Blood is a bodily fluid in humans and other animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells. In vertebrates, it is composed of blood plasma and red blood cells (erythrocytes). White blood cells (leukocytes) and platelets (thrombocytes) are also normally present in circulating blood. Plasma, which constitutes 55% of blood fluid, is mostly water (92% by volume), and contains dissolved proteins, glucose, mineral ions, hormones, carbon dioxide (plasma being the main medium for exchanging respiratory gasses), and platelets. Albumin is the main protein in plasma, globulin is the main class of protein found in plasma with clotting factors.
The functions of blood include:
Transportation: Blood circulates through the body carrying oxygen from the lungs to all its tissues. It also carries carbon dioxide back to the lungs so that it can be exhaled. Along with these gases, nutrients such as glucose and amino acids are transported to where they are needed while wastes such as carbon dioxide and urea are removed from tissues.
Regulation: Blood plays an important role in maintaining homeostasis within the body by transporting hormones around the circulation. Hormones released by endocrine glands regulate many body processes including metabolism, growth & development etc.
Protection: Blood provides immunity against infection by transporting white blood cells around the body to fight infection; antibodies carried in plasma can also neutralise toxins; clotting prevents excessive bleeding following injury.