The mouth chews the food using the teeth to tear, chew and grind the food. The tongue has skeletal muscles (involuntary muscles) that move the food around the mouth for efficient mechanical digestion. Salivary glands beneath and in the back of the tongue secrete saliva which allows for easier swallowing to begin chemical digestion. Saliva is 95.5% water and the remaining 4.5% is a complex mixture of ions, glycoproteins, enzymes, growth factors and waste products.
Saliva contains an enzyme called salivary amylase. This is a primary enzyme in saliva. The amylase starts to breakdown starches in the food. Starches are long chains of sugars that are attached to each other, amylase breaks the bonds along the chain to release maltose sugar molecules.
At a lower temperature, enzymes salivary is deactivated and at a higher temperature, the enzyme is denatured. Therefore, more time is taken by an enzyme to digest starch at a lower or higher temperature. The optimum pH for enzymatic activity of salivary amylase ranges from pH 6 to pH 7. If above or below this range the reaction rate reduces as the enzymes get denatured. At pH 6.8 the enzyme salivary amylase is most active. The stomach has a high level of acidity which causes the salivary amylase to denature and change its shape. Salivary amylase does not function once it enters the stomach. The oesophagus connects the pharynx to the stomach. Involuntary contractions called peristalsis (two layers of smooth muscle, the outer longitudinal and the inner circular) pushes food through a ring of soft muscles that close of the opening of the body called the sphincter towards the lower oesophageal sphincter. All sphincters located in the digestive tract help move along the digested material in one direction.
Once the food has gone through the chemical and mechanical process of the mouth the pharynx carries food to the oesophagus and the larynx. As food is swallowed the epiglottis closes over the trachea to prevent food entering the windpipe that may cause choking.
The stomach is a rounded, hollow organ which is located just inferior to the diaphragm in the left side of the abdominal cavity. It is located between the oesophagus and duodenum, it is a rough crescent-shaped enlargement of the gastrointestinal tract. The inner layer has wrinkles known as gastric folds. The gastric folds allow the stomach to stretch, to accommodate large amounts of food and help to grip and move food during digestion.
When the food reaches the stomach, food undergoes chemical and mechanical digestion. This is when peristaltic contractions (mechanical digestion) start to churn the food. This mixes with strong gastric digestive juices that the stomach lining cells secrete (chemical digestion). There are three layers of smooth muscle that line the stomach wall, arranged in longitudinal, circular and oblique rows. The muscles allow for the stomach to churn and squeeze the food during the mechanical digestion. When the food is being mixed with the gastric juices to produce chyme, enzymes that are present in the gastric juice chemically digest the large molecules into smaller subunits. The gastric lipase splits triglyceride fats into fatty acids and diglycerides. The pepsin breaks the proteins into smaller amino acids. The chemical digestion that begun in the stomach will not be complete until the chyme reaches the intestine, but prepares hard to digest proteins and fats for further digestion. The stomach starts the chemical digestion of protein. The stomach enzyme pepsin starts to break down most of the protein in the food secretion from the stomach lining that consists of around two litres of hydrochloric acid (HCI), pepsin and other fluids that make up gastric digestive fluids each day. This fluid is extremely acidic, which helps to kill of any bacteria and other pathogens that may have been digested. The stomach lining produces a thick mucus to keep the lining from being damaged by the acid. Failure to produce enough mucus, or if too much acid is produced it may lead to peptic ulcers form. The food remains in the stomach for approximately 3 to 4 hours, then moves through another sphincter muscle which passes into the small intestine.
After passing through the stomach and entering the small intestine where the nutritious liquid and the small solid remnants are exposed to more chemical digestion via the enzymes from the pancreas and bile from the liver as peristalsis. The small intestine is nearly 7 meters in length and is folded and curled around in a small area in the abdominal cavity. The surface of the small intestine is covered with projections called villi. A single villus consists of one vein, one artery and a central lacteal, this is a type of capillary that transports lymphatic fluid. The villus is supported by a strand of muscle and connective tissue. There are two types of cells which coat the villi, the goblet cells, these secrete mucus and epithelial cells that are responsible for absorption. The epithelial cells are coated with even smaller projections called microvilli, which increase further absorptive capacity in the small intestine. The microvilli are covered in digestive enzymes, this break down carbohydrates and protein. When food is in the stomach it is broken down and forms a substance called chyme, this is a thick semifluid mass of partially digested food and digestive secretions which is formed in the stomach and intestine during digestion. When it passes into the small intestine it meets the villi. Carbohydrates and protein in the chyme enter the bloodstream via the vein and the artery. The fat is then absorbed by the lacteal into the lymphatic system and eventually empties into the bloodstream. The process called active transport, specialises intestinal cells of the villi allow the passage of B-12, calcium and iron into the blood stream. There are three distinct parts to the small intestine, the duodenum, jejunum and the ileum. Around nine litres of fluid each day enters the duodenum. Majority of the chemical digestion takes place in the duodenum by chemicals that are secreted by the liver, pancreas and small intestine. The other two sections are the jejunum and the ileum, they absorb food molecules by way of the villi directional into the blood stream. It’s at this point the body finally receives the nutrients from the food via the blood stream. The process is highly dependent on activity through a large network of nerves, hormones and muscles. Carbohydrate digestion begins in the mouth with the enzymes in saliva, then continues within the duodenum with the amylase enzyme, that secretes into the duodenum from the pancreas. The intestinal cells use the sucrase, maltase and lactase enzymes. The fat breaks down with the help of lipase enzymes, which secretes from the pancreas. The trypsin enzyme from the pancreas and intestinal peptidase enzymes continue the process. Carbohydrates and fats finish digestion in the second section of the small intestine, while most of the absorption of carbohydrates and fats take place in the duodenum and the jejunum. The main purpose of the jejunum is to absorb the nutrients; therefore, it has the villi and microvilli structure. It also absorbs most proteins as well.
The main function of the ileum is absorption, especially of bile acids, fat soluble vitamins and vitamin B-12. Bile, made in the liver, helps to break down fats. When it absorbs bile acids, it sends them through the bloodstream to the liver to become part of the bile. Fat soluble vitamins that dissolve in fat are vitamin D, A, K and E.
This is that last part of the digestive tract.
It is a “holding place” for the undigested materials. Defecation is the digestive process final stage, known as faeces (undigested waste products) they are carried to the rectum through peristalsis and then eliminated through the anus. It takes around 36 hours for the stool to get through the colon. The stool is made up of mostly food debris and bacteria that live in the colon. The bacteria perform several useful functions, such as synthesising various vitamin, processing the waste products of food particles and protecting against harmful bacteria. When the descending colon is full, it empties its contents into the rectum to begin the process of elimination. There are six parts to the large intestine. The cecum, ascending colon, transverse colon, descending colon, sigmoid, colon and the rectum. The large intestine receives the materials that have been “left over” from the chemical digestion that are nutrient free.
The large intestine job is to remove the liquid from the non-nutritive food waste until it becomes a solid and ready for excretion.
The water is quickly removed from the materials through the villi and is returned to the blood stream.