Transcript The Digestive System (PowerPoint)

The Digestive System
Is the system responsible for:
1. Ingestion of food
2. Digestion of food
3. Absorption of food
4. Egestion (Elimination) of food
The Digestive System
The digestive organs
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The mouth
The pharynx
The esophagus
The stomach
The small intestine
The large intestine
The accessory organs
• The salivary glands
• The liver
• The pancreas
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Digestion: is the breakdown of the complex food molecules
into simpler molecules that could be absorbed by the body.
Digestion
Mechanical (Physical)
Chemical (enzymes)
Mechanical digestion: includes the breakdown of the food
Without any change in its molecular structure i.e.: smaller
particles
Chemical digestion: Bigger molecules are dismantled into
smaller molecules which are different in structure
The optimal functioning of digestive enzymes depends on:
1. The temperature of the medium
2. The pH of the medium
The diagram to the right shows the
optimal pH values for pepsin,
salivary amylase, and trypsin
respectively.
* In case of an increase in the temperature above a certain limit, the enzyme
will be denatured and loses its function. The figure below explains this
concept.
The Digestive System of Humans:
The Mouth (Oral Cavity):
The following processes
take place inside the mouth:
 Mastication (chewing) of
food
 Mixing masticated food
with saliva
 Allowing for the sense of
taste
 Initiation of swallowing by
the tongue
Oral Cavity
2. Saliva: secreted by the salivary glands, it is slightly alkaline and, includes
salivary amylase as well as mucus.
Functions of saliva:
• Moistens the food for easy
swallowing
• Dissolves the food for discerning
taste by the tongue
• Contains salivary amylase which
digests starch into disaccharides
• Act as a natural cleanser for the
Parotid gl.
teeth (isn’t a substitute for toothpaste
and a toothbrush though)
Submaxillary gl.
Sublingual gl.
Three pairs of the salivary glands
3. The Tongue:
This diagram shows the
structure of a papilla as
well as one of the taste
buds along the sides of
the papilla.
The sensory nerve
endings end in
between the sensory
taste cells of the taste
bud.
The dissolved food
enters into the taste
bud through the taste
pore.
The pharynx: The base of the tongue plus the muscular
pharynx result in the involuntary reflex of deglutition
(swallowing) of food towards the esophagus.
Process of deglutition using the base of the tongue and Pharynx (bolus in blue)
The Esophagus:
• Is a muscular tube
that conducts the
food to the stomach
by peristalsis (slow
rythamic contractions)
• Its cells also secrete
mucous that helps
as a lubricant to
move the food
The Stomach:
• Acts as a storage for food
• Is made of three muscle
layers, from the outside to
the inside are the longitudinal
muscle layer, the circular
muscle layer and the oblique
muscle layer
• The inner lining of the
stomach is called the mucosa
it contains the glands of the
stomach and is corrugated
forming the rugae
• Has two sphincters, the
cardiac sphincter and the
pyloric sphincter
• Secretes gastric juice
The Functions of the stomach:
1. Mechanical (physical) digestion: through churning food into
Chyme
2. Controlling the passage of food: through the two sphincters,
contraction of the cardiac sphincter closes the opening to
the stomach located near to the heart, while its relaxation
allows food to enter. A second sphincter, the pyloric
sphincter, regulates the movement of food and stomach
acid to the small intestine
3. Production of gastric juice:
* HCl → breaks down the food
→ kills bacteria
→ converts pepsinogen into pepsin
* Pepsinogen
HCl
Pepsin → incomplete digestion of proteins
* Mucus → protects the stomach inner walls from the acid
The Small Intestine and Pancreas:
• Digestion and absorption take place in the small
intestine
• The small intestine is a muscular tube extending
form the pyloric sphincter to the ileocecal sphincter
about 6-7 metres long
• It is made of three segments namely, the
duodenum, jejunum, and the ileum
• The duodenum is only 20-30 cm long
• The duodenum receives enzymes secreted from
the liver and pancreas
Adjusting the small intestine’s pH (function of secretin):
The Pancreas:
Secretes the following
important enzymes:
1. Trypsinogen
2. Erepsin
3. Pancreatic amylase
4. Pancreatic lipase
Digestion of proteins:
• Starts in the stomach
Proteins
Pepsin
• Continues in the small intestine
Digestion of Carbohydrates:
• Starts in the mouth
Carbohydrates
Salivary
Amylase
Disaccharides
• Continues in the small intestine
Carbohydrates
Pancreatic
Amylase
Disaccharidases
Disaccharides
Disaccharides
Monosaccharides
Digestion of Lipids:
• In the small intestine
Fats
Bile Salts
Small fat droplets
Pancreatic lipase
Small fat droplets
glycerol
+
Fatty acids
The liver and the gall bladder:
• The liver is the largest gland of the body
• The gall bladder is a reservoir for storing Bile secreted by the liver
• Bile comprises, bile salts, phospholipids, cholesterol and bilirubin (a
pigment that results from the breakdown of RBCs)
• The liver has many functions, one of these functions is the emulsification of
fat (breaking large fat globules into smaller fat droplets)
The hormone cholecystokinin and its role in releasing bile salts:
Functions of the liver
Liver and Gall Bladder Problems:
1. Gall bladder stones: Cholesterol, an insoluble component
of bile, acts as a binding agent for the salt crystals found in
bile. The crystals precipitate and form larger crystals called
gallstones. Gallstones can block the bile duct, impairing fat
digestion and causing considerable pain.
2. Jaundice (Icterus): Bile contains a pigment called bilirubin
which results from the breakdown of haemoglobin, any
obstruction of the bile duct or accelerated destruction of red
blood cells can cause jaundice, turning skin and the eye
white yellow in color.
3. Liver Cirrhosis: Alcohol, like many other harmful agents,
can destroy liver tissue. Damaged liver cells are replaced by
connective tissue and fat, which are not able to carry out
normal liver duties. This condition, which can also result
from nutritional deprivation or infection, is referred to as
cirrhosis of the liver.
Digestion in the small intestine:
Mechanical digestion in the small intestine: includes what is called
segmentation which is the movement of the food back and forth (different
than peristalsis which is mainly for the propulsion of the food forwards)
Chemical digestion in the small intestine: through the
secretions received from the pancreas and the liver as well as
other secretions by the glands of the small intestine’s mucosa,
such as erepsin.
Homeostatic control of digestion:
1. Gastrin (stomach): is produced when partially digested
proteins are present in the stomach, gastrin stimulates the
release of gastric juices to digest proteins.
2. Secretin (small
intestine): stimulates the pancreas to
produce HCO3 and adjust the pH from 2 to 9
3. Cholecystokinin (small intestine): stimulates the release
of bile salts from the gall bladder
4. Enterogasterone (small intestine): If the meal is fatty, the
small intestine secretes a digestive hormone
(enterogasterone) that slows peristaltic movements, allowing
time for fat digestion and absorption.
If the meal is fatty,
the small intestine
secretes a
digestive
hormone
(Enterogasterone)
that slows
peristaltic
movements,
allowing time for
fat digestion and
absorption.
Absorption of Materials:
The small intestine:
• The absorption of food takes place in the small intestine
• Long fingerlike tubes called villi (singular: villus) greatly
increase the surface area of the small intestine. The cells
that make up the lining of each villus have microvilli, which
are fine, threadlike extensions of the membrane that further
increase the surface for absorption.
• Villi: small fingerlike projections that extend into the small
intestine which increase surface area for absorption
• Microvilli: microscopic fingerlike outward projections of the
cell membrane
• Each villus is supplied with a capillary network which
intertwines with lymph vessels called lacteals that transport
materials. Some nutrients are absorbed by diffusion, but
some nutrients are actively transported from the digestive
tract. Carbohydrates and amino acids are absorbed into the
capillary networks; fats are absorbed into the lacteals.
The role of the large intestine in absorption and egestion:
• The large intestine consists of the ascending colon, the transverse colon,
the descending colon, the sigmoid colon and the rectum
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Chemical digestion is complete by the time food reaches the large
intestine.
• The colon stores wastes long enough to reabsorb water
• The large intestine houses bacteria (E.Coli) that use waste materials to
synthesize vitamins (B and K).
• Most absorption takes place in the small intestine, however reabsorption
of water plus the absorption of minerals and vitamins takes place in the
large intestine
• Defecation reflex takes place when wastes build up in the large
intestine, receptors in the wall of the intestine provide information to the
central nervous system which, in turn, prompts a bowel movement. The
bowel movement ensures the removal of potentially toxic wastes from the
body. Individuals who do not eat sufficient amounts of cellulose (roughage
or fibre), have fewer bowel movements.
Essential Nutrients
1. The essential amino acids are the eight amino acids that
cannot be synthesized by your body and must be ingested.
• A common source of essential amino acids is animal products.
• The essential amino acids can be obtained in vegetarian diets
by eating complementary foods.
2. Vitamins are either water soluble or fat soluble.
• Water-soluble vitamins dissolve in the blood and are excreted
in urine. Thus they cannot be stored and must be consumed
daily.
• Fat-soluble vitamins can be stored in fatty tissue. Excessive
amounts of fat-soluble vitamins can build up in the body and
cause problems.
3. Antioxidants can make free radicals less toxic.
• Free radicals are created in the body in response to things like
sunlight, X-rays, pollution, etc.
• Free radicals may contribute to cancer and other disorders.