Low blood oxygen levels
Channel: York Cardiology
Date: 2022-01-09

Today’s video is on the subject of blood oxygen levels and in particular causes of low oxygen levels. Since the start of the COVID pandemic, many people have invested in pulse oximeters which are a non-invasive way of telling you how much oxygen is in the blood. This is called oxygen saturation and is usually a percentage and normal value range between 94-100%. The way the pulse oximeter works is that it shines 2 lights throug your fingertip - one red light and one infra-red light. Blood contains lots of oxygen will absorb more infrared light and let more red light pass through it. Blood that does not have as much oxygen will absorb more red light and let more infrared light pass through it - and this way the oximeter can work out how much oxygen is present in the blood. This is great if you have normal oxygen saturations but what does it mean if the oxygen saturations are low and what mechanisms cause low oxygen saturations and this what i will try and discuss in today’s video.

Oxygen is an essential element for life and without oxygen, humans can only survive for a few minutes before their vital tissues start suffocating and dying. It is therefore vital that there is adequate oxygen available to the tissues at all times and that there is the ability within the body to match any extra demand with extra supply of oxygen.

The 2 main systems that are  responsible for maintaining this balance between supply and demand are the respiratory system  and the cardiovascular system.

The respiratory system which consists of the airways and the lungs are responsible for getting air from the atmosphere into the body and loading our blood cells with oxygen and the heart is responsible for pumping that blood around and into our vital organs where the oxygen is unloaded and used by our tissues. If either the lungs or the heart start malfunctioning then our tissues may not get the amount of oxygen that they need and therefore start suffocating and dying. 

The term for having less oxygen in the blood which is predominantly a function of the lungs and the blood supply to the lungs is termed hypoxemia. 

The term for having less oxygen in the tissues which is a function of the lungs, the heart and the blood vessels supplying those tissues in hypoxia.

Whilst it is reasonable to assume that hypoxemia - less oxygen in the blood will lead to hypoxia, it is important to understand that hypoxemia and hypoxia are not synonymous i.e there are situations where you may have less oxygen in the blood but the tissues may still get the amount of the oxygen that they need. This happens for example with altitude training. Here there is less available oxygen and therefore less oxygen is loaded onto each blood cell  but people who have adapted to this will have increased the amount of blood cells and their cardiac output and therefore even though each cell is loaded with less oxygen, there are more cells and therefore the tissues get the amount of blood they need.