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Cardiac muscle
Cardiac muscle
414c Cardiacmuscle.jpg
Cardiac muscle tissue is only found in the heart
Latin Textus muscularis striatus cardiacus

Cardiac muscle (heart muscle) is striated muscle (striped muscle) in the walls of the heart. It makes up the tissue called the myocardium. It is involuntary: a person cannot control it consciously.

Cardiac muscle is one of three main types of muscle, the others being skeletal and smooth muscle. The cells that make up cardiac muscle have one (74%) or two (24.5%) nuclei. The myocardium forms a thick middle layer between the outer epicardium layer and the inner endocardium layer.

Coordinated contractions of cardiac muscle cells in the heart force blood out of the atria and ventricles to the blood vessels of the left/body/systemic and right/lungs/pulmonary circulatory systems. This mechanism illustrates systole (contraction) of the heart.

Cardiac muscle cells, unlike most other tissues in the body, rely on the coronary arteries to deliver oxygen and nutrients and remove waste products directly. There is no time for them to diffuse.


The physiology of cardiac muscle is similar to that of skeletal muscle. The primary function of both muscle types is to contract.

Cardiac cycle

The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next.

It consists of two periods:

1. The heart muscle relaxes and refills with blood (diastole); 2. The heart muscle contracts and pumps blood (systole).

After emptying, the heart immediately relaxes and expands to receive another influx of blood returning from the lungs and other systems of the body. A normally performing heart must be fully expanded before it can efficiently pump again.


Dog cardiac muscle (400X)

It was commonly believed that cardiac muscle cells could not be regenerated. However, this was contradicted by a report published in 2009. Olaf Bergmann and his colleagues at the Karolinska Institute in Stockholm tested samples of heart muscle from people born before 1955 who had very little cardiac muscle around their heart. By using DNA samples from many hearts, the researchers estimated that a 4-year-old renews about 20% of heart muscle cells per year, and about 69 percent of the heart muscle cells of a 50-year-old were generated after he or she was born.

Problems of the myocardium

The heart muscle can become sick and weak. For example, if a person has very high blood pressure (hypertension), part of the heart muscle can get overworked. The heart muscle becomes bigger and cannot do its job as well. This is called hypertrophy.

If a person has a problem with the electrical system that controls the heart, the heart muscle may not beat the way it needs to. It may beat too slowly to get blood out to the body (this is called bradycardia). Or it may beat so fast that the heart does not have time to fill with blood and then squeeze the blood out to the body. This is called tachycardia (there are many kinds).

The heart gets blood through the coronary arteries. These are special blood vessels that carry blood only to the heart. In order to do its job, the heart muscle needs a constant supply of blood and oxygen from the coronary arteries. If these coronary arteries get blocked, blood flow to the heart muscle can stop. Without blood flow, the heart muscle gets no oxygen. If this lasts long enough, the part of the heart muscle which is not getting enough oxygen dies. This is called a myocardial infarction, or a heart attack.

See also

Kids robot.svg In Spanish: Miocardio para niños

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