Textbook in PDF format Every day, millions of people have diagnostic laboratory tests or donate blood. Why is blood so important? Blood reaches every remote corner of our being, provides nutrients, carries oxygen, and removes impurities from tissues. Blood is not only a transport and defense system, but it also plays a crucial role in maintaining the balance of all the other body systems and organs. All living beings (whether they are warm-blooded such as mammals and birds or cold-blooded such as insects, reptiles, amphibians, and invertebrate animals) live thanks to blood, which is essential for life. We all take for granted its existence and importance, without knowing what it contains and its countless and amazing properties