Digestive System Parts and Digestive Glands
Introduction to the Basics of Digestive System
Food is required for getting energy. Food contains carbohydrates, proteins and fats. They are present in complex forms and do not give energy directly. They need to be converted into simpler forms like glucose, amino acids and fatty acids which give energy. This conversion is done by the digestive enzymes which are produced by various glands of the digestive system.
The digestive system helps in the digestion of food. The alimentary canal is wide in some places and narrows in some places. The alimentary canal is lined internally by the mucous membrane.
Parts of the Digestive System:
The digestive system consists of the alimentary canal and the digestive glands. The Alimentary canal contains many parts. The important parts of the alimentary canal are the mouth, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and anus.
Mouth:
The mouth is the first part (front opening) of the alimentary canal and begins with the upper and lower lips. The upper lip and lower lip are protective in function. The mouth opens into a spacious mouth cavity or oral cavity and is bounded by upper and lower jaws.
There is a thick muscular tongue with taste buds on the floor of the mouth cavity. The function of the tongue is to chew food and to feel different tastes with the help of taste buds.
Thirty-two teeth are present in the mouth cavity. Four types of teeth are present in the oral cavity namely incisors, canines, molars and premolars which are arranged in order like incisors (2/2), canines (1/1), premolars (2/2) and molars (3/3). in the upper and lower jaws. This arrangement of teeth is called a dental formula. The teeth are arranged in the sockets of the upper and lower jaws on each side. Sixteen teeth are present on the upper jaw and sixteen are on the lower jaw.
The function of the incisors is to cut the food into pieces. The canines are almost round in shape in human beings. There are two premolars next to the canines and the last three are molars whose function is to grind the food. The last molar teeth are called wisdom teeth and come out of the jaws between the age of seventeen and twenty-five.
Structure of the tooth:
The part of the tooth that is enclosed in the jaw bone is known as the root and which is exposed is known as the crown and is covered by a shiny enamel. The tooth is made up of a substance called dentine and it is harder than bone. There is a cavity inside the tooth that is filled with blood vessels and nerve fibres and is known as the pulp cavity.
The teeth which are developed first in children are known as milk teeth. They are dropped at the age of eight or nine years and a new set of teeth called permanent teeth are developed. Children will have only twenty teeth.
Salivary Glands:
There are three pairs of salivary glands in the oral cavity. These salivary glands secrete saliva into the buccal cavity. The three types of salivary glands are parotid glands, submandibular glands and sublingual glands. Parotid glands are the largest salivary glands. Submandibular glands are located beneath the lower jaws and sublingual glands are located below the tongue. The saliva contains the enzyme ‘ptyalin’ which digests the food partly in the buccal cavity.
Then the food is pushed into the pharynx.
Pharynx
Behind the mouth cavity lies the funnel-like structure known as the pharynx. Pharynx is a wide and muscular chamber that opens into the oesophagus and helps in swallowing. The partly digested food from the pharynx enters the oesophagus with the help of the tongue.
Oesophagus
Oesophagus is a long and narrow muscular structure. Mucous is secreted by the glands which are present in the walls of the oesophagus that helps in the easy passage of food. It passes through the neck, and chest cavity and enters the abdomen where it opens into the stomach. On the top of the oesophagus, a flap-like tissue named epiglottis is present. It prevents food from entering the trachea during swallowing.
Stomach
The stomach is located on the left side of the abdomen below the diaphragm. The diaphragm is a thin layer that separates the body cavity into an upper chest cavity and a lower abdominal cavity. The stomach is a sac-like structure consisting of numerous tiny glands called gastric glands that secrete gastric juice. The stomach opens into the small intestine.
It is a bag-like structure present beneath the oesophagus. The inner walls of the stomach are folded and churn the food and make the food smooth. This food is called chyme.
Glands in Stomach:
The glands which are present in the stomach are called gastric glands which secrete gastric juice. Enzymes are present in the gastric juice and parietal cells in the stomach secrete hydrochloric acid which kills the bacteria present in the food. Proteins are digested and converted to simpler forms like peptones and peptides. This food then enters the small intestine.
Small intestine
The small intestine is about 7 metres long and the first part of it is uncoiled and is called the duodenum and lies parallel to the stomach. The coiled second part is known as the ileum. The opening of the stomach into the duodenum is guarded by the pyloric sphincter which acts as a valve and allows only a little quantities of food at a time from the stomach into the duodenum. There are finger-like folds in the wall of the small intestine which help to increase the surface area for absorption. The ileum opens into the large intestine.
The small intestine is a thin muscular structure and starts with the duodenum. Duodenum is a ‘U’ shaped structure. The next part of the small intestine is the ileum. The ducts from the liver and pancreas open into the loop of the duodenum. Bile juice is secreted by the liver and pancreatic juice is secreted from the pancreas. B(beta) cells of islets of Langerhans of the pancreas secrete insulin and alpha cells secrete glucagon which maintains sugar levels in the blood. The enzymes present in these intestinal juices digest most of the food. The digested food is absorbed mostly from the small intestine into the blood circulation and undigested food reaches the large intestine. The food which is present in the small intestine is called a ‘chyle’.
Large intestine
The large intestine is about 1.5 metres long. The first part of the large intestine is known as the caecum and the opening of the ileum into the caecum is guarded by a sphincter which acts as a valve. There is a finger-like appendix at the base of the caecum. The appendix is a vestigial organ in humans and is reduced and functionless. Caecum is well developed in grazing animals like rabbits and horses and helps in digesting cellulose that is present in the food. The second part of the large intestine is the colon and has an ascending limb, a transverse limb and a descending limb.
Digestion does not take place in the large intestine as it does not contain the digestive glands. Water is absorbed from the large intestine. Undigested food is excreted as faecal matter.
Rectum and Anus
Rectum is the terminal part of the colon and opens to the outside through the anus. Undigested food is sent out through the anus.
Digestive glands:
There are two digestive glands namely the liver and pancreas that secrete juices for the digestion of food.
Liver
The liver is the largest gland in the body and is located on the right side of the upper part of the abdominal cavity. It is reddish brown in colour and weighs about 1.5 kg. The liver has the right and left lobes. The structural and functional units of the liver are hepatic cells. The liver secretes bile juice and it is stored through the bile duct in a sac called as gall bladder. From the gall bladder bile is released into the duodenum.
Pancreas
Pancreas is located below the stomach and it is a leaf-shaped gland with several lobes. Pancreas secretes pancreatic juice that contains digestive enzymes. A specialised group of cells called islets of Langerhans are present in the pancreas that secretes insulin which regulates blood sugar levels. The deficiency of insulin leads to diabetes and excess sugar goes out of the body through urine.
The ducts of the liver and pancreas unite into a common duct and open into the duodenum.
Basic Introduction to The Islets of Langerhans
The pancreas contains many hundreds and thousands of clusters of cells which are randomly distributed and called islets of Langerhans. Each of the little cell clumps contains millions of cells, which is a microscopic endocrine gland.
Hormones secreted by islets of Langerhans:
These glands secrete the hormone insulin and if it is not functioning properly it leads to diabetes mellitus. The islets of Langerhans secrete insulin and also glucagon. Both these hormones are essential for maintaining the glucose in the bloodstream and the rate at which it is to be used by the body. Glucose is essential to get energy for life and living processes. When the secretion of these hormones gets disturbed, the cells’ ability to absorb and use sugar is restricted. The sugar becomes unutilized and it passes out of the body in urine. Now insulin has been manufactured and is available for diabetics. It is to be taken in regulated doses and the person can lead a normal life.