SECTION 5.4
Blood pressure and its measurement
Explain what blood pressure measures, the factors that create it, and how systolic and diastolic values are read.
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Blood pressure is the force that blood exerts on the walls of blood vessels, and it is one of the most frequently measured values in health care. It is expressed in millimetres of mercury (mmHg) and recorded as two numbers, for example 120/80. The first number, systolic pressure, is the peak reached when the left ventricle contracts. The second, diastolic pressure, is the lowest value between beats, when the heart relaxes. Blood pressure is not produced by the heart alone: it depends on how much blood the heart pumps, how much blood is in the circulation, and how much resistance the vessels offer to flow. Measuring it correctly requires the right cuff, the right position and the right technique, because small errors change the reading and can change a treatment decision. This section explains what the numbers mean and how they are obtained.
Key Concepts & Topics
What blood pressure measures
Blood pressure is the force per unit area that blood exerts on a vessel wall. In clinical practice, the term usually means arterial pressure in a systemic artery, because that is the pressure the heart must generate and the pressure that perfuses organs. It is measured in millimetres of mercury, a unit inherited from the mercury manometer used in early instruments. Pressure is highest in the aorta, just after the left ventricle ejects blood, and falls progressively as blood moves through arteries, arterioles, capillaries and veins. The steepest fall occurs across the arterioles, because they are narrow and offer the greatest resistance. By the time blood reaches the veins, pressure is low; this gradient is what keeps blood moving from the heart back to the heart.
For example: A pressure of 120 mmHg means the blood pushes on the artery wall with enough force to support a column of mercury 120 mm high.
Systolic, diastolic and pulse pressure
Arterial pressure rises and falls with each heartbeat. Systolic pressure is the highest value, reached during ventricular contraction when blood is ejected into the aorta. Diastolic pressure is the lowest value, reached during ventricular relaxation when the arteries recoil and keep blood moving. The difference between them is the pulse pressure, which reflects how strongly the heart ejects and how elastic the arteries are. The mean arterial pressure is the average pressure over the cardiac cycle, weighted towards diastole because the heart spends more time relaxing than contracting at a normal rate; it is the value that best reflects organ perfusion. A widening pulse pressure often suggests stiff arteries, while a narrow pulse pressure can suggest reduced output.
For example: At 120/80, the pulse pressure is 40 mmHg — the difference between the peak and the trough of the same cardiac cycle.
The factors that set blood pressure
Blood pressure depends mainly on three factors. Cardiac output is the volume the heart pumps each minute; more output tends to raise pressure. Peripheral resistance is the resistance to flow offered by the vessels, most of it in the arterioles; narrowing them raises pressure. Blood volume is the amount of blood in the circulation; losing volume lowers pressure, and retaining fluid raises it. A useful relationship is that pressure is roughly proportional to cardiac output multiplied by peripheral resistance. Elasticity of the large arteries also matters, because it smooths the pressure wave and keeps diastolic pressure from falling too low. This is why blood pressure can be changed by altering heart rate and stroke volume, by constricting or dilating arterioles, or by changing blood volume.
For example: A patient losing blood has less volume, so pressure falls; the body compensates by narrowing arterioles and raising heart rate.
Measuring blood pressure
Blood pressure is usually measured with a sphygmomanometer and a cuff. The cuff is wrapped around the upper arm at the level of the heart and inflated until it stops arterial flow. As it is released slowly, the person taking the measurement listens with a stethoscope over the brachial artery. The sounds heard are called Korotkoff sounds. The first clear sound marks systolic pressure, because blood is now spurting through the partly opened artery. The sounds change, then become muffled, and finally disappear; the disappearance marks diastolic pressure in most techniques. The reading is recorded as systolic over diastolic. Errors are common and avoidable: a cuff that is too small, an arm below heart level, a crossed leg, talking or a recent coffee or cigarette can all change the result, so standard conditions are part of accurate measurement.
For example: Using a cuff that is too small can falsely raise the reading, which is why the cuff size is chosen to match the arm circumference.
High and low blood pressure
Blood pressure is interpreted as a range, not a single ideal number. Persistently high arterial pressure, called hypertension, increases the work of the heart and damages small vessels over time, raising the risk of stroke, heart attack and kidney disease. It is often called a silent condition because it usually causes no symptoms until damage is advanced, which is why it is checked routinely. Persistently low pressure, called hypotension, may cause dizziness and fainting because organs, especially the brain, are not perfused adequately when standing. Both extremes matter because they change perfusion. Interpretation also depends on context: a pressure that is normal at rest may be high during pain or exercise, and a pressure that is low in a fit young person may be normal for them.
For example: A patient with a headache and a pressure of 190/110 needs urgent assessment, because very high pressure can damage the brain and vessels acutely.
Key facts to remember
- Blood pressure is the force blood exerts on vessel walls, measured in mmHg and usually referring to systemic arterial pressure.
- Systolic pressure is the peak during ventricular contraction; diastolic pressure is the lowest value during relaxation; their difference is pulse pressure.
- Blood pressure depends mainly on cardiac output, peripheral resistance and blood volume, with arterial elasticity smoothing the pressure wave.
- Pressure is highest in the aorta and falls progressively, most steeply across the arterioles, which offer the greatest resistance.
- Blood pressure is measured with a cuff and sphygmomanometer; the first Korotkoff sound marks systolic and their disappearance marks diastolic pressure.
Example · A nurse measures a blood pressure of 150/95 mmHg. The systolic value of 150 means the peak force during ventricular contraction is higher than usual. The diastolic value of 95 means the pressure between beats is also raised, so the arteries are not relaxing to a normal level. The pulse pressure is 55 mmHg, which is wider than the usual 40. Together, these numbers suggest sustained high pressure rather than a single stressful moment, so the nurse repeats the measurement under standard conditions, records it and reports it. The numbers mean little alone; they are interpreted with the patient’s history, symptoms and other measurements.
Read full notes
Read one topic at a time. These notes explain the mechanisms and relationships behind the key facts.
Pressure and flow in the circulation
Blood flows because pressure is higher in one place than another. The heart creates the pressure, and the vessels offer resistance, so the two together determine flow. Because the arterioles are narrow, they account for most of the resistance, and pressure falls steeply as blood passes through them. In the capillaries pressure is low enough for exchange and gentle enough not to damage the vessel, and in the veins it is very low. This pressure gradient is what drives blood from the aorta to the venae cavae. It also explains why a blockage anywhere in the system changes pressures upstream and downstream, and why measuring arterial pressure is a useful window on the whole circulation.
What systolic and diastolic mean
Systolic pressure exists because the left ventricle ejects blood into the aorta faster than it can leave through the arterioles, so the aorta stretches and pressure rises. Diastolic pressure exists because the stretched elastic arteries recoil while the aortic valve is closed, keeping blood moving and pressure from falling to zero. Both values matter clinically. A high systolic value increases the workload of the heart and the stress on vessel walls. A high diastolic value suggests that the small vessels are not relaxing adequately between beats. A low systolic value may mean the heart is ejecting too little, while a low diastolic value may mean the arteries have lost elasticity or that blood is leaking through an incompetent valve. Reading the two together gives more information than either alone.
The factors that set blood pressure
Three factors do most of the work. Cardiac output is the product of heart rate and stroke volume; if either rises without a compensating fall in the other, pressure rises. Peripheral resistance is controlled mainly by the diameter of arterioles; constricting them raises pressure, and dilating them lowers it. Blood volume is regulated largely by the kidneys, which adjust how much water and sodium are retained or excreted. Arterial elasticity is a fourth factor that modifies the shape of the pressure wave rather than its average. Because these factors interact, the same blood pressure can arise from different combinations: a young person may have a normal pressure from a high output and low resistance, while an older person may have the same pressure from a lower output and stiffer, higher-resistance vessels.
How a blood pressure is measured
Accurate measurement depends on controlling the conditions. The patient should rest quietly, sit with the back supported and feet flat, and avoid caffeine, smoking or exercise for at least thirty minutes beforehand. The arm should be supported at heart level, and the cuff should match the arm circumference; a cuff that is too small falsely raises the reading and one that is too large falsely lowers it. The cuff is inflated above the point at which the pulse disappears, then released slowly while the operator listens. The first sound is systolic and the disappearance of sounds is diastolic by the usual technique. Because readings vary, more than one measurement is taken and the average is used, and both arms may be compared at a first assessment. Automatic devices are common but must be validated and used with the correct cuff.
High and low blood pressure
Hypertension is dangerous precisely because it is usually silent. Over years, high pressure damages the small vessels of the brain, heart, kidneys and eyes, and it increases the workload of the left ventricle, which may thicken and eventually fail. Treatment lowers the risk of stroke, heart attack and kidney disease even when the patient feels well, which is why adherence matters. Hypotension is dangerous when it reduces perfusion, particularly to the brain, causing dizziness and fainting, and when it signals blood loss, dehydration, infection or a heart problem. In both cases the number is interpreted with the patient: a pressure that is alarming in one context may be acceptable in another, and a single reading is rarely enough to make a decision.
Explore the diagram
Start with the whole view, then select a numbered structure. Compare the additional illustrations to connect what a structure looks like with what it does.
Changing arteriolar resistance
Reduced motion · use the slider to explore each state.
Relaxed smooth muscle widens the lumen.
Educational schematic · selected major pathways, not to scale. Motion shows a slowed trace or comparison, not real-time physiology or disease progression.
Vessel walls match their jobs
Cross-sections at different enlargements. Compare wall-to-lumen proportions, not absolute diameters. The capillary wall is only one endothelial layer plus its basal support.
A thick smooth-muscle and elastic wall tolerates and modifies pulsatile pressure. Arteries carry blood away from the heart, regardless of oxygen content.
Connect the illustrations to the key ideas
WHAT IT MEASURES · force of blood on artery walls (mmHg)
Pressure is highest in the aorta and falls steeply across the arterioles.
WHAT SETS IT · cardiac output × peripheral resistance; blood volume
Pumping, vessel diameter, blood volume and arterial elasticity together set the value.
SYSTOLIC / DIASTOLIC · peak during contraction; trough during relaxation
Recorded as systolic over diastolic; their difference is the pulse pressure.
Core Concepts Defined
- Blood pressure
- The force blood exerts on vessel walls, usually measured in systemic arteries.
- Systolic pressure
- The highest arterial pressure, reached during ventricular contraction.
- Diastolic pressure
- The lowest arterial pressure, reached during ventricular relaxation.
- Pulse pressure
- The difference between systolic and diastolic pressure.
- Mean arterial pressure
- The average arterial pressure over the cardiac cycle, reflecting organ perfusion.
- mmHg
- Millimetres of mercury, the unit used to express blood pressure.
- Cardiac output
- The volume of blood pumped by the heart each minute.
- Peripheral resistance
- The resistance to blood flow offered by the vessels, mainly the arterioles.
- Blood volume
- The total amount of blood in the circulation, a major determinant of pressure.
- Sphygmomanometer
- The instrument, with a cuff and gauge, used to measure blood pressure.
- Korotkoff sounds
- The sounds heard over an artery as a blood-pressure cuff is released.
- Hypertension
- Persistently high arterial pressure, which damages vessels and organs over time.
- Hypotension
- Persistently low arterial pressure, which can impair organ perfusion.
- Arterial elasticity
- The property of large arteries that smooths the pressure pulse and supports diastolic pressure.
Go deeper · worked example
Reading and interpreting a blood pressure
- Prepare the patient and equipment. Seat the patient quietly for several minutes, support the arm at heart level, choose a cuff that matches the arm circumference and avoid recent caffeine, smoking or exercise.
- Inflate and release the cuff. Inflate until arterial flow stops, then release slowly while listening over the brachial artery.
- Identify the two values. The first clear Korotkoff sound is systolic pressure; the point at which the sounds disappear is diastolic pressure. Record the reading as systolic over diastolic.
- Consider the pulse pressure and mean pressure. Subtracting diastolic from systolic gives the pulse pressure; the mean arterial pressure is the value most closely related to organ perfusion.
- Interpret in context. Compare the values with the patient’s usual readings, symptoms, position and recent events; repeat the measurement if the result is unexpected, and report very high or very low values promptly.
Explain it yourself · Explain why a cuff that is too small gives a falsely high blood pressure reading.
Reveal a model explanation
A cuff transfers pressure to the artery beneath it. If the cuff is too narrow for the arm, the pressure it applies is concentrated over a smaller area and does not fully compress the artery at the pressure the gauge indicates. To stop the flow, the cuff must be inflated higher than the true systolic pressure, so the reading appears higher than it is. Using the correct cuff size is therefore part of accurate measurement, not a detail. Other errors, such as an arm below heart level or talking during the measurement, act in similar ways by changing the pressure the artery actually experiences.
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OpenStax, Anatomy and Physiology 2e, 20.3 Blood Flow, Blood Pressure and Resistance; Tortora & Derrickson, Principles of Anatomy and Physiology, 12e, ch. 21. Author review only; medical and French-language expert review pending.
Original teaching schematics. English and French wording is author-reviewed for study; anatomy and French-language expert review is pending. Checks are practice only, not a graded examination.
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