What Is Blood Pressure and How Does It Work?

Every heartbeat sends blood through a remarkable network of arteries, veins, and smaller blood vessels.

As your heart pumps, it creates pressure that helps drive blood through your arteries and deliver oxygen and nutrients throughout your body.

But what exactly creates blood pressure, and why does a reading such as 120/80 mm Hg contain two different numbers?

Understanding how blood pressure works can help you make better decisions about your cardiovascular health, especially as you get older.

Quick Answer: What Is Blood Pressure?

Blood pressure is the force of blood pushing against the walls of your arteries as your heart pumps blood through your body.

Your heart generates pressure when it contracts and pushes blood into the arteries. Between heartbeats, the elasticity of your arteries and resistance within the circulatory system help maintain pressure and keep blood moving.

Blood pressure is recorded using two measurements:

• Systolic pressure: the pressure in your arteries when the heart contracts and pumps blood.
• Diastolic pressure: the pressure in your arteries between heartbeats, while the heart relaxes and fills with blood.

Doctors record these numbers in millimeters of mercury (mm Hg).

Blood Pressure Glossary

Blood pressure: The force of circulating blood against artery walls.

Systolic pressure: The top number, measured when the heart contracts and pumps blood.

Diastolic pressure: The bottom number, measured while the heart relaxes between beats.

(SOURCE: American Heart Association)

Your Heart Is the Pump Behind Blood Pressure

Think of your cardiovascular system as a transportation network.

Your heart provides the pumping force. Your arteries carry blood away from the heart, while your veins return blood toward it.

The heart does not pump in one continuous motion. Instead, it contracts and relaxes in a repeating cycle.

During contraction, the ventricles push blood into the arteries. During relaxation, the heart chambers fill again before the next heartbeat.

That cycle explains why blood pressure has two numbers.

What Is Systolic Blood Pressure?

Systolic pressure is the first, or top, number in your blood pressure reading.

It represents the pressure inside your arteries when the ventricles contract and push blood out of the heart.

For example, if your reading is 120/80 mm Hg, the 120 represents your systolic pressure.

Because the heart is actively pumping during this phase, arterial pressure reaches its highest point in the cardiac cycle.

What Is Diastolic Blood Pressure?

Diastolic pressure is the second, or bottom, number.

It represents the pressure inside your arteries while your heart relaxes and fills between beats.

Using the same example, the 80 in 120/80 mm Hg represents your diastolic pressure.

During this phase, the elastic walls of the arteries recoil and help maintain pressure and blood flow until the next heartbeat.

Your heart needs this resting and filling phase before the next contraction, which is why diastolic pressure reflects a different part of the cardiac cycle than systolic pressure.

Why Do We Need Blood Pressure in the First Place?

Your organs and tissues need a continuous supply of blood to receive oxygen and nutrients.

The heart creates the pumping force that moves blood into the arteries, and the circulatory system maintains the pressure needed to keep blood moving throughout the body.

Without sufficient blood pressure and blood flow, vital organs such as the brain, heart, and kidneys may not receive the circulation they need.

Blood pressure therefore serves an essential purpose: it helps maintain circulation throughout the body.

Your Arteries Do More Than Carry Blood

Arteries are not simply rigid tubes that transport blood.

Their muscular and elastic walls can change in response to the body’s needs. Blood vessels can narrow or widen, affecting both blood flow and blood pressure.

The nervous system, hormones, kidneys, heart, and blood vessels all play a role in regulating blood pressure.

For example, the kidneys help regulate fluid and sodium balance, which affects blood volume. The nervous system can influence heart rate and blood vessel tone.

Blood pressure is therefore controlled by a coordinated system rather than by one organ alone.

What Happens to Blood Pressure During Exercise?

Your cardiovascular system responds quickly when you exercise.

Working muscles need more oxygen and nutrients, so your heart increases its activity to deliver more blood.

As a result, systolic blood pressure normally rises during physical activity.

At the same time, blood vessels supplying active muscles widen to accommodate increased blood flow.

This response demonstrates an important principle: blood pressure is dynamic, not a fixed number.

It changes as your body’s circulatory needs change.

Why Does Blood Pressure Change During the Day?

Your blood pressure does not remain identical from morning to night.

Physical activity, stress, caffeine, sleep, body position, medications, hydration, and other factors can influence your readings.

For example, exercise and caffeine can temporarily raise blood pressure. Stress can also cause short-term increases as the heart beats faster and blood vessels respond to stress hormones.

Blood pressure also follows normal daily patterns and can vary depending on the time of day.

That is why one isolated measurement does not always provide a complete picture of your blood pressure.

Why Can Blood Pressure Become Too High?

Blood pressure can rise when the circulatory system experiences greater resistance or when changes in blood volume, blood vessel function, heart function, or other regulatory systems increase pressure inside the arteries.

Age also matters.

As we get older, blood vessels naturally tend to become thicker and stiffer. These changes can contribute to rising blood pressure, particularly systolic blood pressure.

Other factors associated with an increased risk of high blood pressure include:

• High sodium intake
• Physical inactivity
• Excess body weight
• Excessive alcohol consumption
• Poor-quality sleep
• Frequent or prolonged stress
• Certain medications
• Kidney disease
• Diabetes
• Family history and genetics

Several factors may occur together, which is why blood pressure risk varies from person to person.

Can Blood Pressure Be Too Low?

Yes.

Your body needs sufficient blood pressure to deliver blood to vital organs.

The National Heart, Lung, and Blood Institute defines low blood pressure, or hypotension, as blood pressure below 90/60 mm Hg.

However, some people naturally have lower blood pressure without experiencing problems.

Low blood pressure becomes more concerning when it causes symptoms, develops suddenly, or is related to an underlying medical condition.

Possible symptoms include:

• Dizziness
• Fainting
• Weakness
• Confusion
• Blurred vision

The significance of a low blood pressure reading depends on the individual, the circumstances, and whether symptoms are present.

Why Does Blood Pressure Matter More as You Enter Middle Age?

Blood pressure tends to rise with age as blood vessels undergo gradual changes.

Middle age can also bring changes in other cardiovascular risk factors. Physical activity may decline, weight may increase, and sleep patterns can change.

Conditions such as diabetes and high cholesterol also become more common with age and may increase overall cardiovascular risk.

Blood pressure interacts with many of these factors. Knowing your numbers gives you another important piece of information about your cardiovascular and vascular health.

Blood Pressure Is Not the Same as Blood Flow

These terms sound similar, but they describe different things.

Blood pressure refers to the force exerted by blood against artery walls.

Blood flow refers to the movement of blood through the circulatory system.

They are closely connected, but they are not interchangeable.

For example, an artery can develop narrowing or blockage that restricts blood flow. A standard blood pressure measurement taken from your arm does not tell you whether a specific artery elsewhere in your body is narrowed or blocked.

A person can therefore have an apparently normal blood pressure reading and still have a vascular condition that affects blood flow.

This distinction is especially important when evaluating symptoms of vascular disease.

The Bottom Line

Blood pressure is essential for maintaining circulation and delivering oxygen-rich blood throughout your body.

Your heart provides the pumping force, while your arteries and other parts of the circulatory system help maintain and regulate pressure as blood travels through your body.

The two numbers in your blood pressure reading represent different parts of the heartbeat: systolic pressure when the heart contracts and diastolic pressure while the heart relaxes between beats.

Blood pressure can change throughout the day, but consistently elevated readings may signal an increased risk of cardiovascular disease.

Knowing your blood pressure—and understanding what the numbers mean—is an important part of protecting your cardiovascular and vascular health.

Frequently Asked Questions About Blood Pressure

Medical and Academic Resources

American Heart Association / American College of Cardiology

• 2025 High Blood Pressure Guideline
• Blood Pressure Explained
• Understanding Blood Pressure Readings
• The Facts About High Blood Pressure

National Heart, Lung, and Blood Institute (NHLBI)

• How the Heart Works: How the Heart Beats
• High Blood Pressure: Causes and Risk Factors
• Low Blood Pressure