Digestive System

Last Updated on Monday, 15 August 2011 04:23 Written by Sandesh Wednesday, 7 April 2010 10:54

DIGESTIVE SYSTEM:

Digestion is the process of breaking down complex food substances into simple substances with the help of enzymes which can be easily absorbed by our body. Digestive system is an important system which provides essential nutrients to the body. It consists of digestive organs( Alimentary canal and digestive glands) and the physiology of digestion.

Alimentary canal: It is a long tubular structure starting from mouth to anus, where food are digested. In human it is about 8 to 10 meters in length. It contains:

1. Mouth and buccal cavity
2. Pharynx
3. Oesophagus
4. Stomach
5. Small intestine
6. Large intestine
7. Anal canal

1. Mouth and buccal cavity: Mouth is surrounded by lips, cheeks, tongue and hard and soft palates. It has immovable upper jaw and movable lower jaw. It is adapted to receive food and to prepare it for digestion by mechanically reducing the size of solid food particles and mixing them with saliva. This action is called mastication.

Hard palate on the roof of the mouth bears transverse ridges called rugae. Soft palate is the posterior soft part of hard palate which has hinder part hanging freely called as uvula.

Tongue: It is a freely movable muscular organ which is lying in the floor of the oral cavity. It has a root, a body and apex. Root is attached posteriorly to hyoid bone. Anterior pointed part is called apex. And remaining part is called body.

Dorsal face of the tongue is rough due to the presence of numerous lingual papillae. Lingual papillae are of three types which  includes vallate papillae, filiform papillae and fungiform papillae. Vallate and fungiform papillae contain taste buds. Filiform papillae contain tough sensitive nerve endings. Vallate papillae are largest and are 8 to 12 in number arranged in ‘V’ shape at the base of the tongue. Fungiform papillae are numerous mushroom shaped present at the tip and lateral edges of the tongue. Filiform papillae are smallest , thread like present at anterior surfaces.

Inferior surface is lined with mucous membrane. Lingual fraenulum connects the inferior surface to the tongue with the floor of the mouth.

Histologically, tongue is lined with stratified squamous epithelium.

Functions: mastication, deglutition (swallowing), taste, articulation of speech, manipulation of food and helps to mix the saliva with the food.

Teeth: Teeth of human are thecodont- embedded in the jaw sockets), heterodont- of 4 different types and diphyodont- errupt twice in the whole life or two sets of teeth are temporary or milk teeth and permanent teeth.  Four types of teeth include the incisors, canine, premolars and molars.

Dental formula of an adult parson is: [I=2/2, C=1/1, PM=2/2, M=3/3]

Structure: Teeth have three parts- crown, root and neck. Crown is the part above gum. Root is embedded in jaw. Neck is between crown and root surrounded by gums. Incisor and canine has 1 root, premolars has 1 to 2 roots, upper molar has 3 roots and lower molar has 2 roots. Each tooth has pulp cavity in centre. Pulp cavity is surrounded by dentine. Pulp cavity contains blood vessels and nerve fibres. Enamel covers the crown while cementum covers the embedded part of the dentine. Enamel is the hardest substance in the body.

Functions: Teeth help in mastication of food and speech production. Incisor teeth help in cutting, canine helps in tearing while premolar and molar helps in grinding food.

2. Pharynx: It is a muscular tube situated posterior to the nasal and oral cavity. It has three parts:

i.          Nasopharynx: behind the nose and superior to the soft palate.

Ii.         Oropharynx: behind the mouth.

Iii.        Laryngopharynx: in front of the larynx.

Nose opens into the nasopharynx through two internal nasal openings. Nasopharynx is connected with middle ear by two Eustachian tubes. Oral cavity opens posteriorly in the oropharynx. The laryngopharynx communicates with the larynx through glottis. Opening of the glottis is guarded by a thin cartilaginous leaf like structure called epiglottis. Epiglottis prevents the entry of food into the glottis. Pharynx leads to oesophagus. Pharynx is lined with stratified squamous epithelium.

3. Oesophagus: it is a narrow muscular tube of about 25 cm in length. It passes through the neck and thoracic cavityand then pierces the diaphragm to open into the stomach.the upper end is closed by upper circo pharyngeal sphincter which prevents the entry of air. The lower end at the junction of the oesophagus and stomach has a gastroesophageal sphincter which prevents the regurgitation of food into the oesophagus. Pharynx and oesophagus donot contribute in the digestion processes but muscular walls helps in swallowing by peristalsis.

4. Stomach: it is a J shaped muscular sac present in the abdominal cavity slightly left. It has inner concave border called lesser curvature and an outer convex border called greater curvature. It is the widest part of the alimentary canal. It about 25 cm long with 2 litres of capacity in adult. Stomach is divided into 4 parts:

• Cardiac
• Fundus
• Body
• Pylorus

Cardiac is the uppermost part continuous with the oesophagus. Fundus is the dome shaped structure lying above the cardiac. Body is between fundus and pylorus. Pylorus is the lowermost funnel shaped part which leads to duodenum.

Opening of Cardiac with oesophagus is provided with the cardiac sphincter. Similarly, Pylorus is provided with pyloric sphincter. These sphincters prevent the back flow of food. Internally, mucosa of an empty stomach is thrown into numerous folds called gastric rugae. These rugae flatten when stomach is distended.the stomach is lined with columnar epithelium containing numerous gastric glands. Gastric glands contain three types of cells:

• Parietal (oxyntic) cells: secrete HCl.
• Chief (peptic) cells: secrete pepsinogen.
• Mucous cells: secrete mucous.

Functions:

• Temporary reservoir of food.

• Digestion of food by gastric enzymes.
• HCl destroys microorganisms present in food.
• Absorption of alcohol, drugs, water etc.

5. Small Intestine: It is a long coiled tube of about 6 meters long which is the longest part of the alimentary canal. Small intestine is coiled in the abdominal cavity which is held together by layer of mesentary. Internally, the intestine is lined with mucosa which contains finger like projections called villi. It increases the surface area for digestion and absorption. Cells of mucosa lining is modified into intestinal glands. Small intestine can be divided into duodenum, jejunum and ileum.

• Duodenum is the first shortest and widest part of small intestine. It is ‘C’ shaped in structure lying just below the stomach. It has an opening of the common bile duct which carry bile and pancreatic juice in to the duodenum.
• Jejunum is the middle part of small intestine of about 2.5 meter long.
• Ileum is the last part of small intestine of about 3.5 meter long. It terminates into caecum.

6. Large Intestine: It is a wide tube extending from ileo-caecal junction to anus. It lies in the periphery of abdomen. It is about 1.5 meter long. It is divided into caecum, colon and rectum.

• Caecum: It is a sac like structure which is connected with ileum. It is guarded by ileocaecal valve. A worm like structure called vermiform appendix is attached to caecum. In human it is vestigial organ.
• Colon: It is a inverted ‘U’ shaped tubular structure running in the periphery of the abdomen. It is divided into ascending colon, transverse colon, descending colon and sigmoid colon. Sigmoid colon is connected with rectum. Wall of colon contains 3 bands of muscles called taeniae coli. The sacculations seen in colon are called haustra.
• Rectum: It is a dilated tube which connects colon with anal canal. It stores faeces temporarily.

Large intestine absorbs water and also helps in defaecation by contraction of muscles. Bacteria present in large intestine produces vitamin K.

7. Anal Canal: Anal canal opens outside through anus. It is about 3.8 cm long. It is provided with external and internal sphincters. Anal canal contains anal columns which are provided with arteries and veins. When the vein are enlarged, it causes piles or haemorhoids as a result of which blood passes with stool.

Digestive glands:

Glands which secrete digestive enzymes are called as digestive glands. Digestive glands are of two types: Associated digestive glands and digestive glands of the alimentary canal.

Associated digestive glands are those glands which are outside but associated structurally and functionally with alimentary canal. They include salivary gland, liver and pancreas.

1. Salivary glands:

These are glands which secrete saliva into the mouth cavity. These are exocrine glands which are under the control of parasympathetic ans sympathetic nervous system. These may be major or minor salivary glands. Major salivary glands include three pairs of glands viz. parotid, sub-maxillary and sub-lingual salivary glands. Where as minor salivary glands are present in the lining of oral mucosa.

• Parotid gland: this is the largest salivary gland lying below and infront of each ear. They pour their secretion through the parotid ducts or Stenson’s ducts. It open into mouth cavity opposite to upper second molar teeth. It secretes about 25% of the saliva.
• Sub-mandibular or sub maxillary glands: This lies all along the body of the mandible in each side. They pour their secretions through the sub mandibular ducts or Wharton’s ducts. They open into mouth cavity under the tongue in each side of lingual fraenulum. They secrete about 70% of the saliva.
• Sub-lingual glands: They are the smallest salivary glands. They are lying in the floor of the mouth between tongue and mandible., more towards the midline. They pour their secretions into the mouth cavity by numerous sub-lingual ducts. They secrete about 5% of the saliva.

Histology: salivary glands are enclosed in a capsule of connective tissue. They are highly branched structure of alveoli which join to form lobules.

Saliva: It is a colourless odoudless transparent fluid. It is slightly acidic pH ranges from 6.35 to 6.85. About 1000 to 1500 ml of saliva are secreted daily. Saliva contains water-95%, solutes-0.5% (bicarbonates, phosphates, Sodium, Potassium), enzymes ( salivary amylase and lingual lipase), lysozyme, immunoglobulins etc.

Functions of saliva:

1. Initiates of digestion of starch(by salivary amylase) and lipid (by lingual lipase)
2. Moistens and lubricates foods.
3. Dissolves soluble food substances.
4. Stimulates taste buds.
5. Keeps mouth and teeth clean.
6. Lysozymes and immunoglobulin prevents from bacteria and microorganisms.

2. Liver:

Adult human liver weigh about 1.5 kg. It is the largest gland of our body. It is reddish brown in color which is enclosed in a fibrous thin fold of peritoneum. It is located on the right side of upper abdomen. It has a high capacity to regenerate. Liver is divided into two lobes- right and left lobes. Right lobe has again quadrate and caudate lobes. A small thin walled pear shaped structure called gall bladder is attached to inferior surface of liver. A duct called cystic duct arises from gall bladder which joins with hepatic duct of liver to form common bile duct which opens into the duodenum.

Histologically, liver is composed of large no. of hexagonal hepatic lobules. These  hepatic lobules are separated by a thin layer of connective tissue called Glisson’s capsule. Each lobule contain a central vein and six portal triads in each corner. Each portal triad contains branches of a hepatic duct, a portal vein and a bile duct. Each lobule contain hepatic cells arranged in chain to form hepatic cords or trabecule extending from central vein to the periphery. Slits between hepatocytes are called bile canaliculi which unite to form bile ductules which further unite to form hepatic ducts and then common hepatic ducts.

Liver contains special kinds of cells –phagocytic  Kupffer’s cells and fat storing Ito cells.

Functions of liver:

• Production of bile: bile is producedby hepatocytes and stored in gall bladder. It is poured in duodenum.
• Metabolism of carbohydrates, proteins and fats: liver is involved in glycogenesis, glycogenolysis and deamination.
• Synthesis of plasma proteins like albumin.
• Detoxification of drugs and poisons.
• Storage of vitamin A,D,E,K and B complex.
• Excretion of drugs and bile pigments.
• Immunity by phagocytic Kupffer’s cells.
• Production of RBC in foetus.

Function of bile:

• Bile neutralize the acidity of chime entering the duodenum.
• It provides alkaline medium which is necessary for the activity of pancreatic enzymes.
• Helps in digestion and absorption of fat and fat soluble vitamins-A,D,E and K.
• Excretion of breakdown product of haemoglobin called haeme in the form of bile pigment.

3. Pancreas:

It is the second largest gland of our body. It is long, extending from depression of duodenum to the spleen. It is divided into head, neck, body and tail.

Pancreas is both exocrine and endocrine gland. Exocrine part contains the pancreatic acini, which are lined with conical cells. These cells secrete pancreatic enzymes. Pancreatic acini contain a lumen in the centre which untie to form many ductules which again join  to form pancreatic ducts. Endocrine part of the pancreas contain islets of Langerhans, which is dispersed between acini. It contains three types of cells: α cells,β cells and δ cells. α cells secrete glucagon hormone, β cells secrete insulin hormone and δ cells secrete somatostatin hormone. These hormones are directly poured into the blood.

Functions of pancreas:

• Secretion of pancreatic enzymes from digestion.
• Secretion of hormones: insulin, glucagon and somatostatin.

Pancreatic juice: it is a clear alkaline fluid secreted by pancreas. It contains water, salts and digestive enzymes. Digestive enzymes include:

• Trypsin: secreted in inactive form trypsinogen ehich is activated by enteropeptidase. It digests protein.
• Chymotrypsin: its is also secreted in inactive form and activated by trypsin. It degests protein.
• Carboxypeptidase A and B: activated by trypsin and digests protein.
• Pancreatic lipase or steapsin: digestion of fat.
• Pancreatic amylase or amylopsin: digestion of starch.
• Ribonuclease and deoxyribonuclease: for the digestion of RNA and DNA.

4. Gastric glands:

Gastric mucosa contains three types of cells which are secretary in function. These are:

Parietal cell- secrete HCl, peptic cell- secrete pepsinogen and mucus cell secrete mucus.

Gastric juice contain water, different salts and gastric enzymes.

HCl kills bacteria, makes food acidic and activate inactivate pepsinogen into active pepsin.

Pepsin digests protein into proteoses and peptones.

Mucus protects mucosa lining fron HCl and pepsin.

5. Intestinal glands:

It includes two types of glands : Bruner’s gland and crypts of Lieberkuhn.

Bruner’s glands are found in duodenum and secrete mucus. crypts of Lieberkuhn are found throughout thesmall intestine and secrete intestinal juice or succus entericus.

Intestinal juice containwater, different salts and intestinal digestive enzymes.

Digestine enzymes includes:

Erepsin: for the digestion of protein.

Enteropeptidase: activates pepsinogen into pepsin.

Maltase: digests maltose in to glucose.

Sucrose: digests sucrose

Lactase: digests lactose.

Lipase: digests lipid.

Physiology of digestion:

1. Digestion in mouth:

Mouth serves for the reception of food. Both mechanical and chemical digestion takes place in mouth.

Mechanical digestion:

• ingested solid food is broken down into smaller particles by the process of mastication
• tongue helps to mix the small food particles with saliva
• food is reduced to a soft flexible bolus that can be easily swallowed

Chemical digestion:

Salivary juice begins the chemical breakdown of food and assists in swallowing the bolus

Lingual lipase starts digestion of dietary fats producing fatty acids and glycerol

Role of saliva:

• water part of saliva softens, moistens and dissolves food
• mucin lubricates food
• Lysosomes kills bacteria
• digestive enzyme, Ptyalin initiates the breakdown of starch and converts it into disaccharide, maltose

ptyalin

starch ( polysaccharide) —————————————–> maltose ( disaccharide)

lingual lipase

lipid ————————————————————->fatty acid + glycerol

2. Digestion in stomach:

Mechanical:

Mechanical digestion is effected by action of the muscular wall of stomach. Few minutes after food enters stomach, the peristaltic movement of stomach called mixing waves performs following functions

• grind food,
• mix it with gastric juice,
• reduce it to semi liquid form called chyme which is forced into the intestine through the pyloric sphincter

Chemical:

• Chemical digestion takes place by the action of gastric juice
• secretion of gastric juice is regulated by nerves and hormones

Role of gastric juice: Gastric juice contains HCl and the digestive enzymes pepsin, rennin and possibly gastric lipase.

HCl:

• maintains acidic pH of gastric juice
• loosens the fibrous components of food
• dissolves food particles and minerals
• kills large number of bacteria which enter the stomach with food and activates pepsinogen into pepsin.

Pepsin:

• Secreted in an inactive form pepsinogen
• Pepsinogen is converted into pepsin by HCl
• Pepsin acts on protein and polypeptides and converts them into peptides
• Pepsin is most effective in acidic environment of stomach and inactive in alkaline medium

HCl

Pepsinogen ( incative) ———————–>Pepsin (active)

pepsin

Protein ( polypeptide) —————————————> Peptides (di-peptide)

Renin (chymosin):

• found only in the stomach of infants
• important in the digestion of milk
• secreted as Prorennin which is converted to Rennin by HCl
• rennin acts on milk protein casein and converts it into a curd called calcium cassinate in the presence of calcium

Gastric lipase: Converts dietary fats into fatty acids and glycerol

1. Digestion in the duodenum and small intestine:

• the acidic chyme when enters the duodenum, gall bladder contracts and the bile is poured into the duodenum.
• pancreatic juice is also secreted in the duodenum
• acidic chyme is neutralized by the pancreatic secretion and pH is changed to alkaline side
• alkaline pH is necessary for the activity of enzymes of pancreatic and intestinal juice

1. Role of Bile juice:

• bile is an alkaline digestive juice secreted by the liver that reaches duodenum through bile duct
• doesn’t contain digestive enzymes
• bile salts such as sobium taurocholate and sodium glycocholate emulsify fat so that pancreatic lipase can cat upon fats
• bile also neutralizes acidity of chyme coming from stomach

1. Role of Pancreatic juice:

• alkaline digestive juice secreted by the lobules of pancreas
• flows into duodenum  through  pancreatic duct
• contains digestive enzymes such as Trypsin, chymotrypsin, carboxypeptidase, amylopsin, steapsin, deoxyribopolynucleotides and ribonucleotides.

1. Trypsin and chymotrypsin: Trypsin acts on proteins and converts them into peptides. Chymotrypsin is secreted as an inactive precursor chymotrypsinogen. It is converted into chymotrypsin by trypsin.

1. Role of intestinal juice:

• alkaline digestive juice secreted by intestinal gland
• contains digestive enzymes such as enterokinase, aminopeptidase, dipeptidase, sucrose or invertase, maltase, lactase, nucleotidase and nucleosidase

1. Enterokinase- It is a non-digestive intestinal enzyme which acts as activator. It activates trypsinogen of the pancreatic juice into trypsin.
2. Aminopeptidase- acts on terminal amino acids at the amino ends of peptides and break them into amino acids
3. Dipeptidase- acts on dipeptides to convert them into amino acids
4. Sucrose- converts sucrose into glucose and fructose
5. Maltase- converts maltose into  two molecules of glucose
6. Lactase- acts on lactose to convert it into glucose and galactose
7. Nucleotidase- acts on nucleotides to convert it into phosphate and nucleoside
8. Nucleosidase- acts on nucleoside to convert it into pentose sugar, purine and pyramidine.

Thus the intestinal juice plays an important role in the digestion of protein, carbohydrates, nucleotides and nucleosides.

2. Absorption:

All the digested nutrients like glucose, amino acid, fatty acids, glycerol, vitamins etc are absorbed from lumen of small intestine to the body. The walls of the small intestine are provided networks of blood capillaries into which the nutrients are transported. Musosa lining of small intestine are also modified into numerous fine fingerlike processes called as microvilli which increases the surface area for absorption. Nutrients are absorbed in two ways:

• Passive absorption
• Active absorption

• Passive absorption: Absorption of nutrients from higher concentration area towards the lower concentration area i.e. according to the concentration gradient is called as passive absorption. This process takes place from lumen to blood capillaries of the intestine. It is a slow process and doesn;t require any energy. It occurs until the concentration on both side becomes equal and then it stops. Complete absorption is not possible by this process.
• Active absorption: Absorption of nutrients from lower concentration area towards the higher concentration area i.e. against the concentration gradient is called as active absorption. This process is a rapid process which is taken place by the help of carrier proteins of the plasma membrane. It requires energy which is provided by ATP. Complete absorption of the nutrients takes place by this process.

Nutrients like glucose, amino acid, vitamins are absorbed directly from lumen of intestine to the blood capillaries. But, fatty acids,glycerol, cholesterol etc are absorbed into the lymph channel(lacteals) in the form of micelles ( after mixing with bile) and chyle. These are milky fluid which is soluble form of fat.

Absorption of water takes place in large intestine where the undigested waste are solidified and finally stored in rectum.

3. Defaecation:

The undigested waste or faeces which are stored in rectum are egested out when the rectal muscles contract to push faeces out of the body through anus.