Lecture 4
The Digestive System
➢
Digestion is the Chemical breakdown of complex biological molecules into
their component parts.
➢
-
Lipids to fatty acids
-
Proteins to amino acids
-
Carbohydrates to simple sugars
The Digestive System consist of the gastrointestinal tract plus the
accessory organs of digestion (the tongue, salivary glands, pancreas, liver,
and gallbladder)
Function of Digestive System
➢
Produces various chemicals to break down the food.
➢
Filters out harmful substances.
➢
Get rid of solid wastes
Phases of Digestion
1.
Ingestion
2.
Movement
3.
Digestion
4.
Absorption
5.
Further digestion
Path of Digestion
➢
Mouth
➢
Pharynx
➢
Esophagus
➢
Stomach
➢
Small intestine
➢
Large intestine
➢
Anus
2 Major parts of Digestive System
➢
➢
Major organs
-
Mouth
-
Esophagus
-
Stomach
-
Small intestine
-
Large intestine
Accessory organs
-
Liver
-
Gallbladder
-
Pancreas
Major Organs
➢
Mouth
-
The 1st part of the digestive system
-
The entry point of food with a ph of 7
-
Its function is for mechanical digestion
-
Structures in the mouth that aids in digestion:
•
Teeth - cut, tear, crush and grind food
•
Salivary gland - produce and secrete saliva into the oral cavity
•
Saliva - moistens the food, it contains
enzymes ptyalin and salivary amylase. (that begins digestion
of starch into smaller polysaccharides.)
➢
Esophagus
-
A tube connecting the mouth to the stomach
-
Running along the thoracic cavity
-
Location: lies behind the windpipe (trachea)
-
The trachea has an epiglottis preventing food from entering the
windpipe moving the food to the esophagus for swallowing.
-
Food travels down the esophagus, through a series of involuntary
rhythmic contractions (wave-like) called peristalsis (propels food and
liquid slowly down the esophagus into the stomach)
-
Function:
•
To support movement of food
•
Lining of esophagus secretes mucous
•
Lubricating
-
Esophageal sphincter
•
Bolus reaches the stomach
•
Must pass through a muscular ringed valve called the
esophageal sphincter (cardiac sphincter)
•
Function: Prevents stomach acid from back flowing into the
esophagus
➢ Stomach
-
J- shaped muscular sac
-
Has inner folds (rugae)
-
Increase surface area of the stomach
-
Ph. of 1.5
-
Function: Stomach performs mechanical digestion
-
How: By churning the bolus and mixing it with the gastric juices
secreted by the lining of the stomach.
-
Gastric Juices:
•
HCL, salts, enzymes, water and mucous
•
HCL, helps break down of food and kills bacteria that came
along with food
•
The bolus is now called chyme (liquid)
-
Pepsin: Major enzyme, converts proteins into peptides in the
presence of HCl
-
Mucus:
•
Lubricates food and protects the gastric lining from
strong digestive juices
•
Then chyme passes through the pyloric sphincter into
the small intestine
-
Pyloric Sphincter
•
As chyme moves from the stomach to the small intestine
it passes through a muscular ringed sphincter called the
pyloric sphincter
➢
Small Intestine
-
Responsible for the complete digestion of all macromolecules,
-
90% of the digestion and absorption of food occurs other 10% taking
place in the stomach and large intestine.
-
And the absorption of their component molecules
-
Like: glucose, glycerol, fatty acids, amino acids, nucleotides.
-
Parts: Duodenum, Jejunum, ileum
-
Duodenum
•
Approximately 30cm.
•
Area completes most of the digestion processes.
•
Enzymes are secreted into the duodenum from the pancreas
and the gall bladder. It is lined by folds of tissue called villi
•
The villi are covered by fine brush-like microvilli
•
The folds increase the surface area of the small
intestine increase the rate of absorption
-
Jejunum
•
Approximately 2.5m long
•
Some digestion is completed here, it has
more villi and microvilli its role is absorption and nutrients
-
Ileum
•
Approximately 3m long
•
Fewer villi and microvilli
•
Responsible more on pushing the waste materials into the
large intestine than absorption
➢
Large Intestine
-
-
Composed of several very distinctive parts:
•
Cecum
•
Colon
•
Ascending Colon
•
Transverse Colon
•
Descending Colon
•
Sigmoid Colon
•
Rectum
Function:
1.
Absorb Water
•
One of the primary function is to absorb water
•
Prepare the waste as a solid stool that will be expelled from
the body.
2.
Absorb Vitamin
•
Role in breaking down undigested sugars and fibers into fatty
acids.
3.
Reduce Acidity
•
The large intestine produces alkaline solutions reduce the
acidity and balance the PH in the intestine.
4.
Protect from infections
•
Help boots immunity
•
It is believed that the appendix may have been a major
producer of antibodies at some point in the evolutionary
process.
5.
Produces antibodies
•
The mucous lining of the large intestine acts as a protective
layer.
•
Prevents harmful bacteria from being reabsorbed into the
body.
-
Cecum
•
1st section of the large intestine, looks like a pouch
•
2 inches long
•
Function: Taking in digested liquid from ileum and passes it
on to the colon
-
Colon
•
Major section of the large intestine
•
Function: Water absorption, absorbs salts when needed
•
Components:
Ascending colon
Transverse colon
Descending colon
Sigmoid colon – empties into the rectum
-
Rectum
•
Final section
•
Measures from 1-1.6 inches
•
Left over waste collects
here expanding the rectum
•
Emptied through anus
The Accessory Organs
➢
Support the digestive system but are not part of the digestive tract
➢
These organs secrete fluids into the digestive tract and are connected by
ducts
➢
➢
The accessory organs includes:
•
Liver
•
Gall bladder
•
Pancreas
•
Larges of the accessory organ
•
1.5kg
•
It produces bile
•
Greenish yellow pigment made up bile pigments and bile
Liver
salts
•
Breakdown old red blood cells
•
Storage of blood
•
Breaks down excess RBC
➢
Gall Bladder
•
A storage sac for bile
•
How it works?
Food containing fat enters the digestive tract.
Salts are secreted into the small intestine to digest fats.
Bile emulsifies fats in partly digested food.
Assisting their absorption
➢ Pancreas
•
The pancreas secretes a number of different enzymes into
the small intestine
•
It is an endocrine gland and exocrine gland
•
Role is to digest carbohydrates lipids and proteins completely
•
It also secretes bicarbonate ions
•
Role: Neutralize the HCL from the stomach change the pH of
the small intestine to a pH of 8