1. A Blood Pressure Primer

How Your Heart Functions

Think of your heart as a pump, about the size of a fist. It consists of two upper chambers (the left and right atria) and two lower chambers (the left and right ventricles) that work in synchrony. Overall, the average heart contracts and relaxes—what we know as a heartbeat—about 100,000 times a day and, in the process, pumps approximately 2,000 gallons of blood daily.

  • Pulmonary circulation (on the right side): The right atrium receives oxygen-depleted blood from the inferior and superior vena cavae (the two largest veins in the body). The atrium contracts and pushes the blood through the tricuspid valve into the right ventricle. Once full, the right ventricle pumps blood through the pulmonary artery to the lungs, where the blood is re-oxygenated.
  • Peripheral circulation system (on the left side): The oxygenated blood returns via the pulmonary veins to the left atrium. From there, the atrium contracts and pushes blood through the mitral valve into the left ventricle. Now filled with oxygenated blood, the left ventricle pumps it out through the aorta and to the network of arteries, capillaries, and veins that channel blood throughout your body.

Your Vast Vasculature

It’s estimated that the human vascular system contains about 60,000 miles of blood vessels—enough to circle the Earth twice—that begin and end at the heart.


Arteries are elastic blood vessels that transport oxygenated blood away from the heart. The largest artery, the aorta, branches off into smaller arteries, which in turn divide into even smaller arteries, eventually leading to tiny blood vessels known as arterioles. The arterioles help manage the flow of blood into the microscopic capillaries found in your body’s tissues.

The arteries contain a wall of connective tissue and several layers of smooth muscle cells. The outermost layer (tunica externa) attaches the blood vessel to surrounding tissue. The middle layer, or tunica media, consists mainly of smooth muscle and is responsible for changing the diameter of the artery to help regulate blood pressure. The tunica intima, the innermost layer of the artery, includes the epithelium, a thin layer of tissue that helps regulate the transport of materials into and out of the bloodstream.


Capillaries, the tiniest and most abundant blood vessels in the body, form a bridge between the arteries and veins. They serve primarily as conduits allowing for the exchange of oxygen from blood in the arteries to tissue cells, and also the removal of carbon dioxide from the tissue cells into the veins for transport to the lungs and eventual removal from the body.


Veins, the other larger blood vessels in the body, transport blood from the bodily tissues back to the heart and then to the lungs to be re-oxygenated. The walls of the veins contain the same three layers as the arteries, but they’re thinner, partly because blood pressure in the veins is lower than it is in the arteries. However, the veins hold much more blood than the arteries—about 60 to 70 percent of your body’s total blood volume.

What Is Blood Pressure?

Simply put, blood pressure is a measure of the force of blood pressing against the walls of your arteries as your heart pumps out blood, with the highest pressures found in the arteries nearer to your heart.

But the process by which your body regulates your blood pressure is much more complex and includes a number of physiologic functions involving the cardiovascular, neurological, endocrine, and renal systems. Understandably, a major regulator of blood pressure is your heart—more specifically, your heart rate, the number of times your heart beats per minute.

Typically, if your heart rate rises, so does your blood pressure, and as your heart rate declines, your blood pressure usually lessens. An elevated heart rate increases your cardiac output, or the amount of blood pumped from your heart each minute, causing blood pressure to rise.

Among its many functions, the autonomic nervous system controls your heart rate and regulates the width of your blood vessels. Narrower arteries create more resistance to blood flow and thus increase blood pressure, while expanded, or dilated, arteries reduce blood pressure. In response to your heart rate, your arteries contract or expand to prevent major swings in pressure.

Most of this narrowing and expansion occurs in the smaller arteries known as the arterioles. However, your veins also expand and contract to influence blood pressure levels. When the veins expand, they hold more blood, which means less blood is delivered back to the heart to be pumped into the arteries. When the veins narrow, they push more blood back to the heart, resulting in more blood pumped into the arteries and higher blood pressure.

On the endocrine side, the renin-angiotensin-aldosterone system plays a significant role in blood pressure regulation. Renin is an enzyme created by the kidneys that helps to produce the hormone angiotensin. Angiotensin is converted in the bloodstream to angiotensin II, the main hormone that constricts blood vessels.

Angiotensin also triggers the adrenal glands to produce aldosterone. This hormone prompts the kidneys to excrete more sodium and water into the bloodstream, thus increasing its volume (the kidneys must maintain the right balance of sodium and fluid in the body to keep blood pressure normal). A higher blood volume forces the heart to pump more forcefully, resulting in increased blood pressure.

Blood Pressure: By the Numbers

Blood pressure is measured in millimeters of mercury—abbreviated mmHg.

Each blood pressure reading includes two numbers:

  • Systolic blood pressure (the top number): Each heartbeat that pumps blood through your cardiovascular system places pressure on the walls of your arteries. The systolic pressure reflects the amount of force placed on your arterial walls when your heart beats.
  • Diastolic blood pressure (the bottom number): This number indicates the pressure that blood is placing on your arterial walls as your heart rests between beats and fills with oxygenated blood.

Historically, more emphasis has been placed on systolic blood pressure as an indicator of cardiovascular risk. That’s because with advancing age systolic blood pressure tends to rise as your arteries stiffen and accumulate more fatty, cholesterol-laden plaque, a process known as atherosclerosis, or “hardening” of the arteries.

However, that doesn’t mean you can ignore your diastolic blood pressure. Elevations in either your systolic or diastolic blood pressure increase your risk of cardiovascular complications and can be used to diagnose hypertension. In fact, evidence suggests that among people ages 40 to 89, the odds of atherosclerotic heart disease and stroke double with each 20 mmHg systolic or 10 mmHg diastolic increase in blood pressure, according to the American Heart Association.

Interpreting Your Blood Pressure Readings

Most medical organizations, including the National Heart, Lung, and Blood Institute and the American Heart Association, categorize blood pressure as follows:

  • Normal: A systolic blood pressure of less than 120 mmHg and a diastolic pressure of less than 80 mmHg.
  • Prehypertension: A systolic blood pressure consistently ranging from 120 to 139 mmHg or a diastolic pressure consistently ranging from 80 to 89 mmHg; you’re at increased risk of hypertension if you don’t take action to control your blood pressure.
  • Stage 1 hypertension: A systolic blood pressure consistently ranging from 140 to 159 mmHg or a diastolic pressure consistently ranging from 90 to 99 mmHg.
  • Stage 2 hypertension: A systolic blood pressure consistently ranging from 160 mmHg or higher, or a diastolic pressure consistently ranging from 100 mmHg or higher.

Experts also recognize hypertensive crisis, a sharp, rapid rise in blood pressure requiring immediate medical attention. If your reading is higher than 180 mmHg systolic or 110 mmHg diastolic and you have no symptoms—such as severe headache, chest pain, nosebleeds, shortness of breath, back pain, vision changes, or numbness/weakness—check your blood pressure again in a few minutes. If it remains that high or rises even more, call 9-1-1 immediately.

Defining hypertension is a bit more complicated nowadays, as health experts are divided on exactly which blood pressure targets are appropriate for everybody. That’s because some studies suggest that people over age 50 should aim for a systolic blood pressure of 120 mmHg to achieve the best outcomes, but other research suggests that a systolic pressure of 140 mmHg may be just fine for older adults.

Your Changing Blood Pressure

It’s only natural to experience some changes in your blood pressure, especially with each passing year. As you age, blood pressure typically rises in response to changes in the health and function of your arteries.

But, blood pressure also changes from hour to hour, following a predictable, daily pattern of rising and falling coinciding with your body’s circadian rhythms—physiologic changes that follow a roughly 24-hour cycle. Your blood pressure typically begins to rise in the hours before you wake up each day, surges upon awakening, and continues its climb until midafternoon. By late afternoon or early evening, your blood pressure starts to dip, reaching a low point while you sleep at night.

On average, blood pressure is about 10 percent lower at night than it is in the daytime. People who experience these normal, healthy dips in blood pressure are known as “dippers,” while those whose blood pressure fails to drop by at least 10 percent at night are referred to as “non-dippers.” Research suggests that people who are non-dippers or whose blood pressure remains elevated at night may face a greater risk of cardiovascular and cerebrovascular disease, insulin resistance (a precursor to type 2 diabetes), and kidney disease.

Seasonal Changes in Blood Pressure

Just as temperatures change with the seasons, so does your blood pressure. Researchers have found that blood pressure and other cardiovascular disease risk factors show a seasonal pattern, with your cardiovascular risk peaking in winter. In fact, studies have found that the highest rates of cardiovascular events, such as heart attack and stroke, occur in the cold winter months.

In one investigation (Heart, October 2014), researchers reviewed data from 24 studies involving a total of 237,979 people from 15 countries to examine the seasonality of cardiovascular risk factors. They found that blood pressure readings in the northern hemisphere were highest in December and lowest in June: Systolic blood pressure, on average, was 2.9 mmHg higher in December versus June, while diastolic blood pressure averaged 1.42 mmHg higher in December than in June.

In the southern hemisphere, systolic blood pressure was 3.4 mmHg higher in July compared with January, while diastolic blood pressure was 0.8 mmHg higher in May versus November.

“Interpretation of individual risk factors levels might be different depending on season: A low value in winter is definitely a low ‘yearly’ value, but a low value in summer does not mean a low value in winter,” according to the study.

One reason why blood pressure may rise in winter is because as temperatures drop, your blood vessels constrict, your sweat output declines, and you retain more sodium. Adrenaline and other hormones that regulate blood pressure also vary according to season, which might explain seasonal blood pressure variations.

Another potential explanation is that people in northern climates with limited wintertime sun exposure are more prone to deficiencies in vitamin D, which your body produces with exposure to the sun’s ultraviolet rays. Some research has linked low vitamin D levels with an increased risk of blood pressure elevations.

Also, your blood pressure and other cardiovascular risk factors may increase when the weather is cold simply because your lifestyle changes.

When it’s cold, you spend less time outdoors, so the walking, yard work, and other physical activity you normally enjoy when it’s warm may decline. And, while you’re hibernating indoors, you tend to consume more fat and sodium in your diet, especially around the holidays.

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