The human body is a truly amazing piece of machinery as it is able to take a physiological stimulus like exercise and adapt to it to improve performance. When we take part in a structured exercise program we know that we get “fitter”. We may even have a general understanding that one reason we get fitter is because our heart gets “stronger”. But what is actually happening to our cardiovascular system to give us this boost in performance over time?

the size of the heart increases

Firstly the size of the heart increases, particularly the size of the left ventricle, which pushes blood through the aorta into the rest of the body. Both the thickness and internal dimensions of the left ventricle increase allowing for a more forceful contraction and increased ejection of blood into circulation with each beat of the heart. This is known as stroke volume. You may also notice that as you get fitter both your resting heart rate and exercise heart rate decrease. This is due to the increase in stroke volume which means your heart doesn’t have to beat as fast to supply working muscles with enough oxygen.


Moving away from the heart through the blood vessels we see more changes occurring to improve efficiency.  For oxygen and fuel to pass from the blood into working muscles it must travel through a network of capillaries or tiny blood vessels. When we exercise regularly our bodies begin to produce more capillaries through a process called capilliarisation. This allows for an increase in blood flow and nutrients to working muscles. Additionally, this increased network of blood vessels helps to lower our blood pressure, as there is less resistance in our vascular system. Our bodies also begin to produce more red blood cells, which increases the amount of oxygen that can be transported by our blood.

mitochondria and their enzymes increase in concentration

Finally, at the level of our muscles, there are tiny structures called mitochondria, which utilise oxygen to metabolise fuels through reactions with enzymes to make our muscles contract. After regular exercise, these mitochondria and their enzymes increase in concentration allowing for more fuel to be metabolised therefore increasing our performance. What’s even more amazing is that some of these adaptations begin to occur after your first workout. However, if your exercise frequency drops the body goes through a process of detraining, which is the gradual reversal of all these wonderful adaptations. So next time you take notice of how much faster you can move and how much more endurance you have in your workouts, just appreciate the incredible inner workings of the human-machine.

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