10.13: Understanding and Measuring Blood Pressure

Blood pressure without further specification usually refers to systemic arterial pressure measured at the upper arm. Understanding how it is produced, measured, and interpreted is key to assessing cardiovascular health.

Master this section and you'll be able to:

• Describe what systemic blood pressure represents and how systolic and diastolic pressures are defined.
• Explain how blood pressure changes as blood moves through different vessel types.
• Outline the basic steps of manual blood pressure measurement and identify Korotkoff sounds.
• Calculate pulse pressure from systolic and diastolic values and determine if it is normal, narrow, or wide.

What Is Systemic Blood Pressure?

Imagine your circulatory system as a giant, branching river network. Blood leaves the heart under high pressure, rushes through the large “main rivers” (arteries), slows down as it winds through tiny “streams” (arterioles and capillaries), and then gathers into the wider “return channels” (venules and veins) on its way back to the heart. Blood naturally flows from high pressure to low pressure. How smoothly that journey goes depends on several variables, including forces that push blood forward and forces that slow it down. Those slowing forces are called resistance, and you will explore them in this section.

What We Mean by “Blood Pressure”

All fluids push outward on the walls of their container — this is hydrostatic pressure. In the cardiovascular system, this outward push is what we call blood pressure. Although pressure exists everywhere in your vessels, the “blood pressure” that clinicians talk about refers specifically to systemic arterial pressure: the pressure in the major arteries that deliver blood to the body.

This pressure is measured in millimeters of mercury (mmHg) and is traditionally taken at the brachial artery in the upper arm. A reading such as 120/80 mmHg gives a snapshot of how forcefully the heart is pumping and how well the arteries are handling that pressure.

Systolic vs. Diastolic Pressure

Systemic blood pressure is reported as two numbers:

• Systolic pressure (the first number) is the pressure when the left ventricle contracts and sends blood surging into the arteries.

• Diastolic pressure (the second number) is the pressure when the ventricle relaxes and the arteries recoil.

Together, they show how effectively blood is being delivered to the tissues.
 

Blood Pressure Patterns in the Systemic Circulation

As blood leaves the heart and enters the aorta and large arteries, it is under high pressure and shows strong, rhythmic pulses created by each heartbeat. These vessels are elastic, so they stretch with each surge of blood and then recoil, helping maintain pressure between beats. As blood continues into smaller muscular arteries and especially into the arterioles, pressure begins to drop. Arterioles provide the greatest resistance to flow because their diameter is small and can be adjusted by smooth muscle. This makes them the main regulators of how much pressure is delivered to downstream capillaries and therefore the tissues.

By the time blood reaches the capillaries, the pressure has fallen enough that the strong pulsations disappear. This lower, steady pressure is essential because capillaries are thin-walled and designed for exchange, not withstanding high force. Pressure continues to decline through the venules and veins, which serve mainly as low-pressure return vessels bringing blood back to the heart. Although venous pressure is very low, features such as valves and skeletal muscle contractions help keep blood moving forward. This gradual decline in pressure across the system ensures efficient flow while protecting delicate capillary networks.

Figure \(\PageIndex{1}\): Changes in Blood Pressure Across the Systemic Circulation. Blood pressure is highest and most pulsatile in the aorta and major arteries, where each heartbeat produces distinct systolic peaks and diastolic troughs. As blood travels into smaller vessels, especially the arterioles, pressure drops sharply and the pulse pressure narrows. By the time blood reaches the capillaries and then the veins, the pulsatile pattern has nearly disappeared and pressures are much lower. This gradual decline supports forward blood flow while allowing efficient exchange of gases, nutrients, and wastes in the capillary beds.

 

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How Blood Pressure Is Measured

To measure blood pressure manually, a clinician uses a stethoscope and a sphygmomanometer (blood pressure cuff). The cuff is wrapped around the upper arm and inflated until it briefly stops blood flow through the brachial artery. As the cuff slowly deflates, the clinician listens for Korotkoff sounds — the tapping noises first described by Dr. Nikolai Korotkoff.

• The first Korotkoff sound marks the systolic pressure.

• The last sound heard before silence marks the diastolic pressure.

To ensure accuracy, the cuff is usually inflated to about 30 mmHg above the point where the radial pulse disappears, and then deflated at a slow, steady rate of 2–3 mmHg per second.

Figure \(\PageIndex{2}\): Manual Blood Pressure Cuff (Sphygmomanometer).
This standard adult-size manual blood pressure cuff is wrapped around a patient’s upper arm during blood pressure measurement. The inflatable cuff is connected by tubing to a hand bulb used to pump air into the cuff and to an analog pressure gauge (manometer) that displays pressure in millimeters of mercury (mm Hg). When the cuff is inflated, it temporarily stops blood flow through the brachial artery, allowing clinicians to determine systolic and diastolic pressures as the cuff slowly deflates.

Why BP Numbers Matter

Normal adult blood pressure is below 120/80 mmHg. Persistent readings of 130/80 mmHg or higher are classified as hypertension, which signals that the heart and vessels are under extra strain. Before diagnosing hypertension, health care providers take multiple readings on more than one occasion to ensure that the average truly reflects the patient’s baseline.

 

Blood Pressure Categories Based on Systolic and Diastolic Values

Blood Pressure Category	Systolic mm Hg (upper #)	Diastolic mm Hg (lower #)
Normal	< 120	< 80
Elevated	120 - 129	< 80
Stage 1	130-139	80-89
Stage 2	At least 140	At least 90
Hypertensive Crisis	> 180	> 120

 

Interactive Element

In this short instructional video, a nurse demonstrates how to take an accurate manual blood pressure using a sphygmomanometer and stethoscope. The video walks you through each step of the technique, from gathering supplies and positioning the patient to locating the brachial artery and checking for the auscultatory gap, a temporary “silent” period that can cause incorrect readings if missed. You learn how to estimate the systolic pressure by palpation, properly inflate and deflate the cuff, and identify the systolic and diastolic values by listening for the first and last Korotkoff sounds. The demonstration concludes with cuff removal, equipment cleaning, and documentation of the final blood pressure reading.

 

Pulse Pressure (PP)

Pulse pressure (PP) is the difference between systolic blood pressure (SBP) and diastolic blood pressure (DBP) —  see Figure \(\PageIndex{1}\). A normal resting pulse pressure is about 30 to 40 mm Hg in most healthy adults, typically at least one quarter of the systolic value.

Pulse pressures consistently below 25 percent of the systolic value are considered narrow (or low). Narrow pulse pressures often reflect a low stroke volume, which can occur in conditions such as congestive heart failure, aortic valve stenosis, or significant blood loss after trauma.

Pulse pressure/approaching or exceeding 100 mm Hg at rest are considered wide (or high). A wide PP can appear in healthy individuals after strenuous exercise, when an ordinary resting pulse pressure of 30 to 40 mm Hg may rise temporarily to about 100 mm Hg. However, a persistently elevated resting pulse pressure at or above 100 mm Hg may indicate increased arterial resistance and can be associated with several cardiovascular disorders. Chronic high pulse pressure places stress on the heart, brain, and kidneys, and usually requires medical evaluation and management.

 

Normal Range for PP is 40–60 mmHg. Formula for calculating pulse pressure: PP = SBP - DBP 

Examples:

• If a person’s blood pressure is 120 over 80 mm Hg, the pulse pressure is 40 mm Hg and therefore normal. 
• If the person has a BP of 160/75 mm Hg, the puls pressure would be 160 - 75 = 85 mmHg which means this person has a wide (or high) PP.

Why Pulse Pressure Matters

Pulse pressure is more than just a math problem between systolic and diastolic pressure; it is a meaningful window into cardiovascular health. A wide pulse pressure often means the arteries are becoming stiff, which raises the risk for heart disease, heart attack, and stroke, especially in older adults or people with diabetes. It may also point to specific conditions such as aortic regurgitation, arteriosclerosis, severe anemia, or hyperthyroidism.

In contrast, a narrow pulse pressure can signal that the heart is struggling to pump effectively, as seen in shock or congestive heart failure. Because pulse pressure reflects both how forcefully the heart beats and how well the arteries can stretch, it sometimes predicts future cardiovascular problems even better than systolic or diastolic pressure alone. For clinicians, tracking pulse pressure helps guide decisions about treatment, lifestyle changes, and how aggressively to manage cardiovascular risk.