Digestion & Absorption Notes
Digestion & Absorption Notes
Digestion & Absorption Notes
DIGESTIVE SYSTEM
Digestive system consists of :
1. Alimentary canal
2. Associated glands
Alimentary Canal
begins with an anterior opening – mouth
ends with a posterior opening – anus
Mouth opens into the buccal/oral cavity. Oral cavity has a number of
teeth & a muscular tongue.
Tooth is embedded into sockets of jaw bone. Such attachment is
called THECODONT.
The type of dentition in which the organism has a set of
temporary/milk/deciduous teeth replaced by a set of
permanent/adult teeth is called DIPHYODONT.
Humans have HETERODONT DENTITION namely incisors, canines,
premolars & molars.
The hard chewing/external surface of the teeth is called enamel.
Teeth helps in the mastication of food.
The tongue is a freely moveable muscular organ attached to the
floor of the oral cavity by the frenulum.
The upper surface of a tongue having small papillae-like projections
are called taste buds.
The oral cavity leads into a small passage called pharynx which is a
common passage for food & air. From the pharynx, the oesophagus
(food pipe) & the trachea (wind pipe) open.
EPIGLOTTIS, a cartilaginous flap at the opening of the trachea,
prevents the entry bolus into the respiratory system during the
process of swallowing. (glottis = the opening of the windpipe)
Oesophagus is a thin, long tube which extends posteriorly passing
through the neck, thorax & diaphragm, leading to the stomach.
Stomach is a J-shaped muscular bag, located in the upper left
portion of the abdominal cavity.
The GASTRO-OESOPHAGEAL SPHINCTER regulates the opening of
the oesophagus into the stomach.
Parts of a stomach :
CARDIAC PORTION – into which the oesophagus opens
FUNDIC REGION
BODY/MAIN CENTRAL REGION
PYLORIC PORTION – from which the stomach opens into the
first part of small intestine (duodenum)
Pyloric sphincter guards the opening into the duodenum from the
stomach.
Parts of small intestine :
DUODENUM – C-shaped
JEJUNUM – long coiled middle portion
ILEUM – highly coiled, opens into the large intestine
DIGESTIVE GLANDS
The digestive glands associated with the alimentary canal are :
Salivary glands
Liver
Pancreas
Salivary glands :
Saliva is mainly produced by :
Parotid glands – cheeks
Sub-mandibular/sub-maxillary glands – lower jaw
Sub-lingual glands – below the tongue
The above glands situated before the buccal cavity secretes salivary
juice into the buccal cavity.
Liver :
The liver is the largest gland in the body weighing about 1.2-1.5kg in
an adult human.
It is situated in the abdominal cavity, just below the diaphragm.
It has two lobes.
*Hepatic Lobules :
Hepatic lobules are small divisions of the liver defined at the
microscopic (histological) scale.
HEPATIC LOBULES are the structural & functional units of liver.
They contain hepatic cells arranged in the form of cords.
Each lobule is covered by a thin connective tissue sheath called the
GLISSON’S CAPSULE.
Gall Bladder :
The bile secreted by the hepatic cells passes through the hepatic
ducts, then stored & concentrated in the gall bladder.
Gall bladder is a thin muscular sac.
The cystic duct from gall bladder & the hepatic duct from the liver
forms the common bile duct.
(cystic duct(gall bladder) + hepatic duct(liver) = BILE DUCT)
The bile duct & the pancreatic duct open into the duodenum as the
common hepato-pancreatic duct.
(bile duct + pancreatic duct = HEPATO-PANCREATIC DUCT)
It is guarded by the sphincter of Oddi.
Pancreas :
The pancreas is a compound, elongated organ situated between the
limbs of the C-shaped duodenum.
It is both an exocrine & endocrine organ.
The exocrine portion secretes an alkaline pancreatic juice containing
enzymes. The endocrine portion secretes hormones (insulin &
glucagon).
DIGESTION OF FOOD
(In the buccal cavity)
Function of the buccal cavity :
Mastication of food
Facilitation of swallowing
*mechanical process*
The teeth & the tongue with the help of saliva, masticate and mix the
food thoroughly into a BOLUS.
The mucus in saliva helps in lubricating & adhering the masticated
food particles.
The bolus is conveyed into the pharynx and then into the
oesophagus by swallowing.
Swallowing is also called DEGLUTITION.
PERISTALSIS is the successive waves of muscular contractions in the
oesophagus for the passing down of bolus into the stomach.
The bolus passes down through the oesphagus into the stomach by
the process of peristalsis.
No digestion occurs in the oesophagus.
*chemical process*
The saliva secreted into the oral cavity contains :
Electrolytes – Na+ , K+ , Cl- , HCO-3
Enzymes – a. Salivary amylase
b. Lysozyme
a. Salivary amylase :
The salivary amylase is a “carbohydrate splitting enzyme”.
The chemical process of digestion is initiated by the hydrolytic action
of the salivary amylase.
About 30% of starch present in the bolus is hydrolysed in the buccal
cavity by the salivary amylase, into a ‘disaccharide’ called maltose.
(salivary amylase has an optimum pH 6.8)
salivary amylase
Starch (pH 6.8) → Maltose
b. Lysozyme :
It acts as an antibacterial agent and prevents infections.
(In the stomach)
Gastric glands are present in the mucosa of stomach.
Types of cells in gastric glands :
MUCUS NECK CELLS – secrete mucus
PEPTIC/CHIEF CELLS – secrete pepsinogen (proenzyme)
PARIETAL/OXYNTIC CELLS – secrete HCl & intrinsic factor
[Intrinsic factor – factor essential for the absorption of vitamin B 12]
[Proenzyme – inactive enzyme]
Small amounts of lipases are also secreted by gastric glands.
*mechanical process*
The stomach stores the food for 4-5hours.
The food mixes thoroughly with the acidic gastric juice of the
stomach by the churning movements of its muscular wall. Such
mixture is called chyme.
The mucus and & bicarbonates present in the gastric juice help in
‘lubrication’ and ‘protection’ of the mucosal epithelium from
“excoriation” by HCl.
*chemical process*
The pepsinogen (proenzyme) secreted by the peptic/chief cells gets
converted into pepsin (active enzyme), on exposure with HCl.
HCl provides an optimum pH 1.8 for pepsins.
Pepsin is a proteolytic enzyme of the stomach.
[Proteolytic enzyme – enzymes that break down proteins]
Now, pepsin breaks down “proteins” into peptides, i.e. proteoses &
peptones.
Peptones are partially hydrolysed proteins.
Rennin, also a proteolytic enzyme, is found in the gastric juice of an
infant. It helps in the digestion of milk proteins.
HCl
Pepsinogen(inactive) (pH 1.8) Pepsin(active)
Pepsin
Proteins Peptides(proteoses & peptones)
(In the small intestine)
*mechanical process*
Various types of movements occur in the muscularis layer of the
small intestine which facilitates digestion by thoroughly mixing up
the food with the intestinal secretions.
*chemical process*
Secretions released into the small intestine :
Bile
Pancreatic juice
Intestinal juice
PANCREATIC JUICE & BILE are released through the hepato-
pancreatic duct.
ENTEROKINASE is an enzyme which is secreted by the intestinal
mucosa.
Pancreatic juice
The pancreatic juice contains the following ‘inactive’ enzymes :
Trypsinogen
Chymotrypsinogen
Procarboxypeptidases
Amylases
Lipases
Nucleases
Inactive trypsinogen is converted to active ‘trypsin’ by enterokinase.
Enterokinase
Trypsinogen(inactive) Trypsin(active)
Trypsin activates other enzymes in the pancreatic juice.
Trypsin
Chymotrypsinogen(inactive) Chymotrypsin(active)
Trypsin
Procarboxypeptidases(inactive) Carboxypeptidase(active)
Bile
The bile contains :
Bile pigments – bilirubin & biliverdin
Bile salts
Cholesterol
Phospholipids
“Bile does not contain any enzyme.”
Function of bile :
Emulsification of fats into very small micelles
Also activates lipids
Intestinal juice (succus entericus)
The intestinal mucosal epithelium has goblet cells which secrete
“mucus”.
SUCCUS ENTERICUS constitutes of the secretions of :
Brush border cells of the ‘mucosa’
Goblet cells
Succus entericus consists of the following enzymes :
Disaccharides – maltase
Dipeptidases
Lipases
Nucleosidases
The mucus & the bicarbonates from the pancreas :
Protects the intestinal mucosa from acids
Provides an alkaline medium for enzymatic activities
Sub-mucosal glands (BRUNNER’S GLANDS) also help in the above.
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