Digestive System
The chemical substance present in the food is called nutrients.Depending upon the quantity or functions nutrients are of two types:
i) Macronutrients or Proximate Principles of food which provide energy,eg. Carbohydrates,Lipids and Proteins.
ii) Micronutrients or Protective Principles of food.They do not provide energy but by their deficiency can cause specific diseases,eg.Minerals,Vitamins and Water.
2. Kinds of Heterotrophic Nutrients
Animals which feed on the high energy organic molecules as food are called Heterotrophs.Their mode of nutrition is known as heterotrophic nutrition.
i)Holozoic Nutrition: It involves taking in of the whole or part of a plant or an animal either in solid or in liquid state,eg.most of free-living Protozoans and animal.
ii)Saprozoic Nutrition: A few animals secrete digestive enzymes directly into their food outside the body.Thus food is digested outside the body is then sucked.This is called Saprozoic Nutrition,eg.Spiders,House fly etc.
iii) Parasitic Nutrition: Parasites take food from the host,eg.Plasmodium,Trypanosoma,Taenia,Ascaris etc.
iv)Symbiotic Nutrition (=Mutualism): Two organism derive food from each other,eg.Escherichia Coli of human intestine synthesises vitamin B12 to be used by man and E. Coli receives food from human intestine.
3. Holozoic Animals
i) Herbivores: Plant eating animals are called Herbivores,eg.Cow,Horse,Rabbit etc.
ii) Carnivores: Flesh eating animals are called Carnivores,eg.Lion,Tiger etc.
iii) Omnivores: These animals feed on plant as well as flesh,eg.Cockroach,Crows,Rats,Bears,Man etc.
Other variations are: i) Insectivores (Insect eaters)-Toads,Lizards etc.
ii)Frugivores(Fruit eaters)-Birds,Bats,Monkeys etc.
iii)Sanguivores(Feeding on blood)-Leeches,Female Mosquitoes,Bedbugs,Vampire Bats etc.
iv)Detritivores(Feeding on decaying organic matters)-Earthworms
v) Fluid Feeders-Butterflies,Male Mosquitoes etc.
vi)Coprophagous(Eat their own faecal matter)-Rabbit
vii) Cannibals (Eat their own fellow or species)-Cockroach,Bedbug etc.
Holozoic nutrition involves four main steps:
i)Ingestion- In take of food
ii)Digestion-Breaking down of complex organic food molecules into simpler and absorbable molecules.Digestion may be Intracellular(in Amoeba) or Extracellular(in man).
iii) Absorption-Simpler organic molecules are absorbed.
iv)Egestion-Elimination of undigested food from the body.
The digestive system of man consists of two main parts: Alimentary Canal and Digestive Glands.
Alimentary Canal
Human Digestive System
The digestive system consists of two main parts: Alimentary Canal and Digestive Glands.
(A)Alimentary Canal
1. Oral Cavity.The roof of oral cavity is called Palate.Anterior part of the palate is Hard Palate and with transverse ridges called rugea.Posterior part of the palate is Soft Palate.The hinder free part of the Soft Palate hangs down as a small flap,the Uvula.
Floor of oral cavity has tongue that has four types of papillae (little projections)
i)Vallate Papillae-Largest
ii)Filiform Papillae-Smallest and most numerous
iii)Fungifirm Papillae-Less numerous than Filiform
iv) Foliate Papillae-Not developed in human tonue.
Teeth: Most of mammals have diphyodont teeth (i.e. two sets of teeth-milk or deciduous and permanent).
-Thecodont teeth (i.e. teeth are embedded in the sockets of the jaw bone)
-Heterodont teeth (i.e. different types of teeth)
-Four kinds of teeth (Incisor, Canine, Premolar and Molar)
-Last molar teeth in human being are called wisdom teeth.
-Milk teeth of man (Incisors-8, Canines-4, Molars-8)
-Permanent teeth of man (Incisors-8, Canines-4, Premolars-8, Molars-12)
-The Incisors and Canines have one root.
-The upper Premolars have two roots.
-The lower Premolars usually have only one root.
-The upper Molars have three roots and the lower Molars have two roots.
-Enamel is the hardest part of the human body.
-Odontoblast(Dentine forming cells)
-Ameloblast(Enamel forming cells)
-First permanent teeth to appear is first Molar.
-Last permanent teeth to appear are third Molars.
The teeth of the lower jaw usually appear before those of the upper jaw.
-Teeth in females appear earlier than in males.
Pharynx (Throat):
-Divided into three parts for easily descriptive purpose
i)Nasopharynx
-Lies behind the Nasal Chamber
-The Eustachian Tube (Auditory Tube) connects Nasopharynx with Middle Ear.
ii)Oropharynx
-Lies behind the Oral Cavity (Buccal Cavity)
iii)Laryngopharynx
-Lies behind the opening into the Larynx and posterior surface of Larynx.
Oesophagus(Food Pipe):
-Passes through the diaphragm and opens into the Stomach.
-Human Oesophagus is about 25cm long.
-Have three parts
i) Cervical part (in the neck)
ii) Thoracic part (in the Thorax)
iii) Abdominal part (in the Abdomen)
Stomach (Gaster):
-Widest part of the Alimentary Canal
-Divided into four parts
i) Cardiac part
-Upper part of the stomach
-Cardiac sphincter (not true valve, functional sphincter) lies in the opening between Oesophagus and Stomach
ii)Fundus part
-Middle part of Stomach
-It extends superiorly from the Cardiac part
-It is commonly filled with air or gas
iii)Body part
-Main part of the Stomach
iv)Pyloric part
-Distal part of the Stomach
Function of Stomach:
-It stores food for some time
-It churns and breaks up the food into smaller particles and mixes with gastric juice secreted by Stomach
-It secretes gastrin (hormone) and Castle’s intrinsic factor (Glycoprotein)
-Castle’s intrinsic factor is important for the absorption of Vit. B12 absorbed in small intestine
-partial digestion of food(protein and fat) takes place here
-Alcohol, Aspirin, some lipid-soluble drug,moderate amount of sugar and water are absorbed by the Stomach wall.
Small Intestine:
-It has small diameter
-Length is correlated with the height of the individual and not the weight.
-Longest part of the Alimentary Canal (about 6.25m long)
Divided into three parts
i)Duodenum(about 25cm long)
-Shortest and widest part of the small intestine
-The Hepatopancreatic Ampulla(Ampulla of Vater) opens into it.
-This Ampulla receives bile duct from Liver and Pancreatic Duct from Pancreas.
ii)Jejunum(about 2.5m long)
-Middle part of the small intestine
iii)Ileum(about 3.5m long)
-Longest part of the small intestine.
-Small nodules of Lymphatic tissue can be seen along the entire length of small intestine. In some places, particularly along the Ileum , these nodules are clustered together in groups called Peyer’s Patches or Lymph nodules. Peyer’s patches are the distinguishing feature of Ileum, which produce Lymphocytes(a type of WBC).Though less conspicuous , lymph nodes are found throughout the small intestine. Finger-like projection of mucosa, the Villi are present in the small intestine.Villi are absent over the Peyer’s patches. The villi increase the surface area of the small intestine.Villi are absent over the Peyer’s patches. The villi increase the surface area of the small intestine.Each villus is covered with epithelium and contains a lymph capillary (lacteal) and blood capillaries.The entire small intestine has circular folds of the submucosa, the plicae circularis (‘Valves’ of Kerkring). These folds are more prominent in the Jejunum. They further increase the absorptive surface considerably.
The digestive system consists of two main parts: Alimentary Canal and Digestive Glands.
(A)Alimentary Canal
Floor of oral cavity has tongue that has four types of papillae (little projections)
i)Vallate Papillae-Largest
ii)Filiform Papillae-Smallest and most numerous
iii)Fungifirm Papillae-Less numerous than Filiform
iv) Foliate Papillae-Not developed in human tonue.
-Thecodont teeth (i.e. teeth are embedded in the sockets of the jaw bone)
-Heterodont teeth (i.e. different types of teeth)
-Four kinds of teeth (Incisor, Canine, Premolar and Molar)
-Last molar teeth in human being are called wisdom teeth.
-Milk teeth of man (Incisors-8, Canines-4, Molars-8)
-Permanent teeth of man (Incisors-8, Canines-4, Premolars-8, Molars-12)
-The Incisors and Canines have one root.
-The upper Premolars have two roots.
-The lower Premolars usually have only one root.
-The upper Molars have three roots and the lower Molars have two roots.
-Enamel is the hardest part of the human body.
-Odontoblast(Dentine forming cells)
-Ameloblast(Enamel forming cells)
-First permanent teeth to appear is first Molar.
-Last permanent teeth to appear are third Molars.
The teeth of the lower jaw usually appear before those of the upper jaw.
-Teeth in females appear earlier than in males.
Pharynx (Throat):
-Divided into three parts for easily descriptive purpose
i)Nasopharynx
-Lies behind the Nasal Chamber
-The Eustachian Tube (Auditory Tube) connects Nasopharynx with Middle Ear.
ii)Oropharynx
-Lies behind the Oral Cavity (Buccal Cavity)
iii)Laryngopharynx
-Lies behind the opening into the Larynx and posterior surface of Larynx.
Oesophagus(Food Pipe):
-Passes through the diaphragm and opens into the Stomach.
-Human Oesophagus is about 25cm long.
-Have three parts
i) Cervical part (in the neck)
ii) Thoracic part (in the Thorax)
Stomach (Gaster):
-Widest part of the Alimentary Canal
-Divided into four parts
i) Cardiac part
-Upper part of the stomach
-Cardiac sphincter (not true valve, functional sphincter) lies in the opening between Oesophagus and Stomach
ii)Fundus part
-Middle part of Stomach
-It extends superiorly from the Cardiac part
-It is commonly filled with air or gas
iii)Body part
-Main part of the Stomach
iv)Pyloric part
-Distal part of the Stomach
Function of Stomach:
-It stores food for some time
-It churns and breaks up the food into smaller particles and mixes with gastric juice secreted by Stomach
-It secretes gastrin (hormone) and Castle’s intrinsic factor (Glycoprotein)
-Castle’s intrinsic factor is important for the absorption of Vit. B12 absorbed in small intestine
-partial digestion of food(protein and fat) takes place here
-Alcohol, Aspirin, some lipid-soluble drug,moderate amount of sugar and water are absorbed by the Stomach wall.
-It has small diameter
-Length is correlated with the height of the individual and not the weight.
-Longest part of the Alimentary Canal (about 6.25m long)
Divided into three parts
i)Duodenum(about 25cm long)
-Shortest and widest part of the small intestine
-The Hepatopancreatic Ampulla(Ampulla of Vater) opens into it.
-This Ampulla receives bile duct from Liver and Pancreatic Duct from Pancreas.
ii)Jejunum(about 2.5m long)
-Middle part of the small intestine
iii)Ileum(about 3.5m long)
-Longest part of the small intestine.
-Small nodules of Lymphatic tissue can be seen along the entire length of small intestine. In some places, particularly along the Ileum , these nodules are clustered together in groups called Peyer’s Patches or Lymph nodules. Peyer’s patches are the distinguishing feature of Ileum, which produce Lymphocytes(a type of WBC).Though less conspicuous , lymph nodes are found throughout the small intestine. Finger-like projection of mucosa, the Villi are present in the small intestine.Villi are absent over the Peyer’s patches. The villi increase the surface area of the small intestine.Villi are absent over the Peyer’s patches. The villi increase the surface area of the small intestine.Each villus is covered with epithelium and contains a lymph capillary (lacteal) and blood capillaries.The entire small intestine has circular folds of the submucosa, the plicae circularis (‘Valves’ of Kerkring). These folds are more prominent in the Jejunum. They further increase the absorptive surface considerably.
Function of Small Intestine:
-Small Intestine completes the digestion of Proteins, Carbohydrates, Nucleic Acid and Fats.
-It absorbs nutrient materials into the blood and lymph.
-It secretes some hormones such as Cholecystokinin, Secretin, Enterogasterone, Duocrinin, Enterocrinin and Villikinin.
Large Intestine:
-Its diameter is larger than small intestine.
-Divisible into three parts
i)Caecum and Vermiform Appendix:
-Vermiform Appendix(Appendix) is an outgrowth of Caecum.
Appendix is thought to be vestigial but according to some scientists Appendix takes part in Immunity System.
-Caecum is well developed in Herbivores like Rabbit, Horse etc.
ii)Colon
-It is divided into four regions
a)Ascending Colon
b)Transverse Colon
c)Descending Colon
d)Sigmoid Colon(Pelvic Colon
-The colon has tree longitudinal bands called taniae coli.
-Taniae coli contract and draw the remainder of the wall into small pouches called haustra.
iii)Rectum
-Two centimeter anal canal comprises rectum.
Opening of anal canal is called anus.
-Anus has an internal anal sphincter composed of smooth muscle fibre and external anal sphincter comprised of striped(Voluntary) muscle fibres.
Function of Large Intestine:
-Absorption of water and elimination of solid wastes is the main function.
-Moderate amount of Vit. K and Vit. B-complex are manufactured by bacteria which are found in the large intestine.
General Histology of Gut
General Histology of Gut(Alimentary Canal):
-Consists of four basic layers.
i)Visceral Peritoneum(Serous Membrane or Serosa)
-Outer most layer made up of Squamous Epithelium
ii)Muscular Layer
-Composed of outer longitudinal and inner circular muscle fibres.
-Both muscles are smooth muscle fibres
-Helps in peristalsis.
-Plexus of Auerbach is present in between the two layers.
iii)Submucosa
-Consists of Loose Connective Tissue and richly supplied with blood and lymphatic vessels and in some area with glands.
-Plexus of Meissner is present in this area.
iv)Mucosa
-Secretes mucus(so called mucosa) that lubricate the inner lining of the Gut.
-Composed of three layers.
a)Muscularis Mucosa(outer)
b)Lamina Propria(Middle)
c)Epithelium(inner)
-forms gastric gland in stomach villi and intestinal gland in small intestine.
Digestive Glands
Digestive Glands:
1. Salivary Gland
-Three pairs in man, secrete saliva into oral cavity.
-Saliva is slightly Acidic (pH 6.8)
-Everyday 1 to 1.5 liter of saliva is secreted in man
i)Parotid Gland
-Largest and situated near the ears.
-Their ducts open into the oral cavity near the upper second Molar teeth.
-The duct of Parotid Gland is called Stenson’s Duct.
ii)Sublingual Gland
-Situated beneath the tongue.
-Their ducts are called Sublingual ducts or duct of Rivinus.
-Their ducts open into the floor of oral cavity.
iii)Submandibular(Submaxillary) Gland
-Located at the angle of lower jaw.
-Their ducts open into the oral cavity near the lower central Incisor.
-Their ducts are called Whartson’s Duct.
-In few mammals including man and Pig, saliva contains starch digesting enzyme(Salivary Amylase also called Ptyalin)
-The Parotid Salivary Glands secrete much of salivary amylase.
-Sublingual and Submandibular salivary glands secrete salivary amylase and mucus.
NOTE:The disease Mumps is a viral infection that may involve one or more of the Parotid salivary gland.
Digestion
Deglutition(Swallowing):
The food is tasted in the oral cavity and mixed with saliva.The bolus (mass of food) is pushed inward through the pharynx into the oesophagus.This process is called swallowing or deglutition.It involves three stages : (i)The Voluntary Stage. The bolus is forced to move from the oral cavity into the pharynx(oropharynx).(ii)The Pharyngeal Stage.This is involuntary stage.The bolus is passed from the pharynx into the oesophagus.(iii)The Oesophageal Stage.This also represents the involuntary stage.The bolus passes through the laryngopharynx and enters the oesophagus in 1 to 2 seconds.Swallow is controlled by a swallowing center located in the Medulla Oblongata and lower Pons Varolli of the brain.
Peristalsis:
It is a reflex wave of relaxation in front of the content following a wave of contraction traveling down the tubular viscera by which contents are pushed through.It occurs in tubular viscera(i.e. oesophagus,stomach,intestine, etc.). Peristalsis is controlled by Medulla Oblongata.
Action of Digestive Enzymes in Human :
Saliva or Salivary Juice(Salivary Gland):
Salivary Amylase(Ptyalin) converts starch(polysaccharide) into Disaccharides in mouth.
Gastric Juice(Stomach):
i)Pepsin converts proteins into large peptides.
ii)Renin converts casein(milk protein) into calcium paracaseinate in the child’s stomach.
iii)Gastric Lipase converts small amount of fat into Glycerol and fatty acids.
Pancreatic Juice(Pancreas) –Site of action is Small Intestine.
i)Pancreatic α-amylase converts starch into Disaccharides
ii)Trypsin converts proteins into large peptides.
iii)Chymotrypsin converts proteins into large peptides.
iv)Elastase converts elastin(protein) into large peptides.
v)Carboxypeptidases converts large peptides into dipeptides and amino acid.
vi)Pancreatic lipase converts fats(Triglycerides) into fatty acids and glycerol.
vii)Nuclease:(a)Deoxyribonuclease converts DNA into deoxyribonucleotides.(b)Ribonuclease converts RNA into Ribonucleotides.
Intestinal Juice(Small Intestine)- Site of action is small intestine.
i)Enterokinase converts Trypsinogen into Trypsin.
ii)Aminopeptidases converts Large peptides into Dipeptides and amino acid.
iii)Dipeptidases converts Dipeptides into Amino Acids.
iv)Disaccharidases converts Disaccharides into Monosaccharides.
v)Intestinal Lipase converts Fats(Glycerides) into Fatty acids and Glycerol.
vi)Nucleotidases converts Nucleotides into Nucleosides and Inorganic Phosphate.
vii)Nucleosidases converts Nucleosides into Nitrogenous bases and Pentose sugar.
Action of Gastrointestinal Hormones:
i)When food enters the stomach then distention of stomach occurs resulting the production of Gastrin hormone by Pyloric stomach and Duodenum.Gastrin stimulates gastric gland to secrete and release the Gastric Juice.It also stimulates gastric mobility.
ii)When chyme enters into the Duodenum then Enterogasterone(Gastric Inhibitory Peptide-GIP) hormone is released by Duodenum that inhibits the gastricsecretion and mobility(slow gastric contraction) of stomach.
iii)Secretin(1st hormone discovered by scientist) issecreted by Duodenum and Jejunum when acidic chymeenter into the Duodenum.This hormone release bicarbonate in the pancreatic juice,increase secretion of bile,decreases gastric secretion.
iv)Cholecystokinin-pancreozymin(CCK-Pz) secreted by small intestine and is stimulus to presence of fats in the Duodenum.It stimulates the contraction of Gall Bladder to release bile,stimulates Pancreas to secrete and release digestive enzymes in the pancreatic juice.
v)Duocrinin is secreted by Duodenum and is stimulus to presence of acidic chyme in intestine.It stimulates the Brunner’s glands to release mucus and enzymes into the intestinal juice.
vi)Enterocrinin secreted by small intestine and is stimulus to presence of acidic chyme in small intestine.It stimulates the Cryps of Lieberkuhn to release enzymes into the intestinal juice.
vii)Vasoactive Intestinal Peptide(VIP) is secreted by small intestine and is stimulus to presence of food in the small intestine.It stimulates stomach and small intestine to dilate peripheral blood vessels of gut and inhibits gastric acid secretion.
viii)Villikinin is secreted by small intestine and is stimulus to presence of food in small intestine.It accelerates the villi movement of small intestine.
