What’s the release of a liquid substance from blood, cells or tissues

Unicellular organisms, primitive multicellular animals, and the early embryos of higher forms of life lack a circulatory system. Because of their small size, these organisms can absorb oxygen and nutrients and can discharge wastes directly into their surrounding medium by simple diffusion. 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 larger and more-complex animals, transport of adequate amounts of oxygen and other substances requires some type of blood circulation. 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 and disposes of carbon dioxide.

Blood, fluid that transports oxygen and nutrients to the cells and carries away carbon dioxide and other waste products. 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 specialized cells that serve particular functions. These cells are suspended in a liquid matrix (plasma), which makes the blood a fluid. If blood flow ceases, death will occur within minutes because of the effects of an unfavourable environment on highly susceptible cells.

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