Liver:
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Liver is the largest gland of the body
Weighing about 1.2 to 1.5 kg in an adult human.
It is situated in the abdominal cavity, just below the diaphragm and has two lobes.
Liver lobes are separated by Falciform ligament
The hepatic lobules (hexagonal in shape) are the structural and functional units of liver containing
hepatic cells (hepatocytes) arranged in plate like pattern.
Each lobule is covered by a thin connective tissue sheath called the Glisson’s capsule.
The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and
concentrated in a thin muscular sac called the gall bladder.
Hepatic lobule: Functional unit of liver
It mainly consists
1. Hepatocytes: secrete bile.
2. Hepatic sinusoids: blood capillaries present between rows of hepatocytes that receive oxygenated
blood from hepatic artery and nutrient-rich deoxygenated blood from hepatic portal vein.
3. Hepatic macrophages or kupffer cells: present in the hepatic sinusoids, stationary phagocytic cell,
which destroy worn-out cells, bacteria, and other foreign matter in the blood draining from the
gastrointestinal tract.
4. Bile canaliculi: are small ducts between hepatocytes that collect bile produced by the hepatocytes.
5. A bile duct, branch of the hepatic artery, and branch of the hepatic vein, together are referred to as
a portal triad.
6. Central vein is present at the center of the lobule.
From bile canaliculi, bile passes into bile ducts.
Blood flows toward a central vein while bile flows in the opposite direction.
Function of liver:
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Liver detoxifies and purify blood.
Liver is the storehouse of many fat soluble vitamins like Vitamin A, D, and K.
Liver is the storehouse of glycogen, minerals etc.
Liver regulates the metabolism of fats, proteins, and carbohydrates
Liver activates a number of inactive enzymes (Zymogens).
They help in excretion of bilirubin.
Hepatocyte cells of liver synthesize Bile. Bile helps in the digestion of fat.
Hepatocyte cells of liver synthesize many proteins like clotting factors, albumin.
Hepatocyte cells of liver synthesize cholesterol.
Bile:
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A yellow or olive-green liquid secreted by hepatocytes and stored in gallbladder.
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The hormones secretin and CCK stimulate contraction of the gall bladder and relaxation of the
hepatopancreatic sphincter, expelling both bile and pancreatic juice through the duodenal papilla
into the duodenum
Bile has a pH of around 8 and between 500 and 1000 mL is secreted daily.
It consists of:
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water
mineral salts
mucus
bile salts (Sodium bicarbonate, Sodium taurocholate, Sodium glycocholate)
bile pigments, mainly bilirubin
cholesterol
Release from the gall bladder After a meal, Secretion is markedly increased when chyme entering the
duodenum contains a high proportion of fat.
Function of bile:
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Emulsification of fats in the small intestine (bile salts), which help their digestion by pancreatic
lipases.
Making cholesterol and fatty acids soluble, enabling their absorption along with the fat-soluble
vitamins - bile salts
Excretion of bilirubin (a waste product from the breakdown of red blood cells), most of which is in
the form of stercobilin.
Movements of GIT
Mouth:
1. Chewing/mastication:
• Food substances are torn or cut into small particles and crushed or ground into a soft bolus
by teeth.
• Lubrication and moistening of dry food by saliva, so that the bolus can be easily swallowed
• Appreciation of taste of the food.
Pharynx and oesophagus:
2. Deglutition or swallowing:
Process by which food moves from mouth into stomach.
Deglutition occurs in three stages:
I.
Oral stage: food moves from mouth to pharynx: voluntary
II.
Pharyngeal stage: food moves from pharynx to oesophagus:
soft palate and uvula move upward to close of the nasopharynx, which prevents swallowed
foods and liquids from entering the nasal cavity.
The epiglottis closes of the opening to the larynx, which prevents the bolus from entering
the respiratory tract.
III.
Oesophageal stage: food moves from oesophagus to stomach via peristalsis.
Peristalsis: coordinated contractions and relaxations of the circular and longitudinal layers
of the muscularis called peristalsis, it pushes the food onward.
Stomach:
3. Gentle peristaltic movements: occurs in fundus and body part of stomach, food is mainly stored
here.
4. Strong peristaltic movements (propulsion):
• Occurs in pyloric part of stomach
• Churns and physically breaks down food and mixes it with gastric juice, forming chyme (soupy
liquid).
• Forces chyme through pyloric sphincter, responsible for gastric emptying into duodenum.
Small intestine:
5. Segmentation (Mixing)
• Type of peristalsis:
• alternating contractions of circular smooth muscle fibers that produce segmentation and
resegmentation of sections of small intestine;
• Mixes chyme with digestive juices and brings food into contact with mucosa for absorption.
6. Migrating motility complex (MMC)
• Type of peristalsis:
• Waves of contraction and relaxation of circular and longitudinal smooth muscle fibers
passing the length of the small intestine;
• Moves chyme toward ileocecal sphincter
Large intestine:
7. Haustral churning (mixing/segmentation). Moves contents from haustrum (pouch) to haustrum by
muscular contractions.
8. Peristalsis. Moves contents along length of colon by contractions of circular and longitudinal
muscles.
9. Mass peristalsis. Forces contents into sigmoid colon and rectum.
10. Defecation reflex. Eliminates faeces by contractions in sigmoid colon and rectum.
Digestion and absorption of nutrients
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The process of digestion is accomplished by mechanical and chemical processes
The digestion of food starts from the mouth itself. Mouth helps in mastication of food and
facilitation of swallowing.
The masticated food mixed with saliva makes a small mass of food called a bolus. The bolus moves
to pharynx and oesophagus by the process of deglutition (swallowing).
There are various enzymes that get mixed with the food at different parts of the alimentary canal and
facilitate digestion
Parts of
alimentary
canal
Activity
Results
Lysozyme: act as antimicrobial agent that prevents
infections.
Salivary amylase: 30 per cent of starch is hydrolysed
(optimum pH 6.8) into a disaccharide – maltose.
1. Salivary glands secrete
saliva
Mucus in saliva helps in lubricating and adhering the
masticated food particles into a bolus.
Mouth
2. Lingual glands secrete
lingual lipase- it works
in stomach at acidic
pH.
Lingual lipase: Triglycerides (fats) broken down into
fatty acids and diglycerides
1. Mucus neck cell
secretes mucus
2. Parietal cells Secrete
intrinsic factor
Forms protective barrier that prevents stomach wall
from HCl.
Needed for absorption of vitamin B12 (used in red
blood cell formation, or erythropoiesis
Kills microbes in food; denatures proteins; converts
pepsinogen into pepsin.
hydrochloric acid
3. Chief cells Secrete
pepsinogen
(Protein digesting enzyme)
Stomach
(stores the
food for 4-5
hours.)
Pepsin (activated form) breaks down proteins into
peptides
Protein → proteoses + Peptones (pH-1.8)
(Rennin is a proteolytic enzyme found in gastric
juice of infants which helps in the digestion of milk
proteins)
Gastric lipase
4. G cells
Secrete gastrin
5. Churning movements
1. Pancreatic juice:
Mucus with
bicarbonates
Pancreatic amylase
Splits triglycerides (fats) into fatty acids and
monoglycerides.
Stimulates parietal cells to secrete HCl and chief cells
to secrete pepsinogen; increases motility of stomach
The food mixes thoroughly with the acidic gastric juice
of the stomach by the churning movements of its
muscular wall and is called the chyme.
Maintains alkaline pH (7.8) and protects intestinal
mucosa from HCl
Carbohydrates in the chyme are hydrolysed into
disaccharides.
Trypsin (activated from
trypsinogen by
enterokinase)
Small
intestine
(duodenum)
Chymotrypsin
(activated from
chymotrypsinogen by
trypsin)
Elastase (activated
from proelastase by
trypsin)
Carboxypeptidase
(activated from
procarboxypeptidase
by trypsin)
Fats that have been emulsified by bile are broken into
diglycerides and monoglycerides
Pancreatic lipase
Nucleases in the pancreatic juice acts on nucleic acids
(DNA and RNA) to form nucleotides and nucleosides
Nucleases (Ribonuclease
Deoxyribonuclease)
2. Bile
3. Brush-border enzymes
Dipeptidase
Aminopeptidase
Maltase
Small
intestine
(duodenum)
Lactase
Sucrase
Nucleotidases
Nucleosidases
Lipases
Bile helps in emulsification of fats, i.e., breaking down
of the fats into very small micelles.
Major hormones that control digestion
HORMONE
Gastrin
Secretin.
Cholecystokinin (CCK)
STIMULUS AND SITE OF
SECRETION
Distension of stomach, and high
pH of stomach chyme stimulate
gastrin secretion
ACTIONS
Promotes secretion of gastric
juice, increases gastric motility.
Acidic (high H+ level) chyme that
enters small intestine stimulates
secretion of secretin
Stimulates secretion of
pancreatic juice and bile that are
rich in HCO3 − (bicarbonate ions),
Inhibits secretion of gastric juice
Partially digested proteins
Stimulates secretion of
(amino acids), fats and fatty acids pancreatic juice, causes ejection
that enter small intestine
of bile from gallbladder.
stimulate secretion of CCK
Digestion process convert carbohydrates to monosaccharides (glucose, fructose, and galactose); proteins
to single amino acids, dipeptides, and tripeptides; and triglycerides (fats) to fatty acids, glycerol, and
monoglycerides.
Absorption:
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Absorption is the process by which the end products of digestion pass through the intestinal
mucosa into the blood or lymph.
It is carried out by passive, active or facilitated transport mechanisms
Transport of water depends upon the osmotic gradient.
Absorption of carbohydrate
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All carbohydrates are absorbed as monosaccharides (glucose, fructose, and galactose)
Monosaccharides enters into absorptive cells via facilitated diffusion or active transport.
Fructose, is transported via facilitated diffusion;
glucose and galactose are transported into absorptive cells of the villi via secondary active transport
that is coupled to the active transport of Na+.
Monosaccharides then move out of the absorptive cells via facilitated diffusion and enter the
capillaries of the villi.
Absorption of proteins
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Proteins are absorbed as amino acids via active transport/secondary active transport processes that
occur mainly in the duodenum and jejunum.
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Some proteins are absorbed as Dipeptide and tripeptide via secondary active transport into
absorptive cell, peptides then are hydrolysed to single amino acids inside the absorptive cells.
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Amino acids move out of the absorptive cells via diffusion and enter blood capillaries.
Absorption of fats:
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All dietary lipids are absorbed via simple diffusion.
Small short-chain fatty acids are absorbed via simple diffusion.
Long chain Fatty acids and glycerol are insoluble in water so cannot be absorbed into the blood.
They form micelles (The bile salts in intestinal chyme surround long chain fatty acids and glycerol, forming
tiny spheres called micelles) → absorption into absorptive cell → formation of chylomicron (protein coated
fat globule)→transported into lacteals→ release of absorbed substance into blood.
Absorption of substances takes place in different parts of the alimentary canal but maximum absorption
occurs in the small intestine.
Organ
absorption
Mouth -
Drugs
Stomach-
water, simple sugars and alcohol
Small intestineLarge intestine-
maximum absorption of nutrients (glucose, fructose, fatty acids,
glycerol and amino acids)
water and some minerals
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The undigested, unabsorbed substances called faeces enters into the caecum of the large intestine
through ileo-caecal valve. It is temporarily stored in the rectum till defaecation.
The absorbed substances finally reach the tissues which utilise them for their activities. This process
is called assimilation.
Disorders of GIT:
1. Jaundice: The liver is affected; skin and eyes turn yellow due to the deposit of bile pigments.
2. Vomiting: It is the ejection of stomach contents through the mouth. This reflex action is controlled
by the vomit centre in the medulla. A feeling of nausea precedes vomiting.
3. Diarrhoea: The abnormal frequency of bowel movement and increased liquidity of the faecal
discharge is known as diarrhoea. It reduces the absorption of food.
4. Constipation: In constipation, the faeces are retained within the colon as the bowel movements
occur irregularly. Indigestion: In this condition, the food is not properly digested leading to a feeling
of fullness. The causes of indigestion are inadequate enzyme secretion, anxiety, food poisoning,
over eating, and spicy food.