Your body needs oxygen to produce the energy needed to function normally. The oxygen you breathe in goes through tiny air sacs in your lungs and into the blood stream, where red blood cells pick it up and carry it throughout the body. This system can be affected by poor air intake, poor blood flow or the poor carrying capacity of the red blood cells, leading to low levels of oxygen in the blood.
For oxygen to reach the red blood cells in the bloodstream, it first needs a clear path to get into and then through the air sacs in your lungs. Among the obstacles that could block that pathway are airway obstructions, such as sleep apnea, according to the American Academy of Sleep Medicine. They occur when the throat muscles relax and gravity causes the tongue to drop back into the throat. This reduces the size of the airway opening, and along with it the amount of oxygen that reaches the lungs, so blood oxygen levels may be lower.
Chronic obstructive pulmonary disease (COPD), emphysema and chronic bronchitis damage the airways or lungs, making it difficult to fully exhale each breath, according to the online Merck Manual. When this happens, the air sacs don't completely empty, so can't take in as much fresh air, and oxygen levels in the blood decrease.
Acute respiratory distress syndrome (ARDS) occurs when the blood vessels in the lungs leak more fluid than normal into the air sacs, according to the American Lung Association, leaving inadequate space for oxygen intake. Similarly, pneumonia or pulmonary edema due to other causes, such as exercising or living at high altitudes, along with some medicines or toxic substances, can cause low blood oxygen due to an excess of fluid that leaves little room for normal air exchange, according to the Mayo Clinic.
Pulmonary fibrosis, according to the National Heart, Lung and Blood Institute, causes lung tissue to become thickened and stiff or scarred, making it difficult for oxygen to be transferred into the blood stream, resulting in low blood oxygen levels.
Poor oxygenation of the blood can also occur when the heart isn't working right. According to the National Heart, Lung and Blood Institute, this happens either because the heart isn't filling with blood adequately or because the heart muscle is weak. Weak heart muscle can't pump the blood to the lungs to pick up oxygen or can't pump forcefully enough to move the oxygenated blood through the body effectively.
Congenital heart disease--heart defects present at birth--and congestive heart failure are common disorders that lead to these weaknesses in the cardiovascular system, causing low blood oxygen levels.
Another possible cause of low blood oxygen saturation is anemia, which occurs when the red blood cells don't have adequate hemoglobin, an iron-rich protein that acts as the oxygen carrier, according to the National Anemia Action Council. Anemia has many causes. Sometimes it's inherited and sometimes it is due to poor nutrition, either due to poor eating habits or as a result of other illnesses, such as cancer and inflammatory bowel disease, affecting the body's ability to absorb nutrients.