Alimentary Canal - Structure, Functions and Organs
The alimentary canal is a major part of the digestive system. It is a continuous muscular tube which runs through the body and it is around 8 to 10 meters long. It is open at 2 ends, with the mouth at the anterior end and anus at the posterior end.
The alimentary canal performs the function of digesting food. It breaks it down to smaller pieces and aids in the absorption of the digested food.
The alimentary canal consists of:
• The Mouth
The mouth is the part where the food enters the alimentary canal. It is the uppermost opening of the alimentary canal and is closed by the upper lip and lower lip. This is where ingestion of food begins.
• Salivary glands
Saliva is produced by the salivary glands and is then released into the buccal cavity when we eat, smell or even think of food! When food enters the mouth, mastication (the process of chewing food) breaks the food into smaller particles, which in turn helps the enzymes in the saliva to act on the broken-down food.
There are three main pairs of salivary glands: the submandibular, the parotid and the sublingual. During the process of mastication (chewing food) the food mixes with the saliva, the end result of the masticated food is known as a bolus.
In an adult, there are 32 teeth present in the buccal cavity. Our teeth help us in cutting down the food and grinding it. Incisors and canines (the front teeth) are used to cut and tear food, while the bicuspids and molars (the teeth further back) crush and grind, which in turn smashes the food into smaller bits that are easier to digest.
The tongue is an organ which is triangular in shape. It is fleshy and muscular. It lies along the floor of the buccal cavity. The tongue aids digestion by mixing the food with saliva. After mixing the food with the saliva, the tongue and soft palate (roof of the mouth) push the chewed food to the pharynx and down the esophagus. The superior (upper) surface of the tongue consists of various projections known as papillae. All these papillae contain sensory receptors in them known as the taste buds. These numerous taste buds help us determine the taste of the food. If it weren't for these microscopic helpers, food wouldn't be that great as we assume it to be!
The pharynx is present at the back of the mouth. It is the area of transition from the mouth to the oesophagus. It is a passage common for both the food we swallow and the air we breathe in. Glottis is the name of the opening of the respiratory tract present in the pharynx and it is protected by the epiglottis, which is a cartilaginous flap. During the ingestion (swallowing) of food, the epiglottis closes the windpipe (trachea). This, in turn, helps to prevent the food from entering the respiratory tract and prevents aspiration of food.
The oesophagus is a muscular, tubular structure that moves the bolus from the pharynx to the stomach in the presence of peristaltic movements. It is around 25 centimeters long in a normal adult. The epithelial cells present in it secrete mucous which aids in smooth movement of the bolus. When the bolus is about to reach the stomach, a valve-like structure which is called the oesophageal sphincter or cardiac sphincter relaxes and lets the bolus pass into the stomach.
The stomach is like an elastic pouch which is made up of highly muscular walls and it is situated right below the diaphragm. The stomach can be divided into 5 parts. They are:
1. The cardiac stomach,
2. The fundus stomach,
3. The body,
4. The antrum stomach and
5. The pyloric stomach.
The walls of the stomach are made up of four layers. (From inside to outside) Mucosa, muscular mucosa, submucosa and muscularis externa. The cardiac part of the stomach is attached to the oesophagus. The junction where the oesophagus opens into the stomach is protected by a sphincter called the cardiac sphincter. This sphincter prevents swallowed food and acid from entering the oesophagus again by constricting as soon as the food enters the stomach. The only time this sphincter relaxes is when a person swallows or vomits.
The stomach's epithelium forms deep pits like structures known as the oxyntic or fundic glands where chief cells produce an inactive form (precursor) of pepsin know as pepsinogen. This pepsinogen helps in the degradation of proteins. The secretion of pepsinogen by the epithelial cells prevents self-digestion of the stomach cells. In addition to the pepsinogen produced by the epithelial cells, the goblet cells also produce mucus that helps prevent self-digestion of the stomach cells.
The Mechanism - The stomach performs chemical, as well as mechanical functions in digestion.
In the mechanical function, the stomach's upper involuntary muscles relax in order to store more food and the lower muscles rhythmically contract in order to mix and churn the food with the gastric juices of the stomach.
In chemical function, hydrochloric acid, which is the main gastric acid and gastric enzymes like Pepsin, Gastric Amylase, Lipase and Gelatinase help in the further breakdown of the food.
This broken-down food transforms into a thick creamy, semi-solid substance known as Chyme, which is then pushed down the small intestine (duodenum).
• Small intestine
The human intestine is divided into the small intestine and large intestine. Hence the small intestine is the upper part of the intestine which comes after the stomach and before the large intestine. It is divided into:
1. The duodenum: It is the part of the small intestine that comes first and it starts at the lower end of the stomach and extends to the jejunum. It is the division in which chemical digestion using enzymes takes place.
2. The jejunum: It is the center division of the small intestine which is highly involved in the absorption of carbohydrates and proteins. Jejunum's inner surface, its mucous membrane, is covered in tiny, finger-like projections called villi, these villi increase the surface area of tissue available to absorb nutrients from contents of the gut.
3. The ileum: It is the part between the jejunum and large intestine. It is the last and most likely the longest part of the small intestine. The main function of the ileum is to absorb Cobalamin (Vitamin B12), bile salts, and any other products of digestion that were not possible for the jejunum to absorb.
• Large intestine
The large intestine begins at the right iliac region of the pelvis, just below or at the right waist; it is then joined to the lower end of the small intestine. It is usually around 1.5 meters (4.9 feet) in length, which is around 1/5th of the entire length of the alimentary canal.
The large intestine aids in the absorption of water from the leftover indigestible food matter and compact faeces before defecation (the process of throwing the nitrogenous waste out of the body through the anal canal).
Parts of the large intestine:
1. Appendix: It is an inner organ with no known use of its own and it is attached to the inferior surface of the cecum.
2. Cecum: It is a pouch-like structure, which is usually peritoneal, and it is considered to be the part where the large intestine begins.
3. Colon: The part of the large intestine is said to be the last part of the digestive tract. This comes after the ileum and before the anus. The parts of the colon are:
a. Ascending Colon: The part of the colon that continues up the abdomen.
b. Transverse Colon: The part of the colon that travels across the width of the abdominal cavity.
c. Descending Colon: The part of the colon that goes downwards (descends) towards the beginning of the sigmoid colon.
d. Sigmoid Colon: It is the S-shaped part of the large intestine, which is also the last part of the colon, ending in the rectum. During constipation, the strain on this colon can put the individual at high risk for diverticulitis.
The rectum can be considered to be the last part of the large intestine in which the faeces is stored temporarily before being passed into the anal canal. It is about 15 centimeters in length and is divided into two parts namely, the rectum proper and the anal canal.
The anal orifice or the anus is the opening at the end of the alimentary canal that facilitates defecation or egestion (the process of bowel waste leaving the body). Circular muscles (sphincters) surround the anus and these sphincters have control over holding and releasing of faecal matter. The internal anal sphincter is involuntary in action, unlike the external anal sphincter. Hence when an individual try to hold defecation, it is the external sphincter one is controlling because the internal sphincter can't be controlled voluntarily.