Physiological processes, then, are internal mechanisms that allow plants and animals to function under varied environmental conditions. Over the next few lectures, we will be analyzing the major organ systems of the body and how they function. The surface of the lungs, the lining of thegastro-intestinal tract, the cells lining the blood vessels, all are includedin this category. Glands, of which there are two types, classified according to where theirproducts are secreted. Exocrine glands, including sweat glands, mammary glands, sebaceous oilglands, secrete their products directly onto the epithelial surface outside ofthe circulatory system.
Connective tissues function primarily in structural roles.
The basic element of these tissues is a long, fibrousprotein called collagen . Cartilage is also comprised primarily of collagen, organized in aneven denser arrangement than tendons. The cells responsible for secreting thecollagen are buried deep inside the connective tissue matrix, isolated fromnutrients and oxygen. As a consequence, cartilage and tendons are very slowto heal from trauma. It often takes many months to recover from a rupturedtendon.