Systems Physiology Lecture 3 March 10 Control of blood pressure Physical vs physiological factors that affect blood pressure Pressure gradients, geometry, resistance and viscosity Reflexes and autonomic systems (remote) Segmental vascular changes in blood pressure - Along the x axis vessels of the cardiovascu l ar system AO - Aorta LA - Large arteries SA - Small arteries ART - Arterioles CAP - Capillaries VEN - Venules SV - Small veins LV - Large Veins VC - Vena Ca va - Along the y axis pressure - Compare AO - LA with SA segments very sharp decline in pressure, greater fluctuation with the AO - LA segment (pre-capillary) . Called pulsa tile blood pressure - In large arteries upstream from the capi llaries, blood pressure is pulsa tile. - From small arteries to arterioles dow nstream, blood pressure becomes non-pulsa tile (little to no fluctuation) . - What causes the pulsa tility? - Upstream to the aorta is the heart (pump) and the heart i s pulsa tile. It has a systolic c ontract ile and a diastolic relaxation phase. -It's the pulsa tility of the cardiac cycles that makes blood pressure pulsa tile upstream of the large arteries. -What causes the absence of pulsa tility downstream of the arterioles? - Due to the arrangement of the cell layers of the walls of blood vessels. - The cross section of the wall of an arteriole vs. aorta. - Smooth muscle cells are much denser than t he smooth muscle cells of the a o r ta. - Vascular smooth muscle cells resist distention of blood vessels. - Very thick-walled vessels that resist being distended. Dampen the pulse. - SA - ART called the resistan ce vessels of the body. They offer the greatest obstacle to forward flow of blood. Huge drop of blood pressure. Physical and physiological factors affecting arterial blood pressure Physiological factors [indirect influences] - Cardiac output - Blood volu me - (Heart rate x Stroke volume) - Stroke volume - volume of blood ejected by left ventricle during each s ystolic phase of the car diac cycle (expressed in mL) [around 50-60 mL in blood for an average 70 kg person] - Ejection fraction - at the end of diastoly when the heart is resting and is fi lled (left ventricle) to its maximum extent it might have a volume of 150 mL of blood. That's c alled end diastolic volume. Through imaging techniques (MRIs, CAT scans) a clinician can measure the size of the chamb er at the end of diastoly. He can measure the chamber at the end of systoly a nd compute the difference in volume (end diastolic volume minus end systolic volume = ejection fraction) - End diastolic volume minus end systolic volume divided by end diastolic volume - Peripheral Resistance Physical factors (mechanical) [direct influences] - Arterial blood volume - Arterial compliance - Compliance - the ability of a tubular structure (blood vessel) to accommodate a given volume of blood at a given pressure. - How distensible the vessel is? - Computed by change in volume over change in pressure. Arterial blood pressure Pressure/volume relationships (compliance) of aorta with aging - Data generated 80 years ago. - Describe how we compute compliance. - Know how much volume it takes to get a known increment of pressure increase. The less volume there is at a give pressure, the less compliant the vessel is. - Decreases with age. It takes more force to put a set amount of blood vessel into an older person t han a younger person. - Nicotine, diet (fattier diets contribute to thickening of vessel walls), and sedate lifestyles contribute to decrease of compliance. - Test by taking a vessel, suturing up both ends, inserting hypodermic needle and inject blood while me asuring increase in pressure. Summary Compliance influences blood pressure Compliance changes with age and disease Volume influences blood pressure Volume is less likely than compliance to change with age and disease Measurement and control of arterial b lood pressure Direc t vs indirect effor t s to measure pressure W hat information is in a pressure waveform What de te rmines pulsatile arterial pressure What is mean arteria l pressure Indirect (top panel) vs direct (bottom panel) estimations of arterial blood pres sure Indirectly -Happens when we go to the clinic for a physical -3 sets of information - Actual procedure, Data generated, Sounds Using a stethoscope detecting sounds to gather information. (Auscultatory) (Palpatory) Finger on radial pulse detecting a pressure pulse as the cuff is inflated and deflated. -Running inside the arm is the brachial artery (major artery that supplies blood to the limb) -As the distensable bag inside the cuff is inflated to a pressure that exceeds systolic pressure the bra chial artery will be compressed and collapse. No blood pressure will occur downstream of this. While this happens, the limb is eschemic. -Inflate the cuff above peak systolic pressure to identify the pressure at which we first begin to hear sounds in the stethoscope. -There are 5 phases/periods of sounds to hear. Tapping tone Murmurs Thumping tone Muffling tone Silence -Systolic hypertension - elevated systolic pressure -Diastolic hypertension - elevated pressure during diastoly -140/90 is the number for systolic/diastolic hypertension -120/80 is a normal number for systolic/diastolic pressure Directly - Isolate and catheterize an artery or impale the brachial artery and attach it to a pres sure transducer, amplifier and recorder. - Can record pressure directly. - To test the accuracy of the i ndirect test, do both tests simultaneously. Arterial pressure waveform - Diastolic blood pressure (Pd) and its determinants - Resistance to blood flow is the most important determinant of blood pressure. - Resistance dete rmines blood runoff (during systoly the stroke volume gets distributed thr ough large conducting arteries out towards smaller arteries and arterioles. If those arterioles are more clamped down and more resistance, less of that blood is going to run off int o the veins downstream than if the arterioles are less resistant) Tonically inactive resistance is lower Tonically active resistance is higher - Systolic blood pressure (Ps) and its determinants - What causes peak systolic pressure and end diastolic pressure? - Compliance determines systolic pressure. The more compliant the arteries, the lower the systolic pressure. - Stroke volume. The greater the stroke volume, the greater the peak s ystolic pressure. Stroke volume is directly affected by strength of contraction. A more vigorous systolic contraction causes a greater systolic pressure. - Mean arterial blood pressure (Pa) and its determinants Estimate mean arterial pressure -> average the highest and lowest values or use a differen tial equation. - Pulsatile blood pressure (P p) and its determinants The difference between systolic and diastolic. Low diastolic means lower sympathetic discharge. Peak systolic pressure, late diastolic pressure, pulsatile pressure (diff between systolic and diastolic), estimate mean arterial pressure. G. Merrill's arterial blood pressure during a period of about 20 years He is not systolic hypertense! The bars indicate the standard errors (dispersion of the data) The good news is the latest numbers he's averaged have been pretty consistent. If he can hold at those leve l s for the next 20 - 30 years, he'll be happy. He's actually very happy now. He's some where be tween 50 (we wouldn't believe it ) and 71 (we'd be surprised). Summary Direct measurement of Pa involves an i ndwelling arterial catheter Indirect measurement of Pa is an estimate and subject to carelessness and continuing debate the pulsatile arteri al waveform yields valuable info about co n dition of heart and vasculature Estimatin g Pa (MAP) is ok, just be consistent Barring genetics, use a prudent diet and a lifetime of activity to control your arterial blood pressure Remote control of blood pressure R emote meaning 'involving the central nervous system ? Remote includes involvement of peripheral and central sensors Remote also includes important reflexes such as 1) the baroreceptor reflex and 2) the Bainbridge reflex Remote control involves reflex adjust ments of many important cardiovascular variables (e.g. HR, contractility, resistance, etc) Central (remote) control of blood pressure and location of baroreceptors - Sensors are located in the circulatory system - High pressure baroreceptors (arterial) The ca rotid sinus (on both sides) Baroreceptors - sensory nerve ending that responds to stretch and de - stretch; pressure receptors The aortic arch - I n the kidneys there is another high pressure baroreceptor - Connected to the central nervous system. Influence of pulsatile aortic pressure on firing rate of a single afferent neuron from the carotid sinus - Mean arterial pressure on y, time on x -Phasic aortic pressure -The sinus nerve carries sensory information from the contracting heart. -Recording elec trode under the sinus nerve record s the number of action potentials -Generally, at blood pressures that are normal, the sinus nerve is active during phases of systoly. There is no activity during diastoly. -If we elevate mean arterial pressure to 125, the si nus nerve becomes continually active throughout the cardiac cycle. -Take home message: Barorecptors respond both to mean arterial pressure and to phasic pulsatile pressure. Remote control of arterial blood pressure. In this example we assume that the ini tial event is an increase in a rterial pressure. Blood pressure is at 100 mm Hg. An event startles you causing an increase in MAP. 1. Increase in mean arterial pressure 2. Detection by baroreceptors. 3. Send afferent signals to the brainstem. 4. Signal reaches the medulla and a bunch of like-behaving neurons (coordinating center/cardiova scular control center) regulate vascular tone. (There are other groups that regulate heart rate) 5. Respond along the efferent pathways to the effectors. 6. The effectors are the heart and the blood vessels. In the heart sympathetic discharge to the h eart decreases; with d rawing sympathetic tone will decrease HR. - P arasympathetic (vagal) discharge will decrease, decreasing HR. - T he withdrawl of sympathetic discharge to the myocardium will decrease contractility . - C ardiac output is decreased (blood f low is decreased), dropping blood pressure In the vessels sympathetic tone to small and large veins decreases, causing the ve in to relax , allowing it to accommodate a larger volume of blood (become a blood reservoir) so cardiac output decreases. - A rterioles relax (vasodilate) 7. Bradycardia (reduced heart rate) and vasodilation counteract increased mean arterial pressure Summary and conclusions Pressure determined by physical and physiological factors Systolic a n d diastolic pressures determined by different variables Direct (most accurate) vs indirect (least accurate) methods for estimating blood pressure Remote control involves baroreflex and the CNS Collect your own data, use law of averages over time
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