The digestive system is a group of organs that breakdown the chemical components of food, with digestive juices, into tiny nutrients which can be absorbed to generate energy for the body.
The Buccal Cavity
Food enters the mouth and is chewed by the teeth, turned over and mixed with saliva by the tongue. The sensations of smell and taste from the food sets up reflexes which stimulate the salivary glands.
The Salivary glands
These glands increase their output of secretions through three pairs of ducts into the oral cavity, and begin the process of digestion. Saliva lubricates the food enabling it to be swallowed and contains the enzyme ptyalin which serves to begin to break down starch.
Situated at the back of the nose and oral cavity receives the softened food mass or bolus by the tongue pushing it against the palate which initiates the swallowing action. At the same time a small flap called the epiglottis moves over the trachea to prevent any food particles getting into the windpipe. From the pharynx onwards the alimentary canal is a simple tube starting with the salivary glands.
The oesophagus travels through the neck and thorax, behind the trachea and in front of the aorta. The food is moved by rhythmical muscular contractions known as peristalsis (wave-like motions) caused by contractions in longitudinal and circular bands of muscle. Antiperistalsis, where the contractions travel upwards, is the reflex action of vomiting and is usually aided by the contraction of the abdominal muscles and diaphragm.
The stomach lies below the diaphragm and to the left of the liver. It is the widest part of the alimentary canal and acts as a reservoir for the food. Here the food is churned over and mixed with various hormones, enzymes including pepsinogen which begins the digestion of protein, hydrochloric acid, and other chemicals; all of which are also secreted further down the digestive tract. The stomach has an average capacity of 1 litre, varies in shape, and is capable of considerable distension. When expanding this sends stimuli to the hypothalamus which is the part of the brain and nervous system controlling hunger and the desire to eat. The wall of the stomach is impermeable to most substances, although does absorb some water, electrolytes, certain drugs, and alcohol. At regular intervals a circular muscle at the lower end of the stomach, the pylorus opens allowing small amounts of food, now known as chyme to enter the small intestine.
The small intestine measures about 7m in an average adult and consists of the duodenum, jejunum, and ileum. Both the bile and pancreatic ducts open into the duodenum together. The small intestine, because of its structure, provides a vast lining through which further absorption takes place. Digestion in the small intestine relies on its own secretions plus those from the pancreas, liver, and gall bladder.
The Pancreas is connected to the duodenum via two ducts and has two main functions:
- To produce enzymes to aid the process of digestion
- To release insulin directly into the blood stream for the purpose of controlling blood sugar levels
Enzymes suspended in the very alkaline pancreatic juices include amylase for breaking down starch into sugar, and lipase which, when activated by bile salts, helps to break down fat. The hormone insulin is produced by specialised cells, the islets of Langerhans, and plays an important role in controlling the level of sugar in the blood and how much is allowed to pass to the cells.
The liver, which acts as a large reservoir and filter for blood, occupies the upper right portion of abdomen and has several important functions:
- Secretion of bile to the gall bladder
- Carbohydrate, protein and fat metabolism
- The storage of glycogen ready for conversion into glucose when energy is required.
- Storage of vitamins
- Phagocytosis - ingestion of worn out red and white blood cells, and some bacteria
The Gall Bladder
The gall bladder stores and concentrates bile which emulsifies fats making them easier to break down by the pancreatic juices.
The Large Intestine
The large intestine averages about 1.5m long and comprises the caecum, appendix, colon, and rectum. After food is passed into the caecum a reflex action in response to the pressure causes the contraction of the ileo-colic valve preventing any food returning to the ileum. Here most of the water is absorbed, much of which was not ingested, but secreted by digestive glands further up the digestive tract. The colon is divided into the ascending, transverse and descending colons, before reaching the anal canal where the indigestible foods are expelled from the body.