Blood pressure is determined by the output of the heart and the resistance to flow in the vascular system. These, in turn, are affected by factors such as the levels of certain hormones (renin, angiotensin II, aldosterone, cortisol, epinephrine, and norepinephrine), physical activity, emotional or physical stress, salt intake, and smoking. Blood pressure typically follows a circadian pattern, dropping at night and rising through the day with small peaks and valleys occurring every one to two hours throughout the day. For this reason, health care providers measure blood pressure at random times on multiple occasions before diagnosing hypertension, or elevated blood pressure.

Nearly one in three adults in the United States has high blood pressure, and of those who have high blood pressure one-third do not know that they have it. Uncontrolled hypertension increases the risk of stroke, mental impairment, heart disease, and kidney disease. The following table shows the guidelines set forth by the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure in 2003.

Table 1
Blood pressure Category
140/90 or higher High
120–139/80–89 Pre-hypertension
119/79 or below Normal

When hypertension is diagnosed, a clinician may prescribe lifestyle changes (e.g., diet, exercise, stress reduction). If these are ineffective, medication is recommended. Different categories of medication act on the heart, peripheral vascular system, central or peripheral nervous system, or hormonal system to lower the blood pressure.

In this experiment, you will take multiple blood pressure readings over the course of two or more days and analyze the variability in these readings. Important: The equipment used in this experiment is for educational purposes only and should not be used to diagnose medical conditions.


  • Obtain graphical representation of blood pressure measured at different times of the day.
  • Analyze the variability of blood pressure readings individually and as a class.
  • Correlate your findings with variables that may have influenced your blood pressure.