In contrast to densely packed epithelial tissues, connective tissues consist of dispersed populations of cells embedded in an extracellular matrix that they secrete. The composition and properties of the matrix differ among types of connective tissues.
An important component of the extracellular matrix secreted by connective tissue cells is protein fibers. The dominant protein in the extracellular matrix is collagen (see Figure 4.26). Collegen is, in fact, the most abundant protein in the human body, representing 25 percent of total body protein. Collagen fibers are strong. They give the connective tissue of skin, tendons, and ligaments resistance to stretch. Similarly, collagen fibers provide a netlike framework for organs, giving them shape and structural strength. Connective tissue that fills spaces between organs has a low density of collagen fibers.
Another type of protein fiber in the extracellular matrix of connective tissues is the stretchable protein elastin. It can be stretched to several times its resting length and then recoil. Fibers composed of elastin are most abundant in tissues that
are regularly stretched, such as the walls of the lungs and the large arteries. Gradual loss of elastin fibers with age causes gradual loss of resiliency of the skin.
Cartilage and bone are connective tissues that provide rigid structural support. In cartilage, a network of collagen fibers is embedded in a flexible matrix consisting of a protein-carbohydrate complex. Cartilage, which lines the joints of vertebrates, is resistant to compressive forces. Since it is flexible, it provides structural support for flexible structures such as external ears and noses. The extracellular matrix in bone also contains many collagen fibers, but it is hardened by the deposition of the mineral calcium phosphate. We will discuss cartilage and bone in greater detail in Chapter 47.
Adipose tissue is a form of loose connective tissue that includes adipose cells, which form and store droplets of lipids. Adipose tissue, or "fat," is a major source of stored energy. It also serves to cushion organs, and layers of adipose tissue under the skin can provide a barrier to heat loss (see Figure 41.15).
Blood is a connective tissue consisting of cells dispersed in an extensive extracellular matrix: the blood plasma. The blood plasma is much more liquid than the extracellular matrices of the other connective tissues, but it too contains an abundance of proteins. Many of the proteins and cellular elements of the blood were presented in Chapter 18, and blood will be discussed again in Chapter 49.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.