The Engine That Keeps You Alive - Cardiopulmonary System: How Your Heart and Lungs Adapt to Exercise

Your body is made of many systems - musculoskeletal, endocrine, integumentary, etc. - that all work in unison to sustain your life. All are very important, but without oxygen delivery to your brain, as well as every other tissue in your body, life is not possible.

This leads us to the topic of the article, the cardiopulmonary system. This system is a combination of your heart and blood vessels (cardiovascular system) and your lungs (pulmonary/respiratory system). As you may already know, we inhale oxygen into our lungs, and exhale multiple byproducts including carbon dioxide and water. The lungs store oxygen, where it diffuses into the blood stream, and the heart does its job from there to pump it through your blood vessels and to tissues such as muscle. When exercising, this process must occur on a larger scale to support your body’s metabolic needs. 

After a few weeks of exercising, you may notice things becoming easier, or yourself become more capable: you are able to sustain a higher pace on the echo bike, you can push the sled for a further distance, or you are able to talk between sets without having to stop and breathe. This is a result of adaptation - a change in response to a stimulus or stressor - in this case, an improvement in ability to exercise and recover. This process of adaptation is a complex one, but here are the essential steps to know:

Side Note: many adaptations occur at the same time, such as strength improvement alongside aerobic improvement. This article will focus on oxygen delivery and its role in sustaining movement and life.

The Heart

The heart itself will adapt to move more blood through the body. Your heart is a muscle and can hypertrophy (increase in size) just like any other muscle. The chambers of your heart can increase in size and density, and the muscular walls of your heart can thicken, leading to increased blood volume and strength of contraction. Increased cardiac workload leads to increased oxygen needs for the heart, which it receives through its own vessels called coronary arteries. These arteries can adapt and improve, which will be covered next.

Blood Vessels

Blood vessels can increase both in quantity and quality in response to exercise. As for quantity, a process called capillarization occurs, where the amount of capillaries - small, thin, web-like vessels where molecules exchange and arteries transition to veins -  increases, leading to more surface area for oxygen, nutrients, and waste products to transfer between muscle tissue and blood vessels. Shifting to qualitative adaptation, vessels can increase in compliance, meaning their ability to dilate and hold more blood during exercise. The arterial wall thickness can also change, providing less resistance (afterload) for the heart to pump against.

The Lungs

The lungs also benefit from capillarization. Your lungs contain small sacs called alveoli, which are thin and are lined in capillaries. The thin walls of the sacs allow for easy diffusion (passive movement of molecules), so inhaled oxygen can enter the bloodstream, and carbon dioxide can exit into the lungs to be exhaled. In response to exercise, capillary count increases to allow this process to happen at a higher rate. The muscles which facilitate breathing can increase in strength and efficiency.

Lastly, the total volume of air your lungs can hold does not necessarily improve with training, but exercise does lead to an acute increased perfusion of the lungs with blood and oxygen which can help keep your lungs healthy. 

Why Does This Matter?

As a result of training, whether it be primarily aerobic, strength, or mobility focussed, your body and its systems will adapt. Think of your brain as a survival machine - it responds to stressors it detects and changes the systems as needed to better handle this stressor better next time, therefore providing higher odds of surviving the encounter.

While programming your sessions trainers use common principles such as progressive overload, periodization, and specificity to ensure the body is receiving the correct stimulus. Realistically, whether you understand the physiology behind these adaptations or not, they are going to happen - especially if following these principles.

Next time your trainer is telling you to go faster on the bike, push the sled further, or do more reps, remember that they are utilizing your physiology to drive you towards adaptation and progress.