HUMAN PHYSIOLOGY

• Main Author: Mason, Elliott B (Author)
• Format: Book
• Language: English
• Published: Menlo Park, Calif. Benjamin/Cummings 1983; ©1983
• Subjects: Human physiology
• Online Access: Click Here to View Status and Holdings.

 Although the organs and tissues of the human body differ from one another in both form and function, they are composed of the same basic materials. The body is made up of chemical elements that interact with one another to form the anatomical structures and carry out the physiological processes characteristic of a living or ganism. CHEMICAL ELEMENTS AND ATOMS A chemical element is a substance that cannot be broken down into simpler material by chemical means. Each chemical element has its own name and a one- or two-letter symbol. For example, the symbol for the element oxygen is O, and the symbol for the element sodium is Na. At the present time, approximately 109 chemical elements are recognized, but only about 24 of these are normally found in the body (Table 2.1). A chemical element is made up of extremely small units of matter called atoms, which are themselves composed of even smaller particles (F2.1). Positively charged particles called protons are located in the central area or nucleus of an atom. Uncharged particles called neutrons, if present, are also located in the nucleus. Negatively charged particles called electrons are in constant mo tion around the nucleus. The number of protons and the number of electrons in an atom and therefore, the number of positive charges and the number of negative charges-are equal. Consequently, an atom has no overall electrical charge and is electrically neutral. Protons, neutrons, and electrons, like the atoms they compose, are matter, and, like all matter, they occupy space and possess mass. Each proton or neutron, however, has over 7800 times the mass of an electron. Thus, most of the mass of an atom is concen trated in the nucleus.