Digestion and Absorption
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Digestion = Process of conversion of complex food substances to simple absorbable forms.
Digestive system consists of: (a) Alimentary canal (b) Associated (digestive) glands (a) Alimentary canal:1. Buccal Cavity (oral cavity).
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- Majority of mammals forms two sets of teeth during their life time (deciduous and adult teeth).
- Type of dentition diphyodont.
- Adult human contains 32 permanent teeth (Heterodont dentition) of four different types: incisors (I),canine (C), premolars (PM) and molars (M) with dental formula 2123/2123.
- The tongue is attached to the buccal cavity by the frenulum.
- Tongue has small projections called papillae, some of which bears taste buds.
2. Pharynx (common passage for food and air).
3. Oesophagus.
- The Gastro-oesophageal sphincter controls the opening of oesophagus into the stomach.
4. Stomach:
Stomach has three major parts:
- cardiac(a portion where oesophagus enters).
- fundic.
- pyloric (a portion which opens to duodenum).
The pyloric sphincter controls the opening of stomach into small intestine.
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5. Small intestine.
The Ileocaecal valve controls the opening of ileum into caecum. 6. Large Intestine. Large intestine consists of : 1. caecum (hosts symbiotic bacteria and contains vermiform appendix - a vestigial organ). 2. colon (ascending, descending and transverse). 3. rectum (opens through anus). |
Wall of alimentary canal:
Alimentary canal from oesophagus to rectum possesses four layers: (1) Serosa (2) muscularis (3) sub-mucosa (4) mucosa.
(1) Serosa: Outermost layer made up of mesothelium (epithelium of visceral organ).
(2) Muscularis: made up of smooth muscles and consists of (a) Outer longitudinal and (b) Inner circular cells.
(3) Sub-mucosa:
(4) Mucosa:
(1) Serosa: Outermost layer made up of mesothelium (epithelium of visceral organ).
(2) Muscularis: made up of smooth muscles and consists of (a) Outer longitudinal and (b) Inner circular cells.
(3) Sub-mucosa:
- Made up of loose connective tissues.
- Contains nerves, blood and lymph vessels.
- In duodenum glands (Brunner’s glands) are present in sub-mucosa.
(4) Mucosa:
- Innermost layer lining the lumen.
- Contains goblet cells that secrete mucus for lubrication.
- Forms irregular folds (rugae) and glands in stomach and small finger-like projections (villi) and crypts between the base of villi (crypts of lieberkuhn).
(b) Associated (Digestive) glands:
Digestive glands associated with alimentary canals are (i) Silivary gland (ii) Liver (iii) Pancreas
(i) Salivary gland: consists of (a) Parotid (cheek) (b) Sub-mandible/Sub-maxillary (lower jaw) (c) Sub lingual (below the tongue).
Salivary gland secretes salivary juice.
(ii) Liver:
(iii) Pancreas:
(i) Salivary gland: consists of (a) Parotid (cheek) (b) Sub-mandible/Sub-maxillary (lower jaw) (c) Sub lingual (below the tongue).
Salivary gland secretes salivary juice.
(ii) Liver:
- Consist of two lobes (structural and functional unit).
- Each hepatic lobule is covered by a sheath of thin connective tissue called glisson’s capsule.
- The bile secreted by the hepatic cells passes through hepatic ducts and is stored and concentrated in a thin muscular sac called gall bladder.
- Duct of gall bladder (cystic duct) and and hepatic duct forms bile duct which joins with the pancreatic duct to form hepato-pancreatic duct.
- Sphincter of Oddi controls the opening of hepato-pancreatic duct into the duodenum.
(iii) Pancreas:
- Is both exocrine and endocrine gland.
- exocrine part secretes pancreatic juice and endocrine part secretes insulin and glucagon.
Digestion:
Buccal cavity:
Stomach:
2. peptic or chief cells which secrete pro-enzyme pepsinogen and
3. parietal or oxyntic cells which secrete HCL and intrinsic factor, (factor essential for absorption of vitamin B12).
Small Intestine:
(a) Bile, (b) pancreatic juice and (c) intestinal juice are secreted into the small intestine.
(a) Bile:
(b) Pancreatic juice:
(c) Intestinal juice or succus entericus:
The pancreatic enzymatic action on food in small intestine (duodenum) is given below:
- The saliva contains electrolytes (Na+, K+, Cl- and HCO-) and enzymes (salivary amylase and lysozyme).
- About 30 percent of starch is hydrolyzed by salivary amylase (otpimum pH 6.8) into disaccharide - maltose.
- Lysozyme acts as antibacterial agent that prevents infections.
- The teeth and tongue with the help of saliva masticate, lubricate and adhere the food into bolus which is conveyed into pharynx by swallowing or deglutition.
- The bolus passes down through the oesophagus peristalsis movement of oesophagus.
- The gastro-oesophageal sphincter controls the movement of food into the stomach.
Stomach:
- Muscular walls of stomach thoroughly mixes the food with acidic gastric juice and is called chyme.
- Gastric glands of stomach present in mucosa has three major types of cells:
2. peptic or chief cells which secrete pro-enzyme pepsinogen and
3. parietal or oxyntic cells which secrete HCL and intrinsic factor, (factor essential for absorption of vitamin B12).
- The mucus and bicarbonates present in the gastric juice lubricate and protect the mucosal epithelium from excoriation by HCl (pH 1.8).
- The pro-enzyme pepsinogen on exposure to HCl gets converted to active enzyme pepsin (optimal pH 1.8), the proteolytic enzyme, that convert proteins into peptones and proteoses (peptides).
- Renin is proteolytic enzyme found in gastric juice in infants which helps in digestion of milk proteins.
- Small amounts of lipases are also secreted by gastric glands.
Small Intestine:
(a) Bile, (b) pancreatic juice and (c) intestinal juice are secreted into the small intestine.
(a) Bile:
- Secreted into small intestine contains, bile pigments (biliribin and bili-verdin), bile salts, cholesterol and phospholipids but no enzymes.
- Bile helps in emulsification of fat and providing alkaline medium (pH 7.8) along with brunner’s gland in submucosal layer of intestine.
(b) Pancreatic juice:
- Contains enzymes - trypsinogen, chymotripsinogen, procarboxypeptidases, amylases, lipases and nucleases.
- Enterokinase, secreted by intestinal mucosa, activates trypsinogen to trypsin which in turn activates the other enzymes in the pancreatic juice.
(c) Intestinal juice or succus entericus:
- Is made up of secretions of brush border cells along with secretions of goblet cells.
- Intestinal juice contains enzymes like disaccharidases (eg maltase), dipeptidases, lipases, nucleosidases, etc.
The pancreatic enzymatic action on food in small intestine (duodenum) is given below:
The intestinal enzymatic action to the end products in small intestine (duodenum) is given below:
- The breakdown of biomacromolecules mentioned above occurs in duodenum region of small intestine and the absorption takes place in jejunum and ilium region of small intestine.
- Undigested and unabsorbed substances are passed on to the large intestine through ileo-caecal valve.
Large intestine:
Functions of large intestine are
- Absorption of some water, minerals and certain drugs.
- Secretion of mucus for adhering and lubrication.
- The undigested food called feces are stored in the rectum until dafaecation.
- The activities of the gastro-intestinal tract are under neural and hormonal control for proper coordination of different parts.
Absorption:
Absorption: is the process by which the end products of digestion passes through the intestinal mucosa in the blood or lymph.
* Absorption of carried out by passive, facilitated or active transportation.
Passive transport: depends on the concentration gradient (high conc. to low conc.)
* does not require energy.
Absorbed materials: Some monosacharides(glucose), amino acids and electrolytes (Cl-).
Facilitated transport: depends on the help of carrier ions like Na+
Absorbed materials: Some monosacharides(fructose), amino acids.
Active transport: digested products are absorbed against the concentration gradient.
* requires energy.
Absorbed materials: Some monosacharides (glucose), amino acids and electrolytes (Na+).
Summary of absorption is given below:
* Absorption of carried out by passive, facilitated or active transportation.
Passive transport: depends on the concentration gradient (high conc. to low conc.)
* does not require energy.
Absorbed materials: Some monosacharides(glucose), amino acids and electrolytes (Cl-).
Facilitated transport: depends on the help of carrier ions like Na+
Absorbed materials: Some monosacharides(fructose), amino acids.
Active transport: digested products are absorbed against the concentration gradient.
* requires energy.
Absorbed materials: Some monosacharides (glucose), amino acids and electrolytes (Na+).
- Absorption of substances takes place in mouth, stomach, small intestine and large intestine.
- Maximum absorption takes place in small intestine.
Summary of absorption is given below:
Assimilation:
Assimilation is a process in which the absorbed substances are utilized by tissues for their activities.
Defecation:
Process of ejection of feces is called defecation.
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On 2014-04-01 04:54:10.071320 by oka ammayi chala bavundi .. inka konchem kavali On 2014-04-01 04:53:51.007190 by oka ammayi chala bavundi .. inka konchem kavali On 2014-02-23 11:13:49.787730 by Ajaya Jena It is very nice for prepairing note but plz explain more about the three regions of small intestine and plz short note the different glands. THANK YOU. On 2013-12-04 07:07:04.459120 by Anonymous its good ....found major points highlightened...thank you.. On 2013-09-19 18:24:17.374670 by Anonymous And, giving it in points is great. But, plz add more information. On 2013-09-19 18:20:46.369390 by Anonymous It would be very nice if digestive system of frog and rabbit is also added. |
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