Blood pressure
Blood pressure is the force exerted by circulating blood against the walls of arteries as the heart pumps it through the body to deliver oxygen and nutrients to tissues.[1] It is typically measured in millimeters of mercury (mmHg) and expressed as two values: the higher systolic pressure, which occurs when the heart contracts to eject blood, and the lower diastolic pressure, which reflects the pressure in arteries when the heart relaxes between beats.[2] A normal blood pressure reading is less than 120 systolic and 80 diastolic mmHg, while readings at or above 130/80 mmHg indicate hypertension, a condition that affects nearly half of adults in the United States[1] and increases risks for heart disease, stroke, and kidney damage if unmanaged.[3] The physiological regulation of blood pressure maintains adequate perfusion of organs while preventing vascular damage, primarily through the interplay of cardiac output—the volume of blood pumped by the heart per minute—and total peripheral resistance, which is influenced by the diameter of arterioles.[2] Key mechanisms include the baroreceptor reflex, where sensors in the carotid arteries and aorta detect pressure changes and signal the autonomic nervous system to adjust heart rate and vessel tone; the renin-angiotensin-aldosterone system (RAAS), which promotes vasoconstriction and fluid retention to raise pressure when it falls; and antidiuretic hormone (ADH), which similarly increases blood volume and resistance.[2] These short-term and long-term controls ensure blood pressure remains stable under varying conditions, such as exercise or stress, with deviations often linked to factors like age, genetics, diet, and lifestyle.[2] Clinically, blood pressure is measured using a sphygmomanometer, often with an automated cuff on the upper arm, and regular monitoring is essential since hypertension is frequently asymptomatic, earning it the moniker "silent killer."[1] Elevated pressure can lead to atherosclerosis—plaque buildup in arteries—and heighten the likelihood of cardiovascular events, while low blood pressure (hypotension) may cause dizziness or fainting due to insufficient organ perfusion.[1] Management typically involves lifestyle modifications, such as reducing salt intake and increasing physical activity, alongside medications like ACE inhibitors that target RAAS pathways when necessary.[3]Fundamentals
Definition and Types
Blood pressure is the force exerted by circulating blood against the walls of arteries as the heart pumps it through the body, primarily reflecting the hydrostatic pressure within the major arterial system. This pressure is measured in millimeters of mercury (mmHg) and arises mainly from the mechanical action of the heart ejecting blood into the compliant arterial tree.[4] The hydrostatic component represents the physical force generated by cardiac output and vascular resistance, driving blood flow throughout the circulation.[2] Additionally, osmotic pressure—particularly the oncotic pressure exerted by plasma proteins like albumin—contributes by counteracting fluid leakage from vessels, thereby helping to sustain intravascular volume and indirectly supporting overall blood pressure levels.[5] The primary types of arterial blood pressure are systolic, diastolic, and mean arterial pressure, each characterizing different phases of the cardiac cycle. Systolic pressure denotes the maximum pressure attained in large arteries during ventricular contraction (systole), when blood is forcefully ejected from the heart.[4] Diastolic pressure, in contrast, is the minimum pressure occurring during ventricular relaxation (diastole), when the heart refills between beats.[4] Mean arterial pressure (MAP) provides an average value over the entire cardiac cycle, calculated as the integral of arterial pressure across systole and diastole, and serves as a key indicator of perfusion adequacy to organs.[6] Blood pressure is distinguished into systemic and pulmonary types based on the circulatory pathway. Systemic blood pressure refers to the higher pressures in the arteries distributing oxygenated blood to the body's tissues via the left ventricle.[2] Pulmonary blood pressure, driven by the right ventricle, involves lower pressures in the pulmonary arteries that carry deoxygenated blood to the lungs for gas exchange, typically about one-sixth of systemic levels to accommodate the lung's delicate vasculature.[7] The concept of blood pressure measurement originated in the early 18th century, with English clergyman and scientist Stephen Hales conducting the first direct recording in 1733 by inserting a brass tube into a horse's carotid artery and observing the blood's rise in a vertical glass tube connected to it.[8]Basic Physiology
Blood pressure is fundamentally generated and maintained by the cardiovascular system, which consists of the heart acting as the central pump and the network of blood vessels serving as conduits for blood flow. The heart propels blood through rhythmic contractions, producing a cardiac output that represents the volume of blood ejected per minute, typically around 5 liters in a resting adult. Arteries carry oxygenated blood away from the heart under high pressure, veins return deoxygenated blood to the heart under lower pressure, and capillaries facilitate exchange of nutrients, gases, and wastes between blood and tissues. This closed system ensures continuous circulation, with blood pressure arising from the force exerted by the heart against vascular resistance.[9] Hemodynamics describes the physical principles governing blood flow and pressure, analogous to Ohm's law in electricity, where mean arterial pressure (MAP) equals cardiac output (CO) multiplied by total peripheral resistance (TPR): MAP = CO × TPR. Cardiac output is the product of heart rate and stroke volume, while TPR arises primarily from the arterioles due to their narrow lumens and smooth muscle control, though the entire vascular tree contributes. This relationship highlights how pressure gradients drive flow from high-pressure regions (aorta) to low-pressure areas (right atrium), with blood viscosity and vessel geometry influencing overall dynamics.[9][10] Blood flow in vessels is predominantly laminar, characterized by smooth, layered movement parallel to the vessel walls, which minimizes energy loss and shear stress on endothelial cells. In contrast, turbulent flow involves chaotic eddies and mixing, occurring rarely in normal physiology such as during peak systole in the aorta or at vessel bifurcations, and is quantified by the Reynolds number (Re = ρVD/μ, where ρ is blood density, V is velocity, D is diameter, and μ is viscosity); laminar flow prevails when Re < 2000. Resistance to flow in individual vessels follows Poiseuille's law for laminar conditions in rigid tubes: R = \frac{8 \eta L}{\pi r^4}, where η is blood viscosity, L is vessel length, and r is radius, emphasizing that resistance is inversely proportional to the fourth power of the radius—thus, small changes in vessel diameter profoundly affect pressure.[10][9] Arteries exhibit vascular compliance and elasticity due to their layered structure, including elastin fibers in the media, allowing them to distend during systole and recoil during diastole, thereby buffering the intermittent pressure pulses from the heart to maintain steady downstream flow. This Windkessel effect transforms pulsatile cardiac ejection into more continuous perfusion of capillaries and tissues, with arterial compliance defined as the change in volume per change in pressure (C = ΔV/ΔP). Large elastic arteries like the aorta are particularly adept at this buffering, absorbing up to 50% of stroke volume during contraction.[9][11]Pressure Parameters
Arterial Pressure
Systemic arterial pressure is the force exerted by circulating blood on the walls of the arteries in the systemic circulation, which delivers oxygenated blood from the left ventricle to the body's tissues. It consists of two primary components: systolic pressure, the maximum pressure occurring during ventricular contraction, and diastolic pressure, the minimum pressure during ventricular relaxation. In healthy adults, systolic arterial pressure typically ranges from 90 to 120 mmHg, while diastolic pressure ranges from 60 to 80 mmHg, with values below 90/60 mmHg indicating hypotension and above 120/80 mmHg suggesting elevated pressure.[12][13][14] Aortic pressure, measured in the proximal aorta near the heart, represents the central arterial pressure where blood is ejected directly from the left ventricle. Due to this proximity, the mean arterial pressure is highest in the aorta, calculated as approximately one-third systolic plus two-thirds diastolic, and it declines gradually toward the peripheral arteries owing to frictional losses and increasing vascular resistance along the arterial tree. Although systolic pressure can exhibit amplification in peripheral sites due to wave reflections, the overall pressure profile ensures adequate perfusion while minimizing cardiac workload.[15][16] Pulse pressure, defined as the difference between systolic and diastolic pressures, normally ranges from 30 to 50 mmHg in adults and reflects arterial compliance and stroke volume. It is computed simply as systolic pressure minus diastolic pressure, providing insight into vascular health; for instance, a pulse pressure of 40 mmHg is typical for a reading of 120/80 mmHg. A widened pulse pressure exceeding 60 mmHg often signifies arterial stiffness, where reduced elasticity leads to higher systolic peaks and lower diastolic troughs, increasing cardiovascular risk.[17][16][18]Venous and Pulmonary Pressure
Venous pressure in the systemic circulation is significantly lower than arterial pressure, reflecting the role of veins as capacitance vessels that store and return blood to the heart. Central venous pressure (CVP), measured at the junction of the superior vena cava and right atrium, typically ranges from 0 to 8 mmHg in healthy individuals.[19] This low pressure serves as the primary determinant of venous return, acting as the back pressure that opposes the flow of blood from peripheral veins toward the heart; higher CVP reduces venous return, while lower values facilitate it.[20] In the pulmonary circulation, pressures are also low to accommodate the thin-walled alveoli and minimize the work of breathing. Normal pulmonary artery pressure includes a systolic value of 15 to 30 mmHg, diastolic of 4 to 12 mmHg, and mean of 9 to 18 mmHg.[21] These values are substantially lower than systemic arterial pressures due to the pulmonary vasculature's high compliance and low resistance, which prevent excessive strain on the lung tissue during gas exchange.[22] Mean systemic filling pressure (MSFP) represents the equilibrium pressure throughout the systemic circulation in the absence of blood flow, such as during cardiac arrest, and is approximately 7 mmHg in normovolemic humans.[23] This pressure arises primarily from the elastic recoil of the vascular walls and the volume of blood within the system, serving as a key indicator of circulatory filling status and the driving force for venous return under static conditions.[24] Compared to the arterial system, both venous and pulmonary circulations exhibit lower resistance and higher volume compliance, allowing them to handle larger blood volumes with minimal pressure changes. The venous system's compliance is about 30 times greater than that of arteries, enabling it to store up to 60-70% of total blood volume while maintaining low pressures.[25] Similarly, the pulmonary circuit's low resistance (about one-tenth of systemic) ensures efficient perfusion without compromising oxygenation.[26]Specialized Pressures
In the fetal circulation, systemic arterial blood pressure at term averages approximately 70/45 mmHg, with a mean of 55 mmHg, reflecting the underdeveloped vascular system and reliance on placental gas exchange.[27] Prior to birth, pulmonary vascular resistance remains elevated—approximately 8 to 10 times higher than systemic vascular resistance—due to factors such as fluid-filled lungs, low oxygen tension, and vasoconstrictive mediators, which minimizes pulmonary blood flow and directs most right ventricular output through the ductus arteriosus to the systemic circulation.[28] This high resistance ensures oxygenation via the placenta but requires a dramatic drop in pulmonary pressure postnatally for effective lung perfusion.[29] Portal venous pressure, which governs blood flow through the liver's portal system, normally ranges from 5 to 10 mmHg in healthy adults, exceeding inferior vena caval pressure by about 4 to 5 mmHg to facilitate nutrient-rich blood delivery from the splanchnic organs.[30] This low-pressure system supports sinusoidal filtration without excessive resistance, but elevations beyond 10 mmHg indicate portal hypertension, often linked to liver cirrhosis or thrombosis.[31] Intracranial pressure (ICP) interacts closely with systemic blood pressure to determine cerebral perfusion pressure (CPP), calculated as CPP = mean arterial pressure (MAP) minus ICP, ensuring adequate oxygen delivery to brain tissue.[32] In adults, normal ICP ranges from 7 to 15 mmHg in the supine position, maintained by cerebrospinal fluid dynamics and cerebral blood volume; when ICP rises above 20 mmHg, it can compress cerebral vessels, reducing CPP and risking ischemia unless compensated by elevated MAP.[33] This relationship underscores the brain's autoregulation, where CPP is typically kept between 60 and 150 mmHg to match metabolic demands.[32] In the microcirculation, capillary hydrostatic pressure drives fluid exchange across vessel walls and varies along the capillary length, typically ranging from 10 to 25 mmHg at heart level, with higher values (around 30-35 mmHg) at the arterial end and lower (10-15 mmHg) at the venous end.[34] This gradient, influenced by precapillary sphincter tone and interstitial forces, balances filtration and reabsorption per Starling's principle, preventing edema while nourishing tissues; for instance, in skin capillaries, pressures measured at the apex fall within 10.5 to 22.5 mmHg under normal conditions.[35]Normal Values and Variations
Norms by Age and Population
Blood pressure norms vary significantly across life stages, reflecting developmental changes in cardiovascular physiology. In the fetal period, blood pressure is low, with mean arterial pressure around 30-50 mmHg near term, but it rises rapidly postnatally as the circulatory system adapts to independent oxygenation. For newborns, average systolic blood pressure is approximately 60-80 mmHg, with values typically ranging from 60-90 mmHg in the first days of life, increasing to 70-100 mmHg by one month. Diastolic pressures follow a similar pattern, averaging 40-50 mmHg initially. These early norms are established through population-based studies of term infants, emphasizing the need for age-specific reference ranges to avoid misdiagnosis.[36] During childhood, blood pressure increases gradually with growth and body size, necessitating norms adjusted for age, sex, and height percentiles rather than fixed values. For example, in a 10-year-old child, normal systolic blood pressure falls between 100-120 mmHg and diastolic between 60-80 mmHg at the 50th-90th percentiles, with hypertension defined above the 95th percentile (e.g., >122/78 mmHg for boys of average height).[37] These percentile-based standards, derived from large cohorts like the National Health and Nutrition Examination Survey, account for the linear upward trend in both systolic and diastolic pressures from infancy through adolescence, reaching near-adult levels by late teens.[38] In adulthood, normal blood pressure is generally defined as less than 120/80 mmHg for both sexes, though mean values rise with age, peaking in middle age before a potential decline in extreme old age due to reduced vascular compliance. For adults aged 18-39, average systolic pressure is around 115-120 mmHg and diastolic 75-80 mmHg; by ages 60-79, systolic averages 130-140 mmHg while diastolic stabilizes or slightly decreases to 70-80 mmHg.[39] Women often exhibit lower pressures than men until menopause, after which differences narrow. Ethnic variations are notable, with individuals of African descent showing higher average systolic pressures (e.g., 5-10 mmHg above non-Hispanic whites) and earlier onset of elevated readings across adulthood.[40] In the elderly (aged 80+), average systolic values often reach 130-150 mmHg due to reduced vascular compliance, but normal blood pressure is still defined as less than 120/80 mmHg, with hypertension thresholds applying at 130/80 mmHg.[41] Recent ACC/AHA guidelines, updated in 2025, maintain the threshold for hypertension onset at 130/80 mmHg for adults, including older populations, while emphasizing individualized targets below 130/80 mmHg for most to reduce cardiovascular events, with considerations for frailty and limited life expectancy in those over 80 to ensure benefits outweigh harms. The 2025 guidelines incorporate the PREVENT equations for risk stratification to inform treatment initiation in those with stage 1 hypertension.[42] These standards, informed by trials like SPRINT, underscore the importance of accurate measurement to interpret norms reliably.[43]| Age Group | Typical Average Systolic (mmHg) | Typical Average Diastolic (mmHg) | Key Notes |
|---|---|---|---|
| Newborn (0-1 day) | 60-90 | 40-50 | Rapid postnatal rise; averages 64/41. Normal BP defined as <120/<80 mmHg for adults; age-specific for children. |
| Child (10 years) | 100-120 | 60-80 | 50th-90th percentiles; height-adjusted.[37] |
| Adult (18-39 years) | <120 | <80 | Lower in women pre-menopause. Normal BP defined as <120/<80 mmHg.[39] |
| Elderly (60+ years) | 120-140 | 70-80 | Systolic peaks mid-life, higher in African descent. Normal BP defined as <120/<80 mmHg.[40][41] |