How Blood Flows through the Human Heart

5 of 8 in Series: The Essentials of Biological Processes

Human hearts, as well as the hearts and circulatory systems of some other mammals, are complex. They need to have a higher blood pressure to get the blood circulated throughout their entire bodies. Blood pressure is a force that sends the blood through the circulatory system.

The structures of the human heart

The human heart has four chambers: two ventricles, each of which is a muscular chamber that squeezes blood out of the heart and into the blood vessels, and two atria, each of which is a muscular chamber that drains and then squeezes blood into the ventricles. The two atria reside at the top of the heart; the two ventricles are at the bottom. And, the heart is divided into left and right halves, so there is a left atrium and left ventricle, as well as a right atrium and right ventricle.

The reason that the heart is divided into halves is because of the two-circuit circulatory system. The right side of the heart can pump blood to the lungs, while the left side of the heart pumps blood to the rest of the body. Blood goes in both directions on each and every pump.

The cardiac cycle

Every minute of your life, your heart pumps about 70 times. Every minute of your life, your heart pumps the entire amount of blood that is in the body — 5 liters, which is equivalent to 2-1/2 big bottles of soda. The heart never stops working from the time that it starts to beat when humans are nothing but wee little embryos in their mother’s wombs until the moment they die.

The 8/10th of a second that a heart beats is called the cardiac cycle. During that 0.8-second period, the heart forces blood into the blood vessels plus it takes a quick nap. Here’s what happens in those 0.8 seconds:

The left and right atria contract.
The left and right ventricles contract.
The atria and ventricles rest.

When the atria and ventricles are resting, the muscle fibers within them are not contracting, or squeezing. Therefore, the relaxed atria allow the blood within them to drain into the ventricles beneath them. This period of relaxation in the heart muscle is called diastole.

With most of the blood from the atria now in the ventricles, the atria contract to squeeze any remaining blood down into the ventricles. Then, the ventricles immediately contract to force blood into the blood vessels. This period of contraction in the heart muscle is called systole.

Blood pressure

If the terms systole and diastole sound familiar, it is probably because you have heard the terms systolic blood pressure and diastolic blood pressure. In a blood pressure reading, such as the normal value of 120/80 mm Hg, 120 is the systolic blood pressure, or the pressure at which blood is forced from the ventricles into the arteries when the ventricles contract; 80 is the diastolic blood pressure, the pressure in the blood vessels when the muscle fibers are relaxed. The “mm Hg” stands for millimeters of mercury (Hg is the chemical symbol for mercury).

If your blood pressure is 140/90 mm Hg, which is the borderline value between normal and high, that means your heart is working harder to pump blood through your body (140 versus 120), and it is not relaxing as well between pumps (90 versus 80).

A blood pressure reading of 140/90 mm Hg indicates that something is causing your heart to have to work at a much higher level all the time to keep blood flowing through your body, which stresses the heart. The “something” that may be the culprit could be any of the following:

A hormonal imbalance
A dietary problem, such as too much sodium or caffeine
A mechanical problem in the heart
A side effect of medication
Blockages in the blood vessels

The high pressure in the “pipes” also may lead to damage. Physical damage from high blood pressure is part of a hypothesis of how fibrous plaques are formed in coronary arteries.