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 gastric secretion and mobility(slow gastric contraction) of stomach.

iii)Secretin(1^st hormone discovered by scientist) is secreted by Duodenum and Jejunum when acidic chyme enter 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.

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.

2.Gastric Gland:

-Four types of gland cells are found in the gastric gland.

i)Peptic(Chief or Zymogenic) Cells

-Secrete proenzyme(Pepsinogen and Prorenin)

ii)Oxyntic(Parietal)

-Secrete HCl and Castle intrinsic factor.

iii)Mucous Cell

-Secrete watery substance, the mucus, which has some protein-like material called mucin.

iv)Argentaffin Cells

-May produce Somatostatin, 5-Hydroxytryptamine(a powerful stimulant) and Histamine.

Gastric Juice is composed of these all cells secretion(pH 2-3.7)

Infant’s gastric juice ph is 5.0

2 to 3 liter of Gastric Juice is secreted everyday.

3.Liver(Heper):

-Largest gland of the body.

-Heavier in male(1.4-1.8 Kg) than in female(1.2-1.4Kg).

-Covered by two sheaths

i)Serous Capsule(outer) consists of visceral peritoneum.

ii)Glisson’s Capsule(inner) consists of thin layer of dense connective tissue.­

-The Liver is divided into two main lobes(Right and Left Lobe)separated by the

  Falciform Ligament.

The Right Lobe of the Liver is differentiated into Right Lobe Propria, a Quadrate Lobe and a Caudate Lobe on the posterior surface.

-Gall Bladder found in the Liver stores bile secreted by the Liver.

-Rat and Horse do not have Gall Bladder.

-Right and Left Hepatic duct arises from the corresponding lobes and join to form the

  Common Hepatic duct. Common Hepatic duct joins with Cystic duct of Gall Bladder and form Bile duct.

-Bile duct passes downwardly and joins with Pancreatic duct forming Hepatopancreatic Ampulla(Ampulla of Vater).

-Ampulla of Vater opens into the Duodenum and the opening is guarded by Sphincter of Oddi.

-The sphincter of Boyden surrounds the opening of Bile duct before it is joined with pancreatic duct.

-Hepatic lobules are found in the Liver which are the functional and structural unit of the Liver.

-At the periphery of the lobule the bile canaliculi empty intoHering’s canals which are walled by cuboidal epithelium.

-Hepatic sinusoids are wide capillary like blood vessels lined by incomplete endothelium.

-Kupffer cells(hepatic macrophages or stellate cells) are also present in the Liver.

Kupffer cells are phagocytic in nature which eat bacteria and foreign substances.

-Liver has high power of regeneration.

-Blood enters the Liver through the Hepatic Artery and Hepatic Portal Vein.

Function of Liver:

1. Production of Bile (pH 8.6) -about 500ml to 1000ml bile is secreted everyday. Bile is stored in the Gall Bladder.

2. Deamination-It is a process by which the amino group(-NH₂) is removed from the amino acids resulting in the production of ammonia which is converted into Urea.Deamination is carried out by Liver cells.

3. Excretion-Liver synthesizes Urea with the help of Ammonia and Carbon dioxide.The bile contains bile pigments(billirubin-yellow and biliverdin-green) the are excretory product.Liver also eliminate certain other wastes like cholesterol, metal ions and waste product of haemoglobin.

4. Glycogenesis- It is the conversion of excess of Glucose into Glycogen by Liver cells with the help of Insulin secreted by the Pancreas.

5. Glycogenolysis- It is the conversion of glycogen into glucose by the liver cells with help of glucagons secreted by the pancreas.

6. Lipogenesis- Conversion of excess of glucose and amino acid into fats.

7. Gluconeogenesis- It is the formation of glucose or glycogen from non-carbohydrate sources.It also occurs in the kidneys and striped muscles.

8. Detoxification- Liver converts toxic substance into harmless substance eg.harmful prussic acid, formed during metabolism in all body cells, is neutralized and rendered harmless by Liver cells.

9. Haemopoiesis-Process of formation of blood corpuscles.Liver produces red blood corpuscles (RBC) in the embryo.

10.Synthesis of Blood Proteins-Liver produces blood proteins such as prothrombin and fibrinogen that help in the clotting of blood.

11.Secretion of Heparin(anticoagulant)

12.Lymph formation

13.Synthesis of Vit. A -Liver synthesizes Vitamin A from β-carotene(orange-yellow substance of carrot)

14.Secretion of Enzyme-Liver secretes certain enzymes which play important roles in the metabolism of proteins, fats and carbohydrates in the body.

15.Destruction of RBC- Old RBCs are broken down and haemoglobin is changed into bile pigment.

16.Phagocytosis- The Kupffer’s cells of Liver engulf the disease causing micro-organism,dead cells and foreign matter.

17.Osmoregulation-Liver produces angiotensinogen(a protein) which helps kidneys in maintaining body fluid osmoregulation.

18.Production of Heat- due to high metabolic activities enough heat is generated which is essential for maintaining the optimum body temperature.

19.Storage-It stores glycogen, fats, Vitamins(A,D,E,K and B₁₂), bile in the gall bladder, blood, water, iron, copper and potassium.

Function of Bile:

1.Neutralization of HCl-Its sodium bicarbonate neutralizes HCl of chyme(semi fluid food found in stomach)

2.Emulsification-sodium glycocholate and sodium taurocholate break the large fat droplets into the smaller ones.

3.Absorption of fat and fat-soluble vitamins-its salts help in the absorption of fat(fatty acids and glycerol) and fat-soluble vitamins(A,D,E and K)

4.Excetion-Bile pigments(bilirubin and biliverdin) are excretory products.

5.Prevention of decomposition-Bile is alkaline hence it prevents the decomposition of food by preventing the growth of bacteria on it.

6.Stimulation of Peristalsis-Bile increases peristalsis of the intestine.

7.Activation of Lipase-Bile contains no enzyme but activates the enzyme Lipase.

4.Pancreas:

-Soft,lobulated,grayish-pink gland weighs about 60grams.

-2.5 cm wide 12 to 15 cm long.

-Located posterior to the stomach in the abdominal cavity.

-The main pancreatic duct (duct of wirsung) opens into the hepatopancreatic ampulla(ampulla of vater).

-an accessory pancreatic duct(duct of santorini) is also present in the pancreas and opens directly into the duodenum.

-It is a mixed type of gland.

-Consists of two parts:   Exocrine part and Endocrine part

i)Exocrine Part

-consists of rounded lobules(acini) that secret an alkaline pancreatic juice with pH 8.4

-500ml to 800ml of pancreatic juice is secreted daily and is carried by main pancreatic duct into duodenum through hepatopancreatic ampulla.

-The accessory pancreatic duct directly pours the pancreatic juice into duodenum.

-The pancreatic juice contains NaHCO₃ ,three proenzymes; trypsinogen, chymotrypsinogen and procarboxypeptidase and some enzymes such as elastase, pancreatic α-amylase, DNAase, RNAase and pancreatic lipase.

-The pancreatic juice helps in the digestion of starch, protein, nucleic acids and fats.

ii)Endocrine part

-Consists of Islets of Langerhans.

-The human pancreas has about 1 million islets.Most numerous in the tail of pancreas.

-Each Islets of Langerhans consists of the following types fo cells which secret hormones to be passed into the circulating blood.

a)Alpha cells(α-cells)-more numerous towards the periphery of islets. Constitute 15% of islets of langerhans. Produce Glucagon hormone.

b)Beta cells(β-cells)-More numerous towards the middle of the islet.Constitute 65% of islets of langerhans.Produce insulin hormone.Dificiency of insulin causes diabetes mellitus.

c)Delta cell(δ-cells)-Constitute 5% of islets of langerhans.Secret Somatostatin(SS) hormone.Its secretion inhibits the secretion of glucagons and to a lesser extent secretion of insulin.It also inhibits the secretion of growth hormone by anterior pituitary gland.

d)Pancreatic polypeptide cells(PP cells or F cells)-It constitute 15% of islets.Secretes pancreatic polypeptide which inhibits production of pancreatic juice.

-Pancreas performs two main function ,i.e. secretion of pancreatic juice(digestive enzyme) and production of hormones.

5.Intestinal Glands:

-These are of two types; Crypts of Lieberkuhn and Brunner’s Glands.

i)Crypts of Lieberkuhn

-occurs throughout the small intestine between the villi.

-secrete digestive enzymes and mucus.

ii)Brunner’s Gland

-found only in the duodenum and are located in the submucosa.

-secrete a little enzyme and mucus.

The secretion of intestinal glands is called intestinal juice or succus intericus with pH 7.6.About two to three liters of intestinal juice is secreted daily.The intestinal juice contains many enzymes –maltase,isomaltase,sucrase,lactase,α-dextrinase,enterokinase,aminopeptidase,dipeptidases,nucleotidases,nucleosidases and intestinal lipase.

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.

Alimentary Canal

4. 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.

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(shortest)

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.

Digestive System

1. Nutrients
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.