Arterial blood pressure is the force that blood exerts against the walls of your major arteries as it flows through them. It’s measured in millimeters of mercury (mmHg) and expressed as two numbers: a top number (systolic) and a bottom number (diastolic). These two values together reveal how hard your heart is working and how much resistance your blood vessels are putting up. A reading of 120/80 mmHg is considered normal for adults.
What the Two Numbers Mean
The top number, systolic pressure, captures the peak force in your arteries at the moment your heart contracts and pushes blood out. The bottom number, diastolic pressure, is the lowest pressure in your arteries just before the next heartbeat, when the heart is relaxed and refilling with blood. Both numbers matter, but they can tell different stories about your cardiovascular health depending on your age and overall condition.
The gap between these two numbers is called pulse pressure. If your reading is 120/80, your pulse pressure is 40 mmHg, which is considered healthy. A pulse pressure consistently above 40 is generally considered unhealthy, and a pulse pressure above 60 is a recognized risk factor for heart disease, particularly in older adults. A widening pulse pressure often signals that large arteries are becoming stiffer and losing their ability to absorb the impact of each heartbeat.
What Determines Your Blood Pressure
Your arterial blood pressure comes down to two things: how much blood your heart pumps per minute (cardiac output) and how much resistance your blood vessels create as that blood flows through them (vascular resistance). Blood pressure equals cardiac output multiplied by vascular resistance. Anything that changes either of these factors changes your blood pressure.
Cardiac output depends on how fast your heart beats and how much blood it ejects with each beat. Exercise, stress, or dehydration all shift these variables. Vascular resistance depends on the diameter and flexibility of your smaller arteries. When these vessels tighten, resistance rises and blood pressure goes up. When they relax, resistance drops and blood pressure falls. This interplay is constantly being adjusted by your nervous system and hormones.
How Your Body Regulates Pressure Second by Second
Your body has built-in pressure sensors called baroreceptors, located in the walls of arteries near the heart and in the neck. These sensors detect how much the artery wall is being stretched with each heartbeat and fire signals to a control center in the brainstem. When blood pressure rises, the sensors fire more rapidly, triggering your nervous system to relax blood vessels and slow the heart. When pressure drops, the sensors fire less, prompting your body to constrict blood vessels and speed up the heart.
This reflex works beat by beat. It’s why you don’t pass out every time you stand up from a chair. The moment gravity pulls blood toward your legs and pressure drops in your upper body, these sensors detect the change and trigger a rapid correction. The response happens within seconds, carried by fast-conducting nerve fibers designed for exactly this kind of real-time adjustment.
Long-Term Pressure Control Through the Kidneys
While baroreceptors handle moment-to-moment changes, your kidneys manage blood pressure over hours, days, and weeks by controlling how much fluid stays in your bloodstream. They do this through a hormone cascade known as the renin-angiotensin-aldosterone system.
When blood pressure or blood flow to the kidneys drops, specialized kidney cells release an enzyme that sets off a chain reaction. The end product is a powerful hormone that raises blood pressure in several ways at once: it tightens blood vessels, tells the adrenal glands to release aldosterone (which makes the kidneys hold onto sodium and water), increases sodium reabsorption directly, and even stimulates thirst so you take in more fluid. All of these actions increase blood volume and vascular resistance, pushing pressure back up. When pressure is too high, this system dials down, allowing the kidneys to release more sodium and water, reducing blood volume.
Current Blood Pressure Categories
The 2025 guidelines from the American Heart Association and American College of Cardiology define four categories for adults:
- Normal: below 120/80 mmHg
- Elevated: 120 to 129 systolic with diastolic still below 80
- Stage 1 hypertension: 130 to 139 systolic or 80 to 89 diastolic
- Stage 2 hypertension: 140 or higher systolic, or 90 or higher diastolic
If your systolic and diastolic numbers fall into different categories, the higher category applies. These thresholds are based on the point at which cardiovascular risk starts to climb meaningfully.
On the low end, there’s no single cutoff for “too low” the way there is for high blood pressure. However, your organs need a minimum average pressure of about 60 mmHg to receive adequate blood flow. That average, called mean arterial pressure, is roughly estimated by adding one-third of your pulse pressure to your diastolic reading. For a person with a blood pressure of 120/80, that comes out to about 93 mmHg, well above the danger zone. Orthostatic hypotension, the dizzy feeling some people get when standing up, is diagnosed when systolic pressure drops by at least 20 mmHg or diastolic drops by at least 10 mmHg within three minutes of standing.
How Blood Pressure Is Measured
A standard blood pressure reading uses an inflatable cuff wrapped around the upper arm. The cuff is inflated until it temporarily stops blood flow in the artery, then slowly deflated. As blood begins flowing again, it creates sounds (called Korotkoff sounds) that pass through five distinct phases. The first clear tapping sound marks the systolic pressure. As the cuff continues to deflate, the sounds change in quality, becoming softer, then sharper, then muffled, and finally disappearing altogether. The point where all sound vanishes marks the diastolic pressure.
Automated devices use sensors instead of a stethoscope, but the principle is the same. For an accurate reading, you should be seated with your arm supported at heart level, legs uncrossed, and resting quietly for at least five minutes beforehand. A single reading can be misleading, since blood pressure fluctuates throughout the day with activity, stress, caffeine, and even conversation.
How Aging Affects Arterial Pressure
As you age, the walls of your large arteries gradually lose their elasticity. The flexible fibers (elastin) that allow arteries to stretch and recoil with each heartbeat break down over time and are replaced by stiffer collagen fibers. This happens independently of plaque buildup or atherosclerosis.
The practical effect is significant. Healthy, elastic arteries act like a shock absorber: they expand when the heart pumps and gently push blood forward between beats. When arteries stiffen, they can no longer buffer the force of each heartbeat effectively. Systolic pressure climbs because the stiff vessel can’t expand to absorb the impact, while diastolic pressure stays the same or even drops because the vessel can’t recoil to maintain pressure between beats. This pattern, called isolated systolic hypertension, is the most common form of high blood pressure in people over 60. It’s also why pulse pressure tends to widen with age, and why a rising top number deserves attention even if the bottom number looks fine.