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