Why don't heart and lung muscle get tired?

[gib65]

I was just thinking about how our heart never stops beating and our lungs never stop breathing. They have to keep it up for the entirety of our lives. That's a long time for muscles to keep working without a moment's break. How do they not get exhausted?

[senexa]

Well, the heart is not one muscle but a muscle group. It contracts in stages. The right and left atria contract simultaneously, followed by contraction of the ventricles. All the muscles relax brief before the next round.

 

Every muscle rests between bursts of activity, even if only for a nanosecond.

 

The pauses can be increased in length by illness, causing arrhythmias; but can also be increased by direct training such as yoga excercises to increase longevity.

[mezarashi]

Every muscle rests between bursts of activity' date=' even if only for a nanosecond.

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I think the mystery really is even with the rest, it seems like an exhausting life-time worth of use. All your other muscles get very very tired even with these rests. Try doing 1000 squats with 2 second rests inbetween each.

[Chumley]

There are fast twitch muscles and slow twitch muscles.

 

Slow twitch muscles are for endurance and fast twitch are for short bursts of strength. The heart is a slow twitch muscle and will continue to beat for years on end, needing only to slow down once in a while to rest.

 

Runners develope a lot of slow twitch muscle in their legs and can do marathon runs, but if you ask them to do squats with a squat bar, they will not be able to do very many--unless of course, they have been doing a lot of squats in addition to running.

 

Look at a chicken. The white meat in the breast is fast twitch and the dark meat in the legs is slow twitch. The chiken can walk around all day but can only fly short distances.

[Yggdrasil]

Muscles get tired mainly because of the buildup of lactic acid from anaerobic metabolism. My guess is that since the lungs and heart are so close to the sites of gas exchange and since they work at such a regular rate, the constant supply of oxygen to the heart and lungs is enough to power their aerobic metabolism and they do not need to switch to anaerobic metabolism in order to provide the necessary energy to perform their work.

[badchad]

I had a related, yet slightly off topic question:

 

We all know that (in a basic sense) when we exercise, our hearts get stronger. But I was curious, would it be possible to strengthen the hear through drug use?

 

For instance, a dose of cocaine will increase the heart rate. So, theoretically speaking, will the heart get stronger through use of drugs which increase our heart rate? (caffein, amphetamines etc.)

[Mayflower]

For instance' date=' a dose of cocaine will increase the heart rate. So, theoretically speaking, will the heart get stronger through use of drugs which increase our heart rate? (caffein, amphetamines etc.)[/quote']

 

The amount of cocaine needed to get a person's heart rate as high as they would in aerobic exercise would probably be impossible for the body to handle.

[Glider]

Well, the heart is not one muscle but a muscle group. It contracts in stages. The right and left atria contract simultaneously, followed by contraction of the ventricles. All the muscles relax brief before the next round.

This is not strictly true. The heart is really a single muscle. The difference in contraction between the atria and the ventricles is attributable to the difference in propogation times of electrical signals through the muscle.

 

A signal propogates from the sinoatrial node causing the atria to contract first, from top to bottom, squeezing blood into the ventricles. At the atrioventricular node there is a short delay before the signal propogates across the AV node and down the main tract (bundle of His) in the septum to the apex of the heart (bottom of the ventricles). The signal then propogates out into the left and right bundle branches, causing the ventricles to contract from the bottom up, squeezing blood into the the ascending aorta (left ventricle) and the pulmonary artery (right ventricle).

 

The reason heart muscle does not tire is that it is unique, insofar as it is a mixture, i.e. has the properties of, both smooth and striated muscle. It has the fast twitch capability of striated (skeletal) muscle, but the long term 'stamina' of smooth muscle. Each cardiac muscle cell contracts autonomously (due to 'leaky' sodium channels) and at its own rhythm, and will continue to do so invitro. Groups of these cells joined together will come into phase. The electrical impulses associated with a heartbeat are triggered at the sinoatrial node by the Vagus nerve which, as the heart will beat autonomously, even outside the body, is only responsible for controlling the timing, according to demand.