Systems Physiology Lecture 14 Control of blood pressure Physical vs physiological factors that affect blood pressure Pressure gradients, geometry, resistance and viscosity Remote control of blood pressure Physical and physiological factors affecting arterial blood pressure Arterial blood pressure is determined directly by two major physical factors, the arterial blood volume and the arterial compliance. These physical factors are affected in turn by certain physiological factors, primarily the heart rate, stroke volume, cardiac output (heart rate x stroke volume), peripheral resistance Direct and indirect determinance Direct ? physical factors (volume of blood in artery at any point in time) and the compliance of that segment Indirect ? the volume of blood is determined by 2 phenomenon, rate of flow into and out of vessel (bloodflow/cardiac output), Peripheral resistance Flow and resistance: opposition to the tonicity of blood vessel (degree which vessel is relaxed or constricted) Arterial pressure waveform Diastolic blood pressure (Pd) and its determinants Systolic blood pressure (Ps) and its determinants Mean aterial blood pressure (Pa) and its determinants ? represents the area under the arterial pressure curve Pulsatile blood pressure (Pp) and its determinants ? difference between peak (systolic) dystolic and late/end diastolic pressure Segmental vascular changes in blood pressure Pulsatile systemic arterial blood pressure decreases as we go from the aorta (large arteries) throughout the system From arterioles to small venioles ? appreciable resistance in the small vessels Small veins, large veins, and vena cava ? capacitance/compliance ? little resistance to flow accommodate large volumes of blood Measurement and regulation of arterial blood pressure Direct vs indirect efforts to measure pressure Maintaining blood pressure Remote (central) regulation of blood pressure Indirect vs direct estimations of arterial blood pressure Note: not shown here are the techniques for calibrating the pressure transducer to ensure accuracy of data Indirect: Blood pressure of cuff: Points nurse are looking for are the first appearance of sound and the last disappearance of sound Direct: Catheter is inserted into artery connected to a strain gauge on flexible ? fluid filled cap and membrane that is movable and body with electrical components ( fed into an amplifier and recorder to print out , drawbacks if there is any air in the system ? air is compressible and blood is not compressible, an air bubble will underestimate the pressure. Cant record pressure absolutely accurately G. Merrill?s arterial blood pressure during a period of about 20 years. 1. 140 ( guideline for systolic hypertension 2. 90 and above ( diastolic hypertension guideline 3. Relatively constant blood pressure Usually taken on right side, pressure taken on the right side No physiological explanation why they differ ( due to human error Neither techniques allow for accurate measurements to the nearest degree Supplemental reading for Systems Phsiology Effect of reduced arterial compliance on pulse pressure for given change in stroke volume Compliance: change in volume over change in pressure For a given volume increment a reduced arterial compliance results in an increased pulse pressure Pressure/volume relationships (compliance) of aorta with aging Young person of 20 to 24 years or age ? maximum compliance and most elastic, will become less compliant with aging Summary and conclusions Pressure determined by physical and physiological factors Systolic and diastolic pressures determined by different variables Direct (most accurate) vs indirect (least accurate) methods for estimating blood pressure Collect data, use law of averages over time Central (remote) conrol of blood pressure and location of baroreceptors Aortic baroreceptors are less sensitive and data is less reliable than carotid baroreceptors Baroreceptors respond both to phasic changes in pressure (pulsatile) and to Pa 5 curves show mean arterial pressures 1. action potential occurs during the systolic phase of the cardiac cycle 2. if you compare the frequencies at a mean arterial pressure that is low with a mean arterial pressure that is normal, carotid Sirius nerves are more active at 100 than 50, carotid Sirius nerves are phasicly active, predominantly active during systole.. As mean arterial pressure increases, reach a point where carotid Sirius nerves are preactive throughout the whole the deformation of the wall of the sinus that is the central stimulus that the baroreceptors respond to, not the absolute pressure Remote (CNS) control of systemic arterial blood pressure subsequent to a fall in sinus pressure Arterial pressure has been reduced (loss of 15-20% of blood) will be sensed by arterial carotid sinus baroreceptors Results in a reduction of frequency of action potentials being conducted centerally to the brain stem (medulla ? several cardiovascular regulatory centers) When the pressure in the carotid sinus ( decreased discharge of the PSNerves that are coming out of cardiocenters ( increased heart rate ? increased activity in vagus nerve / withdrawal of vagus tone ( increased sympathetic activity ( has the same effect at SA node as decreasing PS nerves. ( doubling of heart rate Ventricular myocardium Increased contractility (increase in volume of blood that ejects with each cycle (stroke volume) ( increase in flow Veins ? venomotor tone increases, compliance decreases ( large vein in body are considered compliance reservoirs (60-70%) or total circulating volume is in the veins) . innervated by sympathetic nerves. When they contract the volume of blood goes back to heart, distributed from venous side to arterial side ? will have greater fraction of blood than veins ( increase venous pressure ( increases mean arterial pressure ( sympathetic nerves will cause arterioles to constrict ( peripheral resistance will increase Arterioles ? peripheral resistance will increase ? vasoconstriction ( total peripheral resistance increases EDV ? end diastolic volume increases This system restores arterial blood pressure to near normal ? rapidly responding designed for emergencies SNS and PSNS work cooperatively for the cardiovascular system Summary and conclusions Remote means control via the CNS Sensors are in the carotid sinuses and elsewhere immediate, intermediate and long-term regulation of blood pressure blood pressure measured indirectly and directly (apply the law of averages) Barring genetics, use diet and exercise to control your blood pressure
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