Reasons for Low Oxygen in the Blood


Oxygen is in many ways the life-force of animal and human life on this planet, and is necessary for a variety of functions. Without oxygen for instance our brain becomes starved and neurons begin to die, causing eventually irreparable brain damage. Oxygen is used to fuel many of our organs and provide our body with energy (as it is used to burn glucose), and so a lack of oxygen for more than a few minutes can be fatal.

Likewise though a reduction in oxygen is not good for us either, and can leave us feeling breathless, looking pale and feeling weak and lethargic.

This occurs when there is not enough oxygen in the blood. This is normally how our oxygen is transported and taken to our organs, and this is the main purpose of the heart – to pump the blood around the body. Thus, when there is low oxygen in the blood, this results in a number of unpleasant symptoms. This condition is called ‘hypoxia’ and it can occur for many reasons. Here we will look at what some of those reasons are.

Normal Variation

Blood levels of oxygen vary from time to time naturally and this is not necessarily a sign of something ‘wrong’. For instance after continued exertion your body can use up the oxygen in your body more quickly resulting in a lower overall blood pressure of oxygen, and this is why we pant after we’ve gone running or done other exercise. Similarly as we get oxygen from the air around us, a low oxygen content in our blood may be representative and a result of low oxygen in the surrounding environment. This occurs for instance in high altitude where there is lower oxygen already and can lead to altitude poisoning in many people. In other cases very low temperatures can also reduce the amount of available oxygen and can be a problem depending on the individual’s VO2 Max.


Alternatively there are a number of health conditions that can lead to a low amount of oxygen in the blood. For instance, in anemia can reduce the amount of oxygen in the blood by reducing the iron which is used to transport that oxygen in the red blood cells. However not all definitions will include anemia as a cause of hypoxia, as this does not decrease the oxygen pressure. Depending on your definition then, anemia – defined as a low content of iron in the blood – could potentially cause of hypoxemia, but it is certainly a cause of low oxygen in general.

Smoking meanwhile can also reduce the amount of oxygen in the blood, by increasing the amount of CO or ‘Carbon Monoxide’ and thereby meaning it is more taken up with this gas and there is less room for oxygen. Along with the damage that smoking does to the lungs this affects a smoker’s ability to hold their breath etc.

Other Causes

Other causes though might include the following…

Alveolar Hypoventilation

This occurs when the alveolar ventilation is low so that less oxygen gets to the alveolar each minute. This will usually be due to an obstruction outside the lungs, such as an airway obstruction, brain damage to muscle weakness. In other words, this term is used to explain cases where the oxygen is not getting into the lungs for whatever reason.


‘Shunting’ occurs when blood is moved suddenly. In the case of a right to left shunt, where the blood is shunted from the right to the left of the circulation, this can be intracardiac or intrapulmonary. It is the only cause of hypoxemia not alleviated by the administration of 100% Oxygen.

Lung Disease

Lung disease may cause ventilation perfusion inequality, meaning that areas of the lungs have a ventilation/perfusion ratio of under one. This is considered a form of shunt.

Impaired Diffusion

If diffusion across the blood/gas membrane in the lungs is impaired, then the oxygen cannot get through into the blood, of course resulting in less oxygen in the stream.

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Gary Wickman


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Gary Wickman