What's Actually in Your Blood

The heart pumps blood around the body, but what is actually inside the blood itself? It turns out blood is more like a delivery, security and repair service all running in the same vehicle.

Believe it or not, most of the cells in your entire body are not muscle, skin, or brain cells. They are red blood cells, quietly outnumbering every other type combined. We know the heart pumps blood and the vessels carry it. Now it's time to look at what's actually being pumped, because blood is far more than the simple red liquid it appears to be. Blood isn't a simple fluid. It's a mixture of a liquid and three very different kinds of cell, each with a completely separate job. Understanding what those jobs are is what makes a blood test result readable, rather than a page of numbers that means nothing. The liquid: plasma Just over half your blood, about 55%, is a pale yellowish liquid called plasma . It's mostly water, but dissolved in it are some extraordinary things: proteins, hormones, nutrients, clotting factors, antibodies, and waste products on their way to be filtered out. Plasma is the vehicle that carries everything else. Without it, your blood cells would have nowhere to travel. Red blood cells: the oxygen delivery vans The remaining 45% is almost entirely made up of red blood cells , and they have one job: carrying oxygen. Each one is packed with a protein called haemoglobin, which grabs oxygen in the lungs and carries it to every cell in the body. On the way back, it picks up the waste gas carbon dioxide. The iron in haemoglobin is what makes blood red. Red blood cells are made in the bone marrow, live for about 120 days, then get quietly broken down and replaced. Your body makes millions of new ones every second. When you don't have enough of them, or their haemoglobin isn't working properly, your cells don't get the oxygen they need. That's anaemia, and it explains why the main symptom is tiredness. White blood cells: the security team White blood cells make up less than 1% of your blood by volume, but they're your immune system's frontline. They patrol constantly, looking for bacteria, viruses, and anything that shouldn't be there. Different types do different things: some attack invaders directly, some remember past infections so the response is faster next time, some raise the alarm. When you have an infection, your body rapidly makes more of them. That is why a raised white cell count on a blood test is often a sign something is fighting something. Platelets: the repair crew Platelets are tiny cell fragments, far smaller than red or white cells. They mostly spend their time drifting through the bloodstream until a blood vessel is damaged. Then they rush to the site, clump together, and form a temporary plug to stop bleeding while the proper repair work begins. That process is clotting, and without it, even a small cut would be a serious problem. The full picture So when blood flows through the heart and out to the body, it's carrying all four of these at once. Plasma carrying nutrients, hormones and waste. Red cells ferrying oxygen out and carbon dioxide back. White cells on patrol. Platelets ready for emergencies. A whole logistics operation, crammed into every drop. All of this exists to serve the cells. Every red cell, every platelet, every protein in the plasma is ultimately working to keep those 30 trillion individual cells alive and doing their jobs. The circulation is the road network. Blood is what actually travels on it. What a doctor checks when assessing your blood A doctor cannot see red cells, white cells or platelets with the naked eye, but several checks give early clues about what is happening inside your blood. Looking. A doctor may look inside the lower eyelids and at the palms for paleness, a sign of low red blood cell numbers. A yellowish tinge to the skin or the whites of the eyes, jaundice, can suggest red cells are breaking down faster than usual. Unexplained bruising or tiny pin point red spots on the skin, called petechiae, can point to a problem with platelets or clotting. Feeling. The spleen helps filter old blood cells and store some of the immune system's white cells. A doctor may gently feel the abdomen to check whether it is enlarged, which can happen when the blood or immune system is under strain. Special tests. Beyond what can be seen or felt, the full picture usually needs a blood test: the full blood count you read about above. A doctor may also check temperature, which can be raised in some infections, and blood pressure. What doctors learn. These checks, combined with a blood test, help a doctor work out which blood component is behind a person's symptoms. They also show how urgently that needs following up. When this system is affected When something is wrong with the number or function of blood cells, people may notice unusual tiredness, pale skin, or shortness of breath during physical activity. Other signs include frequent infections, unexplained bruising, and bleeding that takes longer than usual to stop. These symptoms are explored further in the lesson "Persistent Tiredness and Fatigue", with unexplained bruising covered in a future lesson.

Blood is really four things working together to serve every cell: a liquid carrier, oxygen carrying red cells, infection fighting white cells, and clot forming platelets.