The Human Body


Anatomy and physiology is the study of the human body. Anatomy is concerned with the structure of a part. For example, the stomach is a J-shaped, pouchlike organ (Fig. 1.1). The stomach wall has thick folds, which disappear as the stomach expands to increase its capacity. Physiology is concerned with the function of a part. For example, the stomach temporarily stores food, secretes digestive juices, and passes on partially digested food to the small intestine. Anatomy and physiology are closely connected in that the structure of an organ suits its function. For example, the stomach’s pouchlike shape and ability to expand are suitable to its function of storing food. In addition, the microscopic structure of the stomach wall is suitable to its secretion of digestive juices.

Organization of Body Parts


The structure of the body can be studied at different levels of organization (Fig. 1.1). First, all substances, including body parts, are composed of chemicals made up of submicroscopic particles called atoms. Atoms join to form molecules, which can in turn join to form macromolecules. For example, molecules called amino acids join to form a macromolecule called protein, which makes up the bulk of our muscles. Macromolecules are found in all cells, the basic units of all living things. Within cells are organelles, tiny structures that perform cellular functions. For example, the organelle called the nucleus is especially concerned with cell reproduction; another organelle, called the mitochondrion, supplies the cell with energy. Tissues are the next level of organization. A tissue is composed of similar types of cells and performs a specific function. An organ is composed of several types of tissues and performs a particular function within an organ system. For example, the stomach is an organ that is a part of the digestive system. It has a specific role in this system, whose overall function is to supply the body with the nutrients needed for growth and repair. The other systems of the body also have specific functions. All of the body systems together make up the organism- such as, a human being. Human beings are complex animals, but this complexity can be broken down and studied at ever simpler levels. Each simpler level is organized and constructed in a particular way.
Levels of organization of the human body
Figure 1.1 Levels of organization of the human body. Each level is more complex than the previous level.
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