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The easiest way to understand the digestive system is to divide its organs into two main categories. The first group is the organs that make up the alimentary canal. These organs are part of the "tube" our food travels through from the mouth to the anus. Accessory digestive organs comprise the second group. Food never enters or passes through these organs, but they are critical for orchestrating the breakdown of food. Accessory digestive organs, despite their name, are critical to the function of the digestive system.
Also called the gastrointestinal (GI) tract or gut, the alimentary canal (aliment- = “to nourish”) is a one-way tube about 25 feet in length The main function of the organs of the alimentary canal is to nourish the body. This tube begins at the mouth and terminates at the anus. Between those two points, the canal is modified as the pharynx (throat), esophagus, stomach, and small and large intestines to fit the functional needs of the body. Both the mouth and anus are open to the external environment; thus, food and wastes within the alimentary canal are technically considered to be outside the body. Only through the process of absorption do the nutrients in food enter into and nourish the body’s “inner space.”
Each accessory digestive organ aids in the breakdown of food ( [link] ). The salivary glands, in the mouth, begin the chemical digestion of food. Once food products enter the small intestine, the gallbladder, liver, and pancreas release secretions—such as bile and enzymes—essential for digestion to continue. Together, these are called accessory organs because although they are important, food does not pass through them. You could not live without their vital contributions, and many significant diseases result from their malfunction.
Throughout its length, the alimentary tract is composed of the same four tissue layers; the details of their structural arrangements vary to fit their specific functions. Starting from the lumen and moving outwards, these layers are the mucosa, submucosa, muscularis, and serosa, which is continuous with the mesentery (see [link] ).
The mucosa is referred to as a mucous membrane, because mucus production is a characteristic feature of this layer. The membrane consists of epithelium, which is in direct contact with ingested food. Epithelium —is a type of tissue, found in the mouth, stomach, intestines, pharynx, esophagus,and anal canal, The beginning and end of tube is lined with of a type of epithelium called stratified squamous epithelium . The stomach and intestines are lined simple columnar epithelium . Notice that the epithelium is in direct contact with the space inside the alimentary canal called the lumen .
As its name implies, the submucosa lies immediately beneath the mucosa. It is composed of another tissue known as dense connective tissue . The mucosa is referred to as a mucous membrane, because mucus production is a characteristic feature of this layer. It includes blood and lymphatic vessels (which transport absorbed nutrients).
The third layer of the alimentary canal is the muscularis . The muscularis in the small intestine is made up of a double layer of smooth muscle. The contractions of these layers promote mechanical digestion, expose more of the food to digestive chemicals, and move the food along the canal.
The serosa is the outermost layer of the alimentary canal, superficial to the muscularis. Instead of serosa, the mouth, pharynx, and esophagus have a dense sheath of collagen fibers called the adventitia. These tissues serve to hold the alimentary canal in place near the ventral surface of the vertebral column.
As soon as food enters the mouth, it is detected by receptors that send impulses along the sensory neurons of cranial nerves. Without these nerves, not only would your food be without taste, but you would also be unable to feel either the food or the structures of your mouth, and you would be unable to avoid biting yourself as you chew, an action enabled by the motor branches of cranial nerves. In addition the nervous system controls the movement of food through the alimentary tube and controls the release of enzymes and hormones that are important in the digestion and absorption of food
The blood vessels serving the digestive system have two functions. They transport the protein and carbohydrate nutrients absorbed by cells after food is digested in the lumen. Lipids are absorbed via lacteals , tiny structures of the lymphatic system. The blood vessels’ second function is to supply the organs of the alimentary canal with the nutrients and oxygen needed to drive their cellular processes.
The veins that collect nutrient-rich blood from the small intestine (where most absorption occurs) empty into the hepatic portal system . This network takes the blood into the liver where the nutrients are either processed or stored for later use. Only then does the blood circulate back to the heart.
The digestive system includes the organs of the alimentary canal and accessory structures. The alimentary canal forms a continuous tube that is open to the outside environment at both ends. The organs of the alimentary canal are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory digestive structures include the teeth, tongue, salivary glands, liver, pancreas, and gallbladder. The wall of the alimentary canal is composed of four basic tissue layers: mucosa, submucosa, muscularis, and serosa. The enteric nervous system provides intrinsic innervation, and the autonomic nervous system provides extrinsic innervation.
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