The digestive system is a system of organs working together for the uptake of food, its digestion and to eliminate the indigestible waste products out of the body. It is also known as the Alimentary canal.
What is digestion?
Digestion is defined as the breakdown of complex organic food materials into simpler compounds and thus process, releasing energy required by the body.
The complex food substances are carbohydrates, fats, proteins, vitamins, minerals etc. Digestion involves two processes: Mechanical digestion in which food is broken down by the peristaltic movement of the organs involved in digestion and chemical digestion in which digestive juices act on the complex food materials to convert it into simpler compounds.
Essential compounds of diet
- Water: Water is a very important compound which can be obtained from all food and drinks and is also released in the body due to oxidation of food. The optimum requirement of water for an adult is 2,000ml.
- Carbohydrates: Carbohydrates can be obtained from food sources like cereals, bread, fruits etc. They help in providing energy.
- Protein: Proteins help in growth and maintenance of the body. It also helps in formation of enzymes. Proteins can be obtained from pulses, milk, meat, eggs, cheese etc.
- Fats/Lipids: It also helps in providing energy. It can be obtained from milk, butter, ghee, oil, creams, nuts etc.
- Vitamins: Vitamins help in prevention of deficiency diseases and regulation of metabolic activities. The vitamins required by our body are A, B, B6, B12, C, D, E and K.
- Minerals: Different minerals have different functions in the human body. Some of the important minerals are-
- Sodium: Helps in osmotic balance, muscle contraction and nerve impulse conduction. It can be obtained from table salt and vegetables.
- Iron: Oxygen transport as part of haemoglobin. Can be obtained from leafy vegetables and iron supplements.
- Iodine: It helps in metabolic control of hormone thyroxine.
- Potassium: Helps in muscle contraction and nerve impulse conduction. Can be obtained from vegetables.
The human digestive system consists of the alimentary canal and associated digestive glands.
Histology. The alimentary canal is lined with muscular layers. It consists of four layers.
- Serosa: It is the outermost layer. It is formed of a single layer of cells.
- Muscularised: It is formed by two layers of cells and consists of a network of nerve cells. It controls the muscular contractions.
- Submucosa: It consists of highly vascular connective tissue. It also has another network of nerve fibres, it controls the secretion of intestinal juice.
- Mucosa: It is the innermost layer and consists of further 3 layers. It forms the digestive juices.
Parts of the alimentary canal
Buccal cavity: It is the space which is bounded to the sides by jaws, top by the palate and below by the throat. The buccal cavity consists of the tongue which is a highly muscular organ and consists of papillae. The tongue helps in mixing saliva with food and facilitates in swallowing. It also helps in telling the taste of the food.
Teeth: Teeth are embedded in both the upper and lower jaw and helps in chewing, cutting and piercing the food. There are four types of teeth present in humans – Incisors, canines, premolars and molars. Adult human being consists of 32 teeth in the permanent set.
A tooth consists of 3 regions- Crown, neck and root. The root is embedded completely in the jaw and consists of nerve endings and blood vessels. The neck is surrounded by the gum which is soft and fleshy skin. The crown is the exposes part of the tooth and is covered by a shiny material called enamel.
Pharynx: The food and air crosses the pharynx to reach the oesophagus. It has voluntary muscles which contract and help in swallowing.
Oesophagus: It is the long, muscular straight tube which connects the pharynx to the stomach. The major function of the oesophagus is to pass the food from pharynx to the stomach by peristaltic movement.
Stomach. The stomach is a J shaped muscular sac which stores the food for some time. It is also involved in mechanical churning of food, partial digestion of food by gastric juices and regulation of passage of food in the small intestine.
It has 3 parts- The cardiac, fundic and pyloric. The inner surface of the stomach consists of various folding known as gastric rugae which help in increasing the surface area for maximum storage of food. Stomach also secretes the hormone gastrin.
Small intestine: Divided into 3 major parts- duodenum, jejunum and ileum, the major function of the small intestine is absorption of food. It also consists of microscopic finger like projections known as villi which increases surface area for effective absorption. Small intestine also secretes some hormones.
Large intestine: The large intestine is divided into 3 major parts- Caecum, Colon and Rectum. Its major role is absorption of water, formation of faeces, and production of mucus for the lubrication of mucosa.
Anus: The function of the anus is elimination of faeces. It consists of two anal sphincters: the internal anal sphincter and the external anal sphincter.
Human digestive glands include salivary glands, gastric glands, liver, pancreas and intestinal glands.
- Salivary glands: The function of salivary glands is to secrete saliva which is digestive in function. Saliva is secreted by 3 pairs of salivary glands-
- Parotid glands which lie on the sides of the face. The saliva produced by these glands is carried by Stensen’s duct.
- Submaxillary glands which lie at a certain angle of the lower jaw. It has submaxillary ducts which open under the tongue.
- Submandibular glands: These glands are present under the front part of the tongue. Ducts of Rivinus which carries the saliva produced by these glands open under the tongue.
- The Saliva secreted by the salivary glands has a pH of 7 and contains salivary amylase which is an enzyme. Saliva also contains lysozyme which is anti bacterial in function.
- Gastric glands: gastric glands are present in the stomach and is acidic having a pH of 2. There are 3 types of gastric glands present in the stomach:
- Mucous cells which secrete mucus that helps in protection of the internal wall of the stomach from the gastric acid.
- Peptic or chief cells which secrete pepsinogen which is the precursor of enzyme pepsin.
- Oxyntic cells which secrete hydrochloric acid.
Liver: It is the largest gland of the body consisting of hepatocytes, bile canaliculi and hepatic sinusoids. The liver weighs 1.6kg. There are two lobes in the liver. Right and left lobe.
The liver forms bile which is stored in concentrated form in the gall bladder. Liver also has a function in detoxification of poison or toxic substances in the body.
Pancreas: The pancreas is a gland which is both endocrine and exocrine in function. The endocrine part secretes hormones namely, insulin, glucagon and somatostatin.
The exocrine part secretes pancreatic juice which consists of proenzyme trypsinogen, chymotrypsinogen and procarboxypeptidase. There are other enzymes also such as pancreatic lipase, nucleases and pancreatic amylase.
Intestinal juice: the wall of villi present in the small intestine contains small, microscopic glands, Brunner’s glands and Crypts if Lieberkühn.
The both secrete enzymes, mucus and alkaline watery fluid. The mixture of all these secretions is known as the intestinal juice or succus entericus.
Digestion of food
The organs churn the food by mass peristaltic movement and the digestive glands pour their secretions to facilitate the process of digestion. The processes of digestion in various organs are as follows:
Mouth and buccal cavity: Digestion of starch starts in the mouth where starch is converted into maltose by the action of enzyme, salivary amylase. The saliva also contains various electrolytes such as sodium, potassium, chlorine etc. 30% of starch is hydrolysed here and the food is converted into a bolus which is further passed down to the Oesophagus.
Oesophagus: It does not contain any digestive gland so it does not aid in digestion. It only helps in passage of food from the buccal cavity to the stomach.
Stomach. The stomach consists of Gastric glands which secrete gastric juice. The gastric juice gets mixed with the bolus by the churning movements of the stomach. HCl helps in conversion of proenzyme pepsinogen into active form, pepsin. Pepsin helps in hydrolysis of proteins into peptides. Digestion of casein present in milk also takes place in the stomach by the action of enzyme rennin.
Small intestine: The small intestine plays a major role in both digestion and absorption of food. The pancreatic juice and bile from the gall bladder are released into the small intestine. Bile juice helps in emulsification of fats. It also coats each small fat droplet to avoid their merging together. The Pancreatic amylase hydrolyses starch and glycogen into maltose and dextrin’s.
Enzyme trypsinogen gets activated into trypsin by enterokinase. Trypsin converts proteins to peptides. It also converts chymotrypsinogen to chymotrypsin. Trypsin also helps in activation of procarboxypeptidase into carboxypeptidases which further converts peptides into amino acids.
After digestion, food is absorption of nutrition from food and its passage into the blood and the lymph. In mouth, very little absorption takes place. Absorption of few drugs takes place in the mouth into the buccal mucosa. Absorption of simple sugars, water, and alcohol takes place in the stomach.
Major absorption takes place in the small intestine. The final products after digestion such as glucose, fatty acids, amino acids etc. get absorbed into the blood lymph. The large intestine is also not much involved in absorption. It only absorbs some minerals and water.
Egestion is the last process in the complete digestion process. It is the elimination of undigested waste materials from the alimentary canal through the anal canal. Faecal matter is composed of undigested wastes, bacteria, fat, protein, dead cells, mucus and brown pigments stercobilinogen and stercobilin.