Oops! Be careful Leon! Now you're bleeding! Yes. So...
what is blood actually? Why is it so red? Well, without blood you wouldn't be able to sit here and carve the stick. Blood does a lot of important things. Think of it as a motorway!
Yes, really. A motorway. Blood is a clever system for transporting things. Here comes oxygen. Blood transports oxygen from the lungs to the cells.
And here comes a load of glucose molecules. The blood has collected glucose from food in the intestines. The glucose goes to the cells as well. Glucose gives the cells energy, and oxygen is needed for the cells to use it. The cells metabolise glucose with help of oxygen.
Here's some transport in the other direction: a load of carbon dioxide. This carbon dioxide was left over after the cells had metabolised the glucose. It will go to the lungs, and leave the body with the breath. In the blood, a lot of information is moved around, too. It comes in the form of substances that have been made by the body - that transmit signals between different parts of the body.
It is these transmitters that make things happen in the body. They can for example: change the heart's rhythm, constrict blood vessels, control the amount of sugar in the blood, or affect the body's immune system. Defences, yes, are also present in the blood. Among them are many cells on the lookout for intruders, such as bacteria or viruses. The defence cells are called white blood cells.
Most blood cells are red blood cells - that transport oxygen. Red blood cells contain the protein haemoglobin, which can bind oxygen. Hemoglobin contains a lot of iron, and that is what makes the blood so red. A full-grown human has about five litres of blood. Much of this consists of red blood cells.
White blood cells - defence cells - only make up a small percentage of the blood. An even smaller part of the blood consists of platelets. They're needed so the blood can solidify, coagulate, if we get hurt. Platelets are much smaller than white and red blood cells. Slightly less than half of your blood consists of different blood cells.
The rest is blood plasma. It consists of mostly water, salt, and other substances. The heart pumps blood around the body, and when you move, it pumps harder, so that circulation increases: ... the transportation goes faster. If you move your body, much and often, it's like making the roads wider, with fewer traffic jams: your body's transport system works better.
Blood pressure decreases. And it is good for your body. So the answer to the question "What is blood", is... A system for transporting things around between different parts of the body. Our bodies simply would not work at all... ...