If the Heart Is a Muscle, Why Doesn’t It Get Tired?

The heart is the most important muscle, its function being to circulate blood around the body to supply oxygen and nutrients such as glucose, which is required for respiration, to the tissues and remove carbon dioxide and other waste products such as urea. On average the heart beats 60 to 100 times every minute of your life, that’s at least 31,536,000 times a year, so why doesn’t it get tired and need to rest like our other muscles?

The human body consists of mainly three types of muscle: skeletal, smooth and cardiac muscle. Skeletal muscles are striated, attached to bones, and allow voluntary movement of the body such as locomotion (the ability to move from one place to another) and maintaining posture. Skeletal muscle, as well as cardiac and smooth muscle, require energy to contract. This energy comes from the hydrolysis (‘breaking down’) of adenosine triphosphate (ATP) into adenosine diphosphate and a phosphate group.

Muscles can get ATP from a series of chemical reactions – in which glucose is required – in two ways. One being aerobic respiration (involving oxygen) which occurs in the mitochondria, an organelle (a structure within a living cell specialised for a specific function). It is the slower but more efficient way, producing 38 ATP molecules for every glucose molecule. However, when you’re out of breath anaerobic respiration (without oxygen), otherwise known as anaerobic glycolysis, occurs. Although, while anaerobic respiration happens at twice the rate of aerobic respiration, only 2 ATP molecules are produced per glucose molecule used. Anaerobic respiration also produces lactic acid, a toxic substance, which causes muscle fatigue and post-exercise muscle soreness. When out of breath you fatigue more quickly, this is because lactic acid also interferes with anaerobic respiration if it accumulates faster than it can be removed, as well as this it is producing relatively small amounts of ATP for the increasing amount of glucose used. Depending on the individual, after a minute or so the skeletal muscles will be unable to produce ATP at the rate its being used.

The more mitochondria there are, the more ATP can potentially be produced. Depending on a person’s level of physical fitness, about 2-8% of the volume of skeletal muscle is mitochondria.

Smooth muscle, unlike skeletal muscle, is an involuntary muscle which has no striations. It forms part of the walls of your hollow organs (except the heart) such as the stomach, intestines, bladder and blood vessels. It helps dilate your pupils and aids digestion by contracting the stomach and small intestine to mix and move chyme (fluid consisting of gastric juices and partly digested food) down the alimentary canal. The volume of smooth muscle is about 3-5% mitochondria, similar to that of skeletal muscle.

Cardiac muscle is the striated and involuntary muscle found only in the vertebrate heart. In comparison to the other muscles, cardiac muscle can have 10 times the density of mitochondria, about 35% of its volume being mitochondria. This means cardiac muscle can produce more ATP per cell than skeletal muscle. The large number of mitochondria means there is a greater amount of energy available for the cardiac muscle, preventing fatigue. Even though the heart must work constantly to pump blood around the body to keep us alive, individual muscle cells in the heart do get regular rest because of the way the heartbeat works. In fact, about 60-70% of your life part of your heart is in a resting state.

Cardiac muscle also has the advantage of having the oxygen rich blood supply from the coronary artery. This makes it unnecessary for the heart to switch to anaerobic respiration to produce ATP and so wouldn’t produce lactic acid which gives muscles the feeling of fatigue. Unlike skeletal muscle, which often must resort to anaerobic respiration when the muscle is very active (during physical exercise) as you’re short of breath causing the supply of oxygen via the blood stream to be too low for aerobic respiration to occur.

However, a limitation of needing so much ATP is that the heart must rely on aerobic respiration and in turn oxygen. Should the supply of oxygen be cut off for some reason, such as when blockages develop in the coronary arteries, insufficient blood and as a result insufficient oxygen is flowing to the cardiac muscles. This causes chest pain (angina) and in more severe cases when the coronary artery is completely blocked, the lack of oxygen will cause some of the heart muscle to die (a heart attack also known as a myocardial infarction). On the other hand, you can simply stop using skeletal muscles, allowing them to recover the needed ATP over time.

The micro-rests that cardiac muscles take, their extremely large volume of mitochondria, and the heart’s oxygen rich blood supply, which allows it to use aerobic respiration, provides the cardiac muscle with all the things it needs to produce enough ATP and in turn energy for the heart to not get tired, provided you aren’t in an extreme state of starvation or doing any abnormal extreme forms of exercise for extended periods.

Because of these physiological differences between skeletal and cardiac muscle, cardiac muscle is less prone to fatigue and doesn’t get tired like skeletal muscle does.

Eva Roseblade, Year 12.