Digestive system

Digestive system or alimentary canal is a long tube connecting the mouth to the anus. All the food ingested is either absorbed (in a liquid state) or egested (as a solid). This tube is not uniform all along its length. In some places it is quite narrow, being either straight or coiled, in other places it is wide and more capacious.

Composed mainly of: Mouth Oesophagus Stomach Small intestine Large intestine Rectum

Accessory organs: Salivary glands Teeth Tongue Liver Gall bladder Pancreas

First step in the process: The mouth

Mouth and swallowing Tongue Teeth Salivary glands Mechanical ingestion occurs when your teeth are used to break food apart by mastication and then swallowing it. There are many small salivary glands that secrete mucus continuously. However more important ones secrete mucus, salts and salivary amylase (an enzyme which helps to the hydrolysis of starch). These, can be stimulated by sight, smell, taste.

Teeth The shape of the teeth will determine their function and use. Incisors are used to help cut food. Canine are used to cut food and hold food in place. Premolar are used to tear and grind food. Molar are used for chewing and grinding food.

Tooth structure Teeth are divided into two parts. Crown and root. The crown ins the part that is seen. The root is the part that is not seen, it is inside the gum. The crown is covered by a very hard layer of enamel. This same layer is replaced by cement in the root. This cement enables the tooth to grip to its bony socket. Below the enamel there is a layer of dentine. Dentine is softer than enamel and below it there is a pulp cavity containing nerves and blood vessels.

Tooth decay Tooth decay begins with small holes in the enamel. Cavities are usually caused by bacteria that lie on the surface. They feed off the sugar and in return they excrete an acid that dissolved the calcium of the tooth enamel. After eating a sweets, the pH of the mouth drops down to acid levels (between 5.5 and 6). The enamel is under acid attack for about 10 minutes Brushing is important to avoid gum disease but less effective to control cavity formation.

Gum disease There is usually a layer of saliva and mucus over the teeth. This layer contains bacteria that live in the food residues in the mouth, building up a coating on the teeth called plaque. If the plaque is not removed, mineral salts of calcium and magnesium are deposited on it forming a layer of “tartar”. Brushing removes this tartar if not it spreads down the tooth into the narrow gap between the gum and enamel. It will cause an inflammation “gingivitis” (redness and bleeding of the gums).

Gum disease Left untreated it may progress to periodontitis. This is when the fibres that hold the tooth to the jaw are destroyed and the tooth becomes loose. So it must be taken out or it will fall out on its own.

Swallowing The tongue presses upwards and back against the roof of the mouth forcing the bolus to the back of the mouth. The soft palate closes the nasal cavity. The larynx cartilage round the top of the windpipe is pulled upwards so that the opening of the windpipe lies under the back of the tongue. The glottis partly closes The epiglottis, closes of access to the windpipe

Oesophagus: 25 cm long, straight line. Thick-walled muscular tube Glandular epithelium Upper part contains voluntary muscle Middle contains both voluntary and involuntary muscle Lower part involuntary muscle. Movement through the oesophagus is know as peristalsis The epithelium lines the the inside of the oesophagus.

Stomach: J shaped muscular bag, highly coiled. The lining of the muscular wall lies in folds when empty. It distends up to 5 dm3 when eaten a large meal. It is a place of partial digestion. A mixed meal can remain in the stomach up to 4 hrs. Epithelium lines the inside of the stomach, new cells are produced constantly to replace the ones that have been worn away by stomach acids. Mucus cells are in charge of producing mucus which is a slimy liquid that covers the epithelium cells to protect them from the acid conditions.

Functioning of the Stomach: