Human Digestive system

Oral cavity -

1)     Palate- the roof of oral cavity is called as palate. Anterior part of palate is called as hard palate which bears transverse ridges the rugae. The posterior part of palate is smooth and is termed as soft palate. The hinder free part of the soft palate freely hangs down as a small flap which is called uvula.

2)     Tongue- it is a voluntary muscular and glandular structure which occupies the flour of the mouth it is attached to the floor of the mouth by a fold called as lingual frenulum. An inverted v shape furrow termed as sulcus terminalis divide the upper surface of tongue into anterior oral part and posterior oral part. The apex of the sulcus terminalis projects backwards and is marked by a small medium pit named the foramen caecum.

3)     Papillae- the upper part of tongue has four types of papillae:

a)      Vallated papillae or Circumvallate papillae- they are usually 8 to 12 in number and are arranged in an inverted v-shaped row towards the base of tongue they are the largest of the four types of papillae.

b)     Filiform papillae- they are the smallest papillae and most numerous of all four types. They are found mainly near the centre and most of the upper surface of tongue.

c)      Fungiform papillae- they are less numerous than filiform papillae. They are round but smaller than vallate but larger than filiform. They are mainly found near the tip of tongue. Each fungiform papillae contains five taste bud.

d)     Foliated papillae- (not developed in humans) these are leaf like and are situated at the side of base of tongue on each border these are four to five vertical folds.

 

4)     Teeth- most of the mammals have diphyodont (two sets of teeth – milk teeth & permanent teeth), thecodont (teeth are embedded in the sockets of the jaw bones) and heterodont

5)      teeth (different types of teeth).

There are four kind of teeths in human- Incisors, canines, premolars and molars.

Incisors are chisel shaped and posses sharp cutting edge and located anteriorly. They are usually specialised for cutting. Canines lie immediately behind the incisors they are also used for cutting. Premolars and molars are called cheek teeth which are broad, strong crushing teeth. Third molar in human are called as wisdom teeth.

Crown projects above gum of tooth. The crown is covered by enamel. The enamel is the hardest substance in body.

Dental formulae of milk teeth: -              212/212 x2 = Total 20

Dental formulae of permanent teeth: - 2123/2123 x2 = Total 32

 

6)     Waldeyers Ring-

a)      Nasopharyngeal tonsil- refers to nasopharynx. In children’s nasopharyngeal tonsil may become enlarged and is referred to as adenoids. Resulting swelling may be a cause for obstruction in normal breathing.

b)     Tubal tonsil- refers to nasopharynx.

c)      Palatine tonsil- refers to oropharynx. The palatine tonsil is often infected (tonsillitis) leading to soar throat. Such enlarged tonsil may become focus of infection and their surgical removal (tonsillectomy) become necessary.

d)     Lingual tonsil- refer to tongue.

These all lymphoid tissue are active in production of IgA which forms an important part of immune system.

Oesophagus- Human oesophagus is 25cm long. It lies behind the trachea and the heart. Oesophagus pass through diaphragm and opens into stomach.

Stomach- it is located in the upper left portion of abdomen.

a)      Cardiac part- it is called so because it is present near the heart. The gastroesophageal spinchter lies in opening between oesophagus & stomach.

b)     Fundic part- it extends superiorly from the cardiac part. Commonly filled with air or gas.

c)      Pyloric part- it is last part of stomach which opens into small intestine.

d)     Body- it is the main part of stomach.

Small intestine- It is the longest part of alimentary canal (about 6.25 meter).

a)      Duodenum- About 25cm long and is the shortest and widest part of small intestine. The hepatopancreatic ampula (ampula of vatar) opens into duodenum. This ampulla receives the secretion of both bile duct from liver and main pancreatic duct from pancreas.

b)     Jejunum- it is about 2.5-meter-long and 4 cm wide. It is thick walled.

c)      Ileum- it is longest part of intestine which is 3.5-meter-long and 3.5 cm wide.

Small intestine completes digestion of protein, carbohydrate nucleic acid and fat. It absorbs nutrition int blood and lymph.

Large intestine- it is shorter than small intestine & is basically for absorption of water and discharge of undigested food.

a)      Caecum- it is the blind sac and is present where illum joins large intestine there is a finger like projection called vermiform appendix which gets affected during appendicitis. Caecum is large and spacious which is a food storage unit in herbivores where cellulose fermentation takes place.

b)     Colon- it is 5- 6 feet long and 3 inches in diameter. Ascending colon is the first part on the right side that moves upward from caecum. Decending colon is next region which moves from left to right. Pelvic colon is the last part which continues into rectum. Food can remain long in colon before being passed to rectum.

 

Digestive glands-

1)     Salivary gland- 3-pairs of salivary gland.

a)      Paratid gland- they are largest of salivary gland present on each side of face in front of ears. The ducts open on sides of chicks.

b)     Submaxillary gland- they are located at the posterior part at the floor of the mouth. Duct opens at flour of mouth.

c)      Sublingual gland- they are located at anterior part below the tongue.

The secretion of salivary gland is called as saliva which is watery and contains the enzyme salivary amylase along with mucin and other substance. Saliva not only helps in digestion of food but also helps in lubricating food easier to swallow. Every day 2.5 liters of saliva is secreted for digestion.

2)     Liver- it is the largest gland in human body situated at the upper right portion of abdominal cavity. It weights about 1.5 KG in adult human. Liver is covered by two layer, outer layer is called as serous capsule consisting of visceral peritonium and inner layer is called as gilsons capsule consisting thin layers of dense connective tissue. Liver is divided into two lobes right lobe and left liver lobe separated by falciparum ligament. Mammalian liver also contains kupffers cell that are phagocytic in nature and eat worn out WBC, RBC and Bacteria.

 

Gall bladder- it is like a sac which is attached to liver. It stores bile secreted by liver.

 

Function of liver-

a)      Deamination- it is the process of removal of amino group from amino acid resulting in production of ammonia which is converted into urea.

b)     Glycogenolysis- it is the process of conversion of excess of glycogen into glucose with the help of glucagon secreted by pancreas.

c)      Glycogenesis- it is process of conversion of excess of glucose into glycogen with the help of insulin secreted by pancreas.

d)     Lipogenesis- it is process of conversion of excess of glucose and amino acid into fat which also takes place in liver.

e)     Phagocytosis- Kupffer cell of liver engulf the disease-causing microorganism worn out WBC, RBC and foreign matter.

f)       Synthesis of blood protein such as prothrombin and fibrinogen.

g)      Detoxification of toxic substance.

Function of Bile-

a)      Bile emulsifies fat (sodium glycolate and sodium taurocholate convert large fat droplets into small ones)

b)     Neutralises the acidity of chyme (sodium bicarbonate)

c)      Bile secretes bile pigment (bilirubin- yellow, & biliverdin- green)

d)     Bile salt helps in absorption of fat (sodium glycolate, sodium bicarbonate, sodium taurocholate).

e)     It provides an alkaline medium for action of enzymes.

Histology of human gut-

Serosa (Viceral Peritonium)- it is the outer most layer made up of squamus epithelium and connective tissue.

Sub mucosa- it consist of loose connective tissue rich supplied with blood and lymphatic vessel.

Mucosa- it is the inner most lining of alimentary canal it is named so because it secrets mucus to lubricate the inner lining of gut.

Muscularis- it is composed of outer longitudinal and inner circulatory muscle fibre. In between these two muscle fibre there is a network of nerve fibre and parasympathetic nerve fibre called Auerbach’s plexus (=myenteric plexus). Another network of nerve cell and symphatic nerve fibre called Meissner’s plexus (=submucosa plexus), is present between muscular coat and submucosal layer.

 

Digestion of carbohydrate-

1)     Digestion in mouth-

Food gets mixed with saliva. It is acted upon by ptyaline or salivary amylase which brings about hydrolysis of starch.

 

Starch-------(salivary amylase) ---à Maltose + Isomaltose + Limit dextrin.

 

Chewing breaks the food into small pieces and increased surface area for enzyme action. Salivary amylase functions only in alkaline medium of PH 7.5 to 8 cooking of starchy food breaks the cell wall and makes it easier for enzymatic action. Food remains in mouth for a short while so the digestion is not completed (30%). Salivary amylase acts in oesophagus but stops acting in stomach because the content of stomach is acidic & low PH, so carbohydrate digestion stops in stomach and starts again in intestine.

 

2)     Digestion in intestine- food gets mixed with pancreatic juice and intestinal juice, so digestion of carbohydrate in small intestine starts.

Pancreatic juice-

Starch------(pancreatic amylase)-à Maltose + iso maltose + limit dextrin.

Intestinal juice-

Maltose---(Maltase)---------------------------------à 2Glucose

Limit dextrin----(Limit dextrinase) -------------à Glucose

Iso maltose-----(Iso maltase) ---------------------à 2 Glucose

Sucrose------(Sucrase)-------------------------------àglucose + Fructose

Lactose -------(lactase)------------------------------àglucose + Glactose

Digestion of protein- food that enters into stomach has an alkaline PH and become acidic as it mixes with gastric juices.

Function of HCL-

1)     It makes PH highly acidic between 1& 2 for optimum pepsin action.

2)     It converts inactive pepsinogen into active pepsin.

3)     Due to high acidic PH it kills many Germs and Bacteria.

4)     Denaturation of many food protein and also facillates action of pepsin.

Pepsinogen -----(HCL)---------------------------à Pepsin

Prorennin ------(HCL)----------------------------àRennin

Protease are secreted in inactive form and only gets activated at the site of action. Their action along with HCL is so strong that it could destroy the living tissue of stomach lining. The mucus provides a barrier between stomach lining and gastric juice. After conversion of pepsinogen into pepsin by HCL, pepsin auto catalyses rest pepsinogen.

Protein------(Pepsin)---------------------------à Protease + peptone.

Milk Casein-----(Rennin)-----à Paracaesin-------àCalcium paracacinate-------àPolypeptide.

Rennin reacts with milk protein and splits into fluids protein called whey and solid portion called casein.

Enzyme pepsin acts on casein and breaks into amino acid.

 

Digestion of protein in Intestine-

a)      Pancreatic juice contains trypsin, chymotrypsin and carboxy peptidase.

Trypsinogen-----Enterokinase peptide) ---------------------------à Tripsin.

Chymotrypsinogen------(trypsin)------------------------------------à chymotrypsin.

Pro carboxypeptidase-----(trypsin)---------------------------------à carboxypeptidase.

Peptones------(trypsin, chymotrypsin, carboxypeptidase) ---àPolypeptide + Proteos.

Caesin---------(chymotrypsin)-----------------------------------------à Polypeptide.

b)     Intestinal juices-

Small polypeptide---(Polypeptidase + Aminopeptidase) ---àamino acid.

Nucleotide-------(nucleotidase)---------------------------àphosphate, Nucleoside.

Nucleoside-------(nucleosidase)---------------------------àPentose sugar, Nitrogenous base.

Peptides------(aminopeptidase)---------------------------à Amino acids

Dipeptides--------(Dipeptidase-----------------------------àAmino acids.

 

Digestion of Nuclic Acid-

a)      Pancreatic juice-

It contains nucleas : deoxyribonuclease (DNAase) and ribonuclease (RNAase)

DNA----(Dnase)---à deoxyribonucleotides

RNA-----(RNAase)--à ribonucleotides

b)     Intestinal juice-

It contains enzymes, nucleotidase and nucleosidase.

Nucleotides (Deoxyribonuclotides/ribonucleotides) ---(nucleotidase)----à Nucleosides(deoxyribonucleosides/ribonucleosides) + inorganic phosphate.

 

Nucleosides(deoxyribonucleosides/ribonucleosides) ----(nucleosidase)-à nitrogenous base (purine/ pyrimidines) + pentose sugar (Deoxyribose/ ribose)

 

 

Digestion of fat-

Fat do not mix in water but lipase the fat digesting enzyme are present in aqueous solution fat needed to be broken down to small droplet to increase the surface area.

Bile juice secreted by liver has bile salt which makes the medium alkaline necessary for lipase activity. Bile salt also helps in emulsification of fat into droplets to increase the surface area for lipase action.

Fat------(gastric lipase) ---à disaccharides and monosaccharides

Fat(triglycerides)------(pancreatic lipase) --à diglycerides and monoglycerides

Fat (diglycerides and monoglycerids)------(intestinal lipase)----àfattyacid and glycerol

 

Absorption of Fat-

Fatty acid is absorbed by lacteals and not by the blood capillaries. Fatty acid is first converted into small spherical water-soluble droplets called miscells by the help of bile salt. Fat soluble vitamin and steroids are absorbed by intestinal cell. Fatty acid passes through brush bordered columnar cells of villi in these cells free fatty acid is resynthesized into triglycerides. These triglycerides droplets called chylomicrons. The lacteals open into thoracic lymphatic duct which ultimately pour all the lymph into the left subclavian vein and hence chylomicrons get into blood stream.

Chyle- it is a white or pale yellow fluid taken up by the lacteals from the intestine during digestion. It mainly consists of absorbed fat.

Passive absorption-

Monosaccharides like glucose and amino acid may be absorbed passively through simple diffusion as concentration of intestinal lumen is higher than the intestinal cells. Also, all the nutrition is not absorbed by simply diffusion. Fructose and mannose are absorbed by simple facillated diffusion i.e., by the help of carrier molecule but along the concentration gradient.

Active absorption-

It is the absorption of nutrient against the concentration gradient i.e., through concentration of nutrient is less in lumen and much higher in blood the nutrients are still absorbed from lumen to blood.