Blood is an important fluid that keeps us alive. We cannot live without it. The heart pumps blood to all parts of the body and brings them oxygen and food. At the same time blood carries all the substances we don't need away from us. Blood fights infections, keeps our body temperature the same and carries chemicals that control body functions. Finally, blood has substances that repair broken blood vessels so that we don't bleed to death. Plasma is the liquid part of our blood. It makes up about 50-60% of it. Plasma consists mostly of water but many other substances are in it. It contains dissolved food, chemicals that control our growth and do other jobs, proteins, minerals and waste products. Red blood cells look like flat round discs. They contain haemoglobin, a protein that carries oxygen to the body and gives blood its red colour. Each drop of blood has about 300 million of these red cells. White blood cells, also called leukocytes, fight infections and harmful substances that invade the body. Most of these cells are round and colourless. They have different sizes and shapes. White blood cells are not as numerous as red ones. For every 700 red blood cells there is only one white blood cell.Not only does blood deliver oxygen and nutrients, but it also plays a crucial role in maintaining homeostasis and removing waste products from our cells. As blood reaches various tissues and organs, it collects metabolic waste, such as carbon dioxide and other byproducts, and carries them away, preventing their accumulation and potential harm. Moreover, blood acts as a carrier of hormones, enzymes, and immune cells, effectively facilitating communication and defense mechanisms within the body.