Digestive System
You are what you eat
The Old-School Food Pyramid
Digestive System
• Digestion is the mechanical
and chemical breakdown of
foods and the absorption of
the resulting nutrients.
• Mechanical digestion breaks
large pieces into smaller
ones without altering their
chemical composition.
• Chemical digestion breaks
food into simpler chemicals.
Digestive System
• The digestive system consists
of the alimentary canal, which
extends about 8 m from the
mouth to the anus, and the
accessory organs which
secrete substances used in the
process of digestion.
Alimentary Canal
• The alimentary canal includes the mouth,
pharynx, esophagus, stomach, small intestine,
large intestine, rectum, and anus.
• The accessory organs of the digestive system
include the salivary glands, the liver,
gallbladder, and pancreas.
• Overall, the digestive system has a surface
area of over 186 square meters.
Alimentary Canal
• Muscular tube that passes through body’s
ventral cavity.
• Wall has four distinct layers:
– Mucosa: carries out secretion and absorption.
– Submucosa: carries away absorbed materials.
– Muscular layer: produces movements of the tube
– Serosa: outer covering.
Alimentary Canal
• Two movements:
• Mixing – rhythmic
contractions. Mix
food with digestive
juices that mucosa
secretes.
• Propelling –
peristalsis wave
pushes tubular
contents ahead.
Mouth
• Mouth – receives food and
begins digestion by
mechanically reducing size of
food.
• Oral cavity – chamber
between palate and tongue
• Vestibule – space between
teeth, cheeks, and lips.
C. megalodon, extinct shark
Mouth
• Cheeks –pads of
subcutaneous fat,
muscles associated with
expression and chewing
• Lips –surround the mouth opening.
• Tongue – Nearly fills oral cavity. Frenulum
connects tongue to floor of mouth. Mixes food
particles with saliva during chewing and moves
food to pharynx during swallowing.
Mouth
• Papillae on tongue provide friction and bare
tastes buds.
• Palate forms the roof of the mouth. Consists
of hard and soft palate.
• Soft palate to back of mouth and forms uvula.
• During swallowing, muscles draw soft palate
and uvula upward, allowing closure of nasal
cavity and pharynx.
– Why?
Mouth
• Palatine tonsils are
masses of lymphatic
tissue located at the
back of the mouth.
• Pharyngeal tonsils, or
adenoids, are also in
the back of the mouth.
– Why are all of these
lymphatic tissues in the
mouth?
– Have any of you had a tonsillectomy? What could
possible consequences be later in life?
Mouth
• Two different sets of teeth form during
development: primary (deciduous) and
secondary (permanent) teeth.
• Teeth begin mechanical digestion by breaking
pieces of food into smaller pieces.
– Why?
– Increases surface area of food particles, allowing
digestive enzymes to react more effectively.
Salivary Glands
• The salivary glands secrete saliva.
• Saliva moistens food particles, binds them,
and begins chemical digestion.
• Amylase, a digestive enzyme splits starch and
glycogen down into disaccharides.
• Mucus is secreted by mucosa cells that bind
food and lubricates during swallowing.
Salivary Glands
• What happens if your body quits making saliva
(xerostomia)?
• “Meth Mouth”
is the result of
no spit, poor
dental hygiene,
tooth grinding,
and clenching.
• Want to kiss this?
• Didn’t think so
Pharynx (throat) [Look familiar?]
• Made of muscle and
mucous lining into 3
divisions
• Nasopharynx – posterior
nares to soft palate
• Oropharynx – soft palate
to hyoid bone; adenoid
tonsils
• Laryngopharynx – hyoid
bone to esophagus;
palatine and lingual
tonsils
Esophagus
•
•
•
•
A straight, collapsible tube about 25 cm long
Food passageway from pharynx to stomach
Secretions moisten and lubricate tube
The cardiac sphincter (lower esophageal
sphincter) remains contracting, closing the
entrance to the stomach.
– Why is this important?
– To keep the stomach contents from regurgitating into
esophagus.
– When peristaltic waves hit stomach, these fibers relax
and allow food to enter.
Stomach
• The stomach is an organ that:
• receives food from the
esophagus
• mixes the food with gastric
juices
• begins chemical digestion
• moves food on to the small
intestine.
• The pH of the stomach, due to
gastric acids, can go as low as 1,
and ranges up to 3-4 when food
enters.
– Why do you think it’s important
to have such a low pH in the
stomach?
Stomach
• Gastric glands in the stomach excrete digestive
enzymes such as:
• Pepsin, formed from pepsinogen upon contact
with HCl from parietal cells. Pepsin begins
digestion of proteins.
• Intrinsic factor which helps the small intestine
absorb vitamin B12.
• When a person tastes, smells or sees appetizing
food, the brain releases Ach, which stimulates
gastric glands to secrete gastric juices.
Stomach
• After a meal, the mixing movements of the
stomach wall and a mix of gastric juice and
food particles form chyme.
• Chyme is what leaves the stomach and moves
into the duodenum (the first part of the small
intestine), where secretions from other organs
are added.
Pancreas
• The pancreas has both
endocrine and exocrine
functions. It secretes a
digestive juice – pancreatic
juice.
– What are the differences
between endocrine and
exocrine glands?
– Endocrine glands secrete
hormones or other chemicals
into the local environment,
while exocrine glands secrete
their products into ducts which
lead to other parts of the body.
Pancreas
• Pancreatic juice has several enzymes including:
• Pancreatic amylase – splits molecules of srach
or glycogen into disaccharides.
• Pancreatic lipase – breaks down triglycerides
into fatty acids.
• Nucleases – break down nucleic acids into
nucleotides.
• Proteolytic enzymes – break down proteins.
The Liver
• The liver is divided into
lobes – left and right.
• The liver functions in
many ways including
with carbohydrate, lipid,
and protein metabolism,
storage, blood filtering,
detoxification, and the
secretion of bile.
The Liver
• The most vital function of the liver is its work
in protein metabolism.
• This protein metabolism involves deaminating
amino acids, forming urea, and synthesizing
plasma proteins (such as clotting factors).
• The liver also stores iron, glycogen, vitamins,
and breaks down fats that can be stored in
adipose tissue throughout the body.
The Liver
• Bile – yellowish-green
liquid secreted from
hepatic cells in the liver.
Bile contains water, salts,
bile pigments (bilirubin
and biliveridin),
cholesterol, and
electrolytes.
Gallbladder
• The gallbladder is a sac
located on the liver’s
surface.
• It stores bile between
meals, reabsorbs water
to concentrate the bile,
and contracts to release
bile.
Small Intestine
• The small intestine
receives secretions
from the pancreas and
liver and completes the
digestion of the
nutrients in chyme,
absorbs products of
digestion and
transports remains to
the large intestine.
Small Intestine
• Intestinal villi are tiny projections that greatly
increase the surface area of the intestine.
– Why would this be important in this organ?
– Aids the absorption of digestive products.
• Digestive enzymes are released from these villi:
– Peptidase – breaks down peptides into amino acids
– Sucrase – breaks down sucrose
– Maltase – breaks down maltose
– Lactase – breaks down lactose
• People who are lactose intolerant lack lactase in intestine
Small Intestine
• Small intestine is the
most important
absorbing organ of the
alimentary canal.
• Nutrients are absorbed
into the villi and
transported through the
blood to various body
parts.
– Example: cholesterol
transported to liver.
Main Squeeze
• In snakes that feed as
constrictors, the small
intestine stretches
after meals to
increase its mass by
over 40%, therefore
increasing surface
area to help with the
absorption of
nutrients.
Green Anaconda (Eunectus murinus) constricting
a Caiman
Large Intestine
• The large intestine is so
named because it has a
larger diameter than the
small intestine.
• The cecum is the beginning
of the large intestine.
• From the cecum hangs the
veriform appendix.
• The appendix is a vestigial
organ.
– What do you think the
appendix might have been
used for in our ancestors?
Large Intestine
• The large intestine absorbs
water and electrolytes
from chyme remaining the
alimentary canal.
• It also forms and stores
feces.
• 1 - cecum
• 2 - ascending colon
• 3 - transverse colon
• 4 - descending colon
• 5 - Sigmoid colon
• 6 - rectum
Large Intestine
•
•
•
•
The large intestine wall lacks villi.
The large intestine does not secrete enzymes.
Only secretion is mucus.
Instead, large intestine absorbs water and
electrolytes and remainder becomes feces.
• Intestinal flora, the bacteria that inhabit the
large intestine, break down molecules in food,
such as cellulose.
Large Intestine
• The human colon is
home to over 100
trillion bacteria.
• Flatus, intestinal gas,
is the result of
bacterial actions in the
large intestine.
Escherichia coli in human large intestine
Large Intestine
• Peristaltic waves and mixing motions produce
mass movements in the large intestine forcing
the contents towards the rectum.
• Feces include material that were not digested or
absorbed, plus water, electrolytes, mucus, and
bacteria. Feces are usually about 75% water.
• …Or if you ask the girl in Donnie Darko, “What are
feces?”
– “Baby mice.”
Nutrition
• Four Basic Food Groups
• Nutrition is the study of nutrients and how the
body uses them.
• Nutrients include carbohydrates, lipids,
proteins, vitamins, minerals, and water.
• Macronutrients – proteins, lipids,
carbohydrates. Required in large amounts.
• Micronutrients – vitamins and minerals.
Nutrition
• Macronutrients provide potential energy that can be
expressed in calories (unit of heat).
• Calorie – the amount of heat required to raise the
temperature of a gram of water by 1° C.
• The “calorie” used to measure food energy is actually
the kilocalorie, but is often written Calorie.
• 1 gram of carbohydrate or 1 gram of protein yields
about 4 calories.
• 1 gram of fat yields about 9 calories.
• Nutrients that human cells cannot synthesize, such as
some amino acids, are called essential nutrients.
Nutrition
• What
is an example of a
cellular process in which a
carbohydrate is being used?
• Carbohydrates are
organic compounds
used to supply
energy for cellular
processes.
• Starch, mono- and
disaccharides,
glycogen, and
cellulose are all
carbohydrates.
Carbohydrates
• Monosaccharides absorbed from
the digestive tract such as fructose,
galactose and glucose are broken
down and used in cellular
respiration.
• Excess glucose is stored as glycogen
in liver. Glycogen can be converted
to glucose when needed. Some
excess glucose is stored as fat.
• Ribose and deoxyribose are
required for the production of
these…
– Nucleic acids RNA and DNA
Lipids
• Lipids are organic
compounds that include
fats, oil, waxes, and
cholesterol.
• Main function is to supply
energy.
• Saturated fats are found
mainly in meat, eggs, milk
but also in coconut and
palm oil.
• Unsaturated fats are in
seeds, nuts, and plant oils.
Lipids
• Saturated fats are “saturated” with hydrogen
atoms because they lack double bonds.
• Unsaturated fats have a cis double bond.
• Trans fats are unsaturated but have a trans
double bond. This adds a kink to the chain.
• Often this occurs due to hydrogenation, a process
that adds hydrogen atoms and results in the trans
bond. This allows the unsaturated fat to remain
solid at room temperature, thus acting as a
saturated fat and being able to be preserved
longer.
Lipids
• So what?
• Trans fats are not
essential to the
diet.
• Also, they can
raise LDL
cholesterol levels
and lower HDL
cholesterol,
causing problems.
Protein
• Proteins are polymers of amino acids.
• Proteins have a variety of functions. Many are
enzymes. These and other proteins function in:
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Metabolic rate
Clotting
Kertain in skin and hair
Elastin and collagen of connective tissue
Water balance
Muscle components (actin, myosin)
Hormones
Antibodies
Protein
• Food sources of protein:
– Meat
– Fish
– Cheese
– Milk
– Nuts
– Eggs
– Cereal
– Legumes
Protein
• Amino acids that the body can synthesize are
called nonessential.
• Those that it cannot synthesize are called
essential amino acids.
• Complete proteins supply all of the essential
amino acids. These include: milk, eggs, meat.
Eggs are the only 100% complete protein.
• Incomplete proteins must be combined.
– Example: rice and beans.
Vitamins and Minerals
• Vitamins are organic compounds that are
required in small amounts and which the cells
cannot synthesize in adequate amounts.
• Fat-soluble vitamins: A, D, E, K
• Water-soluble vitamins: B, C
• Minerals are elements other
than carbon that are essential
in human metabolism.
Stomach
• Stomach is divided into four parts:
– Cardiac – near heart/esophagus
– Fundic – balloons up
– Body – main part
– Pyloric – near small intestine
• Taste, smell, or see food, Ach stimulates gastric
glands to secrete gastric juice (HCl, pepsin, etc).
• Gastrin also released which aids gastric juice
• Churning of food produces chyme.
Pancreas
• As chyme enters duodenum, hormone
secretin released, pancreatic juice excreted.
• Pancreatic juice includes:
– Pancreatic amylase
– Pancreatic lipase
– Nucleases
– Proteolytic enzymes
Small Intestine
• Three parts: duodenum, jejunum,Ileum
• Villi – project from mucous membrane.
– Increase surface area
– Lacteal – lymphatic capillary carries away
absorbed nutrients