Why does blood pressure have two numbers?

The question

The blood pressure machine shows 118 over 72. Why are there two numbers, and what does each one mean?

Understand

Blood pressure is written as two numbers, like 118 over 72. The top number (systolic) is the peak pressure in your arteries as the heart squeezes. The bottom number (diastolic) is the lowest it falls between beats, while the heart relaxes and refills. Why doesn't it fall to zero? Three reasons: the big arteries are stretchy, so between beats they spring back and keep pressing on the blood; a valve at the heart's exit shuts, so the blood can't escape back into the heart; and the tiny arteries (arterioles) are narrow, so it can only drain out slowly. The pressure keeps falling between beats, but the next beat arrives before it gets anywhere near zero. Blood pressure is measured in millimetres of mercury (mmHg). The usual adult range on screen (about 110-130 over 70-80) is from Healthify NZ. Alex, Mrs Smith and 118/72 are fictional examples, not a personal target, and the drawings are simplified teaching schematics. Your doctor or nurse can tell you what's right for you.

1. Each squeeze stretches the artery walls and the pressure peaks: the top number, systolic.
2. As the heart relaxes and refills, the pressure falls. Its lowest point is the bottom number, diastolic.
3. It never reaches zero: arteries spring back, a valve shuts, and tiny arteries drain slowly.

Key takeaways

• Blood pressure is written systolic over diastolic, in mmHg: 118/72 mmHg.
• Top: the peak as the heart squeezes. Bottom: the lowest point between beats, as the heart relaxes and refills.
• Usual adult ranges are about 110–130 over 70–80 mmHg. Each person's ideal is set by their doctor.
• Compare with earlier readings and report changes, as your local policy says.

In practice

• Write it top over bottom, with the unit: 118/72 mmHg.
• Compare with the person's earlier readings, not only the usual range.
• A reading has changed, or you're unsure? Recheck and report. Follow your local policy.

Check yourself

Between beats the heart isn't pushing. Why doesn't the pressure fall to zero?

Words to know

Systolic

the top number: pressure as the heart squeezes (systole (contraction))

Diastolic

the bottom number: pressure between beats (diastole (relaxing, filling))

mmHg

millimetres of mercury, the unit of blood pressure

Arteriole

a tiny artery that slows the outflow (arteri- (artery) + -ole (small))

Transcript

Sources

1. Heart Foundation (Australia) - Blood pressure and your heart: systolic is the pressure in your arteries when your heart muscle contracts; diastolic is your blood pressure when your heart muscle is between beats; your doctor will tell you what your ideal blood pressure should be; measured with an inflatable arm cuff (read 11 October 2026)
2. Healthify He Puna Waiora (NZ) - Blood pressure: systolic is the highest pressure against your arteries as your heart pumps (usually 110-130 mmHg); diastolic is the pressure as your heart relaxes and fills with blood (usually 70-80 mmHg); mmHg = millimetres of mercury; written systolic over diastolic (read 11 October 2026)
3. OpenStax Anatomy and Physiology 2e, 20.1 Structure and Function of Blood Vessels: elastic arteries expand as blood pumped from the ventricles passes through them, then recoil after the surge has passed; elastic recoil helps to maintain the pressure gradient; arterioles are critical in slowing down (resisting) blood flow and are known as resistance vessels (read 11 October 2026)
4. OpenStax Anatomy and Physiology 2e, 20.2 Blood Flow, Blood Pressure, and Resistance: systolic is the higher value, diastolic the lower; recorded as two numbers, e.g. 120/80; diastolic is the arterial pressure during ventricular relaxation; arterioles are the site of greatest resistance (read 11 October 2026)
5. Klabunde RE. CV Physiology - Arterial blood pressure: as the left ventricle ejects blood the aortic pressure rises to its peak systolic value and the aortic walls expand; as blood leaves the aorta and the pressure falls, the walls recoil until the next cardiac ejection (read 11 October 2026)
6. Mayet J, Hughes A. Cardiac and vascular pathophysiology in hypertension. Heart 2003: damping is due mainly to the elastic behaviour of large elastic arteries such as the aorta; their walls recoil and the elastic energy is reconverted into pressure, which accounts to a large extent for the diastolic component of arterial pressure (read 11 October 2026)
7. OpenStax Anatomy and Physiology 2e, 19.3 Cardiac Cycle: the semilunar valves close to prevent backflow into the heart (read 11 October 2026)
8. Ospina-Tascón GA, Hernández G, Bakker J. Diastolic shock index (DSI) works... Ann Intensive Care 2020;10:109: vascular tone of the resistance arteries and arterioles determines peripheral vascular resistance; diastolic pressure is mainly determined by the diastolic time constant RC (arterial resistance times compliance); a shorter diastole is associated with higher diastolic pressure (read 11 October 2026)
9. Yartsev A. Deranged Physiology - Systolic, diastolic and mean arterial blood pressure: in diastole the stored volume is slowly squeezed out into the distal vessels as the arteries recoil; peripheral vascular resistance influences the rate of diastolic emptying (read 11 October 2026)
10. Klabunde RE. CV Physiology - Valvular insufficiency (regurgitation): in aortic regurgitation the aortic diastolic pressure is much lower than normal because blood leaves the aorta more rapidly because of backward flow into the left ventricle (read 11 October 2026)

Checked against these sources at the time of publishing. Sources change, so check the date above and your local policy. Spotted an error? Tell us and we'll correct it.