Final answer:
Exercise is more tiring at higher altitudes mainly due to lower atmospheric pressure leading to decreased oxygen availability which makes it harder for the body to achieve oxygen saturation, resulting in symptoms of altitude sickness during physical exertion.
Step-by-step explanation:
Exercise seems more tiring at altitude than at sea level primarily due to decreased oxygen availability. At higher altitudes, the atmospheric pressure is lower than at sea level, which means the partial pressure of oxygen is also lower. Because of this, it becomes more difficult to achieve the same level of oxygen saturation in the blood at high altitudes. Hemoglobin saturation, which refers to the amount of oxygen carried by the hemoglobin in the blood, is significantly lower at high altitudes. For instance, at 19,000 feet above sea level, hemoglobin saturation might be around 67%, compared with about 98% at sea level. Additionally, while at rest, tissues at high altitudes receive almost the same amount of oxygen as at sea level, during increased exercise, the body doesn't have as much oxygen reserve to draw from. This can lead to symptoms of altitude sickness, such as fatigue and shortness of breath, when engaging in physical activities.