How the body adapts - transporting oxygen

Building on our previous article about the cardiovascular adaptations to exercise (found here), this current article focuses on how oxygen is transported from the lungs into the tissues of our cells along with how our cells use oxygen. 

Let’s begin with the air in the atmosphere which we breathe. We inhale air which contains oxygen (20.95% O₂, 0.035% CO₂, 78.08% N₂, 0.93% Ar). This oxygen is then diffused from the alveoli (air sacs in lungs) to the pulmonary capillaries (get it into the bloodstream). The transport of oxygen follows the pressure gradient from the alveoli to arterial (respiratory to vascular, i.e., lungs to bloodstream). As oxygen is transported, it follows a kind of cascade (think waterfall) and we lose partial pressure of oxygen as we get further into our cardiovascular system, capillaries, and cells.

Once oxygen is in the bloodstream at the pulmonary capillaries, blood moves to the left atrium of the heart and through the mitral/bicuspid valve to the left ventricle, where is it ejected from the heart and into the vascular system (aortic arch, descending aorta, arteries, arterioles, capillaries). The ability of oxygen to move through the bloodstream is dependent upon the oxygen carrying capacity of the blood. Blood contains hemoglobin, the oxygen carrying molecule in the blood. Oxygen transport then depends on how well the hemoglobin is saturated with oxygen (i.e., oxygen saturation, which is often measured on fingertip or earlobe, SpO₂). Exercise reduces the hemoglobin affinity to oxygen due to higher temperature, increased CO₂, lower pH (by-products of glycolysis – which is our anaerobic metabolism that is engaged during higher intensity efforts which allows us to produce energy without the presence of oxygen). This is quite interesting, as the lower affinity between hemoglobin and oxygen make it harder to transport oxygen - but this is also a stimulus as this stress enhances the unloading of oxygen to the tissues to meet the demand of oxygen (in this case exercise).

We should also discuss the global delivery of oxygen from lungs to tissues, which relies on the cardiovascular system. In general, this level of the cascade is about the oxygen extraction, how much utilization in comparison with delivery (oxygen extraction ratio = consumption/delivery). The delivery of oxygen is made available with the driving force from cardiac output (heart rate multiplied by stroke volume); related to oxygen consumption or uptake (VO₂) – for more insight here refer to previous article on cardiovascular adaptations. Delivery of oxygen is increased during exercise (maybe even 20+ x higher than at rest), as the demand for oxygen increases due to more working tissues and the skeletal muscles demand oxygen for the ability to generate ATP and allow contraction and relaxation to occur. We also know that during exercise, our vessels vasodilate allowing more blood flow and delivery of oxygen. 

On a regional level of distribution of oxygen delivery, the supply of blood to organs (delivery) is highest in kidneys, followed by the brain and skeletal muscle, then GI tract and other organs. Whereas more oxygen is consumed in skeletal muscle followed by the brain and then GI tract. 

Moving further along our oxygen cascade, the next step is diffusion from the capillaries (smallest vessels that exchange oxygen and carbon dioxide) to the cells. Factors affecting the oxygen extraction ratio include rate of oxygen delivery to capillary, hemoglobin saturation with oxygen, size of capillary relative to the cellular pressure gradient of oxygen, diffusion distance from capillary to cell (how deep it must travel), rate of oxygen utilization (metabolic rate). Cells have varying usage of oxygen which is highly dependent on the metabolic rate. With the metabolic rate, we are thinking about what system our body uses to produce energy: aerobic metabolism – oxidative/fat oxidation – where we generate energy by ATP with use of oxygen; glycolysis or anaerobic metabolism – use of carbohydrates and generation of energy without the presence of oxygen - but with by-products such as CO₂, lactate, and lower pH, etc. Factors that affect the metabolic rate include temperature, inflammation, trauma, sympathetic activation (pain, shivering), interventions (procedures or therapy), medications, nutrition (especially high levels of glucose). 

Knowing we need oxygen to sustain life is critical, and now we understand more about how oxygen travels through our system and at least briefly of how it works to allow our cells to function properly. This article will now shift the focus to mitochondria, the “powerhouse” of the cell. Mitochondria are super important as they extract energy from food in a process called cellular respiration. Along the lines that our respiration from our lungs provides us oxygen, the respiration of our cells provides mitochondria with energy (they use oxygen and glucose to obtain and package as adenosine triphosphate, ATP). For clarity, cellular respiration is a process that combines oxygen and molecules from the food we eat (like glucose) and converts that chemical energy into life sustaining activities. Mitochondria have other functions besides producing energy such as storing calcium for cellular signaling, generating heat, and mediating cell growth and death. We will aim our focus on mitochondria’s ability to produce energy. 

If mitochondria need oxygen to produce ATP (energy which is involved in muscle contraction and relaxation) and during exercise we perform a lot of muscle contraction/relaxation, then it is most logical to want more 1) oxygen and 2) mitochondria. With more oxygen, mitochondria can provide more energy and drive all cellular biological processes. With more mitochondria, we can more efficiently convert energy into ATP, and this will allow us to train/compete faster and have more endurance for longer duration exercise. As mentioned earlier, we need ATP both to contract our muscles and to relax our muscles. Interestingly, the muscle tissue, liver, and brain are the organs that require the most mitochondria for function. Can you believe that there are more than 5,000 mitochondria per cell in our heart muscles? This is the greatest number of mitochondria anywhere in our body. 

At this point, you might be wondering, how can we increase the number of mitochondria in our cells? Exercise, specifically higher intensity training in aerobic exercise (such as skiing, running, cycling) utilizes greater amounts of oxygen – especially when the exercise lasts longer than 5 minutes. This allows cells to make more proteins which are used for energy-producing mitochondria OR protein-building ribosomes. By the stimulus of exercise, we demand more oxygen and thus stress our mitochondria to convert energy (from oxygen and glucose) into ATP. Higher intensity exercise increases this stimulus and over time will allow for building the number and thus density of mitochondria within our cells. Again, the more mitochondria we have, the more efficient we are at performing work. A general guideline for adaptations to mitochondrial density would be about 8-12 weeks into training, or into a more specific training block. I would refer you back to review a previous article on the adaptation process. 

Now we understand that by training more endurance but also intensity over a period of several years, our bodies will make adaptations and continue to improve our ability to transport oxygen efficiently (uptake, delivery, extraction, utilization). This is clearly one of the main components of our training adaptations that will make us faster each time we train and emphasizes the importance of consistency in our training.