You have probably had a doctor take your pulse rate before. Have you thought about how much work your heart carries out to keep you going? With a pulse rate of 80, that turns out to be over 42 million heart beats per year! Without this pump our brain, kidneys, and other vital organs would miss out on the nutrients and oxygen they need to function.


Pulse rate is normally maintained at 60-100 beats per minute by our natural pacemaker, the sinoatrial (SA) node. The SA node sends electrical messages through nerve fibers in the heart muscle at regular intervals. These nerves help coordinate the heart muscle to contract and eject nutrient-rich blood into our circulation.

Once the contraction is complete, the heart relaxes. During this relaxation phase, the heart fills with more nutrient-rich blood. The heart normally works in about a 1:2 work to relax ratio, which means 2/3 of the time our heart is relaxing.

If the heart only works around 1/3 of the time, how much blood does it actually pump? Well, stroke volume, the amount of blood ejected with each heart contraction, is about 60 milliliters. Using our example of 80 beats per minute, this works out to just under 5 liters of blood pumped per minute. That is pretty impressive for an organ that only makes up 0.4% of our body weight.


We have three different types of blood vessels that make up our circulation; arteries, capillaries, and veins.

Our heart pumps blood into our thick-walled arteries where blood flows to various parts of our body. Think “A” for Artery moving blood Away from the heart. Blood flows from our arteries to our smallest vessels, our capillaries. These capillaries have thin walls that allow for gases like oxygen to easily pass from the bloodstream into our body tissues. Finally, our veins are responsible for moving blood back to our heart.

The blood in our veins is referred to as ‘deoxygenated’, which may sound like blood without any oxygen in it, but in fact it is still 75% saturated with oxygen. I know, a bit misleading, let’s take a closer look.

Oxygen moves through our circulation attached to hemoglobin proteins. Each hemoglobin protein has an important job to carry oxygen molecules and ‘release’ some of them from the bloodstream in capillary beds. The key is that for every four molecules it carries, the goal is usually to give up no more than one. Hence, three of four, or 75%.

Blood enters the heart before becoming ‘re-oxygenated’. In the diagram below, the blue areas represent where our ‘deoxygenated’ blood flows and the red where oxygen rich blood flows.


Blood enters the heart in the right atrium (RA, top left chamber in the image) and right ventricle (RV, bottom left chamber in the image) before being pumped into lung circulation. The capillary beds in lungs allow for diffusion of oxygen into the blood stream.

The air around us is only about 20% oxygen, the majority is made up of nitrogen plus a small amount of carbon dioxide, argon, water vapor, and other gases. Thankfully when we breathe in the air around us, our lungs are effective at pulling oxygen into the capillary beds that surround the lungs.  

This now oxygen-rich blood, at nearly 100% oxygen carrying capacity, reenters the heart in the left atrium (LA, top right chamber in the image). Next blood flows into the left ventricle (LV, bottom right chamber in the image). This is the last stop in the heart before going back into our whole-body circulation.

So here we covered some of the basics behind how our heart normally functions. Many things can affect our pulse rate and blood flow, such as exercise training or diseases. We will get into these in these in future blogs. Until then feel free to leave your thoughts, comments, and questions.

Thanks for reading.



Editor(s): Austin Robinson, PhD

Curious to learn more? Check out these related resources.

Measuring your heart rate the ‘old-fashioned’ way

Measuring your heart rate using your cell phone

TED-ED: How the heart actually pumps blood - Edmond Hui

TED-ED: Oxygen’s surprisingly complex journey through your body - Enda Butler



Kenney, Wilmore, and Costill (2015). Physiology of Sport and Exercise, 6th Edition. Human Kinetics.
