How does your blood adapt to high altitudes?

Author: Christi LeMunyon

What is different when you’re at altitude compared to sea level?

Oxygen is an essential molecule for the human body to carry out its normal function. When we breath in, oxygen can enter our lungs and make its way into our circulating blood system through gas exchange between alveoli, small air filled sacs in the lungs, and capillaries, or small blood vessels. When the oxygen enters our blood, it binds to hemoglobin, or the oxygen carrying molecule of red blood cells. These red blood cells travel through our circulatory system and deliver the oxygen to working tissues.

Oxygen makes up 21% of the air. The percentage of oxygen in the air at two miles altitude is essentially the same as at sea level. However, the air pressure is 30% lower at altitude. This means that the molecules are less dense and more spread out. When you arrive at a high altitude, the low pressure makes it difficult for oxygen to enter our vascular system. This results in a condition called hypoxia, or a deprivation of adequate oxygen supply to body tissues.

Hypoxia usually begins as a sudden increase in difficulty of normal tasks, such as walking or climbing a flight of stairs, even for well-trained athletes (Lovett 2016). Other symptoms include lack of appetite, vomiting, headache, distorted vision, and general fatigue ( “Altitude sickness” symptoms can vary in severity from person to person.

What happens when you immediately arrive at a high altitude?

When we travel to high altitude areas, our bodies try and compensate for the decrease in available oxygen by developing inefficient physiological responses (Windsor and Rodway 2007). This includes an immediate increase in cardiac output, or the volume of blood the heart can pump each minute, mostly due to a rise in heart rate, or the number of times the heart beats per minute (Naeije 2010). Blood pressure is also believed to rise sharply as our hearts try to pump harder to get more oxygen to our cells (Naeiji 2010). It is believed that cardiac output changes to exactly match the decrease in blood oxygen content to leave the delivery of oxygen to the tissues virtually unchanged (Naeije 2010). This response is temporary and cardiac output returns to normal within a few days (Mairbäurl 2013).

What happens to our bodies as we acclimatize to the high altitude?

Our bodies are well-equipped to adapt to harsh environments and climates in order to keep us alive. Since cardiac output returns to baseline after a few days at high altitude, there must be an increased oxygen carrying capacity of our blood to makeup for the low levels of oxygen (Naeije 2010). A more efficient response develops as acclimatization, or the process of adjusting to a change in environment, takes place. Exposure to high elevation causes a cascade of changes within the red blood cell that allow them to cope with the low-oxygen conditions (Lovett 2016).

It was initially believed that it took several weeks or even months for our blood to adjust to low levels of oxygen, but recent studies have discovered the human body responds almost immediately. Our body begins to produce more red blood cells. Exposure to hypoxia causes a fast increase in erythropoietin concentrations, one of the body’s hormones (Mairbäurl 2013). These levels peak within the first 2-3 days of altitude exposure (Benjamin et. al 2014). Erythropoietin triggers the productions of more blood cells (Peterson 2016). More blood cells continue to be produced drastically during the first few weeks spent at high altitude (Windsor and Rodway 2007). This provides humans with the ability to compensate for the dramatic drop in oxygen levels by creating more oxygen carrying molecules (Windsor and Rodway 2007). Within two weeks, the body can produce enough red blood cells to make up for the decrease in oxygen (Windsor and Rodway 2007). Like many other things, this process is subject to individual differences. Some people’s bodies may be better equipped to respond better than others.

What happens if I exercise at high altitudes?

Chronic exposure to hypoxic environments will ultimately yield performance benefits. Exercising at high altitudes has been a training method for elite athletes around the world. The United States Olympic Training Center is located in Colorado Springs, Colorado and sits at more than a mile above sea level. These athletes are mostly interested in creating a higher capacity to deliver nutrients, like oxygen, to their muscles (Peterson 2010). Athletes use the idea of hypoxia stimulating the production of more red blood cells to get an “upper -hand” against their competitors. Intense and long duration exercise produces stress on the body. This stress response is exacerbated by high altitudes (Mairbäurl 2013). These conditions are believed to have an additive effect on red blood cell production, meaning training at high altitudes will stimulate more red blood cell production than being inactive at high altitude (Mairbäurl 2013). Though exercise can increase the production of red blood cells, sedentary high altitude residents even have an increased red blood cell count in comparison to the low altitude counterparts (Mairbäurl 2013).

At first, it is more difficult to train at the same intensity as they normally would at sea level because they have to allow their bodies to adjust to lower air pressure (Peterson 2010). Because of this, they often arrive at high altitudes several days before major competitions so that their body can start to adjust before they need to perform. After acclimatization, these athletes can perform at the same level as before, but with less oxygen available in the air (Peterson 2010). When they return to sea level, they can take advantage of their increased red blood cell count and perform at an elevated level.

Will I retain these changes after I return to low altitude?

Upon returning to sea level after successful acclimatization to high altitude, the body has more red blood cells to deliver oxygen to working tissues. The increased concentration of red blood cells is consistent with improved physical performance following decent to low altitudes (D’Alessandro et al. 2016). However, the physiological adaptations that resulted in increased fitness do not last forever. Gains in red blood cell count obtained during acclimation at high altitudes are eventually lost, but the time of this de-acclimatization remain unclear (Benjamin et al. 2014). After descending to low altitude, the changes may last up to 120 days, as that is the life span of the average red blood cell (Lovett 2016).

Scientists continue to study the effects of living at high altitudes. Current research projects are examining how to optimize the altitude training formula of how high to go and how long to stay there (Peterson 2010). Understanding how the body adapts to altitude lays the ground work for not only human life high in the mountains, but maybe places beyond, like space!

“Altitude Sickness.” | Altitude Sickness
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