51 Cardiac cycle and pressure

Systolic and diastolic blood pressures

Arterial blood pressure in the larger vessels varies between systolic and diastolic pressures. When systemic arterial blood pressure is measured, it is recorded as a ratio of two numbers (e.g., 120/80 is a normal adult blood pressure), expressed as systolic pressure over diastolic pressure. The systolic pressure is the higher value (typically around 120 mm Hg) and is caused by ejection of blood from the ventricle. Diastolic pressure is the lower value (usually about 80 mm Hg) and represents the arterial pressure of blood during ventricular relaxation, or diastole. Unlike systolic pressure, diastolic pressure is not caused by the ventricle. Rather, this pressure is caused by the elastic recoil of the arteries. This recoil causes blood to continue to flow even when the ventricles are relaxed and filling back up.

Right and left ventricles

As discussed previously, blood flows from the vena cave into the right side of the heart. Blood in the right side of the heart exits and travels into the pulmonary circulation where gas exchange occurs, before returning back to the left side of the heart. Blood is then ejected out of the left side of the heart where it will travel throughout the systemic circulation before returning to the vena cavae and starting the process all over again.

Generally, the right and left ventricles pump the same amount of blood every minute. However, the pressures created by both ventricles are quite different. During systole, the left ventricle will generate ~120 mmHg of pressure, while the right ventricle will have a systolic pressure that is ~20 mmHg. There are two primary factors responsible for this large difference. First, the left ventricle has to pump blood throughout the entire systemic circuit, which means blood is flowing a much greater distance. It has to get blood back to the right atrium from a person’s big toe, as well as getting blood from the left ventricle back up to the top of the brain. Conversely, the right ventricle only needs to pump blood throughout the pulmonary circuit, meaning blood only needs to flow to/from the top and bottom of the lungs. Additionally, the systemic circuit contains a much larger number of vessels compared to the pulmonary circuit. The fact that the systemic circuit is longer and contains more vessels results in a significantly greater resistance (opposition to flow) that the left ventricle must overcome. The left ventricle compensates for this greater resistance, by generating a much larger pressure than the right ventricle.

Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 19