BLOOD

fluid in multicellular animals that transports oxygen and nutrients to the cells and carries away waste products. In many species it also conveys hormones and disease-fighting substances. Technically, blood is a transport liquid pumped by the heart (or an equivalent structure) to all parts of the body, after which it is returned to the heart to repeat the process. Blood is both a tissue and a fluid. It is a tissue because it is a collection of similar cells that serve a particular function. These cells are suspended in a liquid matrixcalled plasmawhich makes the blood a fluid. In single-celled animals and the smallest multicellular invertebrates there is no blood system. Because of their small size, these animals can absorb oxygen and nutrients and can discharge wastes directly into their surrounding medium. Sponges and coelenterates (e.g., jellyfish and hydras) also lack a blood system; the means to transport foodstuffs and oxygen to all the cells of these larger multicellular animals is provided by water, sea or fresh, pumped through spaces inside the organisms. In animals of increasing size and complexity, contact with the surrounding medium is no longer sufficient to supply the metabolic needs of internal body cells. Such animals possess a true blood system in which a specialized fluidi.e., bloodis pumped around the body. In most such animals the blood passes through a respiratory exchange membrane, which lies in the gills, lungs, or even the skin. There the blood picks up oxygen, which is necessary for cellular metabolism, and disposes of carbon dioxide, which is a waste product of metabolism. The blood also absorbs nutrients from the gastrointestinal tract or from various storage tissues and carries them to cells throughout the body. Another function of blood is the transport of water-soluble toxic wastes to excretory organs for elimination from the body. Blood evolved from seawater, which in primitive organisms was the source of elements essential for growth. Thus, the basic chemical composition of plasma resembles that of seawater, consisting of water and dissolved salts. However, plasma also contains dissolved nutrients, wastes, andin many specieshormones. The cellular composition of blood varies from group to group in the animal kingdom. Most invertebrates have various large blood cells capable of amoeboid movement. Some of these aid in transporting substances; others are capable of surrounding and digesting foreign particles or debris (phagocytosis). Compared with vertebrate blood, however, that of the invertebrates has relatively few cells. Among the vertebrates, there are several classes of amoeboid cells (white cells, or leukocytes), special oxygen-carrying cells (red cells, or erythrocytes), and cells that help stop bleeding (thrombocytes, or platelets). Oxygen requirements have played a major role in determining both the composition of the blood and the architecture of the blood circulatory system. In some of the smallest invertebrates, including certain worms and mollusks, transported oxygen is merely dissolved in the plasma. Larger and more complex animals, which have greater oxygen needs, have evolved respiratory pigments. These specialized compounds are able to carry larger amounts of oxygen because of the ability of metal atoms in the pigments to react with and transport several atoms of oxygen. The red pigment hemoglobin, containing iron, is found in all vertebrates and some invertebrates. The blue pigment hemocyanin, containing copper, is found in some crustaceans, such as crabs and in some mollusks. Some annelids have the iron-containing green pigment chlorocruorin, others the iron-containing red pigment hemerythrin. In many invertebrates the respiratory pigments are carried in solution in the plasma, but in higher animals, including all vertebrates, the pigments are enclosed in cells; if the pigments were freely in solution the pigment concentrations required would cause the blood to be so viscous as to impede circulation. There are two basic types of blood circulatory systemsopen and closed. In an open system, the blood flows out of vessels and directly bathes the internal body tissues. In a closed system, the blood never leaves its network of vessels; exchange of materials between the blood and tissues occurs across the walls of the capillaries, the tiniest blood vessels. Arthropods (insects, spiders, and crustaceans) have an open system, as do most mollusks. The cephalopod mollusks (squids and octopuses), however, have a closed system, as do the annelid worms and all vertebrates. An open system is less efficient in delivering oxygen rapidly. It suffices for less active mollusks such as clams and snails, but the active cephalopods need the greater efficiency of a closed system. The fact that insects, most of which are highly active, have an open system might seem unusual; however, insects do not rely on the blood for delivery of oxygen. An elaborate network of air tubules (tracheae) performs this function in insects. Among the vertebrates, the fishes have a circulation in which blood is pumped out of the heart, through the gills (where oxygen is loaded and carbon dioxide is unloaded), and then on to the body as a whole, without first returning to the heart. All other vertebratesi.e., amphibians, reptiles, birds, and mammalshave a double circulation in which, after traveling through the lungs and becoming oxygenated, the blood returns to the heart to be pumped out again through the body. For maximum efficiency, this double circulation requires the separation of the heart into two discrete pumps, so that oxygenated and deoxygenated blood do not become mixed in its chambers. This separation of the heart is achieved only partially in amphibians and reptiles, but it is complete in birds and mammals. a literary term of British origin referring to a lurid work of fiction, especially a cheap and ill-written book of adventure or crime. The word is a short form of blood-and-thunder book.