Human Digestive System
In the human digestive system, the process of digestion starts in the mouth (oral cavity). Digestion involves the breakdown of food into smaller components, which can be absorbed and assimilated into the body. The secretion of saliva helps us swallow so that the food is passed down the oesophagus and into the stomach.
Saliva contains a catalytic enzyme called Amylase which starts to act on the food. Another digestive enzyme called Lipase is secreted by some papillae to enter the saliva. Digestion is helped by the mastication (chewing) of food by the teeth and the muscular contractions of peristalsis. The gastric juice in the stomach is essential to continue digestion as is the production of mucus in the stomach.
Peristalsis is the rhythmic muscular contraction which begins in the oesophagus and continues along the stomach lining and the rest of the gastrointestinal tract. This initially creates chime, which when fully broken down in the small intestine, is absorbed as chyle into the lymphatic system. Most of the digestion happens in the small intestine. Water and some minerals are reabsorbed back into the blood, in the colon of the large intestine. The waste products of digestion are defecated from the anus via the rectum.
Components of the Digestive system
There are several organs involved in digestion of which the accessory digestive glands are the liver, gall bladder and pancreas. Other components are the mouth, teeth and epiglottis.
The largest part of the digestive system is the gastrointestinal tract (GI tract), which begins at the mouth and ends at the anus, a distance of about 9 metres. The largest part of the GI tract is the colon or large intestine, where water is absorbed and the remaining waste matter is stored prior to defecation. Most of the digestion happens in the small intestine. A major digestive organ is the stomach and within its mucosa are millions of gastric glands. Their secretions are vital to normal functioning of the organ. There are many specialised cells in the GI tract ---- gastric glands, taste cells and pancreatic duct cells.
The mouth is the first part of the GI tract and has many structures that begin the process of digestion. These include salivary glands, teeth and the tongue. Most of the oral cavity is lined with oral mucosa that produces a lubricating mucus. Mucus mainly contains a glycoprotein called mucin. Underlying the mucous membrane is a thin layer of smooth muscle tissue and the loose connection to the membrane gives it its great elasticity.
The roof of the mouth is termed the palate, which separates the oral cavity from the nasal cavity. The hard palate allows for the pressure needed in eating food, to leave the nasal passage clear. Mucus helps in the mastication of food with its ability to soften and collect the food.
Saliva functions initially in the digestive system to moisten and soften the food. The food is further lubricated by the saliva in its passage from the mouth into the oesophagus. Also important are the digestive enzymes in the saliva --- Amylase and Lipase. Amylase starts working on the starch, breaking it down into Maltose and Dextrose that can be further broken down in the small intestine. Saliva in the mouth can account for 30% of this initial starch digestion. Lipase starts working on breaking down the fats. Lipase is further produced in the pancreas where it continues the digestion of fats.
Saliva cleanses the teeth and the mouth and supplies antibodies such as Immunoglobulin, which prevents infections of the salivary glands.
Food enters the mouth where the first stage in digestion takes place, with the tongue and the saliva. The liquid saliva will help soften the food and its enzymes begin to break down the food. The first food to be broken down is the starch.
Taste is a kind of chemoreception in the specialized receptors of taste cells, located in the taste buds of the mouth, mainly on the upper tongue surface. There are taste buds elsewhere in the mouth also. Taste messages are sent via cranial nerves to the brain. The brain can distinguish five basic tastes ---- saltiness, sourness, bitterness and sweetness, and the savouriness termed umami. The olfactory (smelling) receptors are located on cell surfaces in the nose, which bind to chemicals, thus enabling the detection of smells. The signals from taste receptors work, together with the signals from the nose, help us perceive complex food flavours.
They are made of dentin, a bone-like material, covered by the hardest tissue in the body --- enamel. Teeth have different shapes to deal with mastication employed in tearing and chewing the food. The teeth are named after their particular roles — incisors are for cutting or biting of food, canines are for tearing, premolars and molars are for chewing and grinding.
The epiglottis is a flap of elastic cartilage, attached to the entrance of the larynx. It prevents the food from going into the trachea and instead directs it to the oesophagus. During swallowing, the tongue forces the epiglottis to prevent any food from entering the larynx, which leads to the lungs. The stimulation of the larynx by ingested matter produces a strong cough reflex to protect the lungs.
The oesophagus, called the gullet, is a muscular tube through which the food passes from the pharynx to the stomach. Once in the oesophagus, the food travels down to the stomach via rhythmic contraction and relaxation of muscles known as peristalsis. The oesophagus has a mucous membrane of epithelium which protects and provides a smooth surface for the passage of food.
Gastric acid (gastric juice), produced in the stomach plays a vital role in digestive. This juice mainly contains hydrochloric acid and sodium chloride. A hormone called gastrin produced by the gastric glands stimulates the production of gastric juice, which activates the digestive enzymes. Pepsinogen is an enzyme produced by the gastric cells and gastric acid activates it to convert it into Pepsin which begins the digestion of proteins. As these two chemicals can damage the stomach wall, mucus is secreted by innumerable gastric glands in the stomach, to provide a protective layer against it. Also, at the same time, mechanical churning occurs through peristalsis, waves of muscular contractions that move along the stomach wall. This allows the food to further mix with the enzymes. The gastric lipase secreted in the stomach is acidic, in contrast with the alkaline pancreatic lipase. This breaks down the fats to some degree.
The pylorus, the lowest section of the stomach, 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 opens, chyme enters the duodenum to mix with digestive enzymes from the pancreas, and then passes through the small intestine to continue digestion. When the chyme is fully digested, it is absorbed into the blood. 95% of the absorption of nutrients occurs in the small intestine. Water and minerals are re-absorbed back into the blood in the colon, where the environment is slightly acidic. Some vitamins, such as vitamin K produced by bacteria, are also absorbed here.