The human body is arranged in a complex yet orderly fashion (Fig. 3-1). Therefore, a systematic approach to studying it is helpful. Chemistry courses typically study the human body at the atomic and molecular levels. Some biology courses, such as cellular biology, study the body at the cellular level. Other courses, such as anatomy and physiology and medical terminology, in general study the body at the organ and organ-system levels. This will be our approach.
Each cell contains organelles that perform a specific function. Some of the major organelles found in human cells behave like an online shopping service that takes your order, packages the items, and delivers them to your home. Ribosomes process genetic instructions. The Golgi apparatus packages the molecules. The endoplasmic reticulum transports the molecules to their destination, and the mitochondria provide the energy, or fuel, for the transportation.
Cells are the structural units that form all body tissues. Their functions are consistent with the functions of the tissues they comprise. Their walls are composed of a membrane made up of lipids (fats), proteins, and other components that selectively allow certain substances, such as nutrients, to enter and other substances, such as wastes, to leave. Within the cell is a gelatinous substance called cytoplasm. It surrounds a variety of tiny structures called organelles that are important to cellular function. The largest of these is the nucleus. Contained within the nucleus is deoxyribonucleic acid (DNA), which is the genetic material for your body. It includes information that determines your gender, skin, hair and eye colors, and numerous other features.
Tissue is composed of a group of similar cells that perform a specific function. The types of tissue are epithelial, connective, nervous, and muscle.
Epithelial tissue forms the epidermis (top layer of the skin) and surface layer of the membranes. It may be composed of a single layer (simple) or of several layers (stratified). It is also classified according to the cell shape: squamous (flat), cuboidal (cube shaped), or columnar (cylindrical) (Fig. 3-2). Epithelial tissue has many functions, including protection, absorption, and secretion.
Common epithelial cell shapes: (A) squamous cells, (B) cuboidal cells, (C) columnar cells.
Because of its location and the large amount of wear and tear it undergoes, epithelial tissue has an amazing ability to regenerate, or replace itself, sometimes as often as every 24 hours.