A healthy person's, blood oxygen level normally ranges between 95% - 100%. This number tells you what percentage of hemoglobin in your blood is currently carrying oxygen and is able to deliver oxygen to your brain, organs and muscles. For smokers, their normal blood oxygen level is generally lower, around 90%. Medically speaking, 90% is the breakpoint for what is considered normal blood oxygen levels. Anything below 90% is considered hypoxemia. Being in a state of hypoxemia for a prolonged period can make you hypoxic, which means your body is being deprived of adequate oxygen to maintain function. Hypoxia can produce symptoms such as headaches, nausea, fatigue, shortness of breath, and in severe cases, can be fatal.
Temporary Hypoxemia During Exercise
When you do any type of strenuous exercise, you will notice that your breath is quickened, your pulse becomes faster, and your lungs expand. All of these are physiological responses that your body makes to meet the much higher demand for oxygen from the muscles, and to prevent you from feeling hypoxic. When you workout, muscles utilize more oxygen and need to draw more oxygen from the blood, creating an oxygen debt which puts you into a temporary state of hypoxemia. Your body will then self-adjust its respiratory and circulatory systems so that it can maximize the oxygen intake and transport abilities to make up for the low blood oxygen saturation.
Getting More Oxygen from the Air
With every inhale, your lungs draw in a volume of air - only 21% of which is oxygen. On every exhale, 80% of the oxygen you breathe in is breathed out. Therefore, only 1/25 of the air you breathe in is actually available as supply for the blood. When the blood oxygen level drops during exercise, the work load on the lung is multiplied 25 times, so in addition to faster intake of air, your lungs have to expand wider to allow in more air. A larger lung capacity will naturally hold more oxygen to be exchanged with the blood, and is an integral contributor to an efficient respiratory system which will enable you to maintain higher blood oxygen concentration with less effort. This is one of the reasons why a better trained athlete can stay in strenuous activities for a longer time.
More Efficient Delivery of Oxygen
Your blood oxygen level is also affected by how well your body can deliver and utilize the oxygen in your blood. This ability is measured by your maximum oxygen consumption (VO2 max). With better trained athletes, VO2 max is higher, and their muscles are more efficient in acquiring oxygen from the blood and metabolizing oxygen, giving them better stamina and better performance.