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Transcript
6.2 The Human Digestive System
In this section, you will:
identify the main structures and functions
of the digestive system
describe the physical and chemical
processing of food through the digestive
system and into the bloodstream
explain the action of enzymes in chemical
digestion
identify and describe, in general terms,
how digested molecules enter the
bloodstream
Digestive System
The Vertebrate Digestive System
the digestive system functions to break
down the macromolecules found in the
diet into usable forms for the body
digestion is the process by which large
chunks of food are broken down into
molecule-sized pieces
products of digestion are used for energy,
building blocks, and as enzymes and/or
coenzymes
the digestive system is a long tube that extends
from the mouth to the anus, and functions to
perform four functions:
– ingestion (eating)
movement of the food through the tract, which is
accomplished by peristalsis (smooth muscle contractions)
– digestion – the breakdown of food by both
mechanical (chewing and scrunching) and chemical
(enzymatic) means
– absorption – the nutrients are absorbed from the
digestive system into the blood and lymph from the
small and large intestines
– elimination – defecation – the elimination of
indigestible/unabsorbed materials from the body
the digestive system consists of two basic
groups – the GI tract (the tube) and the
accessory structures
– the GI tract is a tube from 6.5 to 9 m long in
adults
Digestive Tract
Secretions of the Digestive Tract
Secretion
Site of production
Function
saliva
mouth
contributes to starch digestion via salivary
amylase; lubricates the inside of the mouth to
assist in swallowing
mucus
mouth, stomach, small
intestine, and large intestine
protects the cells lining the innermost portion of
the digestive tract; lubricates food as it travels
through the digestive tract
enzymes
mouth, stomach, small
intestine, and pancreas
promote digestion of food masses into particles
small enough for absorption into the
bloodstream
acid
stomach
promotes digestion of protein
bile
liver (stored in gall bladder)
suspends fat in water, using bile salts,
cholesterol, and lecithin to aid digestion of fats in
small intestine
bicarbonate
pancreas and small intestine
neutralizes stomach acid when it reaches the
small intestine
hormones
stomach, small intestine, and stimulate production and/or release of acid,
pancreas
enzymes, bile, and bicarbonate; help to regulate
peristalsis
Mouth
– the mouth is where mechanical and chemical
digestion begins
– the food is moistened by saliva produced by
the salivary glands (we produce 1.7 L per
day), which moistens the food to make it
easier to swallow, and also contains the first
digestive enzymes:
amylase which begins the breakdown of starch
into maltose (a disaccharide) and dextrin's (short
glucose chains)
Swallowing
– the tongue is used to push the food to the back of the
mouth – this area is called the pharynx, the presence
of the food initiates the swallowing reflex
– the epiglottis, a small flap of tissue will close over the
trachea, preventing food from entering the lungs
– the bolus of food is then swallowed, and travels down
the 25 cm esophagus to the stomach, helped by
peristalsis (you can swallow while standing on your
head!)
– at the bottom of the esophagus is the lower
esophageal sphincter (or cardiac sphincter) – a small
ring of smooth muscle that keeps food in the stomach
Peristalsis
Stomach
– the stomach holds up to 2L (4L) of food, and will hold food
for from 3 to 6 hours while it is broken down
– There are three important chemicals in the stomach involved
in digestion:
HCl(aq) – secreted by the parietal cells of the stomach,
lowers the pH of the stomach to 2.0, allowing the
activation of pepsin from pepsinogen
Pepsin – secreted as an inactive precursor, pepsinogen
(cannot be active or it would digest cellular proteins) by
the peptic (chief) cells. It is activated by the lowered pH
in the stomach (which changes the shape of the enzyme)
mucous – secreted by the mucous (goblet) cells, protects
the stomach endothelium from the acid and enzymes
Two other secretions:
– gastrin – a hormone that stimulates gastric
secretions (which is stimulated by the
presence of proteins in the stomach), and
relaxes the pyloric sphincter to allow stomach
emptying
– rennin – found principally in children, it is
involved in the digestion of milk – slowing its
emptying from the stomach
physical digestion - the acid along with the
peristaltic motions of the stomach contents helps
to break up tissues
the pepsin will break up proteins into smaller
polypeptides called peptones (protein digestion
will be completed in the small intestine)
most materials are not absorbed in the stomach,
but some drugs (notably aspirin), some water,
electrolytes, alcohol and absorbed in the
stomach
the highly acidic stomach contents, called
chyme is emptied into the small intestine a bit at
a time through the pyloric sphincter
Small Intestine
measures up to 7m in length, but only 2.5 cm in
diameter, divided into three sections
– duodenum ~25 cm long, where most digestion
occurs
– jejunum ~2.3 m long
focusing on absorption
– ileum ~ 3.5 m long
the majority of digestion and absorption (90%)
takes place here – lipid digestion, continuation of
carbohydrate and protein digestion
the accessory organs of digestion, the pancreas,
gall bladder and the liver secrete their juices into
the duodenum to aid in digestion
the small intestine, in order to better digest and
absorb digested materials is highly folded to
increase surface area (by 600x); the folds are
called villi, which are covered in small
cytoplasmic projections called microvilli
the structure of the small intestine (from the
outer surface in) includes:
– villi/microvilli – surface (epithelial) cells
designed for absorption
– lacteal – vessel projection of the lymphatic
system, designed to absorb fats
– capillaries – vessels of the circulatory
system, for absorption of all other nutrients for
immediate transport to the liver for processing
Villi in the Small Intestine
as the chyme enters the duodenum, its acidity
and contents stimulates a number of hormones
to be released, which in turn stimulate the
secretion of digestive and protective chemicals
from accessory organs and the intestinal
epithelium itself
the small intestine produces enzymes to aid in
digestions:
– carbohydrases: sucrase, maltase and lactase which
break down sucrose, maltose and lactose,
respectively, into simple sugars
– peptidases: peptides  smaller peptides and amino
acids
– nucleosidases: nucleotides  bases, sugars, and
phosphates
Hormones
– Gastrin: stimulated by proteins in food causing the
secretion of HCl and pepsinogen
– cholecystokinin (CCK): stimulated by partially
digested proteins and irritants in the chyme,
stimulates pancreatic enzyme release and bile from
the gallbladder
– Secretin: in response to acidic chyme in the
duodenum, inhibits the secretion of gastric juice,
stimulates bicarbonate release by the pancreas to
neutralize acidic chyme
– GIP (gastric inhibitory peptide)
Chemical Digestion
Hormone Secretions at work
Selected Enzymes of the Digestive System
Enzyme
Where enzyme
acts/pH
Substrate (food)
digested
Products of digestion
Origin of enzymes
salivary
amylase
mouth/7
starch, glycogen
maltose (disaccharide)
salivary glands
pancreatic
amylase
small intestine/8
starch, glycogen
maltose
pancreas
carbohydrases
• sucrase
• maltase
• lactase
small intestine/8
pancreatic
lipase
small intestine
sucrose
maltose
lactose
glucose + fructose
glucose
glucose + galactose
small intestine/8
lipids
fatty acids and glycerol
pancreas
proteases
• pepsin
• trypsin
• chymotrypsin
stomach/1–2
small intestine/8
small intestine/8
protein
peptides
peptides
peptides
smaller peptides
smaller peptides
stomach
pancreas
pancreas
peptidases
small intestine/8
peptides
smaller peptides and amino
acids
pancreas and small
intestine
nucleases
small intestine/8
nucleic acids
nucleotides and
components
pancreas
nucleosidases
small intestine/8
nucleotides
bases, sugars, and
phosphates
small intestine
Accessory Organs:
Pancreas
– a soft tubular gland that lies just behind the
stomach, and is connected to the duodenum
by two ducts
– has both a exocrine (secretory) and endocrine
(hormonal) function
– its exocrine functions are to secrete digestive
enzymes and sodium bicarbonate to
neutralize the stomach acid and establish a
pH of 7.1-8.2 which will not only neutralize the
enzyme pepsin, but activate the pancreatic
enzymes
Pancreatic Enzymes:
– pancreatic amylase: digest carbohydrates 
maltose
– trypsin: protein digestion: peptides  smaller
peptides (activated by enterokinase, secreted by the
intestinal wall)
– chymotrypsin: protein digestion: peptides  smaller
peptides (activated by trypsin)
– peptidase: peptides  smaller peptides and amino
acids
– pancreatic lipase: digests fats into fatty acids and
glycerol
– ribonuclease: digests RNA to nucleotides
– deoxyribonuclease: digests DNA to nucleotides
Liver
the liver has over 500 functions, four being
fundamental:
– production of bile
– storage of glucose in the form of glycogen ( fat if
glycogen limits exceeded), conversion of galactose
and fructose to glucose
– detoxification of the blood (makes enzymes to break
down toxins; ex: alcohol, caffeine, nicotine,
barbiturates, poisons, excess hormones)
– deamination of amino acids – removing nitrogen,
producing ammonia and eventually urea (excreted by
the kidneys)
it receives two separate blood supplies
– via the portal vein – bringing freshly absorbed
nutrients from the small intestine
– via the hepatic artery – bringing oxygenated blood
from the lungs/heart
bile
– each day, the liver secretes between 800 – 1000 mL
of bile
– bile is stored in the gallbladder for release on demand
into the small intestine
– consists of water, bile salts, cholesterol and bile
pigments (made from bilirubin – yellow in colour)
– bile acts as an emulsifier, breaking large fat globules
into small ones, allowing lipase more surface area for
digestion
Gallbladder
– is lodged in one of the lobes of the liver
– a light muscular bag that stores and releases bile
– why store bile? We eat large quantities of fats at a
time, so having a store of bile is useful but can lead to
problems – bile salts can crystallize in the gallbladder
forming gallstones
back to digestion….
– the combination of hormones and the presence of
certain foods leads to the secretion of pancreatic
enzymes, bile, and intestinal enzymes (maltase,
lactase, sucrase and enterokinase), all of which are
active in the slightly basic pH of the duodenum
the villi of the small intestine are
specialized to absorb the molecules of
digestion:
– large surface area
– transport proteins on the epithelial cells move
amino acids, glucose, water soluble vitamins,
etc. by facilitated diffusion and active transport
into the capillary system, then on to the liver
immediately for processing
– electrolytes by diffusion, water by osmosis
– fatty acids and glycerol are absorbed into the
epithelial cells and are repackaged as
triglycerides
– being hydrophobic, they are packaged into
protein packs to enable their transport through
the body
– the packaged fats are absorbed into the
lacteals in the interior of the villi, and are
transported through the body’s lymphatic
system
– fat soluble vitamins will move with fats into
lacteals for absorption in the body
Absorption of Nutrients
Top: Glucose is actively transported
into cells of the intestinal wall to
move into the bloodstream.
Middle: Amino acids are actively
transported into the cells of the
intestinal wall to move into the
bloodstream.
Bottom: Glycerol and fatty acid
molecules diffuse into the cells of the
intestinal wall where they are
resynthesized into fats, coated with
proteins, and move into lymph
vessels for eventual transport into
the bloodstream.
Large Intestine
– about 1.5 m in length, a diameter of 6.5 cm.
– at the junction of the small and large intestine,
exists the appendix, which has no real function
given our diet
– the large intestine has three main functions:
absorption of water and electrolytes
production of feces – consisting of water, inorganic
salts, cells from the GI tract, bacteria, bacterial
decomposition and undigested food
housing bacteria, that will use remaining unabsorbed
nutrients or undigested carbohydrates to make vitamin
K and B vitamins for us to absorb (as well as methane
gas)
6.3 Health and the Digestive
System
In this section, you will:
recognize and appreciate the relationship
between health and nutritional decisions
identify conditions that adversely affect
the health of the digestive system and the
technologies that are available to treat
them
Digestive Disorders
Digestive Disorders
Disorders of the digestive system and its
accessory organs include:
–
–
–
–
–
ulcers,
inflammatory bowel disease,
hepatitis,
cirrhosis,
and gallstones.
All disorders that affect digestion, including eating
disorders, can seriously damage overall health by
depriving the body cells of the nutrients they need
to survive.
Chapter 6 Review
Why are some amino acids classified as
essential?
Describe the relationship between the
organs in the digestive tract.
What are the benefits of having stomach
stapling? What are the risks?
Summarize chemical digestion of
macromolecules.
Explain how glucose levels are affected
after meals. Include the effects of different
foods.
Concept Organizer
Chapter 6 Summary
The human body takes in matter from the
environment in the form of food and water.
The human digestive system processes
the food and water in order to obtain the
macromolecules it needs for survival.
Making Healthy Food Choices
Chapter 6 Summary
Food passes through the digestive tract—the
mouth, pharynx, esophagus, stomach, small
intestine, and large intestine—during physical
digestion.
The accessory organs—the salivary glands, liver,
gall bladder, and pancreas—supply chemicals that
also contribute to the digestion of food as it passes
through the digestive tract.
The stomach supplies chemicals to aid digestion as
well as generating physical contractions to
physically break down food.
The food is eventually liquefied into soluble units
that can pass through cell membranes for transport
via the circulatory system to all the cells in the body.
The waste materials from the digestive process
leave the body via the large intestine.
Good Nutrition + Exercise =
Health
Chapter 6 Summary
The nutrients that food supplies include carbohydrates,
lipids (fats), protein, and nucleic acids.
Carbohydrates and lipids are broken down to supply
energy; lipids also supply material for the cell
membranes.
Proteins are more structurally and functionally diverse
than carbohydrates and lipids. They assist in transport,
immunity, and muscle action and are used to make up
most cellular structures.
Nucleic acids direct growth and development. Enzymes
speed up chemical reactions, particularly for the
production of energy.
Vitamins and minerals are organic and inorganic
substances that enable chemical reactions to occur and
aid in tissue development and growth and immunity.
These substances are needed for a healthy, functional
human body.
Chapter 6 Summary