Introduction
Living organisms require energy to sustain life. An alimentary canal and digestive glands in the body help provide energy by ingestion and digestion of carbohydrates, proteins, fats and vitamins.
After absorption, the indigestible material is egested out of the body. During digestion, complex food substances are turned into simple absorbable forms by secretion of different enzymes from the digestive juices in the digestive glands. This entire digestion process by the alimentary canal with the associated digestive tract is called the digestive system.
Alimentary Canal
Food passes through the alimentary canal, which is divided into various parts: Buccal Cavity: Teeth, Tongue & Saliva
The process of taking the food into the body is ingestion. It happens through the mouth, which leads to the buccal cavity. The buccal cavity comprises a muscular tongue and teeth. Most mammals, including human beings, have two sets of teeth during their entire lifetime. A set of temporary milk teeth are also known as deciduous teeth. Permanent or adult teeth then replace the temporary teeth. Adults have 32 permanent teeth comprising four different types;
- Incisors
- Canine
- Premolars
- Molars
These are arranged in each half of the lower and upper jaw as I, C, PM, M. In humans, they are represented using a dental formula- 2123/2123 (2-I, 1-C, 2-PM, 3-M). The hard surface of the teeth is made of enamel which helps in chewing (mastication) of the food. Enamel is also the hardest substance in the human body, containing the highest percentage of minerals.
The tongue is the muscular and fleshy organ attached to the floor of the buccal cavity. It helps in chewing and swallowing food by mixing it with the saliva. In addition, there are small projections situated on the upper surface of the tongue known as papillae. Some of these projections bear the taste buds.
The mouth has salivary glands that secrete saliva, which helps in breaking down the starch into sugars. The saliva secreted into the oral cavity, or the buccal cavity, has electrolytes and enzymes that break down the carbohydrates.
Food Pipe or Oesophagus
The food swallowed goes to the food pipe. A food pipe or oesophagus is a long, thin tube that extends further back, going through the neck and thorax. Thorax is the part between the neck and the abdomen; and the diaphragm leading to the stomach. The mucus in saliva helps lubricate the chewed-up food particles into the bolus, which passes into the pharynx. After the pharynx, the bolus is led into the oesophagus by swallowing. The bolus passes down by successive muscular contractions in the oesophagus, known as peristalsis.
Stomach
The stomach’s inner lining secretes digestive juices, hydrochloric acid, and mucous. The mucous protects the lining of the stomach, and the digestive juices help in the breakdown of the proteins into simpler substances. The stomach has the following major parts –
- A cardiac portion where the oesophagus opens
- The fundic region and the pyloric portion open into the first part of the small intestine
Small Intestine
The small intestine is five metres long and is highly coiled. It secretes juices and receives the secretion from the pancreas and liver. The digested food passes into the blood vessels in the intestinal walls. This process is known as absorption. There are thousands of finger-like outgrowths on the inner wall of the small intestine called villi which increases the surface area for absorptions. The absorbed food is transported to different organs by blood vessels, where it is used to build complex substances like protein. This process is called assimilation. The undigested food then passes to the large intestine.
Large Intestine
The large intestine is about 1.5 metres long, and its function is to absorb some salts and water from the undigested food. The remaining waste passes into the rectum, and it remains there as semi-solid faeces. Through the anus, the faecal material gets removed from the body from time to time. This process is called egestion. The large intestine has the caecum, colon, and rectum.
The caecum is a small blind sac that has some symbiotic microorganisms. It opens into the colon, which divides into three parts – an ascending, a transverse, and a descending part. The rectum is where the descending part opens and further opens out through the anus.
Digestive Glands
Digestive glands associated with the alimentary canal and digestive glands study material are as follows:
Salivary Glands
The three pairs of salivary glands are responsible for the production of saliva – The parotids (cheek), the submaxillary (lower jaw), and the sublinguals (below the tongue). The salivary glands are situated outside the buccal cavity, where they secrete the salivary juices into the buccal cavity.
Liver
The liver is the largest gland in the body, and it secretes the bile juice (helps in digestion of fats) stored in a sac known as the gallbladder. The liver has two lobes. The hepatic lobules are the functional units that contain the hepatic cells. The bile secreted by hepatic cells goes through hepatic ducts and is stored in the gallbladder. The hepatic duct from the liver and gallbladder forms the common bile duct.
Pancreas
The pancreas is located just below the stomach, and the pancreatic juices act on proteins and carbohydrates to change them into simpler substances. The partly digested food goes to the lower part of the small intestine, where the digestion process completes with the help of intestinal juices. In addition, the pancreas secretes hormones, glucagon, insulin, and alkaline pancreatic juices.
Conclusion
The alimentary tract comprises the mouth, pharynx, oesophagus, stomach, small and large intestine, rectum, and anus. The associated digestive glands are salivary glands, liver, gallbladder, and pancreas. The egestion of undigested food happens by the two main parts of the digestive system known as the alimentary canal and digestive glands.