Levels of Organization and Tissue Types
The Levels of Organization
All matter in the universe is composed of one or more unique pure substances called elements; familiar examples are hydrogen, oxygen, carbon, nitrogen, calcium, and iron.
The smallest unit of any of these pure substances (elements) is an atom. Atoms are made up of subatomic particles such as protons, electrons, and neutrons.
Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things. Molecules are the chemical building blocks of all body structures.
A cell is the smallest independently functioning unit of a living organism. All living structures of human anatomy contain cells, and almost all functions of human physiology are performed in cells or are initiated by cells.
A human cell typically consists of flexible membranes that enclose cytoplasm, a water-based cellular fluid, together with a variety of tiny functioning units called organelles.
A tissue is a group of many similar cells (though sometimes composed of a few related types) that work together to perform a specific function.
An organ is an anatomically distinct structure of the body composed of two or more tissue types. Each organ performs one or more specific physiological functions.
An organ system is a group of organs that work together to perform major functions or meet the physiological needs of the body.
Consider the breakdown of the human body into eleven distinct organ systems (Figure 3.2 and Figure 3.3). Assigning organs to organ systems can be imprecise, since organs that “belong” to one system can also have functions integral to another system. In fact, most organs contribute to more than one system.
The organism level is the highest level of organization. An organism is a living being that has a cellular structure and that can independently perform all physiologic functions necessary for life. In multicellular organisms, including humans, all cells, tissues, organs, and organ systems of the body work together to maintain the life and health of the organism.
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Media 3.1. Introduction to Anatomy & Physiology: Crash Course A&P #1 [Online video]. Copyright 2015 by CrashCourse.
Tissue Types
Although there are many types of cells in the human body, they are organized into four broad categories of tissues: epithelial, connective, muscle, and nervous. Each of these categories is characterized by specific functions that contribute to the overall health and maintenance of the body. A disruption of the structure is a sign of injury or disease. Such changes can be detected through histology, the microscopic study of tissue appearance, organization, and function.
Epithelial tissue, also referred to as epithelium, refers to the sheets of cells that cover exterior surfaces of the body, line internal cavities and passageways, and form certain glands. Connective tissue, as its name implies, binds the cells and organs of the body together and function