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Session Objectives : Session Objectives Digestion in Stomach
Digestion in Small Intestine
Accessory Glands
Absorption of digested food
Egestion
The Digestive System of man : The Digestive System of man Alimentary canal andaccessory glands
Histology of alimentary canal : Histology of alimentary canal
Slide 6 : All the four layers show modifications in different parts of the alimentary canal
In duodenum, glands are present in sub-mucosa
Mucosal epithelium has goblet cells which secrete mucus that help in lubrication.
Mucosa also forms glands in the stomach (gastric glands) and crypts in between the bases of villi in the intestine (crypts of Lieberkuhn). Histology of alimentary canal
Stomach : Stomach C-shaped sac located at left side in upper portion of abdominal cavity. It has three regions
Glands in Stomach : Glands in Stomach
Specialized cells in stomach : Specialized cells in stomach
Gastric Glands : Gastric Glands Secrete gastric juice:
Goblet cells: mucus.
Parietal cells: HCl and intrinsic factor.
Chief cells: pepsinogen.
Enterochromaffin-like cells (ECL): histamine and serotonin.
G cells: gastrin.
D cells: somatostatin.
Stomach: ghrelin. Feeling Hungry ?
ghrelin functions to increase hunger though its action on hypothalamic feeding centers
Functions of stomach : Functions of stomach Mixes food with gastric juice( Mucous, HCl, Pepsinogen, gastrin)
Acts as a churn for foodand liquefies it into chyme
Retains food giving enzymes time to act
Digests proteins through action of enzymes
Performs some absorption of salts and alcohol
Produces gastric intrinsic factor (GIF) – Helps in absorption of vitamin B12
Gastric Digestion : Protein containing food stimulate secretion of gastrin from pyloric gland
Gastrin stimulate gastric gland for the secretion of HCl, pepsinogen and mucus
Pepsinogen in acidic medicum forms pepsin which digest proteins
Small quantities of other gastric juice is also secreted – gastric lipase, gastric amylase, gelatinase
Enterogasterone regulates the flow of chyme into intestine Gastric Digestion
Gastric juice : Gastric juice
Illustrative problem : Illustrative problem What are stomach ulcers? An ulcer is an irritation in the gastric mucosa through which gastric juices digest the stomach wall. Solution : What is achlorhydria ? It leads to pernicious anemia. How ?
Slide 15 : What is achlorhydria ? It leads to pernicious anemia. How ? When acid producing parietal cells of the stomach are destroyed, which frequently occurs in chronic gastritis, the person develops achlorhydria (lack of acid secretion).
It leads to pernicious anemia because of the failure of the secretion of GIF leading to failure of maturation of the red blood cells in the absence of vitamin B12 stimulation of the bone marrow.
Structure of intestine : Structure of intestine Duodenum
Jejunum
Ileum
The small intestine - Duodenum : The small intestine - Duodenum C-shaped structure (about 25cm long)
Most fixed part of the intestine
Begins at pyloric sphincter and ends into jejunum
Bile and pancreatic ducts open into it
Possesses Brunner’s glands (help neutralize stomach
acidity) Pancreas lies within its loop
Digestion in small intestine : Digestion in small intestine Three basic stimuli required for Pancreatic secretion :- Acetylcholine : released from parasympathetic vagus nerve
and stimulates acinar cell of pancreas Cholecystokinin : released from duodenal mucosa and
stimulates acinar cell of pancreas and liver
for bile secretion Secretin : released from duodenal mucosa and
stimulates secretion of sodium bicarbonate
for neutralizing the acidic properties of chyme
Pancreas : Pancreas Acinar cells produce –
Pancreatic juice
– Sodium bicarbonate
– Pancreatic amylase Carboxypeptidase
Endopeptidase
Lipase
Trypsin
chymotrypsin
Digestion in small intestine : Digestion in small intestine Proteoses and peptones of chyme excites cholecystokinin which in turn stimulate pancreas for enzymatic secretion
Meanwhile secretin get excited by low pH (<4.5 – 5) and secretes bicarbonate thereby medium becomes basic (pH 8)
Proteolytic pancreatic enzymes are synthesized in the inactive forms like trypsinogen, chymotrypsinogen and procarboxypolypeptidases
Digestion in small intestine : Digestion in small intestine Trypsin becomes active by an enzyme called enterokinase secreted by intestinal mucosa. Trypsinogen can be autocatalytically activated by trypsin
chymotrypsinogen and procarboxypolypeptidases are activated by trypsin
Trysin and chymotrysin are endopeptidases. They split partially digested proteins into peptides of various size but do not causes release of amino acids
Carboxypolypeptiddases is exopeptidases. It splits peptides into individual amino acid
Digestion in small intestine : Digestion in small intestine Digestive enzyme for carbohydratae is pancreatic amylase. It hydrolyzes starches, glycogen and other carbohydrate (except cellulose) into disacchrides
Enzymes for fat digestion are pancreatic lipase, chloesterol esterase and phospholipase. Fatty acids and monoglycerides are formed
Bile : Bile Chloecystokin stimulates bile secretion from Gallbladder. Bile consists of bile salts, chloesterol, lecithin and electrolytes of plasma.
Bile plays an important role in fat digestion. It emulsify large fat into minute particle for the action of enzyme lipase. It also aid in absorption of digested end product of fat by intestinal mucosa. Do You Know ? Bile serves as means of excretion for several waste product from blood. These include especially bilrubin, an end product of haemoglobin destruction and excess of chloesterol
Illustrative Problem : carbohydrates
fats
proteins
All of these If the pancreas is removed, the compound
which remains undigested is Illustrative Problem
Intestinal villi : Intestinal villi Adaptations of villi for absorption
Large surface area for absorption
Thin walled for easy passage of nutrients
Rich supply of blood capillariesand lacteals
Specialized cells in the intestine : Specialized cells in the intestine Crypt’s of Leiberkuhn (intestinal glands)-tubular glands possessing various specialized cells. It lie between intestinal vili.
Intestinal surface of crypts and vili are covered by epithelium consisting of two types of cell – Goblet and enterocytes
Goblet cells-secrete mucous for lubrication and protection of alimentary canal.
Enterocytes – It forms intestinal secretion and helps in digestion and absorption of nutrients, processing of fats, transport of small nutrients.
Specialized cells in the intestine : Specialized cells in the intestine Enterocytes of mucosa covering vili contain digestive enzymes
Several peptidases for splitting small peptides into amino acids
Four enzymes for splitting disacchrides inot monosacchride – sucrase, maltase, isomaltase and lactase
Small amount of intestinal lipase for digestion of fats
The large intestine : The large intestine
The large intestine : The large intestine Extends from ileocaecal junction to anus (about 1.5m long)
The surface bears longitudinal bands of muscles called taenia coli
It bears sacculations (called haustra) which are a characteristic feature of large intestine
It is divided into the caecum, colon, rectum and anus.
The Large intestine : The Large intestine Caecum Blind ending sac between the ileum and colon
The opening of ileum into the caecum is guarded by the iliocolic valve
It is functional in herbivores, but has lost its importance in man
The vermiform appendix is attached to the caecum
It is an elongated (10 cm) projection made up of lymphoid tissue
The Large intestine : The Large intestine Colon - (upto 5ft long) distinguished into
Ascending colon- reabsorbs useful materials
Distal colon (transverse, descending and sigmoid) excretes wastes Absorbs large amounts of water
Reabsorbes bile salts and electrolytes
Powerful storage and waste disposal site in the body
Stores wastes till excretion
Through involuntary contractions pushes wastes to anus. Functions
Functions of the Large Intestine : Functions of the Large Intestine Water absorption
Reabsorption of salts and electrolytes
Fermentation – to breakdown the dietary fibres with the help of intestinal bacteria- Lactobacillus acidophilus, E.coli, Klebsiella Bacterial fermentation produces-
Short chain fatty acids
Protein metabolites
Gases (methane, ammonia and hydrogen sulphide)
Illustrative problem : Illustrative problem What do the intestinal bacteria do ? Solution: They mainly have a protective role.
By inhabiting the large intestine they keep out bacteria that may cause disease.
They take up all the available nutrients and space and crowd out dangerous competitors.
Some also synthesize vitamins K and B12.
They do not contribute much to digestion but form bulk of the faeces (constitute 20 – 50%) of the dry weight of feces).
Diarrhea : Diarrhea Whenever a segment of large intestine is irritated , the mucosa secretes large amount of water and electrolyte. This acts to dilute the irritating factors and cause to rapid movement of feces towards anus. The result is diarrhea.
Digestion of nutrients : Digestion of nutrients
Accessory glands : Accessory glands Liver
Gall bladder
Pancreas
Accessory glands– The Liver : Accessory glands– The Liver For digestion produces bile
Stores glucose as glycogen
Processes fats and amino acids
Stores some vitamins
Detoxifies poisons and drugs(clearing house)
Makes blood proteins
Accessory Gland - The Gall Bladder : Accessory Gland - The Gall Bladder Small pear shaped organ (3-4 inches long)
It is connected to liver by the cystic duct
It stores and concentrates bile
Opens into the duodenum through the bile duct Bile
Is produced from cholesterol in the liver
It emulsifies fats and neutralizes acids in partly digested food.
Bile salts – Glycocholic acid, taurocholic acid.
Absorption and Assimilation : Absorption and Assimilation Digestion products are absorbed through micro-vili of small intestine
Fatty acids, monoglycerrides, chlesterol and other fat soluble substances are absorbed by simple diffusion through brush border of the cells lining the inestinal vili
Water soluble substances (alcohol) are absorbed by osmosis
Hydrophilic lipid insoluble sugar like fructose are absorbed by facilitated diffusion. It occurs with the help of specific transporter protein
The process is powered by coupling sugar transport to electrochemical gradient for Na+ across the brush border.
Model of Glucose Absorption : Model of Glucose Absorption
Absorption and Assimilation : Absorption and Assimilation Amino acids are absorbed by active transport
Fat soluble but water insoluble monoglycerides, fatty acids and glycerol are first incorporated into water soluble droplets called micelles.
From these micelles fatty acids, glycerol, sterols and fat soluble vitamins are absorbed by facilitated diffusion and are reconstructed within the absorptive cells (smooth ER) into triglycerides
Remnant of micelle is left behind and combine with new fatty acids and monoacylglycerols
Reconstructed triglycerides later combine with phospholipid and chloesterol and release into lymph in the form of protein coated water soluble fat droplets called chylomicrons.
These droplets are transported to blood through lymphatic system
Absorption of Fats : Absorption of Fats
Routes of absorbed nutrients : Routes of absorbed nutrients Water soluble substances Fat soluble substances
Slide 44 : Fate of absorbed nutrients Glucose To release energy
Excess glucose is converted to glycogen and stored in liver and muscles
Excess glucose is converted into fat for storage Fats As energy reserve
Stored beneath skin and around internal organs Amino acids Build up body tissues, production of hormones, enzymes and antibodies
Replace worn out tissues
Excess amino acids are deaminated in the liver to form urea.
Egestion : Egestion Peristalsis pushes the slurry of indigestible materials of small intestine into large intestine or colon
The colon absorbs most of the water, electrolytes and ions
Active pumping of Na+ and water ffrom chyme
Bacteria in colon produces vit B12, K, thiamine and riboflavin that are absorbed by the wall of colon
Fill in the blanks : : Fill in the blanks : The gland found in submucosa of deodenum is…………..
Vili is a ………………… layer of alimentary canal
GIF is secreted by………………cell of stomach
Serotonin is secreted in ……………..
Gastrin is secreted in ……………
Enterokinase is secreted by………….
The tubular branched exocrine gland situated in submucosa of alimentary canal is……….
Argentaffine cells secretes………………
Flow of chyme from stomach is regulated by -------
Chylomicrons are formed by triglycerides & …………
Answer: : Answer: The gland found in submucosa of deodenum is Bruner’s gland
Vili is a mucosal layer of alimentary canal
GIF is secreted by parietal cell of stomach
Serotonin is secreted in stomach
Gastrin is secreted in stomach
Enterokinase is secreted by deodenum
The tubular branched exocrine gland situated in submucosa of alimentary canal isBruner’s gland
Argentaffine cells secretes serotonin & histamine
Flow of chyme from stomach is regulated by enterogasterone
Chylomicrons are formed by triglycerides & proteins