Saliva would initially also lubricates the food, making the food slip-and-slide down the ten-inch long oesophagus, the tube that connects the mouth and the stomach. This would initially be about 8m long in adults enabling absorption to take place slowly along the gut. Along the Oesophagus there are circular as well as longitudinal muscles present. Together when they contract and relax they direct any food along in one direction, towards the Stomach. This is called Peristalsis.
When it enters the stomach, the mush from the mouth gets mixed. The stomach is your body's mechanical and chemical food processor. It's a pouch composed of sheets of muscle that encircle the stomach in different directions. When they contract, the stomach can mix the mush this way and that. The stomach muscles twist and churn the food like you would knead bread dough. The lining of the stomach secretes gastric juices, including hydrochloric acid, which dissolves the food, a protein-splitting enzyme called pepsin, and a fat-digesting enzyme called lipase. The Protease in the Stomach wall breaks down the Protein into Peptides. Where also the Lipase breaks down Fats (Lipids) into Fatty acids and glycerol. Like fruits or vegetables pureed in a blender, the food is churned and mixed with the digestive juices until it resembles thick soup. This glob is called chyme. This only occur when the stomach changes the state of food from a Solid into a Liquid.
Hydrochloric acid is strong enough to eat through meat (Chicken) and potential to kill most of the harmful bacteria that may be in food. So the stomach not only digests, it disinfects. It is the body's food processor and the body's food purifier. Yet it does not destroy all the bacteria we ingest. Some of the bacteria that are able to survive the harsh conditions in the stomach eventually take up residence in the intestine where, in return for all they can eat, fulfil an important role in the health and digestive process.
After the Chime is formed, it seeps out of the Stomach in to the Small Intestine. The first part of the Small intestine is known as the duodenum. Water and oil are immiscible and therefore stay in the stomach for a few hours longer. However the Chime (food mush), entering the duodenum would be acidic. So as the Small intestine does not prefer this secretes its own ant-acid, known as bicarbonate. Travelling along the Small Intestine, the now neutralised food is squirted with secretions of digestive juices to further the breaking down of Proteins, Carbohydrates, and Fats (Lipids).
Each nutrient requires specific intestinal juices that work on its molecules in particular ways. Trypsin and peptidases are enzymes that disassemble the protein necklaces into individual amino acids, which enter the bloodstream through individual intestinal lining specially marked for "amino acid entry only". Carbohydrates (nearly digested bread, and Museli) are disassembled into individual sugar molecules by the enzymes lactase, sucrose, maltase, and pancreatic amylase, and the individual sugar molecules enter the bloodstream through the intestinal lining of "sugar only." Around the duodenum blood vessels quickly transfers the nutrients throughout the body where they can be burned for energy or reassembled into tissues.
After a few hours the fats get a squirt of bile from the gallbladder (located in the Liver) and some lipase from the pancreas, after being released from the stomach. The bile emulsifies the fat. Bile does not really dissolve the fat, but rather breaks it down into tiny particles, which are then more easily broken down by the intestinal enzyme lipase for absorption into the bloodstream.
When these food molecules have mostly been broken down they can be absorbed in the ileum. This has a very large surface area partly to do with folds, but also because it is completely covered with villi, that speed up the rate of absorption, within the Small intestine. The total surface area of the Small intestinal absorption area is therefore 300m². And even villi have thousands of microvillus located on their surface! Each villi contains a network of capillaries that, digested food molecules can be absorbed into the blood from. However tiny fat droplets enter a tube in the middle of the Villus known as a Lacteal. These form part of the body's Lymphatic System, which transports a liquid called Lymph. This eventually drains into the blood system too.
The Blood vessels from the ileum join up to form a large blood vessel called the Hepatic Portal Vein, which leads to the Liver. The Liver acts as a food processing system that breaks some molecules down, but stores others up. An example of this is Glucose that is stored up as glycogen, and can be converted back only when the body would require them. Digested food molecules are then distributed around the body through the blood. Soluble food molecules are absorbed by cells that make up tissue, and are used to build up new parts of them. This is called assimilation. Or react with Oxygen to produce Energy.
The waste material mainly consists of Cellulose (fibre), Bacteria, dead cells, and water. The colon which is the first part of the Large intestine absorbs most of the remaining water from the contents, leaving a semi-solid waste, called faeces. This is stored in the rectum before being expelled through your anus, when you go to the toilet.
