The main functions of the digestive system;
The organs of the digestive system prepare food for absorption and for use by the millions of cells in the body. Most food when ingested cannot be absorbed and reach the cells because it cannot pass through the intestinal mucosa into the bloodstream, not that it could it be used by the cells even if it were to reach them.
Food must be modified as to both chemical composition and physical state so that nutrients can be absorbed and utilised by cells of the body.
The organs of the digestive system comprise the system by which these complex changes in food occur.
Part of the digestive system – the large intestine serves as an organ of elimination. Ingested food that cannot be put to an absorbable form becomes waste material (faeces) and is finally eliminated by the body.
The process of altering the physical and chemical composition of food so that it can be absorbed and utilised by cells is the process of digestion. It requires input by both the endocrine system and exocrine secretions and the controlled movement of ingested food materials through the tract so that absorption can occur.
Location of the Salivary Glands
We can see from the above illustration that the parotid duct lies between the skin and the masseter muscle. It feeds the saliva produced in the parotid salivary gland into the mouth. The parotid duct and gland are both situated in front of and below the ear. These glands are the largest of the salivary glands and are pyramidal in shape. They secrete a watery or serous type of saliva that contains digestive enzymes but not mucous.
These are situated just below the mandibular angle. They are mixed or compound glands that contain both serous enzymes and mucous producing elements.
These are the smallest of the glands and lie under the mucous membrane of the mouth. they produce only a mucous type of saliva.
The Structure of the Oral Cavity
The mouth is called the oral cavity and this comprises; lips, cheeks, hard and soft palates and the tongue.
Functions of the Oral Cavity
The lips form the anterior boundary of the oral cavity and when closed they prevent ingestion.
Cheeks are lined by the mucous membrane. the tongue, cheeks and lips all play an important role in keeping the food material between the cutting or grinding surfaces of the teeth when chewing. they also serve to mix the food particles with saliva in preparation for swallowing.
The soft palate closes off the nasal passages while swallowing and the hard palate forms a hard arch in the roof of the mouth that contributes to mastication.
The tongue has intrinsic muscles and is covered by a mucous membrane.This muscle allows for extreme maneuverability and the placement of food between teeth during mastication.
The Main Functions of the Stomach
- Serves as a reservoir, storing food until it can be partially ingested and moved forward along the gastro-intestinal tract.
- Secretes gastric juice containing hydrochloric acid and enzymes (e.g.pepsin) to aid in the digestion of food.
- Through contractions of it’s muscular coat, it churns the food, breaking it down into small particles and mixing it with gastric juice. In time it moves gastric contents through to the duodenum.
- It secretes intrinsic factor. Parietal cells (epithelial cells of the stomach) secrete hydrochloric acid and are thought to produce intrinsic factor. This intrinsic factor binds to Vitamin B12 and escorts it through the stomach, acting to protect the B12 from the digestive juices until it reaches the small intestine. Once vitamin B12 reaches the small intestine the B12 molecules can be absorbed into the internal environment, but only if bound to intrinsic factor. (Those taking acid indigestion tablets long term may struggle to maintain B12 absorption levels as the stomach acid is required to separate the vitamin b12 from the protein to which it is attached). B12 is used in the formation of myelin sheaths and in the formation of red blood cells in the bone marrow. A lack of B12 (like B9), can lead to pernicious anemia.
- A limited amount of absorption can take place in the stomach; some alcohol and water, some drugs, some short chain fatty acids such as butter or milk fat.
- The stomach produces a hormone called gastrin which helps to regulate digestive functions. Products of protein digestion in foods that have reached the pyloric part of the stomach, stimulate its mucosa to secrete gastrin into the blood in the stomach capillaries. When gastrin circulates to the gastric glands, gastrin will greatly accelerate their secretion of gastric juice– ensuring that when there is food in the stomach, there are enough enzymes to digest it.
- The stomach helps to protect the body by destroying pathogenic bacteria swallowed with food or with mucous from the respiratory tract. Hydrochloric acid in the stomach achieves this.
- The stomach releases pepsin to aid in the digestion of proteins.
The Division of the Small Intestine
The small intestine measures 2.5 cm (1 inch) in diameter and is 6m long. It’s coiled loops fill most of the abdominal cavity. The walls of the small intestine are covered with folds of mucosa, which in turn are covered in villi. Each villus is covered with epithelial cells, that offering a huge area of absorption.
There are three divisions; Duodenum. Jejunum and the Ileum.
The duodenum is a ‘C’ shape and is around 25cm long. It nestles around the pancreas and receives food from the pyloric end of the stomach. As it turn forward and downward it joins the;
Jejunum; which is around 2.5 m long. Some nutrient absorption takes place here.This moves into the;
Ileum which is some 3.5m long and where much of the absorption of nutrients takes place.