Overview
The system which breaks down the food into useable form i.e., energy or ATP is digestive system. In humans, it consists of mouth, oesophagus, stomach, glands associated with digestive system like salivary gland, liver, pancreas, small and large intestine. The mouth and oral cavity help in selection of food, grinding or mastication and lubrication. As a result of mastication, the softened, partly digested, slimy food mass is rolled into small oval lump called bolus. The waves of contraction of circular and longitudinal muscles preceded by wave of relaxation thus squeezing the food down along the canal are called peristalsis. After entering stomach, gastric juice is released for digestion. Stomach wall is composed of three layers. After digesting in stomach food is converted into semi-solid mass called chyme. It further digested in small intestine. And absorption of food takes place in ileum which has numerous fingers like outgrowths called villi. In the end, there is large intestine which is composed of caecum, colon and rectum.
Digestive System of Humans
The digestive system of a human includes a long-coiled tube that extends from the mouth to the rectum. The main parts in the direction of passage of food, are the oral or buccal cavity, oesophagus, stomach, small intestine (duodenum, jejunum and ileum), large intestine (ascending colon, transverse colon, descending colon, caecum and rectum).
Connected with the different areas are the glands, especially salivary glands, liver and pancreas. There are 3 sites of digestion in the gastrointestinal system of human – mouth, stomach and small intestine.
Food Digestion in Oral Cavity
There are a number of functions of the mouth, the most apparent being the (a) selection of food, (b) grinding or mastication (c) lubrication and (d) digestion.
Selection of food
When food gets in the oral cavity (the cavity bounded by palate, tongue, teeth and cheeks) it is tasted, smelled and felt. If the taste or smell is unpleasant or if hard items like bone or dirt are present in the food, it is rejected. The oral cavity is helped in choice by the senses of odour, taste and sight.
The tongue being sensory and muscular organ plays the most crucial role in the selection of food through its taste.
Grinding or mastication
After selection, the food is ground by means of molar teeth into smaller sized pieces. This is useful as
- (a) the oesophagus allows fairly small pieces to pass through and
- (b) small pieces have a lot more surface area for the enzyme to attack.
Lubrication and food digestion
These are the primary functions of the oral cavity accomplished by saliva. Saliva is secreted by three pairs of salivary glands particularly sublingual glands situated below the tongue; submaxillary glands behind the jaws and parotid glands in front of the ears. Saliva produced by these glands contains three essential ingredients.
- i) Water and mucus
- ii) Sodium bicarbonate and some other salts,
- iii) Carbohydrates absorbing and digesting enzymes, amylase or ptyalin.
Water and mucus together make a slimy liquid which serves to moisten and lubricate the food so that it can be chewed efficaciously and passed through the oesophagus efficiently.
Sodium bicarbonate and some other salts are a little antiseptic however their primary function is to stabilize the pH of food. Fresh saliva is alkaline with a pH nearly 8, quickly loses CO2 and gets to pH 6. Ptyalin is a carb – digesting enzyme, which digests starch and glycogen to maltose.
Swallowing
As a result of mastication, the softened, partially digested, slimy food mass is rolled into a small oval lump called a bolus, which is then pushed to the back of the mouth by the action of tongue and muscles of the pharynx which ensure that the food does not go into the windpipe. Following are the events which happen during swallowing:
- i) the tongue moves upwards and backwards against the roofing system of the mouth, forcing the bolus to the back of the mouth cavity.
- ii) The backward motion of the tongue presses the soft taste buds up and closes the nasal opening at the back. At the same time, the tongue requires the epiglottis (a flap of cartilage) into basically horizontal position hence closing the opening of the windpipe (the glottis).
- iii) The throat, cartilage round the top of the windpipe moves upward under the back of the tongue.
- iv) The glottis is partly closed by the contraction of a ring of muscle.
- v) The food does not enter the partly open glottis, because the epiglottis diverts the food mass to one side of the opening and safely down the oesophagus. The beginning of the swallowing action is voluntary, once the food reaches the back of the mouth, swallowing ends up being automatic.
The food is then forced down the oesophagus by peristalsis.
Peristalsis
These are particular movements of the gastrointestinal system by which food is moved along the cavity of the canal. It consists of the wave of contraction of the circular and longitudinal muscles preceded by the wave of relaxation therefore squeezing the food down along the canal. Peristalsis starts simply behind the mass of food from the buccal cavity along the oesophagus to the stomach and after that along the whole alimentary canal.
Antiperistalsis
Sometimes, the movements are reversed, with the resulting food might be passed from the intestine back into the stomach and even into the mouth. This movement is called antiperistalsis, causing vomiting.
Hunger Pang
Hunger contractions are peristaltic contractions which are increased by low blood sugar levels and are sufficiently strong to develop an uneasy experience typically called a “hunger pang”. Hunger pangs usually begin 12 to 24 hours after the previous meal or in less time for some people.