DIGESTIVE SYSTEM
PREPARED BY MR.
ABHIJIT DAS
INTRODUCTION
Food is one of the basic requirements of all living
organisms. The major components of our food are carbohydrates, proteins and
fats.
These biomolecules present in food cannot be
utilized by our body in their original form. They should be broken down and
converted into simple substances in the digestive system.
This process of conversion of complex food
substances to simple forms is known as digestion.
HUMAN DIGESTIVE SYSYEM
The human digestive system consists of the
alimentary canal and accessory glands.
ALIMENTARY CANAL
It is also known as the gastro intestinal tract
(GIT).
It is a long tube through which food passes.
The alimentary canal begins with the mouth and
terminates at the anus.
The average range of alimentary canal in adults is
around 5 metres.
The parts of alimentary canal are:
·
Mouth (buccal cavity)
·
Pharynx
·
Esophagus
·
Stomach
·
Small intestine
·
Large intestine
·
Anal canal
ACCESSORY GLANDS
Various secretions are released into alimentary
canal by accessory glands.
They consists of:
·
Salivary gland
·
Pancreas
·
Liver
TEETH
The teeth are embedded in the sockets of the
mandible and maxilla.
FUNCTIONS
Depending on their functions, teeth have different
shapes.
Incisors and canine teeth are the cutting teeth and
are used for biting off pieces of food.
Premolar and molar teeth, with flat surfaces, are
used for grinding or chewing food.
DENTAL FORMULA
An adult human has 32 permanent teeth. The
arrangement of teeth in each half of the upper and lower jaw is represented by
a dental formula.
IN ADULTS
2123/2123
2 incisors
1 canine
2 premolars
3 molars
IN CHILDREN
2102/2102
2 incisors
1 canine
0 premolar
2 molars
TONGUE
The tongue is composed of skeletal muscle which is
attached to the hyoid bone.
The superior surface consists of stratified squamous
epithelium with numerous papillae (raised structures).
Three types of papillae are there in an adult human
such as filiform papillae, fungiform papillae and circumvallate papillae.
Filiform papillae are whitish in colour and they do
not have taste buds. Fungiform papillae have taste buds and circumvallate
papillae have thousands of taste buds.
FUNCTIONS
Tongue helps in
·
Chewing (mastication)
·
Swallowing (deglutition)
·
Speech
·
Taste
SALIVARY GLANDS
The salivary glands release their secretions
(saliva) through ducts that lead into the buccal cavity.
There are three main pairs of salivary glands such
as parotid glands, submandibular glands and sublingual glands.
About 1.5 litres of saliva is produced daily.
COMPOSITION OF SALIVA
·
Water
·
Mucus
·
Ions (Na+, K+, Cl-,
HCo3- )
·
Enzymes (lysozymes and salivary
amylases)
FUNCTIONS OF SALIVA
1. DIGESTION
OF CARBOHYDRATES
Saliva contains the enzyme amylase
which begins the breakdown of 30% carbohydrates into maltose.
2. Saliva
softens the food.
3. Saliva
help in tasting the food.
4. Saliva
also kills microorganisms due to presence of lysozymes.
HISTOLOGY OF ALIMENTARY CANAL
The wall of alimentary canal from esophagus to
rectum possesses four layers namely serosa, muscularis, submucosa and mucosa.
Serosa is the outermost layer made up of connective
tissues.
Muscularis is formed by smooth muscles.
Submucosal layer is formed of loose connective
tissue.
The innermost layer is the mucosa. This layer has
goblet cells which secret mucus that help in lubrication.
STOMACH
The esophagus leads to a ‘J’ shaped bag like
structure called stomach.
The four layers of tissue that make the basic
structure of the alimentary canal are also found in stomach but with some
modifications.
A muscular sphincter called gastro-esophageal
sphincter (or cardiac sphincter) regulates the opening of esophagus into the
stomach.
Similarly, the opening of stomach into the duodenum
is also guarded by the pyloric sphincter.
The stomach has four major parts such as cardia,
fundus, body and pylorus.
The gastric muscle generates a churning action that
breaks down the bolus (food + saliva) and mixes it with gastric juice.
Peristalsis waves in the stomach wall push the
content towards small intestine.
GASTRIC JUICE
About 2 litres of gastric juice are secreted daily
by glands present in mucosa of stomach.
Gastric juice consists of:
·
Mucus secreted by mucus neck cells.
·
Hydrochloric acid (HCl) and castle’s
intrinsic factor secreted by oxyntic cells.
·
Inactive enzyme pepsinogen and active
enzyme rennin and gastric lipase secreted by chief cells.
FUNCTIONS OF GASTRIC JUICE
HYDROCHLORIC ACID (HCl)
HCl converts pepsinogen into pepsin.
HCl also converts Fe3+ to Fe2+
It kills ingested microorganisms.
CASTLE’S INTRINSIC FACTOR
It is necessary for the absorption of castle’s extrinsic factor or vitamin B12.
PEPSINOGENS
Pepsinogens are activated to pepsins by HCl.
Pepsins begin the digestion of proteins by breaking
them into peptides and peptones.
GASTRIC LIPASES
Gastric lipases breakdown triglycerides to
diglycerides.
RENNIN
Rennin breaks casein (milk protein) into paracasein
in infants.
MUCUS
Mucus provides protection.
THE DUCT SYSTEM OF LIVER AND PANCREAS
The bile secreted by the hepatic cells of liver
passes through the hepatic ducts and is stored in a thin muscular bag called
the gall bladder. The duct of gall bladder along with the hepatic duct from the
liver forms the common bile duct.
The common bile duct and the pancreatic duct open
together into the duodenum of small intestine as the hepato-pancreatic duct.
LIVER
Liver is the largest gland of the body weighing about
1.2 to 1.5 kg in an adult human.
Liver has two lobes.
The hepatic lobules are the structural and
functional units of liver containing hepatic cells (hepatocytes).
Hepatic lobules are hexagonal in shape.
Sinusoids are present between two pairs of columns
of hepatocytes which contains blood vessels.
FUNCTIONS OF LIVER
1. CARBOHYDRATE
METABOLISM
After a meal, when glucose levels rise, liver converts glucose to glycogen under the influence of the hormone insulin.
Later when glucose levels fall, the
hormone glucagon stimulates conversion of glycogen into glucose.
2. PROTEIN
METABOLISM
Liver removes amino group from the
amino acid by a process called deamination.
Amino group is then converted to
ammonia. This ammonia enters into the urea cycle and is converted to uric acid.
Uric acid is excreted in the urine.
3. LIPID
METABOLISM
Liver stores some lipids.
Sometimes liver also converts
lipids into glucose by a process known as gluconeogenesis.
4. BREAKDOWN
OF ERYTHROCYTES
Although breakdown of RBCs takes
place in the spleen, the damaged fragments of RBCs will be eaten by kupffer
cells (phagocytic in nature) of liver.
5. Storage
of fat soluble vitamins (vitamin A, vitamin D, vitamin E, vitamin K).
6. Metabolism
of alcohol and drugs.
7. Secretion
of bile.
BILE
Bile is a greenish yellow fluid that is made and
released by the liver and stored in the gall bladder.
Between 500 ml – 1000 ml of bile is secreted by the
liver daily.
COMPOSITION OF BILE
·
Water
·
Bile salts
·
Bile pigments (bilirubin, biliverdin)
·
Cholesterol
·
Phospholipids (lecithin)
FUNCTIONS OF BILE
Emulsifying fats: Bile salts break down fats into smaller droplets, increasing their surface area for easier digestion and absorption.
Absorbing fat-soluble vitamins: Bile helps break down fats so that fat-soluble vitamins (A, D, E, and K) can be absorbed.
Eliminating waste: Bile helps eliminate waste products such as bilirubin from the liver, which gives feces their brown color.
PANCREAS
The pancreas is a creamy pink gland weighing about
60g.
It is a heterocrine gland (both an exocrine and an
endocrine gland).
EXOCRINE FUNCTIONS OF PANCREAS
The exocrine part of pancreas (or Acini) secrete
pancreatic juice which contains different enzymes.
The pancreatic juice contains inactive enzymes-
trypsinogens, chymotrypsinogens, procarboxypeptidases, proelastases, amylases,
lipases, nucleases (DNase, RNase) etc.
First trypsinogen is activated into trypsin which in
turn activates the other enzymes.
Proteins, peptones and peptides will be converted to dipeptides and tripeptides by those proteases enzymes (chymotrypsinogens, carboxypeptidases and elastases).
70 % carbohydrates will be converted to maltose by enzyme
amylases.
Some triglycerides will be converted to diglycerides
and some diglycerides will be converted to monoglycerides by enzyme lipases.
Similarly, nucleases (DNases and RNases) in the
pancreatic juice act on nucleic acids to form nucleotides.
ENDOCRINE FUNCTIONS OF PANCREAS
The endocrine gland of the pancreas is otherwise
called as islets of Langerhans. The endocrine glands of pancreas secretes the
hormones insulin and glucagon which are concerned with control of blood glucose
levels.
SMALL INTESTINE
The small intestine is continuous with the stomach
at the pyloric sphincter.
The small intestine is about 2.5 cm in diameter and
about 5 meters in length.
The small intestine is comprised of three continuous
parts (duodenum, jejunum and ilium).
The mucosa layer of small intestine forms small
finger like foldings called villi. The epithelial tissue lining the villi is
brush border columnar epithelium. This modification increase the surface area
of small intestine.
Villi are supplied with a network of capillaries and
a large lymph vessel called lacteal.
Villi also forms glands called ‘Crypt of
Lieberkuhn’. These glands secrete enzymes of small intestine (enzymes of small
intestine is otherwise known as succus entericus).
EXOCRINE FUNCTIONS OF SMALL INTESTINE
Aminopeptide will be converted to dipeptide by the
enzyme aminopeptidase and dipeptide will be converted to two molecules of amino
acids by the enzyme dipeptidase.
Maltose will be converted to two molecules of
glucose by the enzyme maltase.
Monoglycerides will be converted to glycerol and
fatty acid by the enzyme lipases.
Nucleotides will be converted to nucleosides by the
enzyme phosphatases and nucleosides will be converted to sugars and nitrogenous
bases by the enzyme nucleosidases.
ABSORPTION OF DIGESTED PRODUCTS
Absorption is the process by which the end product
of digestion pass into the blood.
Absorption of substances takes place in different
parts of alimentary canal. However, the simple substances formed are mostly
absorbed in the jejunum and ilium regions of the small intestine.
Basically there are two types of absorption such as
active absorption (which requires ATP) and passive absorption (which doesn’t
require ATP).
LARGE INTESTINE
The large intestine is about 1.5 meters long,
beginning at the caecum and terminating at the rectum.
The large intestine is divided into caecum, colon
and rectum.
CAECUM
It is the first part of large intestine and house of
good bacteria.
The vermiform appendix is a tube which leads from
the caecum. It is about 8-9 cm long.
The appendix has no digestive function but can cause
problems when it becomes inflamed (appendicitis).
COLON
Colon has three parts such as ascending colon,
transverse colon and descending colon.
RECTUM
The digestive wastes are solidified into faeces in
the rectum.
The egestion of faeces to the outside through the
anal opening is known as defaecation.
Defaecation is a voluntary process.
Diagram Credits: Yashobanta Mahanta, Yostnarani Setty