Receptors are considered to be sensory organs. They provide the body with the general senses of touch, temperature, and pain. In addition, these receptors initiate reactions or reflexes in the body to maintain homeostasis. For example, receptors in the skin perceive cold, resulting in goosebumps. This reaction is the body's attempt to maintain internal warmth. Receptors are classified according to location, structure, and types of stimuli activating them. Classified according to location, the three types of receptors are as follows: superficial receptors (exteroceptors), deep receptors (proprioceptors), and internal receptors (visceroceptors). See table 1-4 for receptor locations and the senses resulting from the stimulation of these receptors. THE DIGESTIVE SYSTEM LEARNING OBJECTIVE: Identify the location and function of each part of the digestive system. The digestive system includes the organs that digest and absorb food substances, and eliminate the unused residuals. The digestive system consists of the alimentary canal and several accessory organs. The accessory organs release secretions into the canal. These secretions assist in preparing food for absorption and use by the tissues of the body. Table 1-5 illustrates principal digestive juices (secretions) produced by alimentary and accessory organs. Digestion is both mechanical and chemical. Mechanical digestion occurs when food is chewed, swallowed, and propelled by a wave-like motion called peristalsis. When peristalsis occurs, a ring of contraction appears in the walls of the alimentary canal. At the same time, the muscular wall just ahead of the ring relaxes. This phenomenon is called receptive relaxation. As the wave moves along, it pushes the canal's contents ahead of it. Chemical digestion consists of changing the various food substances, with the aid of digestive enzymes, into solutions and simple compounds. Carbohydrates (starches and sugars) change into simple sugars (glucose); fats change into fatty acids; and proteins change into amino acids. Once the food substances have been broken down into simple compounds, the cells of the body can absorb and use them. THE ALIMENTARY CANAL The alimentary canal (tract) is 9 meters in length, tubular, and includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine (fig. 1-53). Mouth The mouth, which is the first portion of the alimentary canal, is adapted to receive food and prepare it for digestion (fig. 1-53). The mouth mechanically reduces the size of solid particles and mixes them with saliva. This process is called mastication. Saliva, produced by the salivary gland, moistens food making it easier to chew. Saliva also lubricates the food mass to aid swallowing. The tongue assists with both mastication and swallowing. Pharynx The pharynx (covered earlier in “The Respiratory System”) is the passageway between the mouth and the esophagus and is shared with the respiratory tract (fig. 1-53). The epiglottis is a cartilaginous flap that 1-49 TYPES LOCATIONS SENSES Superficial receptors At or near surface of body Touch, pressure, heat, cold, and pain Deep receptors In muscles, tendons, and joints Sense of position and movement Internal receptors In the internal organs and blood vessel walls Usually none (except hunger, nausea, pain from stimuli such as chemicals (e.g., aspirin) and distension (e.g., stomach expansion from gas)) Table 1-4.—Types of Receptors, Their Location, and Affected Sense