Gas Exchange

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Gas Exchange

• Campbell Chapter 42
• Pages 886 - 897

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Gas Exchange in Animals

• Gas exchange taking in molecular oxygen (O2)
  from the environment and disposing of carbon
  dioxide (CO2) to the environment.

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Gas Exchange

• Cellular respiration is the breakdown of organic
  molecules to make ATP. A supply of oxygen is
  needed to convert stored organic energy into
  energy trapped in ATP.
• Carbon dioxide is a by-product of these processes
  and must be removed from the cell.
• There must be an exchange of gases carbon
  dioxide leaving the cell, oxygen entering.

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Air Supply

• The respiratory medium or source of oxygen is
• Terrestrial Animals air
• Aquatic Animals water.
• Atmosphere is 21 oxygen (O2)
• Bodies of water variable oxygen content, but
  much less than in an equal amount of air.

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Respiratory Surface

• Gases are exchanged with the environment at the
  respiratory surface.
• Gas movement is by diffusion.
• Respiratory surfaces are usually thin and have
  large areas as well as adaptations to facilitate
  the exchange.
• Gases are dissolved in water, so respiratory
  surfaces must be moist.

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Diffusion Rate

• The net diffusion rate of a gas across a fluid
  membrane is
• proportional to the difference in partial
  pressure,
• proportional to the area of the membrane and
• inversely proportional to the thickness of the
  membrane.

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Ficks Law

• The rate at which a substance can diffuse is
  given by Fick's law

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Surface to Volume Ratio

• Rate of exchange of substances depends on the
  organism's surface area that is in contact with
  the surroundings.
• The ability to exchange substances depends on the
  surface area volume ratio.
• As organisms get bigger, their volume and surface
  area both get bigger, but volume increases much
  more than surface area.

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Unicellular Organisms

• Single-celled organisms exchange gases directly
  across their cell membrane.
• The slow diffusion rate of oxygen relative to
  carbon dioxide limits the size of single-celled
  organisms.

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Simple Animals

• The cells of sponges, cnidarians, and flatworms
  are in direct contact with environment.
• Simple animals that lack specialized exchange
  surfaces have flattened, tubular, or thin shaped
  body plans, which are the most efficient for gas
  exchange. However, these simple animals are
  rather small in size.

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Simple Animals
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Respiratory Surfaces

• Some animals use their outer surfaces (skin) as
  gas exchange surfaces. (earthworms and some
  annelids)
• Arthropods, annelids, and fish use gills.
• Terrestrial vertebrates utilize internal lungs.

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Outer Surface (Skin)

• Flatworms and annelids use their outer surfaces
  as gas exchange surfaces. Earthworms have a
  series of capillaries. Gas exchange occurs at
  capillaries located throughout the body as well
  as those in the respiratory surface.
• Amphibians use their skin as a respiratory
  surface. Frogs eliminate carbon dioxide 2.5 times
  as fast through their skin as they do through
  their lungs.
• Eels (a fish) obtain 60 of their oxygen through
  their skin.
• Humans exchange only 1 of their carbon dioxide
  through their skin.

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Gills

• Gills have evolved many times in different animal
  groups, and the specific anatomy varies widely.
  But as a general rule, gills consist of fine
  sheets or filaments of tissue that extend outward
  from the body into the water

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Fish Respiration

• Some fish ventilate their gills by swimming with
  mouth and gill slits open e.g. sharks, which die
  of asphyxiation if immobilized.
• But many fish can respire while stationary, and
  do so by swallowing water through their mouths
  and forcibly expelling it through the gills.

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Gills

• Gills are out-foldings of the body surface that
  are suspended in water.
• They increase the surface area for gas exchange.
• They are organized into a series of plates and
  may be internal (as in crabs and fish) or
  external to the body (as in some amphibians).

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Gills
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Who Has Gills?

• Gills are found in a variety of animal groups
• arthropods (including some terrestrial
  crustaceans)
• annelids,
• fish
• amphibians.

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Variety of Gills
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Efficiency of Gills

• Gills are very efficient at removing oxygen from
  water there is only about 1/20 the amount of
  oxygen present in water as in the same volume of
  air.
• Water flows over gills in one direction while
  blood flows in the opposite direction through
  gill capillaries. This countercurrent flow
  maximizes oxygen transfer.

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How Gills Work

• Fish maximize gas exchange in their gills by a
  'design principle' called countercurrent
  exchange.
• Countercurrent exchange requires that two fluids
  (in this case, the external water and the blood
  in the gills) flow past each other in opposite
  directions.

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Counter-Current Exchange

• When a fish swims, water moves over its gills
  from anterior to posterior.
• Blood flow in the gill capillary bed is oriented
  from posterior to anterior. The blood picks up O2
  from the external water (and loses CO2) as it
  flows through the gill capillaries.
• This countercurrent arrangement insures that the
  most O2 -depleted blood (entering the gill) is
  confronted with the most O2 -depleted water
  (leaving the gill), and that the most O2 -rich
  blood (leaving the gill) contacts the most O2
  -rich water (entering the gill). This arrangement
  maximizes O2 absorption.

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Counter-Current Exchange
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Terrestrial Respiratory Systems

• Many terrestrial animals have their respiratory
  surfaces inside the body and connected to the
  outside by a series of tubes.
• Insects, centipedes, and some mites and spiders
  have a tracheal respiratory system.
• Vertebrates have lungs.

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Insect Tracheal Systems

• All insects are aerobic organisms - they must
  obtain oxygen (O2) from their environment in
  order to survive.
• The insect respiratory system is a complex
  network of tubes (tracheal system) that delivers
  O2-containing air to every cell of the body.
• Tracheae are the tubes that carry air directly to
  cells for gas exchange.

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Tracheal System

• Air enters the insect's body through valve-like
  openings (spiracles) in the exoskeleton.   These
  are located laterally along the thorax and
  abdomen of most insects. 
• Air flow is regulated by small muscles that
  operate one or two flap-like valves within each
  spiracle -- contracting to close the spiracle, or
  relaxing to open it.

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• After passing through a spiracle, air enters a
  longitudinal tracheal trunk, eventually diffusing
  throughout a complex, branching network of
  tracheal tube that subdivides into smaller and
  smaller tubes that reach every part of the body.
   
• At the end of each tracheal branch, a special
  cell (the tracheole) provides a thin, moist
  interface for the exchange of gases between
  atmospheric air and a living cell.  
• Oxygen in the tracheal tube first dissolves in
  the liquid of the tracheole and then diffuses
  into the cytoplasm of an adjacent cell.   At the
  same time, carbon dioxide, produced as a waste
  product of cellular respiration, diffuses out of
  the cell and, eventually, out of the body through
  the tracheal system.

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Insect Tracheal System
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Human Respiratory System
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Human Respiratory System

• This system includes the lungs, pathways
  connecting them to the outside environment, and
  structures in the chest involved with moving air
  in and out of the lungs.
• The main task of any respiratory system is to
  take in oxygen and remove carbon dioxide.

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Lungs

• Lungs are ingrowths of the body wall and connect
  to the outside by as series of tubes and small
  openings.
• Lung breathing probably evolved about 400 million
  years ago.
• Lungs are not entirely the sole property of
  vertebrates, some terrestrial snails have a gas
  exchange structures similar to those in frogs.

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• The lungs are large, lobed, paired organs in the
  chest (also known as the thoracic cavity).
• Thin sheets of epithelium (pleura) separate the
  inside of the chest cavity from the outer surface
  of the lungs.
• The bottom of the thoracic cavity is formed by
  the diaphragm.

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Pathway of Air

• Air enters the body through the nose, is warmed,
  filtered, and passed through the nasal cavity.
• Air passes the pharynx (which has the epiglottis
  that prevents food from entering the trachea).
• The upper part of the trachea contains the
  larynx. The vocal cords are two bands of tissue
  that extend across the opening of the larynx.
• After passing the larynx, the air moves into the
  bronchi that carry air in and out of the lungs.

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• Bronchi are reinforced to prevent their collapse
  and are lined with ciliated epithelium and
  mucus-producing cells.
• Bronchi branch into smaller and smaller tubes
  known as bronchioles.
• Bronchioles terminate in grape-like sac clusters
  known as alveoli.
• Alveoli are surrounded by a network of
  thin-walled capillaries. Only about 0.2 µm
  separate the alveoli from the capillaries due to
  the extremely thin walls of both structures.

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Alveoli

• Only in the alveoli does actual gas exchange
  takes place.
• There are some 300 million alveoli in two adult
  lungs.
• These provide a surface area of some 160 m2
  (almost equal to the singles area of a tennis
  court and 80 times the area of our skin!).

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Alveoli
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Diffusion of O2 and CO2
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Alveoli Designed for Rapid Gas Exchange

• After branching repeatedly the bronchioles
  enlarge into millions of alveolar sacs
• This arrangement produces an enormous surface are
  for gas exchange
• Each alveolus is surrounded by a net of
  capillaries
• The diffusion distance from gas in the alveoli to
  blood cells in the capillaries is very short
• Blood takes about 1 second to pass through the
  lung capillaries
• In this time the blood becomes nearly 100
  saturated with oxygen and loses its excess CO2

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Surfactants Prevent the Alveoli From Collapsing

• At air/water interfaces there is a high surface
  tension
• The high surface tension would cause the alveoli
  to collapse, but this is prevented by surfactants
  
• Surfactants are detergent-like phospholipids
  which accumulate at the air/water interface and
  lower the surface tension
• Reduced surfactant causes respiratory distress
  syndrome (seen in premature infants and some
  older persons)

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Pigments

• Respiratory pigments increase the oxygen-carrying
  capacity of the blood. Humans have the
  red-colored pigment hemoglobin as their
  respiratory pigment.
• Hemoglobin increases the oxygen-carrying capacity
  of the blood between 65 and 70 times.
• Each red blood cell has about 250 million
  hemoglobin molecules, and each milliliter of
  blood contains 1.25 X 1015 hemoglobin molecules.
• Oxygen concentration in cells is low (when
  leaving the lungs blood is 97 saturated with
  oxygen), so oxygen diffuses from the blood to the
  cells when it reaches the capillaries.

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Animation

• http//www.mdhs.unimelb.edu.au/bmu/examples/gasxlu
  ng/
• http//science.nhmccd.edu/biol/respiratory/alveoli
  .htm
• http//www.smm.org/heart/lungs/breathing.htm
• http//sprojects.mmi.mcgill.ca/resp/anatomy.swf

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Ventilation

• Ventilation is the mechanics of breathing in and
  out.
• When you inhale, muscles in the chest wall
  contract, lifting the ribs and pulling them,
  outward. The diaphragm at this time moves
  downward enlarging the chest cavity. Reduced air
  pressure in the lungs causes air to enter the
  lungs.
• Exhaling reverses theses steps.

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Negative Pressure Breathing
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Diaphragm Action
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Negative Pressure Animation
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Lung Volume

• Tidal volume is the amount of air that is inhaled
  and exhaled in a normal breath.  
• Vital capacity is the maximum amount of air that
  can be inhaled and exhaled in a single breath.  
• Since the lungs hold more air than the vital
  capacity, the air that remains in the lungs is
  the residual volume.

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Dead Space

• Only the air in the alveoli can exchange O2 and
  CO2 with the blood
• When you breath in the first 150 mL fills tubes
  which are outside of the alveoli (trachea,
  bronchi, bronchioles, etc.)
• This part of the tidal volume is called the
  anatomical dead space- it does not participate in
  gas exchange
• There is also a functional dead space- not all of
  the alveoli are perfused with blood air in these
  alveoli doesn't exchange with the blood and is
  part of the dead space

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Avian Respiration

• The avian respiratory system delivers oxygen from
  the air to the tissues and also removes carbon
  dioxide.
• In addition, the respiratory system plays an
  important role in thermoregulation (maintaining
  normal body temperature).

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• The avian respiratory system is different from
  that of other vertebrates, with birds having
  relatively small lungs plus nine air sacs that
  play an important role in respiration (but are
  not directly involved in the exchange of gases).

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• The air sacs permit a unidirectional flow of air
  through the lungs.
• Unidirectional flow means that air moving through
  bird lungs is largely 'fresh' air has a higher
  oxygen content.
• In contrast, air flow is 'bidirectional' in
  mammals, moving back forth into out of the
  lungs. As a result, air coming into a mammal's 
  lungs is mixed with 'old' air (air that has been
  in the lungs for a while) this 'mixed air' has
  less oxygen. So, in bird lungs, more oxygen is
  available to diffuse into the blood

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Respiration

• http//www.wisc-online.com/objects/framz.asp?objID
  AP2404

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Credits

• All material for this PPT was found on various
  websites or is from Campbell Biology 6e.