All the parts of the body involved in the uptake and digestion of food with the elimination of undigested material constitute the digestive system. Organs of the digestive system are divided into two main groups:
I. Organs within the alimentary canal:
Alimentary canal or GI (gastro-intestinal) tract or gut is the entire length of tube that winds through the body from the mouth to the anus. It digests, breaks down and absorbs food through its lining into the blood. Organs within alimentary canal include mouth, pharynx, oesophagus, stomach, small intestine and large intestine. Alimentary canal is considered outside of the body because it opens to the external environment at each end. (Mouth, anus)
II. Accessory digestive organs:
It includes tongue, teeth, gall bladder, salivary glands, liver, and pancreas. While the teeth and tongue lie within the mouth, the digestive glands and gall bladder actually lie outside of the GI tract and connect to it through pathways called ducts. The digestive glands aid in mechanical breakdown of food by producing several types of secretion.
Tissue layers of Alimentary canal:
There are four major tissue layers that line the length of alimentary canal. The layers vary in thickness and function in different sections of the alimentary canal. The wall of alimentary canal has the same four basic layer also known as tunica- mucosa, submucosa, muscularis externa and serosa. Each layer contains a certain type of tissues that play a crucial role in the breakdown of food. These layers are:
i. The mucosa:
Mucosa also known as mucus membrane is the innermost layer of tissue. It’s a moist epithelial membrane that lines the alimentary canal lumen from mouth to the anus. Mucosa has three major functions:
a) Secrete mucus, digestive enzymes and hormones.
b) Absorb end products of digestion into the blood.
c) Protect against infectious disease.
Alimentary canal, the mucosa may perform one or all three of them functions. Digestive mucosa is made up of three sublayers:
a. A lining epithelium
b. A lamina propia
c. Muscularis mucosa
Except in the mouth, oesophagous and anus where its stratified squamous , the epithelium of mucosa is simple columnar epithelium rich in mucous – secreting cell. The mucous it produces protect certain digestive organs from being digested by enzymes working within the same cavity, it also eases food passage along the GI tract. The lamina propia , which underlies the epithelium is loose areolar connective tissue.
Its capillaries nourish the epithelium and absorb digested nutrients. Its isolated lymphoid follicles, which are a part of MALT) help defend against bacteria and other pathogens, which have free access to our digestive tract. Large collections of lymphoid follicles occur in the pharynx (tonsils) and appendix.
External to the lamina propia is the muscularis mucosae. In the small intestine, this muscle layer’s throws the mucosa into a series of small folds that immensely increases its surface area.
Submucosa is just external to the mucosa in areolar connective tissue, containing a rich supply of blood and lymphatic vessels, lymphoid follicles and nerve fibres which supply the surrounding tissues of the GI tract wall. Its elastic fibres enable the stomach to regain its normal shape after temporarily storing a large meal. It contains secretory glands that produce substance for the alimentary glands.
III. The muscularis externa:
It surrounds the submucosa and consists of inner circular and outer longitudinal layers of smooth muscles. Musculris is responsible for segmentation and peristalsis. In several places, along the tract, the circular layer thickens and form sphincters that act as valve which control food passage from one organ to the next, also prevent backflow.
IV. The Serosa:
It is the outermost layer of the interperitoneal organs. In most alimentary canal organs, it’s made up of areolar connective tissue covered with mesothelium, a single layer of squamous epithelial cells. In the oesophagous, which is located in the thoracic cavity the serosa is replaced by adventitia ordinary fibrous connective tissue that binds the oesophgous to surrounding structure.
Alimentary canal Anatomy:
Human digestive tract or alimentary canal is a long hollow tube which extends from the mouth to the anus. The parts of the digestive system includes- mouth, oral cavity, teeth, oesophagous, pharynx, stomach, small intestine, large intestine, anus.
It is the opening through which intake of food occurs. Oral cavity has three parts: Palate, tongue, teeth.
a) Palate: It forms the roof of the oral cavity. Anterior part is called as hard palate and the posterior part is called soft palate.
b) Tongue: it is a muscular and glandular structure attached to the base of the oral cavity. On the upper surface it has V-shaped furrow called sulcusterminalis. It divides the tongue into the anterior oral part and posterior pharyngeal part. Its upper surface also consists of tiny projections called lingual papillae. They are ofthree kinds in humans;Vallate or circumvallate papillae, have gustatory receptorsto sense tests. Filliform papillae, smallest and contain touch receptors. Fungiform papillae rounded and mostly present at the tip of tongue.
c) Teeth: Humans are diphyodont i.e. they have two sets of teeth- milk or deciduous and permanent teeth. Humans are also heterodont having different types of teeth.
Structure of teeth:
It consists of three parts- crown, neck, and root. The exposed part of the tooth is called crown, the region where it is covered with gums is known as neck and root is embedded in the socket of the jaw bone.
It forms the common passage for food and air.
It is a muscular tube through which small bolus of food passes from the mouth to the stomach. It is simply bent at the point at which it meets the stomach. This prevents the backward movement of food from the stomach into it. Food in the digestive tract moves by an involuntary movement of alternate contraction and relaxation of muscles called peristalsis.
IV. Stomach: It is a muscular bag which has three parts- cardiac, fundus and body.
a) Cardiac: It is so called because it is not close to the heart. The opening of oesophagous to the stomach is regulated by cardiac sphincter.
b) Fundus: It is dome shaped and is usually filled with air.
c) Body: This is the main part of the stomach. The opening of the stomach into small intestine and is regulated by pyloric sphincter.
V. Small Intestine: It is the largest part of the alimentary canal and comprises of three parts:
a) Duodenum: It is C- shaped and receives the hepato- pancreatic duct formed by the union of bile and pancreatic duct.
b) Jejenum: Middle part of small intestine with thick walls and more vasculature.
c) Ileum: Lower part of the small intestine with thin walls and less vasculature.
VI. Large Intestine:
Small intestine leads into large intestine which has three parts:
a) Caecum: It is small sac like structure at the point where the small intestine meets the large intestine.
b) Colon: It is divided into four regions: ascending colon, descending colon, transverse colon and sigmoid colon.
c) Rectum: It opens into the anus.
There are six digestive processes found in our digestive system:
1) Ingestion: It is simply the intake of food through mouth.
2) Propulsion: It is the way through which food enters into the digestive tract. This includes swallowing and peristalsis. Peristalsis is the main mean of propulsion which involves rhythmic contraction and relaxation of muscles which surrounds the organ. Its main purpose is to squeeze food through the gastrointestinal tract.
3) Mechanical breakdown: It includes chewing, mixing food with the tongue, stomach churning and segmentation (constrictions in the small intestine). Mechanical breakdown increases surface area which increases absorption. Segmentation mixes food with digestive juices in the small intestine which also increases absorption.
4) Digestion: Digestion is the chemical breakdown of food. It involves a series of steps, but the main idea is that enzymes are secreted into the alimentary canal by accessory digestive glands and the gall bladder that aids in the breakdown of food material.
5) Absorption: It is the process by which uptake of the end products of digestion into the blood or lymph occur through the walls of GI tract.
6) Defecation: It is the elimination of remaining portions of food that is left after digestion and absorption through the GI tract out of the body through anus as faeces.
Stomach plays a big role in the mechanical breakdown and digestion of food. The small intestine is the main organ which helps in absorption even though large intestine and stomach also absorb certain substances. In contrast, the pharynx and oesophagous only contributes to propulsion and have nothing to do with breakdown and digestion or absorption at all.
Types of Digestion:
There are mainly two types of digestion occur in human- beings:
1) Mechanical Digestion:
This includes physical breakdown of food into smaller pieces. The mouth and the stomach are the primary sites of digestion.
2) Chemical Digestion:
In chemical digestion breakdown of large molecule into small units occur. It is small enough for absorption. Chemical digestion is carried out by hydrolases (enzymes) in the mouth, stomach and small intestine.
Functions of Digestive system:
· Mixing of food; helps in breakdown of food in fluids containing digestive enzymes.
· Forward propulsion of food; occurs through the alimentary canal from the oral cavity to anus.
· It helps in Mechanical and chemical breakdown (hydrolysis) of ingested material into smaller units which can easily get absorbed.
· It involves the secretion of digestive substances that must be actively transported into the alimentary canal.
· Nutrients move from the alimentary canal into the blood or lymph.
Monomer units of nutrients, water, vitamins and electrolytes are absorbed.
Food is an important source for human- beings to derive nutritional energy. Digestive glands of the alimentary canal secrete different enzymes and aids in the process of digestion.
Peristalsis is an involuntary movements of the circular and longitudinal muscles to push the food into the stomach. This is done by peristaltic waves which are generated to pass the food.
Protein is a large biomolecule that is digested by HCl, pepsin, trypsin and chymotrypsin. Later protein is absorbed by small intestine and remaining parts are excrete out in the form of faeces.
The energy value of a food indicates how much energy the human body can gain through metabolism. The energy value specified in KJ/100gm.
Gastrointestinal hormones are chemical messenger that regulate the physiological functions. Gastrointestinal hormone includes gastrin, secretin, motilin, GIP, ghrelin, VIP, BBS.