What Goes on Down the Hatch
DIGESTIVE SYSTEM
The human body requires fuel to power itself. This power is derived from raw material that becomes energy, and this energy is processed in the digestive system. The digestive system can be thought of as a long internal tube (alimentary canal). Food enters your mouth and is processed via mechanical and chemical digestion as it is propelled along your digestive tract. In the final portion of this tube, your intestines stop digesting the material and begin absorbing nutrients into your lymphatic and circulatory systems. The essential materials that your cells and tissues rely on for survival are then distributed throughout the body.
Mouth
The oral cavity (mouth) is defined as the space bounded by your lips (anteriorly), cheeks (laterally), and palate (superiorly). In this space, food is processed by mechanical means (teeth), chemically modified (saliva), and moved into the esophagus by the tongue. Raw materials begin this journey by first passing through the opening defined by the lips.
Teeth
Made up of the hardest substance in the body and similar in many respects to bone, the teeth are the cutting and grinding implements that begin the mechanical breakdown of ingested food. All teeth consist of a portion that rise above the gum line (gingiva), which is called the crown, and the portion below the gum line (root). Covering the crown of the tooth is a brilliant white, calcium-rich material called enamel. Unlike bone, enamel has no cells and cannot repair or produce new enamel. In time, enamel slowly wears away and is degraded by bacterial enzymes (dental caries or cavities). Beneath the enamel is the core of the tooth, which is composed of dentin. This living tissue is similar to, yet harder than, bone and surrounds the pulp cavity where blood vessels and nerves reside. Blood is transported in and out of the pulp cavity via vessels that enter and leave through an opening in the root of the tooth.
Primary Teeth
The first teeth for babies erupt through the gums starting around 6 months of age and continue erupting for the first few years of life. During this time, the first set of teeth (primary teeth) form. The first set of teeth includes 5 pairs per jaw, or 20 in total.
Secondary Teeth
The second and final set of teeth, which must last a lifetime, begin to erupt the gum at around 6 years of age. They may not finish erupting until the early 20s. Structurally, both sets of teeth are the same, although the adult teeth are larger to better facilitate chewing (mastication) in the larger adult mouth. In addition to the same 20 teeth a child’s mouth has, the adult mouth contains a pair of first, second, and third molars for each jaw. The final total of secondary teeth in the adult mouth will be 32.
Tongue
The tongue plays an essential role in digestion. As the jaw muscles and teeth cut, tear, and grind the food, the tongue moves the food back and forth in the mouth (intraoral transport) to help process the food into small pieces. The tongue, along with muscular contractions in the pharynx, moves food into the esophagus.
Made up of skeletal muscle covered with a thick tough epithelium, the tongue is a versatile organ. Individual muscles are arranged into at least five different planes within the tongue, which facilitates movement in a myriad of directions.
Raised extensions of the epithelium can be found on the surface of the tongue, and come in contact with the food (dorsum of the tongue). Some of these have thickened layers of dead cells, making them very tough and abrasive (filiform papillae). Other papillae are more flattened, often with grooves on the lateral boundaries, which can accumulate material and assist in the detection of taste via taste buds embedded in the walls of the papillae.
Salivary Glands
As the teeth and tongue facilitate the mechanical digestion of ingested material, an enzymatic mixture called saliva is added through ducts from three major salivary glands. This substance begins the process of chemical digestion, as well as lubricating the material for passage down the esophagus.
Anatomy of a Word
parenchyma
The secretory portion of salivary glands is called the parenchyma (remember that the parenchyma of an organ is the functional tissue, differentiated from the supporting tissues), and typically consists of either or both serous (watery, enzymatic) or mucous (viscous, slippery) secretions. Histological identification of salivary gland tissue is largely dependent upon the percentage of each type that composes the gland.
Parotid Glands
The parotid is located on the lateral portions of the face near the angle of the jaw and the base of the ear. The parenchyma of the parotid glands is entirely serous in nature and includes enzymes such as salivary amylase that begins the breakdown of carbohydrates. These secretions are transferred to the mouth via Stensen’s duct (a channel that is also called the parotid duct) and empty into the oral cavity near the second molar on each side of the maxilla (upper jaw). Although it is the largest of the salivary glands, the parotid does not produce the bulk of saliva.
Submandibular Glands
As the name implies, these glands are found just inside the lower jaws, and produce around 60 percent of all saliva. Unlike the parotid, the submandibular consists of 50:50 serous and mucous secretionproducing cells. Wharton’s duct transfers the saliva from the gland to the oral cavity where it is released at swellings on either side of the frenulum (the membrane that attaches from the floor of the mouth to the underside of the tongue).
Sublingual Glands
The smallest of the salivary glands is the sublingual gland, composed almost entirely of mucus-producing cells. This material will be added to the serous secretions from the other glands and aid in the lubrication of the food before swallowing.