Chapter 10: The Blood: The Red Train That Carries Oxygen

All Aboard

Imagine a train that never stops. It runs on a track that loops through every part of a huge land, past mountains, through valleys, and into every tiny village. Its cars are loaded with fresh supplies at one big station, and then it sets off, dropping supplies at every stop and picking up rubbish to carry away. When it comes back to the big station, it unloads the rubbish, loads up again, and off it goes. It does this all day and all night, and it has never once taken a holiday.

Your body has a train just like that. It is your blood. The track is made of your blood vessels, the tubes that carry blood to every part of you. The big station is your lungs. The supplies are oxygen, and the rubbish is carbon dioxide. And the engine that keeps the whole train moving is your heart.

In the chapter about the alveoli, you saw oxygen slip through the thin walls of the tiny air sacs and into the blood in the capillaries. That was the moment the oxygen climbed aboard the train.

But oxygen is not much use sitting in your lungs. Your toes need oxygen. Your brain needs oxygen. The muscles in your legs and the cells in your fingertips need oxygen. In this chapter, you will ride along with the blood and see how the oxygen gets delivered, and how the blood helps to guard your body along the way.

How much blood is in your body? A grown-up has about five liters of blood, which is about as much as five one-liter bottles of water. A child has less, perhaps one and a half to three liters, depending on how big the child is. Your body makes more blood as you grow, so that there is always just the right amount for your size.

What Blood Is Made Of

Blood looks like a simple red liquid, but it is really a mixture of many different things. If a doctor took a small tube of blood and let it stand still for a long time, or spun it around very fast in a special machine, the blood would separate into layers. At the top, there would be a clear, pale yellow liquid. At the bottom, there would be a thick, dark red layer. In between, there would be a very thin, whitish layer.

The pale yellow liquid is called plasma, said like "PLAZ-muh." Plasma makes up a little more than half of your blood, and it is mostly water. But plasma carries many important things dissolved in it, the way a cup of sweet tea carries sugar you cannot see.

Plasma carries sugar and other nourishment from your food to your cells. It carries salts that your body needs. It carries chemical messages from one part of the body to another. And it carries much of the carbon dioxide on its way back to the lungs.

The thick, dark red layer at the bottom is made of red blood cells. These are the train cars that carry oxygen. The thin, whitish layer in between holds the white blood cells, the defenders you met in the last chapter, along with tiny pieces called platelets, which help to stop bleeding. You will meet each of these travelers in this chapter.

Here is something to try. Look at the inside of your wrist, where the skin is thin. You may see some faint bluish or greenish lines. These are veins, blood vessels that carry blood back toward your heart.

The blood inside them is not blue, though. Blood is always red. Veins only look blue because of the way light passes through your skin and bounces back to your eyes. If you ever see your blood from a small scrape, you will notice that it is red.

Red Blood Cells and Their Oxygen Seats

Red blood cells are some of the most amazing cells in your body. Each one is shaped like a round, flat disc that is pressed in on both sides, a little like a doughnut whose hole did not go all the way through. This shape gives the cell a large surface for its size, which helps oxygen move in and out quickly. It also makes the cell very bendy.

Red blood cells need to be bendy. Each one is tiny, about seven or eight thousandths of a millimeter across. But some of the smallest capillaries are even narrower than that. To get through, the red blood cells fold and squeeze, like a soft rubber ring being pushed through a narrow pipe. On the other side, they spring back into their usual shape and carry on.

There are more red blood cells in your body than any other kind of cell. A grown-up has about 25 trillion of them. A trillion is a million millions, so this is a number so large that it is almost impossible to imagine. If you could stack all of a grown-up's red blood cells one on top of another, the tower would reach far, far higher than the tallest mountain on Earth.

Red blood cells are made in the bone marrow, the soft, spongy center inside some of your bones. Your bone marrow is a very busy factory. It makes about two million new red blood cells every single second. It has to work this fast because red blood cells do not last forever. Each one travels around your body for about four months, making many thousands of trips through your heart and lungs. Then it wears out.

When a red blood cell gets old and stiff, it can no longer squeeze through the narrow capillaries so easily. Then some old friends from the last chapter step in: macrophages. Macrophages in an organ called the spleen, and in the liver, catch the worn-out red blood cells and break them down. Even then, nothing important is wasted. The macrophages save the iron from the old cells, and the iron is sent back to the bone marrow to help build new red blood cells.

What makes a red blood cell so good at carrying oxygen? Inside each one, there are millions of tiny molecules of a special protein called hemoglobin, said like "HEE-muh-gloh-bin." A molecule is a very, very small particle, far too small to see even with an ordinary microscope. A single red blood cell holds about 270 million molecules of hemoglobin, packed in tightly.

You can think of each hemoglobin molecule as having four seats, and each seat can hold one tiny molecule of oxygen. So each red blood cell has room for about a billion oxygen molecules. Red blood cells even leave out some of the parts that most other cells have, including the center that holds instructions for the cell. This leaves more room for hemoglobin and more room for oxygen.

At the heart of each hemoglobin seat is a tiny bit of iron. Iron is the same kind of metal that is found in nails and in some cooking pots, but in your blood it is in a very small and special form. When oxygen sits down on the iron, the hemoglobin turns bright red. When the oxygen gets off, the hemoglobin turns a darker red. So blood that has just left the lungs, full of oxygen, is bright red. Blood that is on its way back to the lungs, carrying less oxygen, is a darker red.

Your body needs iron to make hemoglobin, and it gets that iron from food. Many foods contain iron, such as beans, lentils, leafy green vegetables, eggs, meat, and fish. Eating a mix of different foods helps your body get the iron it needs to build plenty of healthy red blood cells.

Hemoglobin has a clever way of knowing when to grab oxygen and when to let it go. In the lungs, where there is lots of oxygen, hemoglobin grabs it tightly and fills its seats. In the rest of the body, where cells have been using up oxygen, there is less of it around, and hemoglobin lets go. Hard-working muscles are warm and make lots of carbon dioxide. These changes help hemoglobin let go of even more oxygen. So the oxygen is delivered exactly where it is needed most.

The Heart: Two Pumps Side by Side

A train needs an engine, and the blood's engine is your heart. Your heart is a strong muscle about the size of your own fist. It sits in your chest, between your two lungs, a little toward the left side. Remember how your left lung is a bit smaller to make room for it? The heart and the lungs are close neighbors, and they work as a team every second of your life.

Here is something many people do not know: your heart is really two pumps side by side. The right side of your heart and the left side of your heart are separated by a wall, and each side has its own job. The right side pumps blood to the lungs. The left side pumps blood to the rest of the body. Every drop of blood passes through both sides, one after the other.

Let us follow one red blood cell on its journey. It starts in the lungs, where it fills up with oxygen. From the lungs, it flows into the left side of the heart. The left side gives a strong squeeze and sends the cell out into a big blood vessel, then into smaller and smaller vessels, all the way to a muscle in your leg. There, in the capillaries, the cell lets go of its oxygen and picks up carbon dioxide.

Now the red blood cell begins its trip home. It flows into veins, which carry it back up toward the heart. It enters the right side of the heart, and the right side gives a squeeze that sends the cell into the lungs. In the capillaries around the alveoli, the cell drops off its carbon dioxide and picks up fresh oxygen. Then it is ready to go to the left side of the heart again and start another trip.

So the track of the red train is shaped a little like the number eight. One loop goes from the heart to the lungs and back. The other loop goes from the heart to the whole body and back. The heart sits in the middle, where the two loops cross. When you are resting, a red blood cell can make a whole trip around both loops in about a minute.

The blood vessels that carry blood away from the heart are called arteries, said like "AR-tuh-reez." The blood vessels that carry blood back toward the heart are called veins. Between them, connecting the smallest arteries to the smallest veins, are the capillaries, the narrow vessels where the train makes its deliveries. Every part of your body that is alive has capillaries nearby, so that no cell is ever far from a delivery.

You can feel your heart pushing the blood. Gently place two fingers on the inside of your wrist, just below your thumb. Keep still and wait. You may feel a soft, steady tap, tap, tap under your fingers. That is your pulse. Each tap is a wave of blood being pushed through your arteries by one squeeze of your heart.

Try counting your pulse while you sit quietly. For many children, the heart beats somewhere around 70 to 120 times a minute when resting.

Then, if you have space to do it safely, jump up and down for a little while and count again. Your pulse will be faster. And you will probably notice that you are breathing faster too. When your muscles work hard, they need more oxygen, so your heart pumps faster and your lungs breathe faster. The engine and the station speed up together.

Defenders Riding the Train

The red blood cells are not the only passengers on the train. The white blood cells ride along too. In the last chapter, you learned how macrophages in the lungs ring an alarm when germs arrive, and how neutrophils rush to help. The neutrophils get there by riding in the blood. The blood is the road that carries defenders to any part of the body that needs them.

When the alarm rings, the walls of the nearby blood vessels change. They become a little sticky on the inside. White blood cells rolling past in the blood begin to slow down, like passengers who hear their stop being called. They grab onto the sticky walls, then squeeze out through tiny gaps between the cells of the vessel wall. Once outside, they head straight toward the germs.

The plasma carries defenders too. Antibodies, the special proteins made by lymphocytes, float in the plasma and travel all around the body. Because the blood reaches everywhere, the antibodies can find germs almost anywhere. When an antibody finds a germ it recognizes, it sticks to the germ and marks it, so that macrophages and other defenders can find and destroy it.

The blood also passes through some special checkpoints. One of these is the spleen, an organ about the size of a fist, tucked behind the stomach on the left side of the body. Inside the spleen, the blood flows slowly past many white blood cells and macrophages. They check the blood for germs and for worn-out red blood cells, and they remove what does not belong. The spleen is a little like a security gate that every train must pass through now and then.

Then there are the platelets, said like "PLAYT-lets." Platelets are tiny pieces of cells, even smaller than red blood cells. When you get a cut or a scrape, platelets rush to the spot and stick together to plug the hole. They also help the blood to thicken into a clot, which later dries into a scab. This does more than stop the bleeding. It also seals the opening in your skin, so that germs cannot get in through the cut.

The blood even carries warmth. As blood flows through your busy muscles and organs, it picks up heat and spreads it around the body, keeping you warm all over. When your body fights germs, the brain can turn up your temperature to make a fever, and the blood carries that extra warmth everywhere. You will learn more about how fever helps your body in a later chapter.

Keeping Track of Every Breath

Your blood does one more important job for your breathing. It tells your brain how well your breathing is going. As the blood flows around your body, it collects carbon dioxide from your cells. If you are running hard, your muscles make lots of carbon dioxide, and the amount in your blood starts to go up.

Deep in your brainstem, the lowest part of your brain that you met in Chapter 3, there is a group of cells that keeps watch over your blood. These cells are especially sensitive to carbon dioxide. When they notice that the carbon dioxide in your blood is rising, they send messages telling you to breathe faster and more deeply. When the carbon dioxide level goes back down, they let your breathing slow down again.

You never have to think about any of this. When you are asleep, your brain keeps track of your blood and adjusts your breathing all night long. When you play tag, it speeds up your breathing without you having to decide. This is your body's way of making sure you always get the air you need. You will learn more about this watchful part of your brain in the next chapter.

Think back over the journey of the oxygen you breathe in. It entered through your nose, passed the guards in your throat, went down the windpipe and through the branches of the bronchial tree, and reached the alveoli. There, it slipped into your blood and climbed aboard a red blood cell. Then your heart sent it racing around your body to the cells that needed it. And all along the way, defenders stood guard to keep germs out.

But there is still one question left. How does the air actually get pulled into your lungs and pushed back out again? Your lungs do not have muscles of their own to do this. When the brain sends its message to breathe, who answers? The answer is a strong, dome-shaped muscle under your lungs and a cage of bones around them. In the next chapter, you will meet the diaphragm and the ribs, the pump that makes every breath happen.

Key Words

Blood vessels: The tubes that carry blood around your body. They include arteries, veins, and capillaries.

Plasma: The pale yellow liquid part of blood. It is mostly water and carries nourishment, salts, messages, antibodies, and much of the carbon dioxide.

Red blood cells: Bendy, disc-shaped cells that carry oxygen from your lungs to the rest of your body. They are the most common cells in your body.

Hemoglobin: A protein inside red blood cells that holds on to oxygen. It contains iron, which helps to make blood red.

Bone marrow: The soft, spongy center of some bones, where new red blood cells, white blood cells, and platelets are made.

Heart: A strong muscle in your chest that works as two pumps. The right side pumps blood to the lungs, and the left side pumps blood to the rest of the body.

Arteries and veins: Arteries are blood vessels that carry blood away from the heart. Veins are blood vessels that carry blood back toward the heart.

Pulse: The steady tapping you can feel in places such as your wrist. Each tap is a wave of blood pushed along by one beat of your heart.

Platelets: Tiny pieces of cells in the blood that stick together to plug cuts and help form a clot, which keeps blood in and germs out.

Spleen: An organ near the stomach that checks the blood for germs and removes worn-out red blood cells.