DIGESTION OF CARBOHYDRATES,protein fats-NEW DISHA.pptx

Digestive System
Gastrointestinal tract
•Mouth
•Pharynx
•Oesophagus
•Stomach
•Small Intestine
•Large Intestine
Associated glands
•Salivary glands
•Liver
•Gall Bladder
•Pancreas

Digestive system performs 6 basic digestive
processes:
• Ingestion
• Secretion
• Motility
• Digestion
• Absorption
• Egestion

Digestive tract
5-7 metres long
Layers:
• Serosa
• Muscularis propria
• Submucosa
• Mucosa

Mouth/Buccal Cavity
• Food enters the GI tract by ingestion.
• Food is broken down by mechanical digestion, using mastication.
• One chemical digestive process occur where amylase enzyme in saliva
breaks down polysaccharide into disaccharides.
• The tongue, made of skeletal muscle, manipulates the food during
mastication. it also contains taste buds to detect taste sensations.
• Food particles are mixed with saliva during mastication , resulting in a
moist lump called bolus for easier passage into or pharynx.
bordered above by hard and soft palate

TEETH
Adapted for mechanical digestion (mastication) in the oral cavity .
20 deciduous or primary teeth before the age of 6.
By age 7, 32 permanent or secondary teeth are developed & are
divided into 4 types: Incisors (for cutting), Canines (for tearing) ,
Premolars (for crushing), and Molars (for grinding). these teeth
follow the human dental formula of 2-1-2-3.

SALIVARY GLANDS
• 3 pairs of salivary glands called parotid , submandibular , and
sublingual gland secrete most of the saliva in the oral cavity , using
salivary ducts .
• Saliva helps moisten the food during mastication , dissolve the food
in forming the bolus , and help cleanse the teeth.
• Saliva consists of 99.5% water , the remaining 0.5% is dissolved
substances including amylase enzyme (for chemically digesting
carbohydrate ), bicarbonate ion (HCO3-; maintains pH of saliva at 6.5-
7.5) , and many electrolytes.

STOMACH
 A pouch-like organ primarily designed for food storage (for 2-4
hours) , some mechanical and chemical digestion also occur .
 Contains two sphincters at both ends to regulate food movement
– cardiac sphincter near the esophagus ,and pyloric sphincter
near the small intestine .
 Divided into 4 regions : cardiac stomach (or cardiac), fundic
stomach (or funded) , body of stomach , and pyloric stomach
(or Pylorus).
 Contain thick folds called rugae at its layer , for providing
larger surface area for expansion , secretion , digestion , and
some absorption.

Gastric Secretory Cells
-Chief cells: secrete pepsinogen (an inactive enzyme) and gastric lipase.
-Parietal cells/Oxyntic cells: secrete hydrochloric acid and "intrinsic factor"
(which helps absorption of vitamin B12 in the intestines).
- Mucous cells: secrete mucus and alkaline substances to help neutralize HCl in
the gastric juice .
-G cells: secrete a hormone called gastrin , which stimulates the parietal
cells and overall gastric secretion .

Enteroendocrine cells
D cells: secretes Somatostatin.
EC cells: secretes Atrial Natriuretic factor and Serotonin.
ECL cells: secretes paracrine Histamine

Chemical digestion & absorption in the stomach
- Carbohydrate digestion is continued with gastric amylase , resulting in
disaccharides .
- Protein digestion begins with pepsin (activation of pepsinogen by HCl) , resulting in
peptides (small chains of protein).
- Lipid digestion begins with gastric lipases which can only break down certain lipids
such as butterfat , resulting in fatty acids .
Absorption in the stomach is limited, where only small and fat- soluble substances can
be absorbed—water , alcohol, aspirin, and certain drugs .
The result of all these mixing , chemical digestion , secretion, and absorption is a
yellowish paste called chyme , which will be passed on to the small intestine .

Regulation of Gastric Secretion
 Regulation of gastric secretion and activities is by both nervous and hormonal mechanisms
– food moving along the oral cavity and esophagus stimulates the parasympathetic nerves
to activate the secretion in gastric glands , the gastric hormone from G cells in turn
stimulates the gastric glands for more activities ("positive feedback").

On the other hand , when food is emptying from the stomach , sympathetic nerves
inhibit the gastric glands and a hormone called intestinal gastrin (released by small
intestine) inhibits other gastric activities.
 The above regulations occur in 3 overlapping phases:
 Cephalic Phase, Gastric Phase, & Intestinal Phase.

Cephalic phase
Cephalic phase: involves special senses detect food and
uses parasympathetic nerves in the vagus nerve to stimulate
gastric activities.
1. Sight, Smell , and Taste of food cause stimulation of
vagus nuclei in brain.
2. Vagus stimulates acid secretion.
a. Direct stimulation of parietal cells (major effect).
b. Stimulation of Gastrin secretion (lesser effect).

Gastric phase
Gastric phase involves the distention of stomach and
stimulates its own activities by the vagus nerve. Distension of
stomach (stretch - receptors) stimulates vagus nerve ; vagus
stimulates acid secretion .
Amino acids and peptides in stomach lumen stimulates acid
secretion (chemo - receptors).
Direct stimulation of parietal cells (lesser effect)
Stimulation of gastrin secretion ; gastrin stimulates acid
secretion (major effect)
Gastrin secretion inhibited when pH of gastric juice falls
below 2.5.

Intestinal Phase
 Intestinal phase involves acidic chyme passing into the small intestine which
secretes intestinal gastrin hormone to inhibit gastric activities.
 Neural inhibition of gastric emptying and acid secretion. Arrival of chyme in
duodenum activate a neural reflex that inhibits gastric activity.
 In response to fat in chyme , duodenum secretes the hormone,
secretin that inhibits gastric acid secretion.
 The enterogastric reflex: This reflex begins in the small intestine (entero)
and ends in the stomach (gastro).
 Duodenum fills with chyme. Sensory stretch receptors are stimulated. Sensory
nerve impulses travel to CNS. Nerve impulses from CNS (vagus) inhibit peristalsis
in stomach wall.

Stomach: Neural & Hormonal
Mechanisms

Pancreas
• Pancreas : most pancreatic enzymes are produced as inactivated
molecules , or zymogens , so that the risk of self – digestion within
the pancreas is minimized .
• More than 98% of the pancreas mass is devoted to its exocrine
function: the secretion of pancreatic juice by the pancreatic acini
and their ductile cells. Ductile cells produce Sodium bicarbonate
which helps neutralize the acidic gastric contents .
• Acinar cells of the exocrine pancreas produce a variety of
digestive enzymes to break down food substances into smaller
absorbable molecules .
• Only 2% of pancreas mass is devoted to the islets of langerhans ,
which produce insulin and glucagon , hormones that regulate blood
sugar and carbohydrate metabolism (they have opposite effects) .

Major pancreatic
Enzymes
 -Pancreatic amylase: digest polysaccharides into disaccharides
 - Pancreatic lipases digest triglycerides into fatty acids .
 - Pancreatic nucleases digest nucleic acids into nucleotides .
 -Pancreatic proteinases (all secreted in their inactive forms) digest
peptides into amino acids:
Trypsinogen is activated by enterokinase (secreted by duodenum)
into trypsin , which in turn activates the other 3 enzymes –
chymo- trypsinogen becomes chymotrypisn ,
proaminopeptidase becomes aminopeptidase, and
procarboxypeptidase becomes carboxypeptidase.

Activation of pancreatic proteases in the small intestine

Pancreatic Secretion
1.The parasympathetic nervous system increases pancreatic secretion
2. Two duodenual hormones also influence pancreatic secretion: Secretin and
Cholecystokinin.
3. Food entering the small intestine stimulates the secretion of both hormones.
4. Secretin stimulates the secretion of pancreatic electrolyte – rich fluid , while
CCK enhances the enzymatic secretions of the pancreas .

Regulation of pancreatic Juice
1. Acidic chyme enters duodenum.
2. Secretin is released into blood stream from
intestinal mucosa.
3. Secretin stimulates pancreas.
4. Pancreas secretes pancreatic juice.
5. Pancreatic juice , high in bicarbonate ions ,
neutralizes acidic chyme.

Functions of The Liver
Important in carbohydrate metabolism where hepatic cells conduct glycogenesis
(converting glucose into glycogen) , and glycogenolysis (breaking glycogen
down to glucose).
Also is critical in lipid metabolism where hepatic cells produce bile (for fat
emulsification), oxidize fatty acids , synthesize various forms of lipids ,and
convert glucose to fatty acids (lipogenesis) .
Other functions of the liver include :
- Storage of glycogen, iron , and vitamins A,D,B12.
-Contains phagocytes to destroy damaged erythrocytes and foreign substances,
using phagocytosis .
-Detoxifies harmful substances in the blood .
-Serves as a blood reservoir (contains 7% of blood volume).

Gall Bladder
A small sac located on the inferior , visceral surface of the liver.
Stores and concentrates bile secreted by the liver.
Regulation of Bile Release:
1. Chyme with fat enters small intestine.
2. Cells of intestinal mucosa secrete the hormone Cholecystokinin (CCK) into
the blood stream.
3. CCK stimulates muscular layer of gallbladder wall to contract.
4. Bile passes down the cystic duct and common bile duct to duodenum .
5. Hepatopancreatic sphincter relaxes and bile enters duodenum.

Small Intestine
• A long tube, with a small diameter (about 1 inch), extending
from pyloric sphincter to the ileocecal valve .
• Divided into Duodenum, Jejunum, and ileum.
1. Secretions of small intestine:
a. Intestinal glands secrete a watery fluid that lack digestive
enzymes but provides a vehicle for moving chyme to villi.
Intestinal enzymes include : maltase digests maltose
into glucose. sucrase digests sucrose into glucose and
fructose . lactase digests lactose into glucose and galactose.
peptidases digest peptides into amino acids . lipases digest
triglycerides into fatty acids and glycerol . Nucleases digest
nucleotides into nitrogenous bases. Enterokinase converts
trypsinogen into trypsin.

 b. Digestive enzymes embedded in the surfaces of microvilli
split molecules of sugars, proteins and fats .
 c. Regulation of small intestine secretions: secretion is stimulated
by gastric juice , chyme , and reflex stimulated by distension of the
small intestinal wall .

 d. Each villus contains blood capillaries to absorb water , glucose ,
amino acids , vitamins , minerals , and short-chain fatty acids , and
also contains lymphatic capillaries called lacteals to absorb long –
chain fatty acids in the forms of micelles .
 e. Water is absorbed by osmosis , fatty acids are absorbed by diffusion
(since they are fat-soluble), and most other nutrients (glucose, amino
acids, & minerals) are absorbed by active transport.

Large intestine
 The last segment of the GI
tract , with a large diameter
(2-3 inches) , extending
from the ileocecal valve to
the anus .
 Divided into cecum ,
ascending colon , transverse
colon , descending colon ,
rectum.

 The large intestine has little or no digestive function , although it
secretes mucus. Its mucosa has no villi or microvillus , but
contains numerous goblet cells for secreting mucus to aid in the
formation of feces and maintain an alkaline condition .
 Mechanical stimulation and parasympathetic impulses control
the rate of mucus secretion .
 The large intestine only absorbs water, electrolytes and some
vitamins .
 Many bacteria inhabit the large intestine , where they break
down certain indigestible substances and synthesize certain
vitamins .
 Feces are formed and stored in the large intestine . Defecation
involves a reflex mechanism aided by voluntary contraction of
the diaphragm , abdominal muscles ,and the external anal
sphincter .

Major Hormones of The Digestive Tract
1. Gastrin : (Gastric & intestinal) : released by Gastric cells , in response to the
presence of food. Causes Gastric glands to increase their secretory activity.
2. Somatostatin : (Gastric inhibitory peptides - GIP): Inhibits secretion
of acid by parietal cells.
3. Cholecystokinin : released by intestinal wall cells , in response to the
presence of proteins and fats in the small intestine. It causes gastric glands to
decrease their secretory activity and inhibits gastric motility ; stimulation of
pancreas to secrete digestive enzyme; stimulates gall – bladder to contract and
release bile.
4. Secretin: released by cells in the duodenal wall, in response to acidic
chyme entering the small intestine.

Major Digestive Enzyme
Salivary enzyme: Begins carbohydrates digestion by breaking down starch and
glycogen to disaccharides
Gastric enzymes: Pepsin , from Gastric glands – Begins protein digestion .
Lipase, from Gastric glands – Begins fat digestion .
Pancreatic enzymes: Amylase , from pancreas – breaks down starch and
glycogen into disaccharides. Lipase, from pancreas – breaks down fats into fatty
acids and glycerol .
Proteolytic enzymes :Trypsin, Chymotrypsin, and Carboxypeptidase from
pancreas breaks down peptides into amino acids . Nucleases, from pancreas-
breaks down nucleic acids into nucleotides.

 Intestinal Enzymes: Peptidase, from mucosal cells, breaks down
peptides into amino acids. Sucrase, maltase, and lactase , from
mucosal cells, breaks down disaccharides into monosaccharides.
Lipase, from mucosal cells, breaks down fats into fatty acid and
glycerol. Enterokinase , from mucosal cells, (breaks down) converts
trypsinogen into trypsin .

Anatomical Basis of Absorption
Absorptive Surface of the Small Intestinal Mucosa Villi.
 Figure demonstrates the absorptive surface of the small
intestinal mucosa, showing many folds called valvulae
conniventes (or folds of Kerckring), which increase the
surface area of the absorptive mucosa about threefold.
 These folds extend circularly most of the way around the
intestine and are especially well developed in the
duodenum and jejunum.
 The presence of villi on the mucosal surface enhances the
total absorptive area another 10-fold.
 Thus, the combination of the folds of Kerckring, the villi,
and the microvilli increases the total absorptive area of
the mucosa perhaps 1000-fold.

Fig A :longitudinal section the general organization of the villus, emphasizing
(1) the advantageous arrangement of the vascular system for absorption of fluid and
dissolved material into the portal blood and
(2) the arrangement of the “central lacteal” lymph vessel for absorption into the lymph.
Figure B shows a cross section of the villus

Absorption of Carbohydrates
1) Essentially all the carbohydrates in the food are absorbed in the form of
monosaccharides
2) only a small fraction are absorbed as disaccharides
3) and almost none as larger carbohydrate compounds.
By far the most abundant of the absorbed monosaccharides is glucose, usually accounting
for more than 80 per cent of carbohydrate calories absorbed.
The reason for this is that glucose is the final digestion product of our most abundant
carbohydrate food, the starches.
The remaining 20 per cent of absorbed monosaccharides are composed almost entirely of
galactose and fructose.
The monosaccharides are absorbed by an active transport process.

Absorption of Other Monosaccharides.
Galactose is transported by almost exactly the same
mechanism as glucose.
Conversely, fructose transport does not occur by the sodium
co-transport mechanism. Instead, fructose is transported by
facilitated diffusion all the way through the intestinal
epithelium but not coupled with sodium transport.
Much of the fructose, on entering the cell, becomes
phosphorylated, then converted to glucose, and finally
transported in the form of glucose the rest of the way into the
blood.
Because fructose is not co-transported with sodium, its overall
rate of transport is only about one half that of glucose or
galactose.

Absorption of Proteins
 most proteins, after digestion, are absorbed through the luminal
membranes of the intestinal epithelial cells in the form of dipeptides,
tripeptides, and a few free amino acids.
 The energy for most of this transport is supplied by a sodium co-
transport mechanism in the same way that sodium co-transport of
glucose occurs.
 This is called co-transport (or secondary active transport) of the amino acids
and peptides.
 A few amino acids do not require this sodium co-transport mechanism
but instead are transported by special membrane transport proteins in
the same way that fructose is transported, by facilitated diffusion.

Absorption of Fats
 when fats are digested to form monoglycerides and free fatty acids, both
of these digestive end products first become dissolved in the central lipid
portions of bile micelles.
 In the presence of an abundance of bile micelles, about 97 per cent of
the fat is absorbed;
 In the absence of the bile micelles, only 40 to 50 per cent can be
absorbed.
 After entering the epithelial cell, the fatty acids and monoglycerides are
taken up by the cell’s smooth endoplasmic reticulum;
 here, they are mainly used to form new triglycerides that are
subsequently released in the form of chylomicrons through the base of
the epithelial cell, to flow upward through the thoracic lymph duct and
empty into the circulating blood.

Direct Absorption of Fatty Acids into the
Portal Blood.
• Small quantities of short- and medium-chain fatty acids, such
as those from butterfat, are absorbed directly into the portal
blood rather than being converted into triglycerides and
absorbed by way of the lymphatics.
• The cause of this difference between short- and long-chain
fatty acid absorption is that the short-chain fatty acids are
more water-soluble and mostly are not reconverted into
triglycerides by the endoplasmic reticulum.
• This allows direct diffusion of these short-chain fatty acids
from the intestinal epithelial cells directly into the capillary
blood of the intestinal villi.

Absorption in the Large
Intestine: Formation of Feces
About 1500 milliliters of chyme normally pass through the
ileocecal valve into the large intestine each day.
Most of the water and electrolytes in this chyme are absorbed
in the colon, usually leaving less than 100 milliliters of fluid to
be excreted in the feces.
Most of the absorption in the large intestine occurs in the
proximal one half of the colon, giving this portion the name
absorbing colon,
whereas the distal colon functions principally for feces storage
until a propitious time for feces excretion and is therefore
called the storage colon.

Maximum Absorption Capacity of the Large Intestine.
The large intestine can absorb a maximum of 5 to 8 liters
of fluid and electrolytes each day.
When the total quantity entering the large intestine
through the ileocecal valve or by way of large intestine
secretion exceeds this amount, the excess appears in the
feces as diarrhea.

DIGESTION OF CARBOHYDRATES,protein fats-NEW DISHA.pptx

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