Solid food when taken into the mouth is initially broken down mechanically by the teeth, a process known as mastication. Our dentition has evolved over millions of years to allow us to eat a combination of different foods, both plant and animal based. The food is mixed with saliva which is designed to lubricate and aid swallowing otherwise known as deglutition. Saliva in addition to lubricating contains an enzyme called amylase which can break down starch into its constituent sugar namely glucose. This explains why if you chew bread for long enough it will start to taste sweet.
Swallowing involves pushing the food backwards using the tongue into the pharynx which then joins the oesophagus or food pipe. The oesophagus traverses the chest cavity and as soon as it pierces the diaphragm which is the muscular sheet between the chest and the abdomen it opens up into the stomach. The stomach is a specific organ within the abdomen.
The food remains in the stomach for a variable amount of time depending upon the constituents of the food. A churning action resulting from waves of contraction of the stomach wall, otherwise known as peristalsis, once again aids the breaking up of food material.
The pylorus is the name given to the outlet of the stomach where it joins the first part of the small bowel otherwise known as the duodenum. A meal that is particularly fatty can be retained within the stomach for a couple of hours and this is achieved by closure of the pylorus. This delay in gastric emptying explains why a meal, in particular a fatty meal, can delay the onset of the effects of alcohol. Alcohol is only absorbed from the gut into the blood stream in the small bowel and if taken on an empty stomach then it tends to travel straight through without being delayed by the stomach into the small bowel where its effects are felt almost immediately. Taking alcohol with a fatty meal will delay the onset of its effects.
The first part of the small bowel or small intestine is called the duodenum. Once the food has entered the duodenum here it mixes with the first set of digestive enzymes from the pancreas. The cells of the pancreas produce numerous enzymes and these are collected within the pancreatic duct which joins the gut approximately 2/3 of the way down the duodenum which is about 20cm long. The pancreatic duct shares a common opening with the common bile duct and this is where bile comes in from the liver.
The liver has many important functions and one of them is to make bile which is the green fluid that is essential for digesting fat. The bile is made by the cells of the liver and is collected in a series of tubes within the liver called the bile ducts which then run down into one main tube called the common bile duct.
Off the common bile duct is a side branch called the cystic duct and at the end of the cystic duct is the gallbladder. The gallbladder acts as a storage organ for the bile although we are not quite sure why this storage facility exists. Any bile that is stored within the gallbladder is pushed out on a regular basis by contractions of the gallbladder itself.
Once the food has mixed with the digestive enzymes from the pancreas and the bile from the liver it then passes on down through the next part of the small bowel otherwise known as the jejunum where enzymatic digestion continues. It then passes on to the last part of the small bowel otherwise known as the ileum.
The products of this breakdown process, namely simple sugars, amino acids and fatty acids, are absorbed through the gut wall into the blood stream to be utilised by the body. The breakdown of all digestible food should have been completed by the time it reaches the last part of the small bowel, otherwise known as the terminal ileum.
The large bowel is made up of the colon and the rectum collectively. The first part of the colon is called the caecum and the point at which the terminal ileum joins the cecum is called the ileocecal junction. There is a valve like mechanism at this point which largely prevents reflux of the contents of the colon back into the terminal ileum. The appendix which is an integral part of the bowel is also joined to the caecum just beyond the ileocecal valve.
The ileocecal junction is positioned low down in the abdomen on the right-hand side. The colon consists of numerous parts; firstly, the caecum as previously discussed which then becomes the ascending colon (or right colon) which travels up the right side of the abdomen to just below the rib cage close to the liver. It then becomes the transverse colon which travels transversely across the upper part of the abdomen to a point just below the ribs on the left side and very close to the spleen. It then joins the descending colon or the left colon which travels down the left side of the abdomen at which point it becomes the sigmoid colon. This part of the colon is so called because it is S shaped. It then passed down into the pelvis where it joins the rectum, which is the last part of the gut.
As previously discussed, by the time the food has reached the last part of the small bowel or terminal ileum and the first part of the large bowel or caecum, all of the goodness should have been extracted, so all that is left is the non-digestible element of food, in particular fibre.
When the waste or faeces is in the caecum it is quite liquid but as it travels around the colon it thickens up because water gets extracted from the faeces through the bowel wall back into the blood stream. The colon therefore simply acts as a conduit for the faeces and in the process, it thickens up on its way through the colon and down into the rectum.
Colon transit time is the time taken for the faeces to get from the cecum down into the rectum and for most people with a normal functioning gut this is between 12 and 24 hours.
The function of the rectum is to act as a storage organ for the faeces. As faeces are coming through the colon fairly continuously, to stop us going to the toilet all the time, the rectum stores up the faeces and typically once a day we get the sensation that the rectum is full at which point we proceed with defaecation. The anus and its associated sphincter complex give us control of the faeces.