The primary substance that blood is composed of is plasma, which makes up about 55% of its volume. Plasma is a yellowish liquid that contains water, electrolytes, nutrients, hormones, proteins, and waste products. It serves as the medium for transporting blood cells, nutrients, and other substances throughout the body. The remaining 45% of blood is comprised of blood cells, including red blood cells that carry oxygen, white blood cells that are part of the immune system, and platelets that help with clotting. Blood plays a vital role in maintaining homeostasis, transporting gases, and facilitating immune responses.