To understand blood pressure it is helpful to think of a pump and a system of pipes. The heart is the pump and the pipes are the veins and arteries in the body that make up the vascular system. Blood is the fluid being pumped through the pipe system. The effectiveness of the pump, the size of the pipes and the volume of the fluid flowing through the system are factors that affect blood pressure.
Heart Rate and Rhythm
Heart rate is a factor that affects blood pressure. A regular heart rate of about 70 to 80 beats per minute is sufficient to maintain blood pressure in the average healthy adult. When the average healthy adult exerts himself, for example while running, his heart rate increases because his body needs more oxygen to feed his working muscles. The increased heart rate increases his blood pressure to accomplish this task.
However, when the heart rate gets too fast, low blood pressure, called hypotension, occurs. The heart is made up of four chambers. In between heart beats the chambers fill with blood to prepare for the next heart contraction. A very fast heart beat does not allow the heart chambers enough time to fill or empty properly. Therefore, less blood is ejected with each heart beat. The result is less blood in the pipe system, which causes less pressure.
In the case of a slow heart rate, called bradycardia, there is enough blood in the heart chambers and the vascular system, but the pump is working too slowly to keep the pressure in the pipes up.
Kathleen Ouimet Perrin, Ph.D., author of "Understanding the Essentials of Critical Care Nursing," explains how an irregular heart rhythm affects the pressure in the pipes. When the pump runs erratically the pressure in the vascular system cannot be maintained and hypotension results.
Pump Strength
Donna D. Ignatavicius, M.S., R.N., and M. Linda Workman, Ph.D., authors of "Medical-Surgical Nursing: Critical Thinking for Collaborative Care," discuss how a weak pump cannot maintain blood pressure. Even if the heart rate is fast enough and the blood volume is high enough, a weak pump is not able to push the blood throughout the pipe system. Blood sits in the pipes and pressure drops.
Fluid Volume
Fluid volume in the pipe system affects pressure. Lots of fluid elevates pressure and low fluid levels drop pressure.
Systemic Vascular Resistance
Systemic vascular resistance is the pressure in the pipes that the pump has to overcome to pump the blood throughout the body. Vasoconstriction is narrowing of the blood vessels; narrow pipes increases pressure. Vasodilation, or widening of blood vessels, decreases pipe pressure.
References
- "Medical-Surgical Nursing: Critical Thinking for Collaborative Care"; Donna D. Ignatavicius M.S., R.N., & M. Linda Workman Ph.D.; 2006
- "Understanding the Essentials of Critical Care Nursing"; Kathleen Ouimet Perrin, Ph.D.; 2009
