In last week’ post I made the case that cardio is an important part of overall health and fitness. There are unique benefits to cardio that you are not going to be able to get through other forms of exercise.
In this post I would like to dive deeper into how cardio is providing these benefits so that we can then get into how to target these benefits with specific forms of cardio in later posts.
If you recall from last week’s post the top benefits that cardio provides are
- Improved heart health
- Fat loss, including the very dangerous fat around your organs (visceral fat)
- Improved blood lipids
- Improved endurance capacity
- Better quality of life
The reason we see many of these benefits from cardio exercise has to do with the physiological changes and adaptations that occur from repeated bouts of cardio. Below is a summary of many of the physiological changes and adaptations
When we look at this image describing the benefits we get from cardio exercise, from a heart health point of view we get an increase in cardiac output, increased stroke volume, decreased heart rate (both at rest and during exercise), and increased blood volume. How and why do all these changes occur?
It all comes down to the fact that the heart is a muscle, and just like any muscle, if you put more stress on that muscle it will adapt by growing and getting stronger. When we perform cardio exercise we raise our heart rate and so the heart is working harder. Over time as we continue to apply this stress to the heart (in this case, it is a good stress) the heart adapts, and is able to move more blood through your body for every heartbeat.
The amount of blood moved over time is referred to as stroke volume. The amount of blood moved per heart beat is referred to as cardiac output. So if we can increase cardiac output, and move more blood for every heartbeat then the heart does not have to work as hard and can therefore beat less both at rest and during exercise.
All this boils down to efficiency. You have made your heart stronger by doing cardio and therefore it now does not need to work as hard as it used to…that’s a good thing if you want your heart to work for a long time!
Another efficiency that can also impact cardiac output is the amount of blood you have circulating in your body. As we discuss later, your blood cells carry the nutrients your muscles need to contract and relax and enable you to move. If you increase the amount of blood in your body, you will again increase stroke volume and therefore more nutrients can be delivered to the muscles with less work! Efficiency for the win!
Just doing a single bout of exercise can increase your blood volume by 10-12% in 24 hours! If you continue that exercise for 10-14 days the average person will see a 20-25% increase in blood volume and elite athletes can see as much as a 50% increase in blood volume when compared to untrained individuals !
Why is the heart trying to move blood around the body so efficiently?
A lot of it has to do with the delivery of oxygen and removal of carbon dioxide to/from your muscles. Oxygen is needed to make your muscles contract. Carbon dioxide is a byproduct of this contraction and too much of it can alter the acidity in the muscle, so removing it is critical if you want your muscles to continue to function.
Where does the oxygen come from and where is carbon dioxide expelled?
So let’s take a look at how cardio exercise impacts your lungs.
We all know to some degree that the ability to take in oxygen is going to be a limiting factor in exercise…and life. As the intensity of the exercise we are doing increases we start to breathe harder and harder to consume more oxygen (actually the driving factor to your breath rate has to do with removing carbon dioxide, read more here). The more oxygen we can bring into our lungs and the more efficiently we can do that, the better we will be able to perform.
Cardio exercise will help allow better oxygen intake and carbon dioxide removal in two ways.
The first way has to do with adaptations to the muscles you use to breath. Just like in the discussion with the heart above, when we exercise we stress the muscles that help us breath. Over time with repeated exercise we will develop these muscles so they become stronger and more resistant to fatigue (improving their endurance). The stronger the muscles are, the more air you will be able to take in and exhale .
The strength and endurance of your respiratory muscles is not something people often think about when they are exercising.
If you pick up running, initially you might not be able to run very far before you have to stop and walk. You might think this is because the muscles in your legs are weak. In fact your leg muscles are probably not as weak as you think, and instead the limitation might be in your respiratory muscles. They are not used to having to work that hard for that long, and as soon as you can’t supply enough oxygen (or remove enough carbon dioxide) to your body your brain is going to set off a bunch of alarm bells telling you to stop because there might not be enough oxygen for critical functions to continue (like getting enough oxygen to the brain or your heart).
Yes you are reading that right, the limiting factor initially is your respiratory muscles not the muscles in your legs!
Even with the respiratory muscle adaptations in place those adaptations are only useful if the lungs can then take the additional oxygen, exchange it for carbon dioxide, and get it into the blood cells to be delivered to the muscles.
Again, due to the demands cardio exercise puts on the body, the body adapts to make the exchange of oxygen and carbon dioxide more efficient!
To do this new capillaries (small blood vessels) will be developed around the alveoli, the sacks within the lungs. The more capillaries around the alveoli, the more opportunity there is to get oxygen into the bloodstream and remove carbon dioxide from the body.
The red blood cells that are moving through the capillaries around the alveoli have a protein within them called hemoglobin. Hemoglobin is what the oxygen binds to within the red blood cells. Therefore the more hemoglobin within the red blood cells the more oxygen can be transported. It is no surprise then that cardio exercise will increase the amount of hemoglobin within the red blood cells . Yet another adaptation!
With the oxygen on board within the blood cells it needs to be efficiently delivered to the working muscles. One way to do this is to relax the blood vessels, or make them bigger to allow for more blood flow through the body. This is called vasodilation.
There are several mechanisms that appear to contribute to vasodilation, including changes in the nervous system, vascular system, and various chemicals that get released. Again this is an adaptation that occurs from continuous cardio exercise.
Your arteries literally change their physical shape by decreasing the thickness of the wall and increasing in diameter. In addition we see changes in the nervous system that causes the individual to be more relaxed, i.e. more parasympathetic, which in turns allows the arteries to relax as well. Another process that occurs is the release of nitric oxide which also helps contribute to vasodilation. These are just a few of the contributing factors for vasodilation but the end result is lower blood pressure, both acutely and long term.
At this point we have seen how cardio exercise strengthens the heart, and increases blood volume as well as hemoglobin content. It has also strengthened the muscles around the lungs and allowed for more efficient intake of oxygen and removal of carbon dioxide. To make it easier to get the blood containing the oxygen to the muscles and remove the carbon dioxide, cardio exercise causes the widening of the blood vessels so we can move more blood throughout the body.
The final beneficial adaptations that occur happen at the muscle.
One of these benefits is one we have already talked about. Just like cardio exercise causes an increase in capillary density at the alveoli in the lungs, the same thing happens at the muscle as well. The reason is the more capillaries at the muscle the more efficiently we can exchange oxygen and carbon dioxide.
This efficiency can be seen in a measure of aerobic performance called VO2Max.
As you can see in the graph below, the more fit you are, as determined by measuring your VO2Max, the more capillaries your muscles tend to have.
Once the oxygen and nutrients are delivered to the muscle, they are only useful if the muscle actually takes them and converts them to something used to make the muscle contract.
Inside the muscle there are cells called mitochondria that consume the oxygen and nutrients and turn it into something that the muscle can then use to contract. The more mitochondria in the muscle the more energy that can be produced. Due to the unique demands of cardio exercise, our bodies not only create new mitochondria to meet the imposed demand but also remove old damaged mitochondria resulting in not only improved performance but improved health as well !
If you have made it this far in the blog post I truly congratulate you…you are just as nerdy as me!!!!
I hope at this point you can appreciate all the amazing physiological changes that take place in your body from cardio exercise and how these changes lead to the many health benefits we mentioned in the previous post. At a high level what I hope you take away from this post is that cardio training can elicit all of these changes in our body because it challenges your body to work harder over a long period of time. Strength, power, and speed training are too short in duration in order to elicit many of these adaptations. It’s the sustained duration of cardio exercise that causes the body to adapt in these ways.
If you are interested in how to practically apply all this knowledge in your own life to get all of the benefits we have talked about, you are in luck! The next few blog posts in this series will dive into the different types of cardio training you can do, how to implement them, and how to monitor and make progress towards your goals using them. To be the first to know when the next blog post in the series comes out, jump on my newsletter using the form below and I will send you an email with all the information once the post is available.
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