Alimentary canal:

The digestive system of a human consists of a long tube that extends from the mouth to the anus. This tube is called the alimentary canal. Structure of human alimentary canal: Main sections of the alimentary canal are:

1. Oral cavity 2. Pharynx 3. Oesophagus,                 
2. Stomach 5. Small intestines               6. Large intestine.

Glands associated with the alimentary canal:

In addition, there are many glands associated with the alimentary canal. These are the three pairs of salivary glands, the pancreas, and the liver.

Oral cavity - (Selection, grinding, partial digestion):

Functions of the oral cavity:

1. Food Selection:

The oral cavity is the space behind the mouth and has many important functions in the whole process. Food selection is one of them. When food enters the oral cavity, it is tasted and felt. If the taste of mutton suggests that it is old, we reject it. If the teeth or tongue detect some hard object, such as dirt, we also reject that bite. The senses of smell and vision also help the oral cavity in the selection of food.

2. Grinding of food:

The second function of the oral cavity is the grinding of food by teeth. It is known as chewing or mastication. This is useful first because the esophagus can pass only small pieces and secondly because enzymes cannot act on large pieces of food. They require small pieces with large surface areas to attack.

3. Lubrication and chemical digestion of food:

The third and fourth functions of the oral cavity are the lubrication and chemical digestion of food. The chewing process stimulates the three pairs of salivary glands (under the tongue, behind the jaws, and in front of the ears) to release a juice called saliva in the oral cavity.

Functions of Saliva:

Saliva has two main functions. First, it adds water and mucous to the food which acts as lubricants to ease the passage of food through the esophagus. Second, saliva contains the enzyme salivary amylase, which aids in the partial digestion of starch.

Bolus:

After the processes of chewing, lubrication, and partial digestion the pieces of bread and mutton are rolled up by the tongue into a small, slippery, spherical mass called a bolus. We swallow the bolus and push it into the esophagus through the pharynx.

Pharynx and Oesophagus- (Swallowing and Peristalsis):

The bolus is swallowed and then pushed down by the movement called peristalsis.

Steps in swallowing:

During swallowing the bolus is pushed to the back of the mouth by the tongue. The soft palate moves upward and to the rear to close the opening of the nasal cavity.

When swallowed, the bolus goes to the pharynx, which makes special adaptations to prevent choking or aspiration when food is swallowed. When the larynx (the top of the trachea) moves upward, it forces the. epiglottis (a flap of cartilage) into a more or less horizontal position thus closing the glottis i.e., opening of the windpipe (trachea) The beginning of the swallowing action is voluntary, but once the food reaches the back of the mouth, swallowing becomes automatic.

After being swallowed, the food enters the tube called the esophagus, which connects the pharynx to the stomach. Neither the pharynx nor the esophagus contributes to digestion and the previous digestive actions of saliva continue.

Steps in Peristalsis:

Peristalsis moves the food from the oral cavity to the rectum. It is defined as the rhythmic sequence of waves of contraction in the smooth muscles of the walls of the alimentary canal, thus squeezing the food down along the alimentary canal.

Note:

If due for any reason, the direction of peristalsis reverses, what would be the result? It results would be vomiting.

Stomach - (Digestion, churning, and melting):

The stomach is a dilated part of the alimentary canal. It is J-shaped, located on the left of the abdomen, just beneath the diaphragm.

Portions of Stomach:

The stomach has two main portions.

1. Cardiac portion:

The cardiac portion is present immediately after the esophagus.

1. Pyloric portion:

 The pyloric portion is located beneath the cardiac portion. The stomach has two sphincters (openings that are guarded by muscles).

Cardiac sphincter and pyloric sphincter:

The cardiac sphincter is between the stomach and esophagus while the pyloric sphincter is between the stomach and the small intestine. The bolus enters the stomach from the esophagus through the cardiac sphincter.

Entrance of Food In the stomach:

When food enters the stomach, the gastric glands found in the stomach wall are stimulated to secrete gastric juice. It is composed chiefly of mucous, hydrochloric acid, and a protein-digesting enzyme pepsinogen.

Functions of Hydrochloric acid:

Hydrochloric acid converts the inactive enzyme pepsinogen into its active form called pepsin, HCI also kills microorganisms present in food. Pepsin partially digests the protein portion of the food (the bulk of mutton) into polypeptide and shorter peptide chains.

Churning action:

In the stomach, food is further broken apart through a process of churning. The walls of the stomach contract and relax and these movements help in the thorough mixing of the gastric juice and food. The churning action also produces heat which helps to melt the lipid content of the food.

Note:

The starch in our bite of bread and the protein in mutton have been partially digested and the food has been converted to a soup-like mixture called chyme. After it, the pyloric sphincter allows a little mass of chyme to enter the duodenum.

Small Intestine - (Complete digestion and absorption):

The function of Small Intestine:

Duodenum comprises the first 10 inches (25 cm) of the small intestine and it is the part of the alimentary canal where most of the digestive process occurs. Here the food is further mixed with 3 different secretions.

1. Bile from the liver helps in fat digestion through emulsification.
2. Pancreatic juice from the pancreas contains enzymes trypsin, pancreatic amylase, and lipase which digest proteins, carbohydrates, and lipids respectively
3. Intestinal juice from the intestine walls contains many enzymes for the complete digestion of all kinds of food.

Jejunum:

Next to the duodenum is 2.4 meters long jejunum. It is concerned with the rest of the digestion of the proteins, starch, and lipids of our bite.

The function of Ileum:

Last 3.5 meters long part of the small intestine is the ileum. It is concerned with the absorption of digested food.

The function of Villi:

There are circular folds in the inner wall of the ileum. These folds have numerous fingerlike projections called villi (singular: Villus). The villi increase the surface of the inner walls and it helps a lot in the absorption of digested food. Each villus is richly supplied with blood capillaries and a vessel of the lymphatic system called a lacteal. The walls of the villus are only single-cell thick. The digested molecules i.e., simple sugars and amino acids are absorbed from the intestine into the blood capillaries present in villi. The blood carries them away from the small intestine via the hepatic portal vein and goes to the liver for filtering, removal of toxins, and nutrient processing. Fatty acids and glycerol are absorbed into the lymphatic vessel (lacteal) present in villi, which carries them to the main lymphatic duct, from where they enter the bloodstream.

Large intestine- (Absorption of water and defecation)

After the digested products of our bite have been absorbed in the blood, the remaining mass enters the large intestine.

Parts of Large intestine:

It has 3 parts:

1. The caecum (or pouch that forms the T-junction with the small intestine).
2. The colon
3. The rectum.

From the colon, water is absorbed into the blood. As the water is absorbed, the solid remains of the food are called feces. The feces contain the undigested material A large number of bacteria, sloughed-off cells of the alimentary canal, bile pigments, and water are also part of the feces. Feces are temporarily stored in the rectum, which opens out through the anus. Under normal conditions when the rectum is filled up with feces, it gives rise to a reflex, and the anus is opened for defecation.