The alimentary canal , the tubular portion of the digestive tract, extends from the oral cavity to the anus. Aliquots of food that was swallowed at the level of the oral cavity enters the alimentary canal to be churned, liquefied,…
The digestive system—composed of the oral cavity, alimentary tract, and associated glands—functions in the ingestion, mastication, deglutition (swallowing), digestion, and absorption of food, as well as in the elimination of its indigestible remnants. Regions of the digestive system are modified…
The lungs and the airways leading to the external environment constitute the respiratory system , whose role is to provide oxygen (O 2 ) to and eliminate carbon dioxide (CO 2 ) from the cells of the body. The following…
The integument is composed of skin and its appendages: sweat glands , sebaceous glands , hair , and nails . It is the largest organ, constituting 16% of the body weight. The skin invests the entire body, becoming continuous with…
The endocrine system consists of ductless glands , distinct clusters of cells within certain organs of the body, and individual endocrine cells (known as the diffuse neuroendocrine system [DNES] ), situated in the epithelial lining of the respiratory system and…
The lymphoid system is responsible for the immunological defense of the body. Some of its component organs— bone marrow , lymph nodes , thymus , and spleen —are surrounded by connective tissue capsules, whereas its other components, members of the…
The circulatory system is composed of two separate but related components: the cardiovascular system and the lymphatic vascular system. The function of the cardiovascular system is to carry blood in both directions between the heart and the tissues. The function…
Blood, a specialized connective tissue, is a bright- to dark-red, viscous, slightly alkaline fluid (pH 7.4) that accounts for approximately 7% of the total body weight with a total volume of approximately 5 L in an average adult. It is…
The human nervous system is composed of perhaps a trillion neurons, each with a large number of interconnections. Some of these neurons possess receptors that are specialized for receiving different types of stimuli (e.g., mechanical, chemical, thermal), which are transduced…
Muscle cells are specialized for contraction that permits movement. Organisms harness the contraction of muscle cells and the arrangement of the extracellular components of muscle to permit locomotion, constriction, pumping, and other propulsive movements. Muscle cells are called striated or…