After a thirty-minute jog around your neighborhood, you’re thirsty, out of breath, and most of all, you’re tired. Your legs give way and all you want to do is lay on the ground and rest your tired muscles. But wait a minute, your heart has been pumping this whole time—and seemingly harder than you’ve worked your leg muscles. So why doesn’t your heart feel tired at all?

To answer this question, let’s first recognize that your heart is made up of muscle. But it’s a different type of muscle than the kind in your legs and arms, which are called skeletal muscles. The heart is an organ made of cardiac muscle, and this type is only found in the heart. With this information in mind, let’s look at three of the reasons why your heart never tires out.

1. Sufficient blood supply

You probably already knew that the heart is responsible for pumping blood to the rest of the body, but it’s important to keep in mind that the heart itself benefits from this blood too! Blood serves as a medium by which nutrients and oxygen can be delivered to the tissues in the rest of your body, so being at the center of the circulatory system means that the heart always has access to these things.

Nutrients are important for the health and maintenance of heart cells (called cardiocytes), while oxygen is what actually enables the heart to pump because oxygen is involved in a process that results in the production of energy in the form of ATP, which fuels the heart. Oxygen is a requirement for cardiocytes, whereas skeletal muscles can still function even without oxygen—in that case, skeletal muscle cells produce a substance called lactic acid (skeletal muscle only, not cardiac muscle!), which explains that burning sensation in your legs after your jog and why you feel so tired in those muscles.

2. Heart cells have excellent energy supply

Cardiocytes contain twice the amount of mitochondria compared to skeletal muscle cells. About one-third of the cardiocyte’s volume is attributed to mitochondria! As you’ve probably heard before, mitochondria are the powerhouse of the cell, and that is because they are the site of energy production. Cardiocytes most commonly use fatty acids as the substance that they obtain energy from. These fatty acids are taken out of the bloodstream and broken down in mitochondria. With such a high density of mitochondria (and with the heart having easy access to nutrients in the blood), cardiocytes have excellent energy supply to keep them pumping.

3. Pacemaker cells enable the heart to keep pumping

What are pacemaker cells? They are a specialized group of cells in the heart that are responsible for initiating contractions (i.e. heartbeats; pumping) in the heart. Pacemaker cells spontaneously send out regulated, consistent electrical signals (called action potentials) that travel through the heart and tell it to contract.

So, one last reason why the heart never tires is because it has numerous pacemaker cells that control the heart’s automaticity, and it is meant to be this way simply because of the heart’s function. This essentially means that the heart is under involuntary control to continuously pump.

The next time you go for a jog, just remember that the only reason you sit down to rest is only for the sake of your skeletal muscles and never cardiac muscle—your heart. And that’s not a bad thing at all; exercise benefits your heart by helping it to become more efficient at what it does.