systolic pressure (Ps) the maximum of the pressure during systole Ø Diastole pressure (Pd) the minimum pressure during diastole Ø Pulse pressure the difference between Ps and Pd Ø Mean arterial pressure the average pressure throughout each cardiac cycle. Ø Mean arterial pressure (Pm) = Pd + Pulse pressure / 3 Normal range of arterial pressure Ø At rest, the arterial pressure of Chinese adult young people should be Ø Ps 100 120 mmhg Ø Pd 60 80 mmhg Ø Pulse pressure 30 40 mmhg Measurement of the arterial pressure Ø Direct (inserting a cannula into the artery) Measurement of the arterial pressure Ø Indirect (auscultatory) method Ø Stethoscope 3
Blood Pressure (BP): Methods of measuring BP 1. Auscultatory method it depends on the recognition of Korotkoff s sounds. In hyper dynamic circulations (such as during exercise) diastolic value is taken when the sound is muffled. In rare conditions auscultatory gap can be present. 2. Palpation method it gives rough idea about systolic pr. value only. It is 10-15 mmhg less accurate than the auscultatory method. 3. Factors Determining Arterial Pressure Normal values Ø Stroke volume ---- Ps Ø Heart rate ---- Pd Ø Total peripheral resistance (Ps) Ø Action of Windkessel vessel (aorta and other large arteries) Pulse pressure Ø Mean circulatory filling pressure - Systolic BP 90-130 mmhg - Diastolic BP 60-90 mmhg - Mean BP = Diastolic BP + ⅓ Pulse pressure 100 mmhg Also Mean BP = CO X TPR - The gravitational effect in standing position causes the mean BP in a large artery in the head to be 62 mmhg, whereas it increases the mean BP in the feet to a value close to 180 mmhg. - The arterial pressure is conventionally written as systolic over diastolic pressure. e.g. 120/80. BP tends to rise with age. 4
Integrated regulation of arterial BP Mechanisms responsible for maintaining stable BP can be classified into three groups depending on their time of onset. 1. Immediate control responses they act within seconds, they are; A. Baroreceptor feedback mechanism B. Chemoreceptor mechanism C. CNS ischemic response These mechanisms act through the autonomic nervous control of the circulation (generalized arteriolar constriction, venoconstriction, and direct stimulation of the heart). 2. Intermediate control responses they are; A. Renin-angiotensin vasoconstrictor mechanism B. Stress-relaxation mechanism C. Capillary fluid shift mechanism These three mechanisms become activated within 30 minutes to several hours during which the nervous mechanisms fatigue and become less effective. Their effect can last for days if necessary. 3. Long term mechanism for pr. regulation this mechanism relies on the renal-blood volume (or body fluid) control mechanism. An increase in blood pr. has a direct effect on the glomerulus. It causes the kidney to excrete extracellular fluid (i.e. pressure diuresis), thus reducing the blood volume and causing the pr. to return back to normal. Baroreceptor Feedback Loop for the Regulation of Mean Arterial Blood Pressure stretch mechanoreceptors High pressure baroreceptors respond to stretch in the aortic arch and carotid sinus. Carotid and Aortic Baroreceptors sinus nerve depressor nerve Fig 22-2 5
Carotid nerve fires above and below normal pressures. Aortic nerves are activated above normal pressures. Vagal/glossopharyngeal AFFERENTS NTS Medulla Oblongata Cardio-inhibitory area Vasomotor area Aortic sinus MAP ~ 93.5 mmhg Increased firing of Baroreceptors by stretch Sympathetic and Parasympathetic EFFERENTS The aortic receptors help reinforce the carotid activation above n pressures. MAP TARGET ORGANS -heart -blood vessels -adrenal medulla -glands (skin/sweat) Low Pressure Cardiac Baroreceptors Baroreceptor Bainbridge Cardiac Output Heart rate increases under volume depletion and loading Stroke Volume Heart rate Bainbridge Reflex Unlike activation of the high-pressur baroreceptors, activation of the A an B fibers will INCREASE heart rate. (and also cause renal vasodilation). In volume depletion conditions Baroreceptor response prevails In volume loading conditions Bainbridge response prevails Bainbridge response only HR is affected Respond to fullness or volume Located in Low pressure sites Control the effective circulating volume Indirectly regulate MAP Baroreceptor -Increased stretch on hig pressure decreases stroke volume flattening the Starling respon Sympathetic input - HEART Parasympathetic input - HEART ACTIONS Nerve fibers release NE SA, atria, and ventricles HR and contractility R side SA node L side contractility MECHANISM ß1 receptors pacemaker activity ß1 myocardium contraction ACTIONS Vagus nerve releases ACH SA and myocardium HR and conduction velocity R side SA node (HR) L side contractility MECHANISM Muscarinic receptors (M2) ßγ subunit (HR) Nitric oxide (weak inotropic effect) 6
Net result of Chemoreceptor stimulus is an integration of central and peripheral chemoreceptors Venous return Hemorrhage blood volume SV and CO Atrial volume MAP LP Baroreceptors Stretch of pulmonary receptors cancel peripheral stimulus on cardio-inhibitory area causing ta Central Chemoreceptors Peripheral Chemoreceptors Hormonal response -Angiotensin/Renin -ADH release -ANP (decreased) HP Baroreceptors Heart rate Contractility Medullary Cardiovascular Control Center SYMPATHETIC RESPONSE Vasoconstriction (arteriole/venous) 7