Our blood does a lot of good things. Blood contains white blood cells, our defence fighters, that protect us against different bacteria. Blood also contains red blood cells that transport oxygen to the different parts of our body. We need oxygen to be able to live and it comes into our lungs when we breathe. The oxygen attaches to these red blood cells in our lungs and then travels round the body so all parts of our body get oxygen. We actually have 25 thousand billion (25,000,000,000,000) red blood cells in our blood. And finally, our blood also contains platelets. These stop blood from draining out of the body when we bleed.
An adult person has around five litres of blood in their body. A tiny, newborn baby only has 0.3 litres (30cl) of blood in their body, seventeen times less blood than an adult.
Blood is red because it contains a substance called haemoglobin. It is this haemoglobin that transports oxygen around the body. Haemoglobin contains iron and iron is red when it is joined with haemoglobin and oxygen. It is the oxygen rich haemoglobin that makes blood red.
The arteries that contain oxygen-rich blood are deep inside our body. The veins that contain oxygen-poor blood are closer to our skin and these are the blood vessels we can see. Blood is red when it contains a lot of oxygen. When the blood has left its oxygen in parts of the body and is on its way back to the heart, it is transported through our veins. As the blood gets its red colour when it contains a lot of oxygen, the oxygen-poor blood in the veins becomes a darker colour than the blood in our arteries. If you cut yourself and blood runs out, it will become red again even if it comes out of a vein. This is because the blood quickly binds oxygen from the air into it again.
Blood travels round the body in blood vessels. You could say that these blood vessels are very, very thin tubes that are connected to each other. These blood vessels are called arteries, veins and capillaries. If you were to connect all the blood vessels in a person end to end, you would create a 140,000km ribbon that could be wound round the equator three times.
Blood travels round the body in large and small tubes called blood vessels. If you hurt yourself, the blood vessels can break and blood will then run out. If you also damage your skin, blood will come out of your body. That is when you see the wound is bleeding. If the skin remains intact, the blood stays in the body and you then get a bruise.
You do actually bleed when you break your leg, sometimes so much that it can be dangerous. But if the skin is not broken, you don’t always see that you are bleeding as the blood stays inside your body.
When you hurt yourself and it starts bleeding, the body forms something that is a bit like a fishing net over the wound. This net is made of a substance in the blood called fibrin. When the platelets in the blood stick to the net, the blood forms clumps in the fishing net and these stop the bleeding.
The heart is a large muscle that acts as a pump. When the heart pumps blood round the body, it first contracts so the blood in the heart is pumped out into the body. The heart then relaxes so it can be filled with new blood to pump out. It is when the heart is pumping that it beats. The heart is controlled by the brain. We have nerves, that are a bit like electric wires, that go to and from the heart. The nerves send signals from the brain to the heart and these signals tell the heart to beat. The heart beats more than 4,000 times an hour and does so all our lifes. Over the course of your life, your heart will beat 2,500 million times.
The heart is a large muscle that pumps blood round the body. To ensure the blood is sent in the right direction, there are tiny doors in the heart called heart valves. These valves steer the blood in the right direction. Blood rushes forward very fast in the heart and these valves have to open and close very quickly. The pounding sound you hear is these valves closing. It’s a bit like when you slam a door closed.
The air around us contains a gas called oxygen that we need to be able to live. The cells that are the building blocks of our body need oxygen to be able to work and stay alive. When we breathe in, we take oxygen down into our lungs. The lungs contain loads of blood vessels with blood that transport oxygen around the body and out to the cells. When we breathe out, the body gets rid of carbon dioxide, a waste product that is formed when the cells work. And do you know what? We breathe in and out about 20,000 times every day – on the way to school or work, when we play sports, eat and sleep. A human can survive for a few weeks without food and a few days without water, but only a few minutes without breathing.
When we breathe, air containing oxygen flows down into our throat and through our windpipe. Our windpipe is divided into two airways called bronchi, that branch into our right and left lungs. Inside the lungs, these bronchi further divide into bronchioles that look like tree branches with many different twigs. Around the thinnest twigs at the very end of these branches are tiny air sacs called alveoli that look a bit like tiny balloons. These alveoli are very thin and surrounded by a network of blood vessels. This is where the oxygen from the air we breathe in and the carbon dioxide from the blood vessels are exchanged. The oxygen is then taken around the body in the blood and the carbon dioxide leaves the body with the air we breathe out.
We have loads of tiny sensors in our skin that warn us something dangerous is happening that can injure us. If a needle is inserted through our skin, these sensors think something very dangerous is happening to our body. They then send signals to our brain and the brain then makes us feel it is painful. If the signals are stopped along the way and do not reach the brain, we hardly feel anything or nothing at all. This is how an anaesthetic and anaesthetic plasters work.
We have tiny tear glands behind our eyelids that produce tears. These tears drain into small holes in the eyelids and then travel down into the nose and finally the tummy. The tears prevent your eyes from becoming dry and protect eyes against bacilli and dirt. When you become angry or sad, a lot of things happen in your brain. The nerves in your brain that control your feelings then talk to the nerves that control your tears and signals are sent to your tear glands. When you feel sad, the tear glands produce so many tears that not all of them can drain into the small holes in your eyelids. The tears flood out instead and down your cheeks. This is why your nose also becomes bunged up when you cry, because so many tears are streaming down into your nose. When you cry, you are showing you are sad and need consoling. That is why it is important to cry and not hold back your tears.
The medicine doesn’t know that. However, when you have pain somewhere in your body, different substances are formed that irritate the pain nerves. When these pain nerves are irritated, they send signals to the brain and this is when you feel you are in pain. The medicine you take for the pain, stops the substances that are formed when you have pain from irritating the pain nerves. So you actually still have the pain, but do not feel it any more.
The appendix is about the size of your little finger and looks like a small worm sitting on your large intestine. When we talk about appendicitis, this means the appendix is inflamed, perhaps because something has blocked the entrance to it. Nobody really knows why we have an appendix, but many researchers believe it is a storeroom for bacteria, that is to say, part of our immune system. Just like certain glands in our throat that are also part of our immune system, the appendix can be removed and you will still feel fine.