The Human Digestive System

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The Human Digestive System
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Food - Getting / Nutrient Use

• All organisms need food, and all foods contain
  nutrients. Nutrients are the substances that
  provide the energy and the materials needed for
  growth, repair, regulation, and maintenance of
  the cells.
• Therefore, food is what the organism consumes,
  and nutrients are substances within food that are
  needed by the cells to sustain life.

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Organisms Require Six Basic Nutrients

• 1. Carbohydrates
• Source - plants
• Function - major source of energy in the body
• 2. Lipids (fats)
• Source - ingestion of animal and plant fats
  conversion of carbohydrates into fats
• Function - storage of energy component of cell
  membranes cushion for delicate organs carriers
  for certain vitamins raw materials for important
  chemicals

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• 3. Proteins
• Source - meat, fish, poultry
• Function - broken down into amino acids which are
  used in the construction of human proteins.
  Proteins are essential for the building, repair,
  and maintenance of cell structure.
• The predominant part of muscles, nerves, skin,
  and hair is protein.
• Things such as enzymes and antibodies are
  specialized proteins.

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• 4. Water
• Source - ingestion
• Function - a variety of functions including its
  role as a solvent
• 5. Vitamins
• Source - the various vitamins are each ingested
  from a variety of foods
• Function - various metabolic functions including
  enzymatic activity
• 6. Minerals
• Source - the various minerals are each ingested
  from a variety of foods
• Function - a large variety of bodily functions

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• Carbohydrates, lipids, and proteins require
  digestion.
• Vitamins, water, and minerals do not.

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Research Assignment

• Construct a chart displaying the specific sources
  and functions of each of the following vitamins
  and minerals
• Vitamins - A, B1, B2, Niacin, B12, C, D, E, and K
• Minerals - calcium, phosphorous, sodium,
  chlorine, potassium, magnesium, iodine, iron

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• Foods taken into the body consist of large
  complex organic compounds. Digestion must occur
  in order to release the nutrients contained
  within the food.
• Digestion will reduce the large complex organic
  compounds into smaller, simpler units that can be
  absorbed and used by the cells of the organism.
• This is accomplished through a digestive system.
  In such a system digestion occurs in two ways

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• (A) Mechanical - food is broken down into smaller
  pieces by the teeth. This prepares the food for
  faster chemical digestion by exposing more food
  surface to the action of the digestive enzymes.
  Mechanical digestion also includes the physical
  movement of food.
• (B) Chemical - food is broken down into smaller,
  simpler organic units through the activity of the
  digestive enzymes.
• The human digestive system consists of a tract
  of organs and associated glands.

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Digestive Tract / Alimentary Canal
Organs
Glands
Mouth
Salivary
Pharynx
Esophagus
Stomach
Gastric
Small Intestine
Liver
Intestinal
Large Intestine
Pancreatic
Rectum
Anus
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Mouth

• Structure
• Teeth - 20 infant, followed by 32 adult
• Tongue - location of taste buds, four types being
  salty, sweet, sour and bitter
• Mechanical Digestion
• (1) teeth tear and grind food into smaller pieces
• (2) tongue keeps the food between your teeth and
  pushes it into the pharynx

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• Chemical Digestion
• Six salivary glands release saliva that contains
  salivary amylase (enzyme) and mucin.
• (1) Salivary amylase (enzyme) starts the
  breakdown of starch (polysaccharide) into maltose
  (disaccharide).
• (2) Mucin makes the food slippery and easy to
  swallow. The food particles stick together
  forming a food mass called a bolus which the
  tongue pushes into the pharynx.

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Pharynx

• Structure Made up of muscular walls containing
  two openings
• trachea - leads to the lungs and is covered by
  the epiglottis.
• esophagus - leads to the stomach
• Mechanical Digestion - none
• Chemical Digestion - none

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Esophagus

• Structure It is a tube about 30cm long through
  which food passes from the pharynx to the
  stomach.
• Mechanical Digestion There are a series of
  muscle contractions called peristalsis that push
  food down the esophagus and into the stomach.
• Chemical Digestion - none

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Stomach

• Structure It is a J-shaped tube that lies below
  the diaphragm towards the left side of the
  abdomen.
• It is fitted with two sphincter muscles cardiac
  (entrance) and pyloric (exit)
• These sphincters act as valves and control the
  passage of food into and out of the stomach.
• Mechanical Digestion Peristalsis occurs that
  churns and mixes the food and the digestive
  juices.
• The food stays in the stomach from 2 to 6 hours.
  It can expand to hold more than 2 liters of food
  or liquid.

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• Chemical Digestion
• Gastric glands found in the wall of the stomach
  release gastric juices. A hormone called gastrin
  controls the release of gastric juice. This
  hormones release is triggered by three
  mechanisms
• (1) thought, sight, smell, or taste of food
• (2) food touching the lining of the stomach
• (3) stretching of the stomach wall

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• Gastric juice contains
• (1) Hydrochloric Acid (HCl)
• HCl drops the pH of the stomach to 2. This lower
  pH stops the digestion of starch and is necessary
  for the digestion of protein
• (2) Pepsinogen (inactive enzyme)
• Pepsinogen enters the stomach and comes into
  contact with HCl. It converts to pepsin which is
  an active enzyme. Pepsin breaks protein into
  polypeptides.

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Protection of Stomach

• The stomach protects itself from its own
  digestive juices by way of pyloric glands.
• These glands secrete mucus which covers the
  stomach lining and prevents it from being
  digested.
• When this mucus lining breaks, an ulcer forms.
• When the food is ready to leave the stomach it is
  an acidic mass called chyme.

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Small Intestine

• Structure
• It is a tube 2.5cm in diameter and 7m long. It
  is coiled inside the abdomen and consists of
  three sections
• (1) duodenum - 25cm long
• (2) jejunum - 2m long
• (3) ileum - 5m long
• Digestion is completed here and nutrients are
  absorbed.

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Mechanical Digestion

• (1) Peristalsis
• (2) Fats are emulsified. The liver produces bile
  which is stored in the gall bladder.
• The bile enters the intestine at the duodenum
  through a tube called the common bile duct.
• The bile emulsifies fats. This means fats, oils,
  and waxes are broken down into very small
  droplets increasing the surface area for enzyme
  activity.
• Bile is not an enzyme.

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Chemical Digestion

• There are two important glands involved in
  digestion in the small intestine
• (A) Pancreas
• (B) Intestinal

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(A) Pancreas

• It is located outside the small intestine and is
  attached to the wall of the abdomen.
• It produces pancreatic juices which enter the
  small intestine at the duodenum through the
  pancreatic duct and into the common bile duct.
• The pancreatic juices contain
• (1) Pancreatic amylase - an enzyme that converts
  remaining starch to maltose.
• (2) Lipase - an enzyme that converts fats to
  fatty acids and glycerol.

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• (3) Proteases / Trypsin - enzymes that convert
  the polypeptides produced in the stomach into
  simpler polypeptides.
• (4) Erepsins - enzymes that convert simpler
  polypeptides into amino acids. This is the final
  stage in protein digestion.
• (5) Sodium bicarbonate - raises the pH to 8
  (base) so the above enzymes can function. It
  does this by neutralizing the acidic chyme
  released from the stomach. Bile also aids in the
  process.

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(B) Intestinal Glands

• They are found in the wall of the small
  intestine. They produce and release intestinal
  juices which contain
• (1) Peptidases - enzymes that convert simpler
  polypeptides into amino acids.
• (2) Lipase - enzyme that converts fats to fatty
  acids and glycerol
• (3) A group of enzymes that convert disaccharides
  into glucose
• A. maltase - converts maltose to glucose
• B. sucrase - converts sucrose to glucose
• C. lactase - converts lactose to glucose

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Absorption

• The small intestine contains many finger-like
  projections called villi. They increase the
  surface area of the small intestine by a factor
  of 10. To get the same surface area without the
  villi, your small intestine would have to be 70m
  long.
• The large surface area is ideal for nutrient
  absorption.

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There are two major parts to each villus

• (A) Lacteal - part of the lymph system, located
  in the center of the villus and absorbs fatty
  acids and glycerol.
• (B) Capillaries - part of the circulatory system,
  located around the lacteal and absorbs the
  following
• (i) glucose (simple sugar)
• (ii) amino acids
• (iii) vitamins
• (iv) minerals
• (v) water

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• The capillaries of all villi join together to
  form the hepatic portal vein that travels to the
  liver.
• Once in the liver, glucose is converted into
  glycogen and stored.
• The rest of the nutrients travel in the blood
  around the body to every individual cell.
• The various nutrients enter the villus by the
  various forms of transport discussed earlier

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Passive
diffusion
facilitated diffusion
osmosis
Active
carrier proteins
vesicles
exocytosis
endocytosis
phagocytosis
pinocytosis