Blood. Our blood is a fluid that is also a type of connective tissue. It is composed of blood cells and an aqueous fluid known as plasma. Two major functions of the blood include transporting substances to and from our cells and providing immunity and protection against infectious agents such as bacteria and viruses.lood. Our blood is a fluid that is also a type of connective tissue. It is composed of blood cells and an aqueous fluid known as plasma. Two major functions of the blood include transporting substances to and from our cells and providing immunity and protection against infectious agents such as bacteria and viruses.