What Is Blood Pressure?
Astronaut Jeffrey Williams tests a continuous blood pressure device.
Courtesy of NASA.
- Length: Variable
- Objectives and Standards
- Materials and
- Procedure and
- Handouts and
Students now have learned about the heart, the blood vessels, and blood. But what about blood pressure? Blood behaves like any other liquid and exerts pressure against the vessels in which it is contained. Blood pressure is the force of blood against the walls of blood vessels, specifically the arteries, and is responsible for the movement of blood through the arteries. Blood pressure is much higher in arteries than in veins and capillaries.
Most students have had their blood pressure “taken” with a blood pressure cuff attached to a measuring device (sphygmomanometer). In this common practice, a cuff is secured just above the bend in a person’s elbow and inflated to increase pressure against the artery of the upper arm (brachial artery). A stethoscope is placed on the inside of the elbow to listen for the whooshing or pounding sound of blood flowing through the brachial artery. The cuff is inflated until no pulse or sound can be detected with the stethoscope. At this point, blood flow has stopped. Then, air is slowly released from the cuff, and the stethoscope is used to listen for the first sounds of blood flowing again through the brachial artery.
The force of blood flowing through the artery at this point, known as systolic pressure, is slightly greater than the pressure being exerted against the artery by the cuff. The systolic pressure indicates the amount of pressure in the artery while the heart’s ventricles are contracting. Systolic pressure is the larger (and first) of the two numbers in a reading. For instance, it is the “120” in a blood pressure reading of “120 over 80.”
Once systolic blood pressure is measured, air is slowly released from the cuff until the beating or whooshing sounds no longer can be heard through the stethoscope. Then, another reading is taken to measure diastolic pressure, which is the pressure in the artery while the heart is relaxing and refilling. Diastolic pressure is the smaller (and second) number reported in a blood pressure reading (it is the “80” in a reading of “120 over 80”).
One of the first accurate tools for measuring blood pressure was a mercury manometer (measures pressure with a column of mercury, similar to a thermometer). That’s why today, blood pressure always is reported as millimeters of mercury, even when it is measured by an aneroid device (calibrated dial with a needle) or a digital monitor.
A blood pressure measurement of 100 mm Hg indicates a force of blood pushing against the arteries sufficient to hold up a column of mercury that is 100 millimeters high. And a blood pressure reading of 120/80 (or 120/80 mm Hg), means the systolic and diastolic pressures are 120 and 80 millimeters of mercury, respectively. Knowing what these two values mean is important to health and well-being. Normal blood pressures vary by age.
When a person has pressure in the arteries that is considerably higher than normal during inactivity, we say he or she has high blood pressure (also called hypertension). A doctor must make this diagnosis, but readings higher than 140/90 usually signal high blood pressure. A rise in heart rate, increased blood volume, or a narrowing of the blood vessels all can cause high blood pressure by increasing the force of blood against the artery walls. Uncontrolled high blood pressure is sometimes called the “silent killer,” because the individual who has it feels normal. High blood pressure can damage the arteries, heart, brain, kidneys or eyes in a number of ways.
Many factors contribute to hypertension. Some, such as genetics or age, cannot be changed. A person is more likely to develop high blood pressure if his or her parents have the condition. And the chances for developing hypertension increase with age. High blood pressure also can be caused by medical conditions, such as kidney disease and diabetes. Fortunately, we can control some of the risk factors for high blood pressure. For instance, we can get regular exercise, limit the consumption of alcohol, salt and saturated fats (fats that are solid at room temperature), maintain a healthy body weight, and reduce stress.
Since there are so many negative health effects of high blood pressure, it may seem desirable to have low blood pressure (called hypotension). And it is true that people who exercise regularly tend to have lower blood pressure than those who are not as fit. However, blood pressure that is too low may signal the presence of underlying problems, such as a heart condition, low blood sugar, or even dehydration. Some experts say that readings below 90 systolic or 60 diastolic indicate low blood pressure, but since there are so many factors involved, these numbers can be misleading. What is normal for one person might be considered low for someone else.
Objectives and Standards
Living systems at all levels of organization demonstrate the complementary nature of structure and function. Important levels of organization for structure and function include cells, organs, tissues, organ systems, whole organisms and ecosystems.
The human organism has systems for digestion, respiration, reproduction, circulation, excretion, movement, control and coordination, and for protection from disease. These systems interact with one another to protect us from diseases.
The motion of an object can be described by its position, direction of motion and speed. Motion can be measured and represented on a graph.
Science, Health and Math Skills
Materials and Setup
Teacher Materials (see Setup)
Electronic blood pressure monitor with a self-inflating cuff (sold in drugstores)
Materials per Student
Obtain an electronic blood pressure monitor with a self-inflating cuff. Do not use a manual blood pressure monitor because students easily could over-inflate the cuff and cause injury. Read and follow the manufacturer’s instructions, which can vary between models.
Place the monitor in a central location, where students can take turns measuring their blood pressure. While students are waiting their turn at the blood pressure center, teams may begin to research and discuss the provided questions (see Procedure, Item 6).
If a blood pressure monitor is not available, ask students to measure their blood pressure, under the supervision of their parents/guardians, at a public blood pressure kiosk, usually found in drug or grocery stores.
Have students work in teams of two.
Procedure and Extensions
Time: Two class periods of 45-60 minutes.
Ask, Have you ever had your blood pressure taken? If so, what do you think was being measured? Explain that when a health care provider takes a patient’s blood pressure, he or she briefly restricts the flow of blood through one of the patient’s arteries by applying pressure to the artery. The health care provider then slowly reduces the pressure until he or she hears (using a stethoscope) the sound of blood forcing its way through the vessel. The measurement taken at this point is called the systolic pressure. The health care professional continues to reduce the pressure until he or she no longer hears any sounds. The measurement taken at this point is called the diastolic pressure.
Explain that the top number in a blood pressure reading (systolic) represents the pressure when blood is forced from the ventricles, and the bottom number (diastolic) represents the pressure when the ventricles are at rest, or between beats (filling with blood). Remind students that even when blood is not being forced from the heart, it continues to flow. There always is a certain amount of pressure maintained in the blood vessels.
With a student volunteer, demonstrate how to take a blood pressure reading. Have the student sit in a chair with feet flat on the floor and with shirt sleeves rolled up. Place the monitor cuff just above the bend of the student’s upper arm. Ask the student to raise his or her arm to the level of the heart. Place your arm underneath the student’s arm to support it. Prompt the student to relax his or her arm. Take a reading according to the manufacturer’s instructions for the monitor.
Mention that several factors might lead to an inaccurate blood pressure reading. These include physical activity (standing up quickly, walking fast, etc.), posture, medications, emotions, temperature and diet. Ask, Why would blood pressure be an important measure of a person’s overall health? Do you think it is more dangerous to have high or low blood pressure? Why? Remind students of earlier lessons about the heart and valves. Just as too much air pressure can damage an over-inflated tire, high blood pressure, over time, places additional stress on the heart, valves, arteries and other organs of the body.
Have teams of two students visit the blood pressure center, one team at a time. Students should take turns using the blood pressure monitor and recording their pressure readings in their lab notebooks. Be sure students record their results by writing the higher number on top and the lower number below (for example, 115/75). You may want teams to begin working on their research questions (see Item 6, below) while they wait to use the blood pressure monitor.
Discuss healthy ranges for blood pressure (see table). Remind students that if their readings do not fall within the healthy range, they may want to have their blood pressures checked by a health care professional. You may wish to construct a class graph of students’ blood pressure measurements.
Have each student team investigate one of the following questions related to blood pressure. Each team should conduct its research on the Internet and/or in the library, and then present its findings to the class during the next class period. Teams may want to develop their own topics for investigation. Students should include lists of the sources they consulted.
- What is the relationship between eating high-fat foods and blood pressure?
- How does family history (for example, whether one of your parents has high blood pressure) affect your chances of developing high blood pressure at some time?
- How does diabetes affect a person’s chances of having high blood pressure?
- What effect does walking or running three times per week have on blood pressure?
- Do stress levels influence blood pressure? If so, what is the effect?
- How does heavy alcohol consumption affect blood pressure?
- How does eating a lot of salty food affect blood pressure?
- What types of foods, if any, help to maintain blood pressure in a healthy range?
Benjamin D. Levine, MD, researches exercise programs to learn how astronauts can maintain fitness while living and working in microgravity (podcast with lessons and more).
Students investigate the heart's structure and function, blood pathways, how volumes of blood are moved through the body, and the effects of microgravity on the heart. (9 activities)
This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.