How Oxygen Travels in Your Blood

Oxygen crosses into your blood inside the lungs, but that is only the first step. It still has to reach your brain, your muscles and every other cell in your body, and almost none of it travels the way you might expect.

Oxygen crosses from the air in your lungs into your blood, but dissolving in the watery part of blood carries only a tiny fraction of what your body needs. Something else does almost all the work of transporting it, and understanding that carrier changes how you read one of the most common numbers in medicine. Almost none of it just dissolves If oxygen only dissolved directly into blood the way it dissolves in a glass of water, your blood would carry far too little to keep you alive. Blood needs a dedicated carrier, something that can pick oxygen up in the lungs, hold onto it, and release it again wherever it is needed. Haemoglobin: the oxygen carrier That carrier is haemoglobin, a protein packed inside red blood cells. Each haemoglobin molecule can bind several oxygen molecules at once, and each red blood cell contains hundreds of millions of haemoglobin molecules. This is why almost all the oxygen in your blood is riding inside red blood cells rather than floating free in the plasma around them. As blood passes through the lungs, haemoglobin binds oxygen. As blood reaches tissues that are using oxygen, haemoglobin releases it. Saturation: how full the carriers are Oxygen saturation is a simple idea once you see haemoglobin as the carrier. It is the percentage of haemoglobin's oxygen carrying capacity that is actually full at any moment. A saturation of 98% means that, on average, the haemoglobin passing through is carrying close to as much oxygen as it possibly can. It is not a measurement of how much oxygen is in the air, and it is not a measurement of how many red blood cells you have. It only describes how full the carriers already present are. Two different problems, easily confused Getting oxygen into the blood and carrying oxygen once it is there are two separate jobs, and either one can fail on its own. A lung problem can stop oxygen crossing into the blood in the first place, which lowers saturation because the carriers are not being filled. A blood problem, such as too few red blood cells or too little haemoglobin, does not necessarily lower saturation at all. The carriers that exist may still be filling normally, there are simply not enough of them, which instead shows up as tiredness and breathlessness during physical activity rather than a low saturation reading. What a doctor checks A doctor assessing someone who is breathless or unwell often reaches for a pulse oximeter early in the assessment. Pulse oximetry. A small sensor, usually clipped to a finger, shines light through the skin and estimates oxygen saturation without needing a blood sample. It gives a fast, non invasive read of how well the lungs are loading oxygen onto the blood passing through them. Looking beyond the number. A doctor considers a low saturation alongside how someone looks and how hard they are working to breathe, since a single reading is only one part of a larger picture. A normal saturation does not rule out every problem, since a blood carrying problem may not lower it at all. Blood tests. A full blood count checks the number of red blood cells and the amount of haemoglobin directly, catching a carrying problem that pulse oximetry alone would miss. When this system is affected A low oxygen saturation points a doctor toward the lungs and the process of gas exchange, explored further in the lesson "Feeling Short of Breath". Tiredness with a normal saturation, by contrast, more often points toward the blood itself, explored further in the lesson "Persistent Tiredness and Fatigue".

Oxygen barely dissolves in blood on its own. Almost all of it rides on haemoglobin inside red blood cells, and saturation measures how full those carriers are.