Radiobiology, a branch of science concerned with the action of ionizing radiation on biological tissues and living organisms, is a combination of two disciplines: radiation physics and biology. All living things are made up of protoplasm, which consists of inorganic and organic compounds dissolved or suspended in water. The smallest unit of protoplasm capable of independent existence is the cell. Cells contain inorganic compounds (water and minerals) as well as organic compounds (proteins, carbohydrates, nucleic acids and lipids). The two main constituents of a cell are the cytoplasm, which supports all metabolic functions within the cell, and the nucleus, which contains the genetic information (DNA). Human cells are either somatic cells or germ cells. Cells propagate through division: division of somatic cells is called mitosis, while division of germ cells is called meiosis. When a somatic cell divides, two cells are produced, each carrying a chromosome complement identical to that of the original cell. The new cells themselves may undergo further division, and the process continues. CHAPTER 14 486 Somatic cells are classified as: ● Stem cells, which exist to self-perpetuate and produce cells for a differentiated cell population (e.g. stem cells of the haematopoietic system, epidermis and mucosal lining of the intestine); ● Transit cells, which are cells in movement to another population (e.g. a reticulocyte that is differentiating to become an erythrocyte); ● Mature cells, which are fully differentiated and do not exhibit mitotic activity (e.g. muscle cells and nervous tissue). A group of cells that together perform one or more functions is referred to as tissue. A group of tissues that together perform one or more functions is called an organ. A group of organs that perform one or more functions is a system of organs or an organism. Health effects Ionizing radiation is generally harmful and potentially lethal to living things but can have health benefits in radiation therapy for the treatment of cancer and thyrotoxicosis. Its most common impact is the induction of cancer with a latent period of years or decades after exposure. High doses can cause visually dramatic radiation burns, and/or rapid fatality through acute radiation syndrome. Controlled doses are used for medical imaging and radiotherapy. Some scientists suspect that low doses may have a mild hormetic effect that can improve health.[1] Some effects of ionizing radiation on human health are stochastic, meaning that their probability of occurrence increases with dose, while the severity is independent of dose.[2] Radiation-induced cancer, teratogenesis, cognitive decline, and heart disease are all examples of stochastic effects. Other conditions such as radiation burns, acute radiation syndrome, chronic radiation syndrome, and radiation-induced thyroiditis are deterministic, meaning they reliably occur above a threshold dose, and their severity increases with dose.[2] Deterministic effects are not necessarily more or less serious than stochastic effects; either can ultimately lead to a temporary nuisance or a fatality. Other effects include radiation-induced lung injury, cataracts, and infertility.[2] Quantitative data on the effects of ionizing radiation on human health is relatively limited compared to other medical conditions because of the low number of cases to date, and because of the stochastic nature of some of the effects. Stochastic effects can only be measured through large epidemiological studies where enough data has been collected to remove confounding factors such as smoking habits and other lifestyle factors. The richest source of high-quality data comes from the study of Japanese atomic bomb survivors. In vitro and animal experiments are informative, but radioresistance varies greatly across species.