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Epiglottis
The epiglottis is a flap that is made of elastic cartilage and attached to the entrance of the larynx. It is covered with a mucous membrane and there are taste buds on its lingual surface which faces into the mouth. Its laryngeal surface faces into the larynx. The epiglottis functions to guard the entrance of the glottis, the opening between the vocal folds. It is normally pointed upward during breathing with its underside functioning as part of the pharynx, but during swallowing, the epiglottis folds down to a more horizontal position, with its upper side functioning as part of the pharynx. In this manner it prevents food from going into the trachea and instead directs it to the esophagus, which is posterior. During swallowing, the backward motion of the tongue forces the epiglottis over the glottis' opening to prevent any food from entering the larynx which leads to the lungs; the larynx is also pulled upwards to assist this process. Stimulation of the larynx by ingested matter produces a strong cough reflex in order to protect the lungs.
Pharynx
The pharynx is a part of the digestive system and also a part of the conducting zone of the respiratory system. It is the part of the throat immediately behind the nasal cavity at the back of the mouth and superior to the esophagus and larynx.The pharynx is made up of three parts. The lower two parts–the oropharynx and the laryngopharynx are involved in the digestive system. The laryngopharynx connects to the esophagus and it serves as a passageway for both air and food. Air enters the larynx anteriorly but anything swallowed has priority and the passage of air is temporarily blocked. Muscles in the pharynx push the food into the esophagus.
Esophagus
The esophagus commonly known as the gullet, is an organ which consists of a muscular tube through which food passes from the pharynx to the stomach. The esophagus is continuous with the laryngeal part of the pharynx. Its length averages 25 cm, varying with height . It is divided into cervical, thoracic and abdominal parts. The pharynx joins the esophagus at the esophageal inlet. Generally the esophagus is closed at both ends, by the upper and lower esophageal sphincters. The opening of the upper sphincter is triggered by the swallowing reflex so that food is allowed through. The sphincter also serves to prevent back flow from the esophagus into the pharynx.
The esophagus has a mucous membrane having a protective function which is continuously replaced due to the volume of food that passes inside the esophagus. Once in the esophagus, the bolus travels down to the stomach via rhythmic contraction and relaxation of muscles known as peristalsis .The lower esophageal sphincter is a muscular sphincter which remains constricted at all times other than during swallowing and vomiting to prevent the contents of the stomach from entering the esophagus. Any failure of this sphincter can lead to heartburn.
Diaphragm
The diaphragm is an important part of the body's digestive system. The diaphragm separates the thoracic cavity from the abdominal cavity where most of the digestive organs are located. The suspensory muscle attached helps the digestive system in the easier passage of digesting material. The diaphragm also attaches to the bare area of the liver, which it anchors.
Stomach
Gastric acid (informally gastric juice), produced in the stomach plays a vital role in the digestive process, it mainly contains hydrochloric acid and sodium chloride. A peptide hormone gastrin produced by G cells in the stomach, stimulates the production of gastric juice which activates the digestive enzymes. Pepsinogen is produced by the gastric cells and gastric acid activates the enzyme pepsin which begins the digestion of proteins. As these two chemicals would damage the stomach wall, mucus is secreted by the stomach, to provide a slimy protective layer against the damaging effects of the chemicals. At the same time that protein is being digested, mechanical churning occurs through the action of peristalsis, waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes. Gastric lipase secreted by the glands in the gastric mucosa of the stomach, is an acidic lipase, in contrast with the alkaline pancreatic lipase. This breaks down fats to some degree though is not as efficient as the pancreatic lipase.
The lowest section of the stomach which attaches to the duodenum via the pyloric canal, contains countless glands which secrete digestive enzymes including gastrin. After an hour or two, a thick semi-liquid called chyme is produced. When the pyloric sphincter, or valve opens, chyme enters the duodenum where it mixes further with digestive enzymes from the pancreas, and then passes through the small intestine, where digestion continues. When the chyme is fully digested, it is absorbed into the blood. 95% of absorption of nutrients occurs in the small intestine. Water and minerals are re-absorbed back into the blood in the colon of the large intestine, where the environment is slightly acidic. Some vitamins, such as biotin and vitamin K produced by bacteria in the colon are also absorbed.
The cells in the pit of the stomach, produce a glycoprotein which is essential for the absorption of vitamin B12. Vitamin B12 (cobalamin), is carried to, and through the stomach, bound to a glycoprotein secreted by the salivary glands - transcobalamin I also called haptocorrin, which protects the acid-sensitive vitamin from the acidic stomach contents. Once in the more neutral duodenum, pancreatic enzymes break down the protective glycoprotein. The freed vitamin B12 is then absorbed by the enterocytes in the ileum.
The stomach is a distensible organ and can normally expand to hold about one litre of food. The stomach of a newborn baby will only be able to expand to retain about 30 ml.
