Humans have a complex body plan and hence possess a digestive tube with two openings, a mouth which is the anterior end and an anus which is the posterior end. The food material moves along the alimentary canal in a single direction, passing through a series of specialised organs. Such a tube whose primary function is to facilitate movement of the food particles and ultimately aid in nutrition is referred to as the Alimentary Canal.
The human alimentary canal consists of the buccal cavity, the oesophagus, the stomach, the small and large intestine and the anus. Their functions include digestion of complex food stuff into simpler components which can be readily absorbed and assimilated into the body and then the ejection of the unabsorbed digested material from the body in a sequential manner.
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The Alimentary Canal consists of various parts and regions, an overview of which has been given below. Each region will be discussed in detail in the following sections.
The alimentary canal begins at the Mouth which is also called oral or buccal cavity and comprises structures like teeth and tongue that aid in breakdown of food particles.
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It is commonly known as the throat region. The food material enters the pharynx when it is swallowed.
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It is a long tube that connects the pharynx with the stomach.
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The stomach can be seen as an elongated pouch-like structure between the oesophagus and small intestine. For the ease of absorption, the food is processed into a liquid suspension here.
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It is nick-named as the ‘work-house’ of digestion since nutrient absorption is very high in this region. It is the longest region of the Alimentary CanalIt is nick-named as the ‘work-house’ of digestion since nutrient absorption is very high in this region. It is the longest region of the Alimentary Canal.
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This is the terminal region of the Alimentary Canal. The essential nutrients and water are absorbed from the ‘leftovers’ in this region which is then ejected via the anus.
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It is a long tube that connects the pharynx with the stomach.
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The structure and anatomy of the stomach has been presented in detail in this section.
The curve formed by the stomach in the upper surface towards the right and the lower surface towards the left is known as the lesser and the greater curvature respectively.
It comprises three regions namely the fundus, the body and the pylorus.
The largest region of the stomach is fundus and it can be seen elevated above the level of esophageal opening.
The body and pylorus are the central and lower parts of the stomach respectively. The pylorus can further be divided into two regions namely the proximal antrum and the distal pyloric canal. The distal pyloric canal ends in the pyloric sphincter that opens into the duodenum.
The junction between the body region and the proximal antrum is referred to as incisura angularis.
The wall of the stomach comprises four tissue layers which are a mucous lining, a submucosal layer, a muscular layer and a fibroserous layer.
The innermost layer of the gastrointestinal wall or the mucosa is made up of inner mucus epithelium, a larger loose connective tissue, laminar propria and a thin layer of smooth muscle called the muscularis mucosa.
The innermost epithelial lining of the stomach is thrown into folds called rugae which have depressions called gastric pits. In the fundus region, gastric glands are present below these pits which secrete hydrochloric acid and enzymes.
The submucosal layer comprises connective tissue, blood vessels, alveolar tissue and Meiisner’s nerve plexus.
The muscular layer is thick and wraps the submucosa. It contains the myenteric plexus that lies between the inner circular and outer longitudinal muscle fibres.Its functions include the regulation of movement of digestive tract and secretion.
Serosa, the outermost layer is made of connective tissues and peritoneum.
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| Parts of Alimentary Canal | Structure | Functions |
|---|---|---|
| Buccal Cavity | The mouth consists of the vestibule and oral cavity proper. | Ingestion of food. |
| Roof of the mouth is called a hard palate that separates the oral and nasal cavity. | Tongue helps in producing the sense of taste by detecting chemicals present in food | |
| The soft palate or the uvula helps in preventing swallowed food from entering into the nasal cavity | Teeth chew and grind the food material into smaller pieces. During mastication, incisors are used for cutting | |
| Tongue is a muscular organ that covers the base of the mouth and has many taste buds | Receives secretions from the salivary glands that moistens the food to form the bolus before it gets swallowed. It also partially digests starch and kills germs that may enter along with food | |
| Teeth are made up of dentin and covered by the hardest tissue enamel | ||
| Pharynx | It is a tube-like muscular structure. | Swallowing is carefully coordinated here so that the food particles do not pass into the trachea. |
| It leads to two passageways: the oesophagus and the trachea | ||
| Oesophagus | It is a long, muscular but collapsible, mucus coated tube which extends from pharynx to stomach | The bolus gets pushed towards the stomach by peristaltic waves |
| The oesophagus is guarded by a muscular sphincter at both ends | The upper sphincter prevents entry of air during respiration while the lower or the cardiac sphincter prevents the backflow of food from stomach back to the oesophagus | |
| Stomach | The stomach is located just below the diaphragm. | Secretes hydrochloric acid that aids to dissolve the particulate matter in food and activates inactive pepsinogen to pepsin that helps in digesting proteins. It also plays a role in killing the bacteria along with food. |
| The inner surface of the stomach and glands are covered by secretory cells called chief cells which secrete the enzymes present in gastric juice, parietal cells that secrete hydrochloric acid and intrinsic factors that combine with vitamin b12 to protect the lining of the stomach wall | Helps in storage. It has a very elastic wall and can stretch to accommodate about 2 L of food and fluid | |
| Detailed structure has been discussed in the previous section | SSecretes a digestive fluid known as gastric juice | |
| Small Intestine | The small intestine appears as a long coiled loop in the abdominal cavity. It comprises a short duodenum region, followed by the jejunum region and the ileum region which is the longest. | Microvilli increases the surface area of the small intestine and aids in better absorption of food. |
| The innermost layer of the small intestine called the mucosal layer has many tiny folds called villi. A villi comprises an arteriole, a venule and a lacteal or lymph vessel | Presence of secretory cells at the base of crypts inhibits bacterial growth in the small intestine. | |
| The brush border appearance of the lining of the small intestine is due to microvilli | The food which is now called chyme mixes with digestive secretions from the liver and pancreas in this region. The continuous breakdown of food happens in duodenum whereas jejunum and ileum mainly aid in absorption of digested food into the bloodstream. | |
| In villi and crypts, goblet cells are found which secrete mucus. | ||
| A structure appearing as a worm called the vermiform appendix is present at the posterior end of the small intestine and has no known functional activity. | ||
| Large Intestine | It appears as a large muscular tube that joins the small intestine and the rectum. | Intestinal mucus glands present that produce the lubricating mucus coated in faeces |
| It comprises the caecum,colon, the rectum and the anus. The colon comprises ascending, transverse, descending and sigmoid regions | Undigested food material gets concentrated, water and salts get absorbed here | |
| The caecum is a pouch-like structure.There is a presence of sphincter or valve called the ileocaecal valve. | Rectum serves as a reservoir for the faeces. The rhythmic contraction of rectum relaxes the associated sphincter muscles to expel faeces | |
| Anus has pelvic floor muscles and the internal and external sphincter. The pelvic floor muscles aids in stopping the stool from coming out when it is not supposed to. |
To ensure the proper digestion and absorption of ingested food materials the integration and coordination of the digestive organs is vital and necessary. The physical and chemical changes in the ingested food materials were brought in the oral cavity and the digested food materials were transported to different parts of the alimentary canal. The stomach which aids in digestion passes the digested food material to the small intestine where it gets absorbed and further metabolised. The process gets completed in the large intestine where the undigested remnants are concentrated as faeces and expelled out.
Q1. What is epiglottis?
Ans. It is a flap-like structure that covers the glottis, the entrance of the windpipe during swallowing of food. This prevents the food from entering the windpipe and choke.
Q2. Explain the role of Meiisner’s Nerve Plexus?
Ans. It regulates the secretions and the local blood flow of the gastro-intestinal tract and also helps in the initiation of peristaltic movement.
Q3. What are delta cells and where are they located?
Ans. The delta cells are located in the Islets of Langerhans of the Pancreas. These cells produce somatostatin, a hormone that inhibits the production of other hormones in the body.
Q4. What are the pigments found in bile and what is their source?
Ans. Haemoglobin is broken down from dead red blood cells and are excreted into bile as bile pigments. The bile pigments include yellowish bilirubin and greenish biliverdin.
Q5. What is the role of E. coli in the human intestine?
Ans. E. coli are harmless bacteria that aids in the digestion process, breakdown of food particles and absorption in the small intestine along with Vitamin K production.
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