The ingestion and mechanical breakdown of food starts in the mouth. Your lips and tongue mix up the food with saliva and teeth break up the food into smaller pieces by grinding when you chew. This is called mastication. Saliva, which is released from the salivary glands, softens food making it easier to swallow. When we swallow bolus, which is what is formed from the chewed food, is forced down into the oesophagus as the trachea is now covered by a muscular piece of skin called the epiglottis. Smooth muscles in the oesophagus wall contract rhythmically to force the bolus downwards.
Mechanical breakdown continues in the stomach. Food is mixed and churned with gastric juices by muscular action and is retained there giving enzymes time to act. Chemical breakdown also continues in the stomach, which is a muscular sac under the diaphragm. Food stays in the stomach for as long as 40 minutes and fatty meals remain there longer. Specialised groups of cells in the gastric pits in the mucosa secrete gastric juices. Each type of cell here produces a specific secretion.
Oxyntic cells secrete a solution of hydrochloric acid, which brings the pH of the gastric juice down to between 2.0 and 3.0. This is the optimum pH for pepsin and rennin. Hydrochloric acid denatures protein and softens tough connective tissue in meat, it is also a strong bactericide.
Digesting protein is one of the main functions of the stomach. Pepsin breaks peptide bonds in the middles of the protein chain, which turns protein molecules into polypeptides. This process is complete when exopeptidase enzymes remove amino acids from the ends of the polypeptides.
Mucous cells secrete the mucous that protects the stomach lining from digestive action of its own secretion.
Food then moves from the stomach to the small intestine in the form of chyme. The small intestine is the major site of digestion and consists of the duodenum and the ileum. Enzymes and secretions flow into the duodenum from the pancreas and the liver. Pancreatic juice enters the duodenum via the pancreatic duct, this is an alkali solution and neutralises the strong stomach acid. This fluid contains several enzymes. Trypsin is an endopeptide enzyme that breaks down proteins into polypeptides. Carboxypeptidase is a polypeptide digesting enzyme that converts these into smaller peptides and amino acids. Pancreatic amylase completes the breakdown of starch to maltase. Pancreatic lipase continues the breakdown of fats into fatty acids and glycerol. Liver cells called hepatocytes produce bile, which is stored in the gall bladder, until it is needed at which point it reaches the duodenum via the bile duct. Bile contains water, bile salts, bile pigments and cholesterol. It emulsifies fat increasing its surface area, neutralises the chyme, stimulates peristalsis in the duodenum and the ileum and allows the excretion of cholesterol, bile pigments and fats.
Food absorption takes place mainly in the ileum and is done so through the gut lining. There are folds in the inner surface of the wall here where villi are present. Lining the villi are microvilli on the surface membrane of epithelial cells, which absorb amino acids, monosacharides, fatty acids, glycerol, water, vitamins, nucleic acid, ions and trace elements. Diffusion, facilitated absorption and active transport do this.
The ileum joins the large intestine at the caecum near the appendix. The colon absorbs minerals as ions, which is diffused into the bloodstream, and water, which has been passed into the gut during digestion, from the undigested food that reaches it and what is then left in the colon is faeces and dead cells. Cellulose is also passed out in faeces. The faeces is compacted here and passed out of the anus via the rectum by the act of defecation.
As each phase of the digestive system is reached food is prepared and made smaller for the next part, each part of the alimentary canal prepares the food more for digestion until this is complete and the process starts again. Each structure of the alimentary canal has a particular function that being the ingestion, mechanical breakdown, digestion, absorption or egestion of food. Each structure relates to the one before and the one after and a balanced diet is necessary for this efficiency. The mouth is a hard cavity containing teeth and the tongue, which efficiently breaks down food into a suitable state to travel through the oesophagus. The oesophagus, which is a rigid tube with a smooth surface, pushes the food to the stomach. The stomach, a muscular sac, aids in the mechanical breakdown of food. Cells in the gastric pits of the stomach aid in the chemical breakdown of food. This result is liquidation of the food in preparation for the small intestine. The small intestine is the longest section of the alimentary canal and here the most part of chemical digestion and absorption takes place. This moves the food to the large intestine. These are the final stages of digestion where the food is being prepared for egestion. The last minerals and water are taken here in this tube which is about 1.5 meters long. Finally the food is passed though the rectum and the anus, which are concerned with the compaction of faeces and the act of defecation.
