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and dementia 57 independent of the average blood pressure level recent evidence from clinical trials has also linked variation in blood pressure to mortality 58 59 stroke 60 heart failure 61 and cardiac changes that may give rise to heart failure 62 these data have prompted discussion of whether excessive variation in blood pressure should be treated even among normotensive older adults 63 older individuals and those who had received blood pressure medications are more likely to exhibit larger fluctuations in pressure 64 and there is some evidence that different antihypertensive agents have different effects on blood pressure variability 57 whether these differences translate to benefits in outcome is uncertain 57 physiology edit cardiac systole and diastole blood flow velocity waveforms in the central retinal artery red and vein blue measured by laser doppler imaging in the eye fundus of a healthy volunteer schematic of pressures in the circulation during each heartbeat blood pressure varies between a maximum systolic and a minimum diastolic pressure 65 unreliable medical source the blood pressure in the circulation is principally due to the pumping action of the heart 66 however blood pressure is also regulated by neural regulation from the brain see hypertension and the brain as well as osmotic regulation from the kidney differences in mean blood pressure drive the flow of blood around the circulation the rate of mean blood flow depends on both blood pressure and the resistance to flow presented by the blood vessels in the absence of hydrostatic effects e g standing mean blood pressure decreases as the circulating blood moves away from the heart through arteries and capillaries due to viscous losses of energy mean blood pressure drops over the whole circulation although most of the fall occurs along the small arteries and arterioles 67 pulsatility also diminishes in the smaller elements of the arterial circulation although some transmitted pulsatility is observed in capillaries 68 gravity affects blood pressure via hydrostatic forces e g during standing and valves in veins breathing and pumping from contraction of skeletal muscles also influence blood pressure particularly in veins 66 hemodynamics edit main article hemodynamics a simple view of the hemodynamics of systemic arterial pressure is based around mean arterial pressure map and pulse pressure most influences on blood pressure can be understood in terms of their effect on cardiac output 69 systemic vascular resistance or arterial stiffness the inverse of arterial compliance cardiac output is the product of stroke volume and heart rate stroke volume is influenced by 1 the end diastolic volume or filling pressure of the ventricle acting via the frank starling mechanism this is influenced by blood volume 2 cardiac contractility and 3 afterload the impedance to blood flow presented by the circulation 70 in the short term the greater the blood volume the higher the cardiac output this has been proposed as an explanation of the relationship between high dietary salt intake and increased blood pressure however responses to increased dietary sodium intake vary between individuals and are highly dependent on autonomic nervous system responses and the renin angiotensin system 71 72 73 changes in plasma osmolarity may also be important 74 in the longer term the relationship between volume and blood pressure is more complex 75 in simple terms systemic vascular resistance is mainly determined by the caliber of small arteries and arterioles the resistance attributable to a blood vessel depends on its radius as described by the hagen poiseuille s equation resistance 1 radius 4 hence the smaller the radius the higher the resistance other physical factors that affect resistance include vessel length the longer the vessel the higher the resistance blood viscosity the higher the viscosity the higher the resistance 76 and the number of vessels particularly the smaller numerous arterioles and capillaries the presence of a severe arterial stenosis increases resistance to flow however this increase in resistance rarely increases systemic blood pressure because its contribution to total systemic resistance is small although it may profoundly decrease downstream flow 77 substances called vasoconstrictors reduce the caliber of blood vessels thereby increasing blood pressure vasodilators such as nitroglycerin increase the caliber of blood vessels thereby decreasing arterial pressure in the longer term a process termed remodeling also contributes to changing the caliber of small blood vessels and influencing resistance and reactivity to vasoactive agents 78 79 reductions in capillary density termed capillary rarefaction may also contribute to increased resistance in some circumstances 80 in practice each individual s autonomic nervous system and other systems regulating blood pressure notably the kidney 81 respond to and regulate all these factors so that although the above issues are important they rarely act in isolation and the actual arterial pressure response of a given individual can vary widely in the short and long term pulse pressure edit main article pulse pressure a schematic representation of the arterial pressure waveform over one cardiac cycle the notch in the curve is associated with closing of the aortic valve the pulse pressure is the difference between the measured systolic and diastolic pressures 82 p pulse p sys p dias displaystyle p_ text pulse p_ text sys p_ text dias the pulse pressure is a consequence of the pulsatile nature of the cardiac output i e the heartbeat the magnitude of the pulse pressure is usually attributed to the interaction of the stroke volume of the heart the compliance ability to expand of the arterial system largely attributable to the aorta and large elastic arteries and the resistance to flow in the arterial tree 82 clinical significance of pulse pressure edit a healthy pulse pressure is around 40 mmhg 83 a pulse pressure that is consistently 60 mmhg or greater is likely to be associated with disease and a pulse pressure of 50 mmhg or more increases the risk of cardiovascular disease as well as other complications such as eye and kidney disease 44 pulse pressure is considered low if it is less than 25 of the systolic for example if the systolic pressure is 120 mmhg then the pulse pressure would be considered low if it is less than 30 mmhg since 30 is 25 of 120 84 a very low pulse pressure can be a symptom of disorders such as congestive heart failure 44 elevated pulse pressure has been found to be a stronger independent predictor of cardiovascular events especially in older populations than has systolic diastolic or mean arterial pressure 44 45 this increased risk exists for both men and women and even when no other cardiovascular risk factors are present the increased risk also exists even in cases in which diastolic pressure decreases over time while systolic remains steady 47 46 a meta analysis in 2000 showed that a 10 mmhg increase in pulse pressure was associated with a 20 increased risk of cardiovascular mortality and a 13 increase in risk for all coronary end points the study authors also noted that while risks of cardiovascular end points do increase with higher systolic pressures at any given systolic blood pressure the risk of major cardiovascular end points increases rather than decreases with lower diastolic levels this suggests that interventions that lower diastolic pressure without also lowering systolic pressure and thus lowering pulse pressure could actually be counterproductive 85 there are no drugs currently approved to lower pulse pressure although some antihypertensive drugs may modestly lower pulse pressure while in some cases a drug that lowers overall blood pressure may actually have the counterproductive side effect of raising pulse pressure 86 pulse pressure can both widen or narrow in people with sepsis depending on the degree of hemodynamic compromise a pulse pressure of over 70 mmhg in sepsis is correlated with an increased chance of survival and a more positive response to iv fluids 87 88 mean arterial pressure edit main article mean arterial pressure mean arterial pressure map is the average of blood pressure over a cardiac cycle and is determined by the cardiac output co systemic vascular resistance svr and central venous pressure cvp 89 90 91 map co svr cvp displaystyle text map text co cdot text svr text cvp in practice the contribution of cvp which is small is generally ignored and so map co svr displaystyle text map text co cdot text svr map is often estimated from measurements of the systolic pressure p sys displaystyle p_ text sys and the diastolic pressure p dias displaystyle p_ text dias 91 using the equation map p dias k p sys p dias displaystyle text map approxeq p_ text dias k p_ text sys p_ text dias where k 1 3 although other values for k have been advocated 92 93 this estimation is only accurate when the heart rate is around 60 peats per minute regulation of blood pressure edit see also renin angiotensin system the endogenous homeostatic regulation of arterial pressure is not completely understood but the following mechanisms of regulating arterial pressure have been well characterized baroreceptor reflex baroreceptors in the high pressure receptor zones detect changes in arterial pressure these baroreceptors send signals ultimately to the medulla of the brain stem specifically to the rostral ventrolateral medulla rvlm the medulla by way of the autonomic nervous system adjusts the mean arterial pressure by altering both the force and speed of the heart s contractions as well as the systemic vascular resistance the most important arterial baroreceptors are located in the left and right carotid sinuses and in the aortic arch 94 renin angiotensin system ras this system is generally known for its long term adjustment of arterial pressure this system allows the kidney to compensate for loss in blood volume or drops in arterial pressure by activating an endogenous vasoconstrictor known as angiotensin ii aldosterone release this steroid hormone is released from the adrenal cortex in response to activation of the renin angiotensin system high serum potassium levels or elevated adrenocorticotropic hormone acth renin converts angiotensinogen to angiotensin i which is converted by angiotensin converting enzyme to angiotensin ii angiotensin ii then signals to the adrenal cortex to release aldosterone 95 aldosterone stimulates sodium retention and potassium excretion by the kidneys and the consequent salt and water retention increases plasma volume and indirectly arterial pressure aldosterone may also exert direct pressor effects on vascular smooth muscle and central effects on sympathetic nervous system activity 96 baroreceptors in low pressure receptor zones mainly in the venae cavae and the pulmonary veins and in the atria result in feedback by regulating the secretion of antidiuretic hormone adh vasopressin renin and aldosterone the resultant increase in blood volume results in an increased cardiac output by the frank starling law of the heart in turn increasing arterial blood pressure these different mechanisms are not necessarily independent of each other as indicated by the link between the ras and aldosterone release when blood pressure falls many physiological cascades commence in order to return the blood pressure to a more appropriate level the blood pressure fall is detected by a decrease in blood flow and thus a decrease in glomerular filtration rate gfr decrease in gfr is sensed as a decrease in na levels by the macula densa the macula densa causes an increase in na reabsorption which causes water to follow in via osmosis and leads to an ultimate increase in plasma volume further the macula densa releases adenosine which causes constriction of the afferent arterioles at the same time the juxtaglomerular cells sense the decrease in blood pressure and release renin renin converts angiotensinogen inactive form to angiotensin i active form angiotensin i flows in the bloodstream until it reaches the capillaries of the lungs where angiotensin converting enzyme ace acts on it to convert it into angiotensin ii angiotensin ii is a vasoconstrictor that will increase blood flow to the heart and subsequently the preload ultimately increasing the cardiac output angiotensin ii also causes an increase in the release of aldosterone from the adrenal glands aldosterone further increases the na and h 2 o reabsorption in the distal convoluted tubule of the nephron the ras is targeted pharmacologically by ace inhibitors and angiotensin ii receptor antagonists also known as angiotensin receptor blockers arb the aldosterone system is directly targeted by aldosterone antagonists the fluid retention may be targeted by diuretics the antihypertensive effect of diuretics is due to its effect on blood volume generally the baroreceptor reflex is not targeted in hypertension because if blocked individuals may experience orthostatic hypotension and fainting measurement edit main article blood pressure measurement taking blood pressure with a sphygmomanometer measuring systolic and diastolic blood pressure using a mercury sphygmomanometer arterial pressure is most commonly measured via a sphygmomanometer which uses the height of a column of mercury or an aneroid gauge to reflect the blood pressure by auscultation 97 the most common automated blood pressure measurement technique is based on the oscillometric method 98 fully automated oscillometric measurement has been available since 1981 99 this principle has recently been used to measure blood pressure with a smartphone 100 measuring pressure invasively by penetrating the arterial wall to take the measurement is much less common and usually restricted to a hospital setting novel methods to measure blood pressure without penetrating the arterial wall and without applying any pressure on patient s body are being explored 101 in office blood pressure measurement terminal digit preference is common according to one study approximately 40 of recorded measurements ended with the digit zero whereas without bias 10 20 of measurements are expected to end in zero 102 in animals edit blood pressure levels in non human mammals may vary depending on the species heart rate differs markedly largely depending on the size of the animal larger animals have slower heart rates 103 the giraffe has a distinctly high arterial pressure of about 190 mmhg enabling blood perfusion through the 2 metres 6 ft 7 in long neck to the head 104 in other species subjected to orthostatic blood pressure such as arboreal snakes blood pressure is higher than in non arboreal snakes 105 a heart near to the head short heart to head distance and a long tail with tight integument favor blood perfusion to the head 106 107 as in humans blood pressure in animals differs by age sex 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