Download SBI 3U Digestive System

The Digestive System
The Hierarchy in Biology
• Large organisms have many cells and are more complex, and
therefore they must specialize, to do a more specific function
• Cells that have similar functions are grouped together to form tissues
• There are four main tissues that make up the human body: connective
tissue, epithelial tissue, nervous tissue, and muscle tissue
• Atom < molecule < organelle < cell < tissue < organ < organ system
Building a Digestive
System
• An organ consists of several types of tissues that come
together and coordinate to perform one overall function.
• Examples of organs within the digestive system are the
stomach, liver and small intestine.
• An organ system is a group of several organs that work
together to perform a vital body function.
• Bodies are made of several organ systems which work
together in order to maintain the internal environment
(homeostasis)
Digestive System
What does the digestive
system look like?
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Also called the gastrointestinal tract of the alimentary canal.
Looks like a tube that is open at both ends.
Muscular tube from mouth to the anus
The inner surface, called the lumen is continuous with the outer
surface of the body.
• It also includes some accessory organs: salivary glands, pancreas,
liver, and gall bladder.
• Begins with the oral cavity and goes to the mouth, pharynx
esophagus, stomach, small intestine, large intestine, and finally the
anus.
What are the structures
of the digestive system?
• The inner layer of the small intestine is folded into ridges
and has many small finger-like projections called villi that
help to increase the surface area.
• Each villus is covered in even smaller, microscopic
microvilli.
• Each villus has a network of tiny blood vessels called
capillaries where all nutrients except fats are absorbed into
the bloodstream
• Each villus also has a lacteal vessels which is where
digested fats are absorbed and they travel through the
lymphatic system and then into the bloodstream.
Layers of tissue making up the lumen
The Steps in Digestion
• The main function of the digestive system is to change the
food we eat into chemical forms that our body can use.
• We start by ingesting our foods.
• Two types of digestion: mechanical and chemical.
• Mechanical: when food masses are bitten (using our incisors) torn,
shredded (using our canine teeth), ground (using our molars),
shaken (in stomach)
• Chemical: happens when food is in small pieces (high surface
area). Food is mixed with chemical juices from digestive glands
and then enzymes act on the broken-down food
The Steps in Digestion
continued…
• Next, we absorb our food. Usually done by the small
intestine. The large intestine generally only absorbs
water and vitamins.
• Elimination is what happens through the rectum and
anus.
• The nervous and endocrine systems help the digestive
system by providing impulses and hormones that target
and stimulate the digestive organs and glands.
The Organs of the Digestive Tract
The Oral Cavity
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Takes in the food and dismantles it
Lips, tongue, teeth and jaw muscles.
The tongue manipulates and moves the food around in the mouth.
Saliva contains an enzyme: salivary amylase, which begins the
chemical digestion of carbohydrates.
• At the end, right before swallowing, the food is called a bolus
• Passes through the pharynx, over the epiglottis (trap door that
prevents food going into the trachea), into the esophagus.
The Esophagus
• Muscular tube connecting the pharynx and the stomach
• Ring of smooth muscle just before the stomach called the
lower esophageal sphincter or cardiac sphincter (when
constricted, it prevents a reflux of food going back up the
esophagus)
• Peristalsis is a series of coordinated muscular contractions
that help move the food through the tract not by gravity. Is a
coordinated action between circular and longitudinal muscle
layers and makes a wave of constriction
The Stomach
• J-shaped and stretchable that can hold about
1.5L of food
• The pyloric sphincter controls the passage of
food in the stomach to the small intestine
• The stomach’s muscular walls turn and squeeze
bolus
• Thick layers of smooth muscle and the many
folds in the stomach (called rugae) allow the
stomach to expand
• Gastric glands in the stomach lining contain
cells that secrete hydrochloric acid (can sterilize
the upper digestive tract and destroys invading
microbes) and other cells that secret pepsinogen
that changes into pepsin (an enzyme that breaks
down proteins)
The Stomach
• Mucus is secreted which lines the stomach, forming a protective
coating against the corrosive effects of hydrochloric acid.
• Another enzyme called rennin is found in children that helps to
slow down the breakdown of milk, so they get more absorption of
nutrients.
• At the end, the stomach changes the bolus into a paste called chyme
and sends it to the small intestine through the pyloric sphincter.
• No absorption happens in the stomach
• Ulcers are common disorders that can happen when the
hydrochloric acid creates a hole in the lining of the stomach caused
by excessive hydrochloric acid secretions.
• Some ulcers may be caused by the bacterial infection Helicobacter
pylori. This causes a loss of the mucosal lining as well, but can be
cured with antibiotics.
The Small Intestine
• Major site of digestion and
absorption
• 3 sections: duodenum, jejunum,
ileum
• chyme that enters is further
mechanically digested through
segmentation movements
• Intestinal juices are from the
intestinal glands as well as the
accessory glands: pancreas and
liver
The Small Intestine
• The ileocaecal valve separates the small intestine from the
large intestine.
• Small intestine is smaller in diameter, but is actually a lot
longer than the large intestine.
• Absorption used both active and passive transport in order to
get through the walls of the small intestine.
• The lining has finger-like extensions called villi.
The Large Intestine
• 4 parts: caecum, colon, rectum,
and anus
• Divided into ascending,
transverse, descending, and
sigmoid regions
• Absorbs water, salts, some
vitamins
• Jutting out from the caecum is
the appendix (function is
debatable: probably a part of
the lymphoid system
• Also assembles some vitamins because of the microorganisms that are present.
• Wastes are kept here as the body prepares for elimination
• Defecation is controlled by two anal sphincters and usually
occurs once or twice a day in humans. Feces are about 75%
water and 25% solid.
• Constipation is a common problem of the digestive tract.
Laxatives and fibres can help cure it.
Digestive Accessory Organs
Salivary Glands
• Secrete saliva
• Two types of glands:
• Parotids: secrete watery fluid that
contains salivary amylase (that
breaks down starch)
• Buccal: secrete mucus
Liver and Gall Bladder
• Liver is second largest organ in
the body (after skin)
• Liver makes the bile (bile salts,
bile acids, cholesterol,
phospholipids, fatty acids,
water)
• Two large lobes
• In a recess under the right lobe
is the gallbladder (muscular sac
that stores bile)
• When fats enter the duodenum, endocrine cells release a hormone:
CCK into the blood. CCK causes the gallbladder to contract and
sends bile through the bile duct into the duodenum.
• Liver regulates metabolism (all blood leaving the intestines flows
through liver first so that it can take out any toxins)
• Toxins are decomposed by catalase enzyme
• The body recognizes alcohol as a toxin and therefore tries to
eliminate it from the body.
• After a long time of alcohol abuse, it can cause cirrhosis of the liver.
Cirrhosis is when the tissue is scarred and hinders blood flow and
disrupts the homeostasis.
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The Pancreas
Lies behind the stomach and extends from the duodenum
Finger-shaped and about 15cm long.
Chyme leaving the stomach is acidic, and must be neutralized.
Once some acid is released into the duodenum, a hormone
called secretin is released and tells the pancreas (through the
blood stream) to release bicarbonate ions that will neutralize
and make it slightly basic: pH of 9.
• The basic environment causes the pepsin (from stomach) to
become inactive.
• Pancreas also secretes lipases (break down lipids),
carbohydrases (digest sugars and starches) and proteases
(proteins).
Pancreas continued…
• An example of homeostasis also occurs in the pancreas. The regulation of blood sugar
levels is controlled by Negative Feedback which can be broken into 3 steps:
• A variable is identified that has changed in the body, and is either falling below or rising above its
normal range or set point.
• Receptors detect the change and signal other parts of the body to respond in order to restore the
steady state.
• Organs receive the signal and respond.
• We do not want our blood sugar to go lower, or higher than 4-6 mmol/L (set point). After a
meal, we have a higher blood glucose level, which is detected by receptor cells. These
receptor cells then stimulate the release of insulin from the beta cells in the pancreas.
Insulin helps the body cells take in the glucose and the levels decrease in the blood.
• If we fall below our set point for blood sugar levels, the hormone glucagon from the alpha
cells in the pancreas stimulate reactions for the liver to release energy (glycogen).
Human Digestive Enzymes
• Enzymes are proteins that speed up reactions.
• Produced by two types of glands: those with ducts
(pancreas and salivary) and those without (stomach and
small intestine)
• Carbohydrate digestion begins in the mouth with salivary
amylase (breaks down starch into disaccharides) and
then finishes in the duodenum.
• Pepsin breaks down proteins into shorter amino acid
chains in the stomach
Digestive enzymes
continued…
• Pancreatic enzymes work in a basic environment (hence
the bicarbonate ions)
• Pancreatic Juice is a mixture of several enzymes:
proteases, trypsin (completes protein digestion by
breaking into single amino acids), pancreatic amylase,
lipases, nucleases
Bile
Has 2 functions:
• Emulsifies fat by breaking down into
small globules
• Bile salts help the absorption of lipids
from the intestine.
Secreted by the gallbladder
Gallstones caused when large amounts
of water are absorbed from the bile,
leaving behind solids.
Endoscopy
Tiny camera with a light that is
inserted through the mouth to
look at the upper GI tract.
Can also have a colonoscopy,
which is inserted through the
anus and looks at the lower
GI tract.