Why is blood that is returned to the right side of the heart referred to as deoxygenated, when it really does have oxygen content in it?

The protein in blood cells that carries oxygen is called hemoglobin. Each hemoglobin molecule is oxygenated or deoxygenated; it is either bound to 4 oxygen molecules (and is called “saturated), or it’s bound to no oxygen at all.

But, it’s important to remember that each red blood cell contains millions of hemoglobin proteins. When we talk about deoxygenated hemoglobin, that means that the individual hemoglobin is carrying no oxygen at all; when we refer to deoxygenated blood, however, we’re referring to blood that has a lower oxygen content. That content is determined by the percentage of hemoglobin molecules that are oxygenated.

In oxygenated (arterial) blood, ideally the oxygen content would be between 95-100% - meaning 95-100% of the hemoglobin molecules are saturated. In deoxygenated (venous) blood, the oxygen content for a normal person at rest tends to be about 75%. That oxygen is kept in “reserve,” just in case a situation arises where you need more oxygen – e.g. if you’re exercising.

So basically, the blood that returns to the right side of your heart is called deoxygenated because it has a lower percentage of oxygenated hemoglobin molecules.