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Introduction: All the organisms require energy for various life activities like growth, renewal of body tissues, reproduction etc., Food is the basic need of all the living organisms. The major components of food are carbohydrates, proteins & fats. Vitamins & minerals are also required in small quantities. Food provides the energy required to the body. The chemical constituents of food essential for the body are called nutrients. Most of the food taken by animals or human beings are in the complex form. The body cells can't make use of the complex forms of food directly. The food has to be converted from complex form to simpler form. Digestive System of Man (Gross Anatomy) The human digestive system consists of the Alimentary canal and its associated Digestive glands. I Alimentary Canal : (Digestive tract or gut or gastro intestinal tract). It is an elongated, tubular structure extending from mouth to the anus. It measures about 9 meters in length. Alimentary canal is differentiated the buccal cavity, pharynx, oesophagus, stomach, small intestine and large intestine.
1. Buccal Cavity : It begins with the mouth. It is the first part of the alimentary canal. It is bounded by the upper and lower jaw. It encloses a cavity called mouth or buccal cavity. The roof of the buccal cavity is made up of a bone called palate. The palate is differentiated into anterior hard palate and posterior soft palate. The soft palate posteriorly extends into a conical muscular projection called uvula. It closes the internal nostrils during swallowing the food. The floor of the cavity is occupied by tongue. Tongue (lingula or glossa): The tongue is highly muscular, sensory organ made up of skeletal muscles attached to the floor of the buccal cavity. The dorsal surface of the tongue is lined by stratified epithelium. The tongue is provided with numerous projections called lingual papillae, which are present on the dorsal surface of the tongue. There are three types of papillae, namely circum Vallate (8 to 12), fungi form and filiform paillae. Taste buds are present only in circum vallate & fungi form papillae. The tongue helps in mixing food with saliva, deglutition (swallowing) and in speech. Teeth: Arrangement of teeth on the jaws is called dentition. Teeth help in cutting, tearing and grinding the food materials. Teeth are hard bony structures covered by dentine and perform various functions during feeding. The hard chewing surface of teeth are made up of enamel helps in mastication of food. Totally four types of teeth are present, namely Incisor, Caninne, premolar and molar. The incisors are the cutting teeth. The canines have pointed ends and used for tearing the food. Premolars and
molars have peculiar grinding surfaces which are used for grinding the food materials. The human teeth is said to be thecodont, heterodont & diphyodont type. Thecodont : The conditions in which the teeth are located in the sockets of jaws is called thecodont. Heterodont : The condition in which the different kinds of teeth i.e. incisors, canines, premolar and molar performs different functions is called heterodont. Diphyodont : The condition in which formation of two sets of teeth during the life time of an individual is called diphyodont. The two sets of teeth develop in human beings during their life time. They are primary dentition (milk dentition) and Secondary dentition (permanent dentition). Milk dentition : One set of teeth develops during early life time is called milk dentition. Permanent dentition : Milk teeth are replaced by the permanent teeth is called permanent dentition. Dental formula : The number of teeth present on the jaws is represented by the formula called dental formula. Fig. 1.1 : Human oral cavity. Fig. 1.2 : Arrangement of human permanent teeth The last molars also called wisdom teeth. The milk set are 20 in number and permanent set are 32 in all. Three pairs of salivary glands open into the buccal cavity. They secrete saliva. Hence buccal cavity helps both in mechanical as well chemical digestion.
2. Pharynx (Throat): The buccal cavity leads into the next part called pharynx. The pharynx is a muscular region measures about 12cms in length. It is a common passage for both food and air. The pharynx can be differentiated as follows. Nasopharynx: The upper part of the pharynx adjacent to the nasal chamber is called nasopharynx.
provided with the sphincter called pyloric sphincter (pyloric valve). It prevents the reverse flow of food from duodenum to the stomach. Inner wall of the stomach is provided with a numerous folds called rugae. Millions of gastric glands present in wall of the stomach and open into lumen of the stomach. They secrete gastric juice containing gastric enzymes and HCl. Fig.1.4 : Section of the stomach Fig.1.5 : Areas of the stomach 5.Small intestine: The stomach leads into the small intestine. It is the longest part of the alimentary canal which measures about 6.5 meters in length (22 feet). It is highly coiled muscular tube. It can be differentiated into three regions. i.e. duodenum, jejunum and ileum. Duodenum: It is the first part of the small intestine. It is 10 inches in length. It receives the digestive juice from liver, pancreas and Brunner's glands. Jejunum: It is the middle part of the small intestine. It measures about 2.5 meters in length (10 feet). Ileum: Ileum is the last part of the small intestine, measures about 3.6 meters in length ( feet). Inner wall of jejunum & ileum is provided with number of finger like projections called villi. Each villus is lined with brush like border called microvilli. The Villi and microvilli are responsible for an increase the surface area of absorption. Small intestine helps in digestion and absorption. Intestinal glands (Crypts of Lieberkuhn) are present at the base of villi. These glands secrete intestinal juice (succus entericus) that plays an important role in the digestive process. Ileocaecal valve is present in between the ileum and caecum.
6. Large intestine : Small intestine leads to the large intestine. It measures about 1. meters in length. It is much wider than the small intestine. It is differentiated into caecum, colon and rectum.
Caecum: It is the first part of large intestine. At the junction of small and large intestine (ileum & caecum) a finger like outgrowth is present it is called Vermiform appendix. It has no function. It is a vestigial organ. It is about 12cms, in length. Infection and inflammation of the vermiform appendix is called Appendicitis. The process of removal of appendix by surgical method called Appendectomy. Colon: It is the middle part of large intestine. is further differentiated into ascending colon, transverse colon and descending colon. The descending colon produces an S-Shaped portion called sigmoid colon. It plays an important role in absorbing the water and minerals from the undigested food. Vitamin B is synthesized by the bacteria in the colon. Rectum: It is the last part of large intestine. It is about 20cms in length. It opens to the exterior by anus. Rectum stores the undigested food and it is thrown out through anus. Functions of the large Intestine : It absorbs the water & electrolytes from the undigested material so that the undigested matter becomes semi solid mass called faeces or stool. It secretes mucin that is useful for lubricating the faeces so that it can easily pass through the rectum and anus. It excretes excess calcium, iron, drugs and other unwanted materials. Bacteria which escape the effect of HCI in the stomach pass through small intestine and multiply in the colon. These bring about the decomposition of the cellulose part coming with the undigested food materials. The translatory movements of the colon help in pushing the water materials into rectum and to outside through anus. Table : 1.1. Summary of the Alimentary canal 1 Buccal cavity It is the first part of the alimentary canal. Its roof is formed by palate. Palate is differentiated into anterior hard and posterior soft palate. Floor of the buccal cavity is formed by the hyoidian muscles and is occupied by the tongue. Tongue is lined by stratified epithelium bearing numerous papillae. 2 Pharynx (^) It is a common passage for both the food and air. It is differentiated into nasopharynx, oropharynx and laryngopharynx. 3 Oesophagus It is a food pipe, meant for the passage of food. 4 Stomach It is a sac like structure differentiated into five parts namely cardiac, fundic, body, antrum and pyloric stomach. 5 Small intestine It is differentiated into duodenum, jejunum and ileum.
2.Gastric glands : Inner wall of the stomach is lined by mucous membrane. Gastric glands are embedded in the wall of the stomach. They secrete gastric juice. Gastric glands contain there types of cells namely, i) Chief(peptic) cells : They secrete enzymes like pepsin, renin &gastric lipase. ii) Parietal cells (Oxynticcells): These cells secrete HCl. iii) Goblet (mucus) cells : These cells secrete mucus. The combined secretion of all the gastric glands forms gastric (gastrin) juice. Gastric juice: Gastric juice is a colourless fluid. The pH of gastric juice range between 1to 2.5. 2 to 2
litres of gastric juice is secreted per day. The secretion of gastric juice is stimulated by the hormone called Gastrin. The gastric juice contain water, mucous, HCl and enzymes like pepsin, renin, gastric lipase and gelatinase. Functions of gastric juice: Water i n the gastric juice moistens the food. Mucus i n the gastric juice protect the stomach wall. HCl in the gastric juice activate the pepsinongen enzyme, prevent the decay of food in stomach, helps in curdling of milk and loosens the fibrous food. Pepsin - It helps in protein digestion. It converts proteins into proteoses and peptones. Renin - It is an enzyme helps in curdling of milk. Gastric Lipase - It is a lipid digesting enzyme Gelatinase- It is an enzyme helps in gelatin digestion. Role of HCl in digestion : HCl present in the gastric juice performs the following functions. HCl plays an important role to convert inactive pepsinogen into active pepsin. It prevents the decay (putrification) of food in the stomach. The HCl kills bacteria and loosens the fibrous food molecules. iv. It releases iron from the food molecules. It brings about the curdling of milk. It is responsible for the acidic nature of gastric juice. Maceration :- The process in which loosening of the fibrous food molecules in the presence of HCl is called Maceration. Functions of Gastric Enzymes : Pepsin is a proteolytic enzyme. It acts on proteins and converts it into proteoses and peptones. Renin is a milk curdling enzyme. It is produced in the form of prorenin. It is converted into active renin by HCl. Renin acts on milk protein called casein. Gastric lipase is ineffective since fats are not emulsified in stomach. Gelatinase liquifies gelatin.
3. Intestinal glands: Intestinal glands are located in the mucous membrane of the intestine. They secrete intestinal juice called succus entericus. There are two types of intestinal glands, namely i) Brunner's (duodenal) glands found only in the duodenum ii) Crypts of Lieberkuhn found throught the small intestine. These glands are stimulated by Enterokinin. Fig. 1.8 : Intestinal glands Succus entericus (Intestinal juice) : It is a yellow coloured fluid. Its pH is 7 to 8. It contains water, mucin and enzymes like Enterokinase (Enteropeptidase), Maltase, Lactase, Sucrase, Aminopeptidase, Tripeptidase and Dipeptidase. Functions of intestinal juice: Water: It moistens food and makes suitable medium for digestion Mucin: It lubricates food for smooth passage. Enterokinase is the non digestive activating enzymes. [Enterokinase enzyme is an activating enzyme helps to convert inactive tripsinogen into an active trypsin] iv. Tripeptidase, dipeptidase & aminopeptidase (Erepsin) enzymes acts on peptides and converts into amino acids. Maltase, sucrase and lactase are the carbohydrses helps to converts disaccharides into monosaccharides. Nuclease enzyme involved in nucleic acid digestion.
acini. The acinar cells to secrete pancreatic juice. Pancreatic duct (duct of wirsung) carries the pancreatic juice from pancreas to the duodenum. Endocrine part of the pancreas is called Islets of Langerhans. It contains different types of cells like a-cells, ß-cells, ö-cells secreting insulin, glucagon and somatostatin respectively. Exocrine part of pancreas secretes pancreatic juice. Pancreatic juice: It is a colourless alkaline fluid containing water, some salts, sodium bicarbonate and enzymes like pancreatic amylase (Amylopsin), Trypsin, Chymotripsin, carboxypeptidase, Pancreatic lipase (Steapsin) and Nuclease. Its pH is 7 to 8.2. Functions of pancreatic juice : Pancreatic amylase acts on starch/glycogen and converts into monosaccharides. i.e helps in Carbohydrate digestion. Pancreatic lipase (Steapsin) acts on lipids helps in the digestion of lipids. Trypsinogen, chymotrypsinogen, carboxypeptidase & elastase are protein digesting enzymes helps in protein digestion. Nuclease enzyme involved in nucleic acid digestion. Table : 1.2. Summary of Digestive glands : 1 Salivary glands There are 3 pairs namely, parotid, sub maxillary and sublingual gland. In addition to these there are numerous small glands called labial, buccal, palatine and lingual glands which are present embedded in the mucous membrane. 2 Gastric glands These are tubular glands containing chief or peptic cells which secrete proenzymes, oxyntic or parietal cells which secrete HCI, Goblet or mucous cells which secrete mucous. 3 Liver It is the largest exocrine gland, weighing about 1.5 kg. It is differentiated into 4 lobes namely, the right, left, quadrate and caudate lobe. 4 Pancreas It is second largest gland. It is double or dual or heterocrine gland. It is both exocrine as well as endocrine in function.
- Exocrine part of the pancreas is called pancreas proper and secretes pancreatic juice.
- Endocrine part of the pancreas is called islets of Langerhans’s and made up of 3 types of cells namely Alpha, beta and delta cells secreting glucagon, insulin and somatostatin respectively. 5 Intestinal glands namely, They are simple tubular glands.
Brunner’s gland Crypts of Lieberkuhn. Brunner’s glands are located in the duodenum. They secrete alkaline fluid and crypts of Lieberkuhn. Secrete alkaline fluid and intestinal juice. Table : 1.3. Summary of digestive juice: Sl. No. Secretion pH, colour & amount Other components Enzymes 1 Saliva 6.7-7, colourless, about 1 to 1.5 litres 98-99% of water, 2% of salts like NaCl, HCO3,mucin Lysozyme, salivary amylase, lingual lipase 2 Gastric juice 1.5-3, colourless, about 2 to 2.5 litres Water, mucous, HCl Pepsin, Renin, gastric lipase, Gelatinase 3 Intestinal juice 7 to 8, straw yellow, about 1 to 1.5 litres Water, mucin Enterokinase, Aminopeptidase, Tripeptidase, dipeptidase,maltase, lactase & sucrase 11 4 Bile 7- 8, greenish yellow, 1/2- liter Water, bile pigments like bilirubin & biliviridin, bile salts like sodium and potassium taurcholate and glycocholate, cholesterol and phospholipids like lecithin Nil 5 Pancreatic juice Above 8, colourless, about 1 to 1.5 litres Water Pancreatic lipase, pancreatic amylase, Trypsinogen, Chymotripsinogen, Carboxypeptidase, Elastase and nuclease
required in small quantities. Physiology of Digestion Digestion: The process in which conversion of complex food molecules into the simpler food molecules by the action of the digestive enzymes which are suitable for absorption and assimilation is called digestion. OR Digestion is the chemical process by which complex and large food molecules are broken down into simple molecules by the action of digestive enzymes which are suitable for absorption and assimilation. Types of Digestion: Based on the location, the digestion is classified into two types. They are as follows. A. Intracellular digestion: The digestion occurs within the cell is called intracellular digestion. Ex: Lower organisms like protozoans, poriferans. etc. B. Extracellular digestion: The digestion occurs outside the cells, but within the lumen of the alimentary canal is called extracellular digestion. Ex: Higher organisms. Based on the mode of digestion, the digestion is classified into two types, namely 1) Mechanical digestion: The process of the fragmentation of food by the mechanical energy (Jaws, tongue, teeth) is called mechanical digestion. 2) Chemical digestion: The process of the fragmentation of food by the action of enzymes is called chemical digestion. Table : 1.5. Summary of Types of Digestion : 1 Intracellular digestion Here digestion occurs within the cell 2 Extracellular digestion Here digestion takes place outside the cell, but within the lumen of the alimentary canal 3 Mechanical digestion It is the fragmentation of food by the mechanical energy 4 Chemical digestion It is fragmentation of food by chemicals called enzymatic hydrolysis Enzymes involved in digestion: Hydrolases : The enzymes which are participating in digestion are collectively termed as hydrolases. There are three types of enzymes involved in the digestion. They are as follows.
- Proteases ( Proteolytic enzymes): The enzymes which acts upon protein molecules are called proteolytic enzymes. Ex: Pepsin, Renin, Trypsin, Chymtrypsin, Aminopeptidase etc.,
- Carbohydrases ( Amylytic enzymes) : The enzymes which acts on carbohydrates called Amylytic enzymes. Ex: Salivary amylase, Pancreatic amylase, Maltase, Lactase, Sucrase.
- Lipases (Lipolytic enzymes): The enzymes which acts upon lipids are called lipolytic enzymes. Ex: Gastric lipase, Pancreatic lipase.
- Nucleases : The enzymes which acts upon nucleic acids are called nucleases. End products of proteins, carbohydrates & lipids (fats): The glucose, fructose and galactose are the end products of carbohydrate digestion. Amino acids are the end products of protein digestion. The fatty acids and glycerol are the end products of fat digestion. Table : 1.6. Summary of the Digestive Enzymes : Sl. No Digestive Enzymes Substrate End products 1 Carbohydrases / Amylases Carbohydrates Glucose, fructose, sucrose and galactose 2 Proteases/proteolytic enzymes Proteins Amino acids 3 Lipases / lipolytic enzymes Fats Fatty acids and glycerol 4 Nucleases Nucleic acid Pentose sugar, nitrogen bases and phosphate Physiology of Digestion : The process of digestion begins from the mouth itself. It involves both mechanical and chemical processes of the digestion of food. The stages in digestion are ingestion, digestion, absorption, assimilation and egestion (defaecation). I. Mechanical digestion : The food is cut into small pieces with the help of teeth, it is called mastication. The tongue mixes the food with saliva it is called insalivation. Fragmentation of the food is also by the rolling action of tongue & peristalsis movement of alimentary canal. II. Chemical digestion : The process of the fragmentation of food by the action of enzymes is called chemical digestion. Digestion of Carbohydrates :
Intestinal Juice : Intestinal juice containing carbohydrate digesting enzymes such as maltase, lactase and sucrase acts upon disaccharides and converting them into monosaccharides. Maltase acts on Maltose and converts it into glucose. Maltase Maltose Glucose + Glucose pH 7t08. Lactase acts on Lactose (milk sugar) and converts it into glucose and galactose. Lactase Lactose Glucose + Galactose pH 7 to 8. Sucrase acts on sucrose (cane sugar) and converts (splits) it into glucose and fructose. Sucrase Sucrose Glucose + Fructose (invertase) The simple sugars like glucose, galactose and fructose are the end products of carbohydrate digestion. They are ready for the absorption by the microvilli present in the small intestine. Digestion of Proteins: The process by which breakdown of the complex protein molecules into simpler amino acids with the help of proteases are called protein digestion. The digestion of protein occurs only in stomach and small intestine.
- Protein digestion in Mouth: Saliva does not contain any proteolytic enzymes, therefore proteins do not undergo any change in the mouth.
- Protein digestion in the stomach: The digestion of protein begins in the stomach. The gastric juice containing two protein digesting enzymes namely pepsin and renin. Pepsin : The pepsin is secreted in the form of pepsinogen. The inactive pepsinogen is converted into active pepsin in the presence of HCl. HCl Pepsinogen pepsin Pepsin Proteins Proteases, peptones and polypeptides.
pH 1.5t Proteins are converted into fragments (pieces) of different sizes like proteases, peptones and polypeptides in the presence of pepsin enzyme. Renin : The Renin is a milk curdling enzyme present in infants. It is secreted as prorenin. It is converted into an active renin by HCl HCl Prorenin Renin Renin acts on the milk protein i.e. casein converting it into paracasein Renin Casein Paracasein. In the presence of calcium (Ca^2 +) of the milk the paracesein is further converted into calcium paracaseinate. Ca2+ Paracasein Calcium Paracaseinate. Calcium Paracaseinate settles down as curds. Then calcium Paracaseinate is acted by pepsin and is converted into proteases, peptones and polypeptides. Calcium paracaseinate Proteases, peptones & polypeptides.
- Protein digestion in small intestine: The partially digested proteins enters from the stomach into the duodenum. Here the proteins come under the action of pancreatic proteases and intestinal proteases. a. Pancreatic proteases : The Trypsin, Chymotrypsin and Carboxypeptidases are the proteolytic enzymes present in the pancreatic juice. Trypsin : It is produced in the form of trypsinogen. The inactive trypsinogen is converted into active trypsin in the presence of Enterokinase (activator enzyme) secreted from the intestinal glands. Enterokinase Trypsinogen Trypsin Trypsin Proteins Proteases, peptones and polypeptides
The end product of protein digestion are free aminoacids. They are ready for the absorption by the microvilli present in the small intestine. Digestion of Fats : The process of breakdown of fat molecule into fatty acids and glycerol in the presence of lipases is called fat digestion. Fats of our diet mostly contains triglycerides. i.e. three molecules of fatty acids attached to the one molecule of glycerol. I. Fat digestion in Mouth: The lingual lipase is secreted from the Ebner's gland of tongue, acts on triglycerides (TGL) and converts it into fatty acid and glycerol. Lingual lipase Triglycerides (TGL) Fatty acid + Diglcerol(DGL) II. Fat digestion in the stomach: Gastric lipase is secreted from the gastric glands of the stomach. It may act on short-chain of lipid molecules. III. Fat digestion in the intestine: The duodenum receives the bile juice from liver and pancreatic juice from pancreas. Digestion of fat requires bile salts for emulsification. The emulsification is a process by which fats are converted into small water soluble miscles by bile salts making them easily accessible to the action offat digesting enzymes (i.e lipases ). Bile Salts Fats Emulsified fat. pH 7 to 8. Emulsified fat is freed from the bile salts by an enzyme called Colipase. It is then acted upon by pancreatic lipase and is converted into fatty acids and glycerol. Pancreatic Lipase Emulsified fat freed from bile salts Fatty acid + glycerol. (Steapsin) Emulsification : Emulsification is a process by which large fat droplets are converted into small droplets (water soluble miscles) of about 0.1mm diameter by the bile salts. OR Emulsification is a process by which fats are converted into small water soluble misccles by bile salts making them easily accessible to the action of fat digesting enzymes (lipases). Physiology of digestion in the Alimentary canal : Digestion of food mainly occurs in mouth, stomach and in small intestine.
1. Digestion in mouth: The saliva contains salivary amylase or Ptyalin. It acts on starch & converts into disaccharides. Salivary amylase
Starch Maltose Only 3-4% of the starch in the food is digested. There is no digestion of proteins in the buccal cavity as there are no protein digesting enzymes in the saliva. It is carried out by lingual lipase, secreted by the Ebners gland of the tongue. Lingual lipase acts on dietary triglycerides and converts it into fatty acid and glycerols. Lingual lipase Triglycerides Fatty acids + Glycerol At the end of the buccal digestion food is converted into a soft mass of ball called bolus. It passes down through oesophagus by muscular contraction called peristalsis and reaches the stomach.
2. Digestion in stomach: The stomach is internally lined by mucous membrane and there are glands in the wall of the stomach called gastric glands. Chief cells or peptic cells which secrete proenzymes. Oxyntic cells or parietal cells which secrete HCl.. The mucous is secreted by the Goblet cells present towards the opening of the glands. The stomach stores the food for 4-5 hours. The food mixes with the acidic gastric juice of the stomach by the churning movement and is called the Chyme. Since there are no starch digesting enzymes in the gastric juice there is no digestion of carbohydrates in the stomach. The gastric juice contains the enzymes apart from HCl and mucin. Pepsin and Renin are the two protein digesting enzymes present in the gastric juice. The gastric juice contains Pepsinogen which is converted into active Pepsin by HCl. The pepsin acts on proteins and converts them into proteoses and peptones. HCl Pepsinogen pepsin Pepsin Proteins Proteoses + peptones. Renin : It is produced as Prorenin and converted into active renin by HCl. It acts on milk protein called Casein. It is a milk curdling enzyme. HCl Prorenin Renin Renin acts on Casien and produces paracasein. Renin Casein Paracasein
