THE DIGESTIVE SYSTEM
There are four (4) stages of food processing:
·
·
·
·
Ingestion (eating): taking in nutrients (important part in food)
Digestion:
using physical and chemical means to break down
complex organic molecules into smaller usable parts
Absorption:
pulling in digested molecules into the cells of the
digestive tract, then into the blood
Egestion:
the removal of waste food materials (e.g. cellulose = fibre)
from the body
These four steps are done by the digestive system. The steps occur in the
GASTROINTESTINAL tract (a long TUBE open at both ends). Accessory glands
such as the liver and pancreas help during the process. The GI tract is also called
the Alimentary tract.
1. Ingestion: The MOUTH
Food enters the gastrointestinal tract through the mouth.
2. Digestion:
A) MOUTH – both physical (mechanical) and chemical digestion begins here
v Physical - Two main structures involved:
i) TEETH -
teeth and tongue
four types (normal adult has 32 teeth on average) see book pg. 184 , fig.3
Tooth type
a) incisors
b) canine
c) bicuspids
d) molars
ii) TONGUE -
Number
8
4
8
12
Function
cut food
pierce and tear food
pierce and tear
crush and grind
both tongue and teeth help to physically break apart food
v Chemical digestion: using ENZYMES to break up food
In the mouth, SALIVARY glands produce SALIVA which contains the enzyme AMYLASE
which acts on starch, breaking it into maltose.
v STARCH à amylase à maltose
BOLUS:
the resulting ball of food in the mouth made by the actions of the teeth,
tongue, and saliva
PERISTALSIS:
the rhythmic muscular contractions that push digested food in one
direction through the gastrointestinal tract
see book page 184, fig. 4
The bolus is swallowed and travels down the esophagus into the stomach. It moves down
the esophagus by peristalsis.
B. STOMACH: physical and chemical digestion
The bolus passes through the CARDIAC SPHINCTER, the top opening in the stomach.
Sphincter: a muscle that controls the opening
i) cardiac: esophagus & stomach
ii) pyloric: stomach and duodenum
v Physical: the stomach is a muscular storage organ that churns the food
v Chemical: GASTRIC juice mixes with bolus
- stomach contents now called CHYME
GASTRIC juice contains:
a) HCl – hydrochloric acid
b) pepsin – enzyme that breaks down PROTEIN
HCl:
- helps breakdown fibrous tissue
- kills bacteria
- helps pepsin work properly (ph = 2 optimal pH for pepsin)
PEPSIN:
- enzyme begins to break down PROTEIN into smaller polypeptides
MUCUS:
- also made by stomach to protect itself from STRONG ACID
Chyme stays in stomach until adequate protein digestion has occurred. This is why
you feel less hungry between meals if you have eaten some protein. The chyme is kept in
the stomach by the pyloric sphincter.
C. SMALL INTESTINE
Final area where MOST digestion occurs and ABSORPTION begins.
·
·
Physical – villi and microvilli help separate food particles
Chemical – where many enzymes complete the digestive process
- many enzymes from accessory glands (liver, gall bladder, pancreas)
are dumped into the SI to help with digestion
SMALL INTESTINE – STRUCTURE
Website : http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/smallgut/anatomy.html
The small intestine is a long tube (7 m) in length and 2.5 cm in diameter with a total
inner surface area of 30 m2 (due to the villi and microvilli).
The SI is also divided into three sections:
Duodenum:
a) duodenum
b) jejunum
c) ileum
- first 25 cm portion
- means 12 fingers in Latin (duo = 2, den = 10)
- pancreas and liver dump enzymes here to help digestion
- most digestion occurs here
- produces intestinal enzymes: maltase à maltose
peptidase à polypeptides
Chyme leaves the stomach through the pyloric sphincter where it is mixed
with intestinal and pancreatic enzymes, bile and sodium bicarbonate, which neutralizes
the acidic chyme.
Accessory glands:
· Pancreas:
- releases sodium bicarbonate to neutralize acidic chyme
(pH 2.5 à 8)
- releases digestive enzymes:
pancreatic amylase à starch
lipase à lipids
trypsin à polypeptides
These enzymes, as well as intestinal enzymes, have an optimal pH = 7.5,
therefore need to have the chyme neutralized before they can work properly.
·
Liver:
- 1.5 kg organ
- secretes BILE (bile salts)
à
these help emulsify (separate) FATS which
increases the surface area, enabling lipases to
break down lipids more efficiently
ex. soap and a greasy dish
v The liver performs a vast number of functions (500), including production of bile from
cholesterol, recycling of red blood cells, glycogen storage, storage of fat-soluble
vitamins, deamination of amino acids, and detoxification of poisons. Everything that
enters the body must go through the liver to be cleared.
·
Gall bladder: - small sac near liver
- stores bile
3. ABSORPTION OF NUTRIENTS
Stomach:
- small amounts of water
- alcohol
- some medicines, e.g. aspirin
Small intestine:
jejunum - where most of the absorption takes place
ileum: where the remaining nutrients are absorbed
Absorption is greatly aided by the folded interior structure of the intestine, particularly
due to the presence of the villi and microvilli.
Structure of Villi:
v
- tiny finger like projections lining the interior of the intestines
- contain two types of vessels:
· capillaries
– tiny blood vessels
- collect water soluble nutrients
à glucose, amino acids
· lacteal
- contain lymph
- collect products of fat digestion
à fatty acids and glycerol
Final products of digestion (or what is absorbed)
Macromolecule
starch (CHO)
protein
lipids
Digestion starts
mouth - amylase
stomach à HCl + pepsin
duodenum à bile + lipase
Digestion complete
duodenum (SI)
duodenum (SI)
duodenum (SI)
Final product
glucose
amino acids
fatty acids +
glycerol
Small molecules like vitamins, minerals, natural glucose and amino acids do not need to be
digested and are easily absorbed by the small intestine.
v Water is absorbed throughout the alimentary (GI) tract but the majority of water
absorption occurs in the LARGE INTESTINE, also known as the COLON.
The COLON – the LARGE INTESTINE
(the BOWEL)
Structure:
- 1.5 m in length, 5 cm in diameter (twice the diameter of SI)
- made up of four sections:
ascending
transverse
descending
sigmoid process
-the colon ends with the rectum
Function:
- main job is to reabsorb water
- bacteria here produce vitamins K and some B
4. EGESTION OF WASTE
CELLULOSE: main component of feces, along with living and dead bacteria and water
- FIBRE, found in plant cell walls, cannot be digested by humans
FECES:
- waste is stored in rectum
- removed by body removing possible toxic materials
v Egestion controlled by anal sphincter which allows feces to leave rectum through the
anus. Toilet training is learning how to control the anal sphincter.
WEBSITES:
http://www.medtropolis.com/VBody.asp
http://www.innerbody.com/htm/body.html
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/smallgut/anatomy.html
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/FramesVersion.html
http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20102/Bio%20102%20lect
ures/Digestive%20System/digestive%20system.htm
http://www.bartelby.com/107/241.html
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/basics/index.html
Digestion REVIEW
Most higher animals, including all vertebrates, have digestive tracts, or alimentary
canals, through which food passes. Human digestion begins in the mouth. There the food is
chewed and mixed with saliva, which adds moisture and contains the enzyme amylase that
begins to break down starches. The tongue kneads the food into a smooth ball (bolus), which
is then swallowed. The bolus passes through the pharynx and esophagus into the stomach,
propelled by peristaltic muscular contractions. In the stomach the food is mixed by peristaltic
contractions (about three per minute) with highly acidic gastric juices secreted into the
stomach. The hormone gastrin stimulates the secretion of these juices, which contain water,
inorganic salts, hydrochloric acid, mucin, and several enzymes, the most abundant of which
is pepsin. Pepsin breaks protein molecules into smaller molecules called polypeptides.
The food, now in a semiliquid state called chyme, passes from the stomach into the
duodenum, the first section of the small intestine, where the greatest part of digestion takes
place. The chyme is subjected to the actions of a large number of enzymes, some secreted
by the pancreas (which is connected to the duodenum by a duct) and some produced by
glands in the intestinal wall. Each enzyme acts on specific food molecules. For example,
amylase, maltase, lactase, and sucrase complete the digestion of carbohydrates; trypsin,
chymotrypsin, carboxypeptidase, aminopeptidase, and dipeptidase break down proteins; and
lipase hydrolizes fat molecules. Bile, which is produced by the liver, also empties into the
duodenum; it contains salts that break up (emulsify) fat globules, thereby exposing fat
molecules to the enzymatic action of lipase. By the time this process has been completed,
the carbohydrates have been broken down into simple sugars (monosaccharides), the
proteins into amino acids, and the fats into glycerol and fatty acids. These simple molecules
are then absorbed into the circulatory system through countless microscopic projections of
the intestinal wall called villi as the material moves through the jejunum and ileum (the
remaining sections of the small intestine).
Substances that cannot be digested, such as cellulose (plant fibre), pass into the
colon, or large intestine. There, water and ions such as sodium and chloride are reabsorbed,
and the remaining solid material (feces) is held until it is expelled through the anus. Common
diseases of the human digestive tract include infections, inflammations, ulcers, and cancers.