digestive system - part 3

INGESTION OF FOOD
Ingestion means intake of food into the buccal cavity through mouth.

DIGESTION OF FOOD
1. digestion of CARBOHYDRATES -
Simple sugars like glucose and fructose can be absorbed and metabolised directly.Disaccharides like sucrose and lactose and Polysaccharides such as starch and glycogen have to be  broken down to monosaccharides .Carbohydrate splitting enzymes are known as carbohydrases

•  digestion in Buccal cavity

                      

Saliva of buccal cavity contains salivary amylase which converts starch of food into maltose(30% digestion of starch occurs in mouth).It  also contains bicarbonate ions and enzyme lysozyme that are bacteriocidal in nature.

•  digestion in stomach 

 Partially digested food passes from oral cavity into stomach via pharynx and oesophagus.No carbohydrate digestion occurs in stomach as gastric juice contains no carbohydrases.

• digestion in small intestine

Small intestine receives following juices -

i. Bile from liver

ii. Pancreatic juice from pancreas

iii. Intestinal juice from intestinal glands

BILE : It contains no enzyme , so has no enzymatic action on food.

PANCREATIC JUICE : It contains Pancreatic amylase which is a carbohydrase.

INTESTINAL JUICE : It contains the enzymes as :

                                     i. Maltase

                                    ii. Sucrase

                                   iii. Lactase

                   

2. digestion of PROTEINS

Protreins are polymers of amino acids linked by peptide bonds. The enzymes acting on proteins are known as proteases or peptidases.

• digestion in stomach

Gastric juice contains two proteaese in the form of proenzymes.They are :

i.Pepsinogen

ii.Prorennin

Proenzyme pepsinogen  Pepsin (active enzyme)

 

• digestion in small intestine

BILE : It contains no enzymes but neutralises the acidic food obtained from stomach.

PANCREATIC JUICE : 

 

    

 INTESTINAL JUICE : 

 3.digestion of FATS

ALmost entire fat portion of our diet consists of triglycerides.Digestion of fars starts and is completed in small intestine.

• digestion in small intestine

BILE : It contains salta such as sodium glycocholate and sodium taurocholate which emulsify the 

            fats. Emulsification increases the action of lipases on fats.

  

PANCREATIC JUICE:

INTESTINAL JUICE:

Intestinal juice also contains intestinal lipase which hydrolyses some triglycerides, diglycerides and monoglycerides into fatty acids and glycerol.

4.digestion of NUCLEIC ACIDS
Digestion of nucleic acids occurs in small intestine.Enzymes which digest nucleic acids are found in pancreatic and intestinal juices.

• PANCREATIC JUICE :

• INTESTINAL JUICE :

         Intestinal juice also acts on nucleosides and convert it into sugar and nitrogenous bases. 

                       

Human digestive system - part 2

DIGESTIVE GLANDS
-These are exocrine glands associated with human alimentary tract.
-They release digestive enzymes which help in the process of digestion.
-Main digestive gland are :
i.Salivary glands  ii.Gastric glands  iii.Liver  iv.Pancreas  v.Intestinal glands

1.SALIVARY GLANDS - There are three pairs of salivary glands
a.Parotid glands  : These are largest salivary glands present in the cheek below the external ear.
                              Their ducts are called parotid ducts or Stenson's ducts open into mouth at the level 
                              of 2nd molar tooth.

b.Sublingual glands : These are smallest sized salivary glands located beneath anterior tongue.They have large number of small ducts called sublingual ducts or Bartholin's ducts or ducts of Rivinus open at the floor of oral cavity.

c.Sub-mandibular glands : They lie one on each side of face under the angle of jaw.Their ducts are
                                           called submandibular or Wharton's ducts open at the floor of oral cavity.
 Saliva : combined secretion from salivary glands and small mucus from the glands of the lining of
              oral cavity.Its pH is 6.8.
             - consists of water , minerals , enzymes : ptyalin or salivary amylase or lyzozyme.

2.GASTRIC GLANDS  - microscopic ,tubular glands in the wall of stomach
                                       - it has three parts :
                                        a.Cardiac part : secretes alkaline mucus
                                        b.Pyloric part : also secrete alkaline mucus
                                        c.Fundic part : secrets mucus and enzymes (gastric juice)

Gastric glands consist of four types of cells -
i.Peptic or chief or Zymogen cells -

•  usually basal in position
•  produce gastric digestive enzymes - prorennin and pepsinogen
• also produce small amount of gastric lipase

ii.Oxyntic or Parietal cells -

• secrete HCl and intrinsic factor of Castle (helps in absorption of B12)

iii.Mucous neck cells or Goblet cells -

• secrete mucus

iv.Argentaffin cells -

• generally located at the base of gland
• secrete somatostatin , serotonin and histamine

Gastric glands also contain endocrine cells (G-cells)which occur in pyloric stomach and produce hormone Gastrin - stimulates gastric glands to secrete HCl and enzymes
Gastric juice consists of water , mucus , HCl and two enzymes pepsinogen and prorennin and sometimes also gastric lipase.

3.LIVER - largest and reddish brown gland
                - locate din upper right side of abdominal activity
                - consists of two main lobes : large right lobe and smaller left lobe
                                    two smaller lobes : quadrate lobe and caudate lobe
-A thin walled sac,the gall bladder, lie in the groove in the lower surface of right lobe.Its function is to store and concentrate bile secreted by liver.
-Bile is carried from liver by the bile duct ( ductus choledochus) - formed by the union of cystic duct from gall bladder and common hepatic duct from right and left liver lobes.
-The bile ducts is joined by the hepatopancreatic duct which is dilate to form hepatopancreatic ampulla or Vater's ampulla.It opens into duodenum and the opening is regulated by sphincter of oddi.

Liver consists of small structural and functional units called lobules and consist of specialized epithelial cells called hepatic cells or hepatocytes.Liver is highly vascular and has many sinusoids.Kuffer cells are found in the walls of sinusoids and they are phagocytic in action.Between hepatocytes there is also the presence of bile canaliculi which unite to form bile ducts.

4.PANCREAS - it is  a heterocrine gland , having exocrine and endocrine part

                         - exocrine part consists of numerous lobules that secrete alkaline pancreatic juice

-Pancreatic juice is carried by pancreatic duct (duct of Wirsung) into hepatopancreatic ampulla.Sometimes an accesory pancreatic duct called duct of Santorini is also present in the pancreas and opens directly into duodenum.

-Pancreatic juice contains sodium bicarbonate , 3 enzymes : trypsinogen , chymotrypsinogen and procarbo-xypeptidase and several others like pancreatic amylase , pancreatic lipase and pancreatic nuclease(DNase and RNase)

-Endocrine part consists of groups of calls called islets of Langerhans.

5.INTESTINAL GLANDS - numerous , microscopic gland in the mucosa of small intestine
                                             - are of two types :
   a.Crypts of Leiberkuhn:

• simple , tubular structures which occur throughout small intestine between the villi
• secrete enzymes and mucus
• at the base they have Paneth cells and argentaffin cells
• paneth cells produce antibacterial substance, lysozyme
• argentaffin cells secrete hormone secretin and vasoconstrictor serotonin

b.Brunner's gland :

• compound tubular glands found only in duodenum
• secrete large amount of viscous , enzyme free a.d watery mucoid fluid.
• secretion enables duodenum to withstand the effect of acidic chyme entering from the stomach
• secretion contains excess of bicarbonate ions and it opens into crypts of leiberkuhn

Secretion of intestinal juice is called intestinal juice or succus entericus.It contains many enzymes:

maltase , isomaltase , limit dextrinase , aminpeptidase , dipeptidase , intestinal amylase , intestinal lipase , nucleotidase , nucleosidase , enterokinase( entero peptidase ).

                                           
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