Hypoxemia is defined as a low oxygen level in the blood. Hypoxia is defined as an abnormally low oxygen level in the tissues.
Cells need oxygen to produce the energy required to maintain their integrity and perform their functions. Without a sufficiently high level of oxygen, cells can no longer perform their functions and most often end up dying. A myocardial infarction (heart attack) is an example of heart muscle cells dying because of a lack of oxygenation caused by a blocked blood vessel supplying the heart.
The most common causes of hypoxemia are acute and chronic respiratory problems (poor oxygenation of the blood in the lungs), heart failure (poor irrigation of blood vessels due to a weak heart) and severe anemias (lack of oxygen in red blood cells to transport oxygen to the tissues). Hypoxemia due to a lung or heart problem is treated by providing the patient with extra oxygen (oxygen mask, ventilator). Hypoxemia due to anemia is treated by increasing the hemoglobin level by providing supplements such as iron and vitamin B12.
The SARS-CoV-2 coronavirus responsible for the COVID-19 pandemic attaches tightly to human lung cells, impeding their proper functioning. Normally, this viral infection should be eliminated by the immune system (antibody production). It appears that, in the elderly, the immune and inflammatory response is inadequate, further impairing oxygen exchange within the lung. Therefore, the response to a SARS-CoV-2 infection is thought to be partly responsible for the severe hypoxemia and increased mortality observed in older patients.