arterial blood pressure. arterial blood pressure “blood pressure” generally refers to arterial...

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Arterial Blood Pressure

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  • Arterial Blood Pressure

  • Arterial blood pressureBlood pressure generally refers to arterial blood pressure

    Definition - B.P. is the lateral pressure exerted by the column of blood on the walls of the arteriesIt is not steadyFluctuates during the Cardiac Cycle

  • Arterial blood pressureDuring ventricular systole B.P is higher than that during diastole

    Systolic B.P -the peak pressure during the ventricular systole

    Diastolic B.P -the minimum pressure during ventricular diastole

  • Arterial blood pressurePulse pressure -the difference between systolic and diastolic B.P

    Mean arterial blood pressure It is the average pressure throughout the cardiac cycle

    Pulse Pressure

  • Arterial blood pressureMean B.P

    Normally diastole is longer than systoleThus mean arterial BP is not the arithmetic mean of systolic and diastolic BP

    Mean BP = Diastolic B.P + 1 of pulse pressure

    Mean BP = SBP + (2xDBP)

    Clinically important BP is SBP & DBP and not mean BP

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  • Arterial blood pressureArterial blood pressure

    Pressure depends on the amount of bloodAmount ofBloodEntering cardiac outputAmount ofBlood leaving determined by resistance to flow

    Determines mainly systolic BPDetermines mainly diastolic BP

  • Arterial blood pressure

    A concept:

    B.P. = Cardiac output x Total peripheral resistance

    BP = CO x TPRThis equation holds true for actual values of the above parameters

    But, clinically neither the CO nor the TPR is measured routinely.

  • Total peripheral resistance

    total peripheral resistance (TPR) refers to the resistance to blood flow offered by all of the systemic vasculature excluding the pulmonary vasculature.

    R = 8Lvr4R = resistance

    L = length of the blood vesselv = viscosity of bloodr = radius of the blood vessel

  • Total peripheral resistance TPR is therefore determined by those factors that influence vascular resistance in individual vascular beds. The main site of vascular resistance within the circulation is at the ARTERIOLAR level.

    TPR is primarily determined by changes in blood vessel diameters vasodilataion increase in vessel diameter vasoconstriction decrease in vessel diameter

    Changes in blood viscosity will also affect TPR

  • Control of arterioles

    The blood vessels are controlled by

    1. Central mechanismsNeural mechanismsCirculating hormones

    2. Local mechanismsLocal factorsTissue factorsEndothelial factorsMyogenic factors

  • Control of arterioles

    Arteriolar toneVascular tone refers to the degree of constriction High arteriolar tone) / dilatation (low arteriolar tone) of an arteriole

  • Control of arteriolar tone central mechanisms

    Central mechanisms are

    Neural and endocrine

    Regulate a systemic factor Blood pressureBody temperature

  • Control of arteriolar tone central mechanismsNeural control

    Mostly sympathetics cause vasoconstriction in most arterioles via alpha 1 receptors. Beta 2 receptors cause vasodilatation e.g. in skeletal muscle

    Parasympathetics in some organs only e.g. external genitalia

  • Control of arteriolar tone central mechanisms2. Hormonal controlAdrenalin binds to the beta 2 adrenoceptors to cause vasodilation in some organsAngiotensin IIAntidiuretic hormoneAtrial natriuretic peptide

  • Control of arteriolar tone Local mechanisms

    Local mechanisms are useful for modifying blood flow in a limited area (e.g. one organ or one area of the body)

    They are dependent on Metabolic activities of that areaPressure within the blood vessels in that areaVascular endothelium

  • Control of arteriolar tone Local mechanismsMetabolic mechanisms of vasodilation

    There is a close coupling between the metabolic activity and blood flow in most organs of the body

    an increase in tissue metabolism, as occurs during muscle contraction, will lead to an increase in blood flow (active hyperaemia)

  • Control of arteriolar tone Local mechanismsLocal factors causing vasodilationHypoxiaAdenosineK+CO2 excess hypercapniaH+Lactic acidInorganic phosphates

  • Control of arteriolar tone Local mechanisms2. Myogenic MechanismsAutoregulationAutoregulation the intrinsic ability of an organ to maintain a constant blood flow despite changes in perfusion pressure. for example, if the pressure within the arterioles of an organ is decreased blood flow will initially fallthis results in dilatation of the arterioles due to the reduction in stretchblood flow then returns towards normal levels over the next few minutesthis autoregulatory response occurs independently of any neural or humoral influences and therefore is intrinsic to the organ

  • Control of arteriolar tone Local mechanisms3. Other factors influencing vascular tone some increase and some decrease arteriolar tone.Endothelial factors prostacyclin - vasodilatationnitric oxide - vasodilatationendothelin - vasoconstriction

    Local hormones/chemical substances thrombaxane, other arachidonic acid metaboliteshistamine Bradykinin

    These factors interact with each other through many mechanisms and help fine-tune the vascular tone

  • Normal BPPhysiological variations in BPAgeGenetic racial differencesGenderBody buildSleepPostureRespirationExerciseEmotion and stressPain

  • Normal BPBecause of the multitude of factors affecting blood pressure it is impossible to determine the normal blood pressure for any individual

    In general, blood pressure is said to be normal if at rest

    systolic is 130 mmHg or lessANDdiastolic is 80 mmHg or less

    A BP of more than 140/90 mmHg is generally considered to be high

  • Regulation of BPThe primary goal of the cardiovascular system is the maintainance adequate tissue perfusion

    Blood pressure is one major factor that affects tissue perfusion

    In order to maintain adequate tissue perfusion the body regulates blood pressure (through the regulation of cardiac output and arteriolar diameter)

  • Regulation of BP

    The blood pressure Mostly maintained constant Increased when necessary (e.g. exercise)Even allowed to fall a little if appropriate (e.g. sleep)

  • Maintenance of BP

    Short term minutes to hours, neuro-humoralLong term hours to days, renal and endocrineMaintenance of a constant BP

  • Short term neuro-humoralMostly neural mechanisms. Hormones (adrenaline) also contributeThis regulation is quick acting (seconds minutes) A negative feedback mechanism

    Maintenance of a constant BP

  • Arterial baroreceptors are located in the carotid sinus (at bifurcation of external and internal carotids) in the aortic arch

    The sinus nerve, a branch of the glossopharyngeal nerve (IX cranial nerve), innervates the carotid sinus. The aortic arch baroreceptors are innervated by the vagus nerve The baroreceptors are tonically activeArterial baroreceptors

  • Arterial baroreceptors are sensitive to stretching of the walls of the vessels in which the nerve endings lie.

    Stretching occurs when arterial pressure increasesArterial baroreceptors

  • The baroreceptors send impulses to the vasomotor centre and cardiac centres (cardioacceleratory & cardioinhibitory) in the medulla oblongata

    Impulses along the vagus and glossopharyngeal nerves inhibit the medullary centres The normal activity of the centres is to activate the sympathetics and inhibit parasympathetics

    Increased baroreceptor activity reduce sympathetic activity and enhance parasympatheticsArterial baroreceptors

  • A decrease in arterial pressure results in decreased baroreceptor firing The CVS centers respond by increasing sympathetic outflow and decreasing parasympathetic outflowArterial baroreceptors

  • Influences on medullary centresBaroreceptorsPeripheral chemoreceptorsPain receptorsTemperature centreProprioceptorsIntracranial pressureO2 and CO2 partial pressuresRespiratory centreTemperature receptorsCerebral cortexBaroreceptors are only one of many factors influencing the medullary centers

  • Local short term regulationSome factors affect the heart directly and influence Heart rate Force of contraction

  • Long term hours to days, renal & endocrine

    through maintaining blood volume

    Renin angiotensin aldosterone mechanismOther renal mechanismsVasopressinAtrial natriuretic peptideThirst

    Regulation Long term

  • Secretion of renin From the Juxta-glomerular apparatus (in kidney)

    Major stimuli -sympathetic stimulation (as does occur in arterial hypotension)renal artery hypotensiondecreased sodium delivery to the distal tubulesRenin angiotensin aldosterone mechanism

  • Renin angiotensin aldosterone mechanismReninAngiotensinogenAngiotensin IAngiotensin IIIncreased aldosterone secretionVasoconstrictionThirstIncreased ADH secretionAngiotensin converting enzyme

  • Kidneys also regulate blood volume by adjusting the excretion of water and sodium into the urine through mechanisms other than renin-angiotensin-aldosterone pathwayOther renal mechanisms

  • Vasopressin (ADH)ADHAngiotensin IIHyperosmolality Decreased blood volumeAbsorption of waterIn kidneyVasoconstrictionThirst

  • Atrial natriuretic peptideAtrial Natriuretic PeptideDecreased renin secretionIncreased excretion of Sodium and waterAtrial distension