Blood is a fluid that transports oxygen from the lungs to body tissue and carbon dioxide from body tissue to the lungs. Blood also transports nourishment from digestion and hormones from glands to all areas of the body. Blood also conveys disease fighting substances to the tissues and waste to the kidneys. Blood is thicker than water and has a slightly salty taste. An adult's body usually has 10 pints of blood in circulation. Blood is composed of billions of living blood cells floating in a liquid called plasma. If you took a small sample of this blood and poured it into a test tube and then put it in a machine called a centrifuge, you would be able to see the layers of this blood. This machine spins the blood around so fast that it separates the red blood cells, from the white blood cells, from the platelets. The red blood cells sink to the bottom because they are the heavier, more solid parts, but the plasma remains at the top because it is lighter. Plasma is 95% water and the other 5% is made up of dissolved substances including salts.
All of the cellular elements of blood, including the red blood cells that transport oxygen, the platelets that trigger blood clotting in damaged tissues, and white blood cells of the immune system, derive ultimately from the same progenitor or precursor cells, the hematopoietic stem cells in the bone marrow. As these stem cells can give rise to all of the different types of blood cells they are often known as pluripotent hematopoirtic stem cells. Initially they give rise to stem cells of more limited potential, which are the immediate progenitors of red blood cells, platelets, and the two main categories of white blood cells.
Blood is a body 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 cells suspended in blood plasma. Plasma, which constitutes 55% of blood fluid, is mostly water (92% by volume), and contains proteins, glucose, mineral ions, hormones, carbon dioxide (plasma being the main medium for excretory product transportation), and blood cells themselves. Albumin is the main protein in plasma, and it functions to regulate the colloidal osmotic pressure of blood. The blood cells are mainly red blood cells (also called RBCs or erythrocytes), white blood cells (also called WBCs or leukocytes) and platelets (also called thrombocytes). The most abundant cells in vertebrate blood are red blood cells. These contain hemoglobin, an iron-containing protein, which facilitates oxygen transport by reversibly binding to this respiratory gas and greatly increasing its solubility in blood. In contrast, carbon dioxide is mostly transported extracellularly as bicarbonate ion transported in plasma.
Vertebrate blood is bright red when its hemoglobin is oxygenated and dark red when it is deoxygenated. Some animals, such as crustaceans and mollusks, use hemocyanin to carry oxygen, instead of hemoglobin. Insects and some mollusks use a fluid called hemolymph instead of blood, the difference being that hemolymph is not contained in a closed circulatory system. In most insects, this "blood" does not contain oxygen-carrying molecules such as hemoglobin because their bodies are small enough for their tracheal system to suffice for supplying oxygen.
Jawed vertebrates have an adaptive immune system, based largely on white blood cells. White blood cells help to resist infections and parasites. Platelets are important in the clotting of blood. Arthropods, using hemolymph, have hemocytes as part of their immune system.
Blood is circulated around the body through blood vessels by the pumping action of the heart. In animals with lungs, arterial blood carries oxygen from inhaled air to the tissues of the body, and venous blood carries carbon dioxide, a waste product of metabolism produced by cells, from the tissues to the lungs to be exhaled.
Medical terms related to blood often begin with hemo- or hemato- (also spelled haemo- and haemato-) from the Greek word αἷμα (haima) for "blood". In terms of anatomy and histology, blood is considered a specialized form of connective tissue, given its origin in the bones and the presence of potential molecular fibers in the form of fibrinogen.