The mouth is the first part of the gastrointestinal tract and is equipped with several structures that begin the first processes of digestion. These include salivary glands, teeth and the tongue. The mouth, consists of two regions, the vestibule and the oral cavity proper. The vestibule is the area between the teeth, lips and cheeks, and the rest is the oral cavity proper. Most of the oral cavity is lined with oral mucosa a mucous membrane that produces a lubricating mucus, of which only a small amount is needed. Mucous membranes vary in structure in the different regions of the body but they all produce lubricating mucus, which is either secreted by surface cells or more usually by underlying glands.
The mucous membrane in the mouth continues as the thin mucosa which lines the bases of the teeth. The main component of mucus is a glycoprotein called mucin and the type secreted varies according to the region involved. Mucin is viscous, clear, and clinging. Underlying the mucous membrane in the mouth is a thin layer of smooth muscle tissue and the loose connection to the membrane gives it its great elasticity. It covers the cheeks, inner surfaces of the lips, and floor of the mouth.
The roof of the mouth is termed the palate and it separates the oral cavity from the nasal cavity. The palate is hard at the front of the mouth since the overlying mucosa is covering a plate of bone; it is softer and more pliable at the back being made of muscle and connective tissue, and it can move to swallow food and liquids. The soft palate ends at the uvula. The surface of the hard palate allows for the pressure needed in eating food, to leave the nasal passage clear. The lips are the mouth's front boundary and the fauces (the passageway between the tonsils, also called the throat), mark its posterior boundary. At either side of the soft palate are the palatoglossus muscles which also reach into regions of the tongue. These muscles raise the back of the tongue and also close both sides of the fauces to enable food to be swallowed. Mucus helps in the mastication of food in its ability to soften and collect the food in the formation of the bolus.
a. Salivary glands
There are three pairs of main salivary glands and between 800 and 1,000 minor salivary glands, all of which mainly serve the digestive process, and also play an important role in the maintenance of dental health and general mouth lubrication, without which speech would be impossible. The main glands are all exocrine glands, secreting via ducts. All of these glands terminate in the mouth. The largest of these are the parotid glands (their secretion is mainly serous). The next pair are underneath the jaw, the submandibular glands, these produce both serous fluid and mucus. They produce about 70% of the oral cavity saliva.
The third pair are the sublingual glands located underneath the tongue their secretion is mainly mucous with a small percentage of saliva. Within the submucosa of the mucous membranes lining the mouth and also on the tongue and palates and mouth floor, are the minor salivary glands. their secretions are mainly mucous and are innervated by the facial nerve, the seventh cranial nerve.
The glands also secrete amylase a first stage in the breakdown of food acting on the carbohydrate in the food to transform the starch content into maltose. There are other glands on the surface of the tongue that encircle taste buds on the back part of the tongue and these produce a serous fluid which contains lipase.
Lipase is a digestive enzyme that catalyses the hydrolysis of lipids. These glands are termed Von Ebner's glands which have also been shown to have another function in the secretion of histatins which offer an early defense against microbes in food, when it makes contact with these glands on the tongue tissue. Sensory information can stimulate the secretion of saliva providing the necessary fluid for the tongue to work with and also to ease swallowing of the food.
Saliva functions initially in the digestive system to moisten and soften food into the formation of a bolus. The bolus is further helped by the lubrication provided by the saliva in its passage from the mouth into the oesophagus. Also of importance is the presence in saliva of the digestive enzymes amylase and lipase. Amylase starts to work on the starch in carbohydrates, breaking it down into the simple sugars of 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 to work on breaking down fats. Lipase is further produced in the pancreas where it is released to continue this digestion of fats. The presence of salivary lipase is of prime importance in young babies whose pancreatic lipase has yet to be developed.
As well as its role in supplying digestive enzymes, saliva has a cleansing action for the teeth and mouth, and has an immunological role in supplying antibodies to the system, such as immunoglobulin A. This is seen to be key in preventing infections of the salivary glands, importantly that of parotitis.
Saliva also contains a glycoprotein called haptocorrin which is a binding protein to vitamin B12. It binds with the vitamin in order to carry it safely through the acidic content of the stomach. When it reaches the duodenum, pancreatic enzymes break down the glycoprotein and free the vitamin which then binds with intrinsic factor.
Food enters the mouth where the first stage in the digestive process takes place, with the action of the tongue and the secretion of saliva. The tongue is a fleshy and muscular sensory organ, and the very first sensory information is received via the taste buds on its surface. If the taste is agreeable the tongue will go into action, manipulating the food in the mouth which stimulates the secretion of saliva from the salivary glands.
The liquid quality of the saliva will help in the softening of the food and its enzyme content will start to break down the food whilst it is still in the mouth. The first part of the food to be broken down is the starch of carbohydrates. The tongue is attached to the floor of the mouth by a ligamentous band called the frenum and this gives it great mobility for the manipulation of food. The range of manipulation is optimally controlled by the action of several muscles and limited in its external range by the stretch of the frenum. The tongue's two sets of muscles, are four intrinsic muscles that originate in the tongue and are involved with its shaping, and four extrinsic muscles originating in bone that are involved with its movement.
Taste is a form of chemoreception, that takes place in the specialised receptors of taste cells, contained in structures called taste buds in the mouth. Taste buds are mainly on the upper surface of the tongue. Taste perception is vital to help prevent harmful or rotten foods from being consumed. This is a function of the gustatory system where the taste buds are at the forefront. There are taste buds elsewhere in the mouth not just on the surface of the tongue. The taste buds are innervated by a branch of the facial nerve the chorda tympani, and the glossopharyngeal nerve. Taste messages are sent via these cranial nerves to the brain.
The brain can distinguish between the chemical qualities of the food. The five basic tastes are referred to as those of saltiness, sourness, bitterness and sweetness, and the most recent addition of a certain savouriness termed umami.
The detection of saltiness and sourness enables the control of salt and acid balance. The detection of bitterness warns of poisons. Many of a plant's defences are of poisonous compounds that are bitter. Sweetness guides to those foods that will supply energy. The initial breakdown of the energy-giving carbohydrates by salivary amylase creates the taste of sweetness since simple sugars are the first result. The taste of umami is thought to signal protein-rich food. Sour tastes are acidic which is often found in bad food. The brain has to decide very quickly whether to eat the food or not.
Teeth are complex structures made of materials specific to them. They are made of a bone like material dentin, which is covered by the hardest tissue in the body enamel. Teeth have different shapes to deal with different aspects of mastication employed in tearing and chewing pieces of food into smaller and smaller pieces.
Incisors: are used for cutting or biting off pieces of food.
Canines: are used for tearing.
Premolars and Molars: for chewing and grinding.
Mastication of the food with the help of saliva and mucus results in the formation of a soft bolus which can then be swallowed to make its way down the upper gastrointestinal tract to the stomach. Dental health is maintained by the salivary secretion of gingival crevical fluid. The digestive enzymes in saliva also help in keeping the teeth clean by breaking down any lodged food particles.
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 oesophagus, which is posterior. During swallowing, the backward motion of the tongue forces the epiglottis over the glottis' opening to prevent any food that is being swallowed 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.
The pharynx is a part 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 oesophagus and larynx.
The pharynx is made up of three part
The lower two parts
Oropharynx: The oropharynx lies behind the oral cavity, extending from the uvula to the level of the hyoid bone.
Laryngopharynx: Is the caudal part of the pharynx. It is the part of the throat that connects to the esophagus.
The oropharynx and the laryngopharynx are involved in the digestive system. The laryngopharynx connects to the oesophagus 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. The pharynx is innervated by the pharyngeal plexus of vagus nerve.
Muscles in the pharynx push the food into the oesophagus. The pharynx joins the oesophagus at the oesophageal inlet which is located behind the cricoid cartilage.
The oesophagus 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 oesophagus is continuous with the laryngeal part of the pharynx. It passes through the posterior mediastinum in the thorax and enters the stomach through a hole in the thoracic diaphragm the oesophageal hiatus, at the level of the tenth thoracic vertebra.
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 attaches the ascending duodenum to the diaphragm. This muscle is thought to be of help in the digestive system in that its attachment offers a wider angle to the duodenojejunal flexure for the easier passage of digesting material.
The diaphragm also attaches to the bare area of the liver, which it anchors. The oesophagus enters the abdomen through a hole in the diaphragm.
The stomach is a muscular J-shaped organ found in the abdominal cavity. Food is temporarily stored in the stomach. The stomach is a muscular organ which performs mechanical digestion by churning the bolus and mixing it with the gastric juices (HCl, salts, enzymes, water and mucus) secreted by the lining of the stomach. The bolus is now called Chyme.
Gastric Juices are a mixture of hydrochloric acid, salts, enzymes,water and mucus that is produced by glands in the stomach to help digest food.
The environment of the stomach is very acidic. HCl is secreted to kill any microbes that are found in the bolus, creating a pH of 2.
Mucus prevents the stomach from digesting itself.
Pepsin is also secreted. This enzyme is responsible for initiating the breakdown of proteins found in the food. Pepsin hydrolyzes proteins to yield polypeptides. Since the pH is 2, the enzyme from the salivary glands stops breaking down carbohydrates.
The stomach does not digest itself because of three protective mechanisms. First the stomach only secretes small amounts of gastric juices until food is present. Second the secretion of mucus coats the lining of the stomach protecting it from the gastric juices. The third mechanism is the digestive enzyme pepsin is secreted in an inactive protein called pepsinogen. Pepsinogen is converted to pepsin in the increased presence of hydrochloric acid.
The chyme moves from the stomach to the small intestine. It passes through a muscular ringed sphincter called the pyloric sphincter.
Chyme is a thick liquid produced in the stomach and made of digested food combined with gastric juice.
Pepsin is an enzyme in gastric juice that helps break down proteins into polypeptides.
a. The Small Intestine:
The small intestine is responsible for the complete digestion of all macromolecules and the absorption of their component molecules (glucose, glycerol, fatty acids, amino acids and nucleotides). The process of absorption allows the component molecules to be diffused into the surrounding intestinal cells and then into the circulatory system for transport to all the cells in the body.
The small intestine is made up of three parts, duodenum, jejunum and the ileum. The first part is the duodenum, u-shaped organ, approximately 30 cm in length. This area completes most of the digestion processes. Enzymes are secreted into the duodenum form the pancreas and the gall bladder.
The duodenum is lined by folds of tissue called villi. The villi are covered by fine brush like microvilli. These folds increase the surface area of the small intestine increase the rate of absorption.
The jejunum is approximately 2.5 m. long. Although some digestion is completed here, it has more villi and microvilli; its role is absorption of nutrients.
The ileum is approximately 3 m. long, and has fewer villi and microvilli than the other two parts. Although absorption also occurs here, it is responsible for pushing the waste materials into the large intestine.
In Small Instestine succus glands release succus entericus. In this enzymes are peptidage, lypage, maltage, lactage, enterokynage, sucrage.
Chemical Digestion and Absorption:
Enzymatic digestion or chemical digestion of macromolecules are performed by carbohydrases or carbohydrates digestion, lipases or lipid digestion, proteases or protein digestion, and nucleases or nucleic acid digestion.
Factors that affect enzyme actions:
There are two factors that affect enzyme function. Temperature and pH. As the temperature increases beyond optimal levels (37°C), the enzyme becomes denatured and no longer functions. As temperature decreases all chemical reactions slow down. Enzymes also function at optimal pH levels. The pH levels vary depending on which organ of the digestive tract is involved. The mouth has a pH of 7, therefore only enzymes suitable work, salivary amylase.
The stomach has a pH of 1. The enzyme pepsin is best suited here to digest proteins.
The small intestine has a pH of 6 - 8, lipase, trypsin, chymotrypsin, peptidase, pancreatic amylase, sucrase, maltase and lactase work.
Absorption in the Small Intestine:
Carbohydrates in the simplest form are monosaccharides. Monosaccharides are absorbed from the small intestine into the circulatory system. Monosaccharides are transported to the liver.
Any monosaccharides other than glucose are converted into glucose. The excess glucose is converted to glycogen, which can be stored in the liver and in smaller amounts in the muscles. When the body requires energy the glycogen is converted by into glucose and is used by the cells in cellular respiration.
Amino acids, the building blocks of proteins, are also absorbed into the circulatory system and transported to the liver. Some amino acids are converted into sugar and used as an energy source. Other amino acids are converted to a waste product called urea. Urea is removed from the body through the excretory system.
Lipids are broken down into glycerol groups and fatty acid chains. The sub units enter the cells of the small intestine and converted to triglycerides and coated with protein to make it water soluble. The triglyceride enters the lymphatic system and eventually to the circulatory system. The triglycerides are then broken down again and used as an energy source by the body.
b. The Large Intestine:
The waste materials move from the small intestine and moves into the large intestine. The large intestine is approximately 1.5 m. in length. Here the 90% of water is reabsorbed back into the blood stream. Anaerobic bacteria in the colon breaks down the waste material producing vitamins, folic acid, B vitamins, and vitamin K, which is transported to the blood stream. The rectum stores the fecal matter until eliminated by the anus.