Causes of Low Blood Oxygen Levels

The cells of the human body require a constant supply of oxygen to produce the energy necessary to accomplish their life-sustaining functions. Oxygen derived from the air is delivered via the bloodstream to the body organs and tissues. A low blood oxygen level, or hypoxemia, can occur due to reduced environmental oxygen or a problem affecting the respiratory or circulatory system. In many people with hypoxemia, several factors contribute to the low blood oxygen level.

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Reduced Environmental Oxygen and Hypoventilation

Breathing in a sufficient amount of oxygen from the air is the first step in ensuring adequate oxygen delivery to the body tissues. A reduced amount of oxygen in the air can result in hypoxemia. This most commonly occurs in people who rapidly ascend to extremely high altitudes, such as mountain climbers. Breathing too slowly or too shallowly -- or hypoventilation -- can also lead to hypoxemia. Hypoventilation leads to inadequate oxygen intake by the lungs and a buildup of carbon dioxide in the blood, both of which contribute to hypoxemia. Hypoventilation can occur for many reasons, including: -- suppressed respiratory signaling from the brain due to drug or alcohol toxicity, or a brain injury -- nervous system conditions that interfere with breathing signals from the brain, such as Guillain-Barre syndrome, Lou Gehrig disease or a spinal cord injury -- chest muscle weakness due to conditions such as muscular dystrophy and myasthenia gravis -- physical impediments to normal breathing, such as extreme obesity, a collapsed lung or multiple broken ribs

Lung Disorders

Oxygen crosses into the bloodstream via tiny air sacs in the lungs called alveoli. A variety of lung diseases can interfere with the uptake of oxygen by the bloodstream leading to hypoxemia. With pneumonia, infection leads to fluid accumulation in the air sacs that impairs oxygen transfer into the bloodstream. Chronic obstructive pulmonary disease (COPD) damages the small airways and alveoli, impairing gas exchange and often leading to low blood oxygen levels. Other lung diseases that lead to persistent inflammation or scarring of the airways, or alveolar damage can also lead to hypoxemia. Examples include cystic fibrosis, poorly controlled asthma, idiopathic pulmonary fibrosis and sarcoidosis.

Circulatory Disorders

The respiratory and circulatory systems work together to ensure that adequate oxygen is transferred from the lungs to the bloodstream and subsequently delivered to the body. Therefore, problems with the circulatory system that interfere with normal blood flow through the lungs can lead to hypoxemia. Heart failure, for example, often triggers abnormally high pressure in the lung circulation. This can provoke fluid buildup in the air sacs and lead to hypoxemia. An abrupt drop in blood oxygen level can occur when a blood clot lodges in the pulmonary circulation. This condition, known as a pulmonary embolism or PE, obstructs blood flow through the lungs, interfering with oxygen uptake. Malformations of the heart or lung blood vessels that lead to blood bypassing the lungs can also lead to varying degrees of hypoxemia.


Red blood cells transport oxygen from the lungs to the body organs and tissues via a carrier molecule called hemoglobin. A deficiency of red blood cells, or anemia, limits the oxygen-carrying capacity of the blood, potentially leading to reduced total oxygen content in the bloodstream. Iron deficiency is the most common cause of anemia, but this condition can occur for many other reasons, including: -- vitamin deficiencies, particularly vitamin B12 and folate -- blood loss due to heavy periods, peptic ulcer disease or injury -- chronic kidney or liver disease -- inherited blood disorders, such as sickle cell disease and thalassemia -- leukemia and other cancers

Warnings and Precautions

Hypoxemia ranges from mild to severe, but even a mild decrease in blood oxygen requires medical evaluation to determine the underlying cause and best treatment. Signs and symptoms develop when a low blood oxygen level leads to an oxygen deficiency in the body tissues, which is known as hypoxia. Signs and symptoms of hypoxia are more likely to occur when the condition is severe and develops rapidly. Seek emergency medical care if you experience difficulty breathing or chest pain. Other warning signs and symptoms of hypoxia include: -- shortness of breath or breathlessness with minimal exertion -- increased heart and breathing rate -- lightheadedness or dizziness -- frequent headaches -- irritability, restlessness or confusion

Reviewed and revised by: Tina M. St. John, M.D.

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