5.1 ANATOMY 64 Even bacteria, which are microscopic, independently living organisms, have a cellular structure. Each bacterium is a single cell. What is DNA? Give a definition. Blood and cartilage are examples of connective tissue. Skin, nails, and hairs make up the integumentary system, whose main function is to protect the body from bacteria, infection, injury and sunlight. Another important organ system is the lymphatic one, which has to protect the body from bacteria, viruses, parasites and fungi. to enclose: racchiudere integumentary: tegumentario smooth: liscio to store: immagazzinare tissue: tessuto The human body is the most complex system ever created. The more we learn about it, the more appreciation we have about what a rich system it is. Bill Gates PHYSIOLOGY AND THE STRUCTURAL ORGANISATION OF THE BODY Physiology is the scientific study of how the human body works and describes the chemistry and physics that govern basic body functions. Structural organisation of the body Every process of the human body is built upon several levels of structural organisation which include the cells, tissues, organs, and organ systems. Higher levels of organisation are built from lower levels. Cell The most fundamental level of organisation is the cellular level. A cell is the smallest independently functioning unit of a living organism , and is capable of self-reproducing. All functions and structures of the human body are performed in cells or are initiated by cells. A cell consists of a flexible external membrane that encloses nucleic acids (such as DNA and RNA) and cytoplasm, which contains structures called organelles that perform various cellular functions, for example the production of energy or the destruction of toxins. Tissue A tissue is a group of many similar cells (for example smooth muscle cells) that work together to perform a specific function. There are four types of tissue in an adult body: muscle tissue is specialised in contraction to generate movement; neural (or nervous) tissue is specialised in carrying messages to and from different parts of the body; epithelial tissue provides a physical barrier that protects the entire body and performs several functions such as secretion, absorption, excretion, filtration, diffusion, and sensory reception; connective tissue is specialised in storing fat, helping move nutrients and other substances between tissues and organs, and repairing damaged tissue. Organ An organ is a structure of the body composed of two or more types of tissue that form a functional unit. Each organ performs one or more specific physiological functions which are essential for life. For example, the brain controls and coordinates all the vital functions of the body, and the heart moves blood throughout the body. Organ systems An organ system is a group of organs that work together to perform major functions or meet physiological needs of the body. The main organ systems of a human body are: skeletal, muscular, nervous, cardiovascular, respiratory, digestive, urinary, endocrine, reproductive, and sensory. 130 SCIENCE AND PHYSICS