Gas Exchange

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Gas Exchange

• By Zoe Kopp-Weber

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Coevolution of circulatory and respiratory systems

• Allowed for vertebrates to develop larger bodies
  and locomotion.
• As these abilities grew, the need for efficient
  delivery of nutrients and O2 and removal of
  wastes and CO2 from the growing mass of tissues
  grew too.

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Coevolution of circulatory and respiratory
systems (cont.)

• Gills developed in fish and with it the 4-chamber
  heart, one of the major evolutionary innovations
  in vertebrates.
• Mammals, birds and crocodiles also have a
  4-chamber heart, with 2 separate atria and 2
  separate ventricles.
• Right atrium receives deoxygenated blood and
  sends it to right ventricle which pumps blood to
  lungs. Left atrium receives oxygenated blood and
  delivers it to the left ventricle to pump the
  blood to the rest of the body.

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• For most multicellular animals, gas exchange
  requires special respiratory organs which provide
  intimate contact between gases in the external
  environment and the circulatory system.

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• Respiration describes the uptake of O2 from the
  environment and disposal of CO2 into the
  environment at a body system level.
• Cellular respiration internal respiration
• Gas exchange external respiration
• Communication between internal and external
  respiration is provided by the circulatory system.

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• Respiration involves processes ranging from the
  mechanics of breathing to the exchange of O2 and
  CO2 in respiratory organs.
• Respiratory organs
• Invertebrates epithelium, trachae and gills
• Fish and larval amphibians gills
• Other amphibians skin or epithelia used as
  supplemental/primary external respiratory organ.
• Mammals, birds, reptiles, adult amphibians lungs

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• Respiration involves the diffusion of gases
  across the plasma membrane
• Which must be surrounded by water to be stable.
• Thus the external environment is always aqueous,
  even in terrestrial animals.

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• Rate of diffusion between 2 sides of the membrane
  has a relationship called Fricks Law of
  Diffusion
• RD x A delta p/d
• R rate
• D diffusion constant
• A area diffusion occurs
• Delta p difference in concentration btw interior
  of organism and external environment
• d distance across diffusion occurs

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• Evolution has optimized R via increased surface
  area, decreased distance and increased
  concentration difference.
• Levels of O2 required cant be obtained by
  diffusion alone over distances greater than 0.5
  mm.
• Vertebrates decreased this distance through the
  development of respiratory organs and bringing
  the external environment closer to the internal
  fluid

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• Dry air is composed of 78.09 N, 20.95 O2, 0.93
  Ar and other inert gases, and 0.03 CO2.
• This composition remains constant at altitudes of
  at least 100 km but the amount of air decreases
  as the altitude goes up.
• Humans dont survive long over 6000 meters,
  though the same composition of O2 is there, the
  atmospheric pressure brings it to only half the
  amount of 02 than whats at sea level.

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• Though gills are effective in aquatic
  environments, there are two reasons terrestrial
  animals replaced gills with other respiratory
  organs.
• 1. Air is less buoyant than water. Gills collapse
  out of water while internal air passages remain
  open because the body provides structural
  support.
• 2. Water diffuses into air via evaporation.
  Terrestrial animals are constantly surrounded by
  air and therefore lose H2O. Gills would provide a
  large surface area for H2O loss.

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Terrestrial respiratory organs

• Trachae used by insects and is a network of
  air-filled tubular passages.
• Lung moves air through branched tubular
  passages. Air is saturated with H2O before
  reaching a thin, wet membrane that allows gas
  exchange.
• All but birds use a uniform pool of air
• Moves in and out of the same airway passages

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• Mammals have higher metabolic rates so they
  require a more efficient respiratory system.
• Lungs are packed with tiny, grape-like sacs
  called alveoli. Air is inhaled through
  mouth/nose, past the pharynx to the larynx where
  it then passes through the glottis and into the
  trachea.

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• The trachea splits into right and left bronchi
  which enter into each lung and subdivide into
  bronchioles that deliver air into the alveoli.
• All gas exchange btw air and blood occurs across
  walls of alveoli.

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• Visceral pleural membrane a thin membrane that
  covers the outside of each lung.
• Parietal pleural membrane lines the inner wall
  of the thoracic cavity.
• Pleural cavity the space between these two
  membranes, very small and filled with fluid.
• Fluid allows membranes to adhere to each other,
  coupling the lungs to the thoracic cavity.
• Pleural membranes package each lung separately so
  if one should collapse, the other can function.

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Mechanics of breathing

• In all terrestrial vertebrates but amphibians,
  air is drawn into the lungs by subatmospheric
  pressure.
• Boyles Law when the volume of a given quantity
  of gas increases, its pressure decreases.
• When inhaling, volume of thorax is increased and
  the lungs expand. Lowered pressure in lungs
  allows air to enter.

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• Diaphragm a muscle that increases thoracic
  volume by contracting.
• When it contracts, it assumes a flattened shape
  and lowers, expanding the volume of the thorax
  and lungs while adding pressure onto the abdomen.
• External intercostal muscles also contributes
  in increasing thoracic volume.
• These muscles between the ribs contract, causing
  the ribcage to expand.

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• The thorax and lungs have a degree of elasticity.
• They resists distension and recoil when
  distending force subsides.

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Breathing measurements

• At rest, each breath moves a tidal volume of 500
  mL of air in and out of the lungs.
• 150 mL in trachea, bronchi and bronchioles where
  no gas exchange occurs.
• Anatomical dead space, air here mixes with fresh
  air during inhalation.
• Maximum amount of air expired after a maximum
  inhalation is called the vital capacity.
• Averages 4.6 liters in young men and 3.1 liters
  in young women.

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• Hypoventilating - when breathing is insufficient
  to maintain normal blood gas measurements.
• Hyperventilating when breathing is excessive
  for a particular metabolic rate.
• Increased breathing after exercise isnt
  necessarily hyperventilating because faster
  breathing is matched to faster metabolic rate and
  blood gas measurements remain normal.

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Mechanism regulating breathing

• Each breath initiated by a respiratory
  controntrol center in the medulla oblongata.
• Neurons send impulses that stimulate muscles to
  contract and expand the chest cavity.
• Though controlled automatically, these controls
  can be overridden by, for example, holding ones
  breath.

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• A fall in blood pH stimulates neurons in aortic
  and carotid bodies
• These are sensory structures known as peripheral
  chemoreceptors in the aorta and carotid artery.
• Send impulses to the respiratory control center
  in the medulla oblongata, which stimulates
  increased breathing.
• responsible for immediate stimulation when the
  blood partial CO2 pressure rises.

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• Central chemoreceptors responsible for
  sustained increase in ventilation if partial CO2
  pressure remains elevated. Increased respiratory
  rate acts to eliminate extra CO2, bringing blood
  pH to normal.

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Hemoglobin and gas transport

• When O2 diffuses from alveoli into blood, the
  circulatory system then delivers the O2 to
  tissues for respiration and carries away the CO2.
• Amount of O2 dissolved in blood plasma depends
  directly on the partial O2 pressure or the air in
  the alveoli.
• When lungs function normally, the blood plasma
  leaving the lungs have almost as much DO as
  possible.
• Whole body carries almost 200 mL/L of O2, most is
  bound to molecules of hemoglobin inside red blood
  cells.

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• Hemoglobin - protein composed of four polypeptide
  chains and four organic compounds (heme groups).
• Each heme group has an iron atom at the center,
  able to bind to a molecule of O2.
• Allows hemoglobin to carry four molecules of O2.

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• Hemoglobin loaded with O2 forms oxyhemoglobin.
• Bright red, tomato juice color
• As blood passes capillaries, some oxyhemoglobin
  releases oxygen, becoming deoxyhemoglobin
• Dark red but gives tissues a bluish tinge.

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• Red color,
• oxygenated
• Blue color,
• oxygen-depleted

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• Hemoglobin is used by all vertebrates, and also
  by many invertebrates
• Other invertebrates use hemocyanin as an
  oxygen-carrier
• O2 binds to copper rather than iron.
• Not found in blood cells but rather dissolved in
  circulating fluid of invertebrates

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Oxygen transport

• As blood travels through the systemic blood
  capillaries, O2 leaves the blood and diffuses
  into tissues.
• 1/5 of O2 is unloaded in tissues, 4/5 in blood as
  a reserve.
• The reserve allows the blood to supply the body
  O2 during exercise.
• Also ensures enough O2 to maintain life 4-5
  minutes if breathing is interrupted or the heart
  stops.

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• O2 transport affected by
• CO2 produced by metabolizing tissues, it
  combines with H2O forming carbonic acid. This
  dissociates into bicarbonate and H, lowering
  blood pH.
• Also reduces hemoglobins affinity for O2 and
  causes it to release O2 more readily.
• This is all called the Bohr effect.
• Increase in temperature has a similar effect.
• Skeletal muscles produce CO2 quicker during
  exercise, producing heat.

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Carbon Dioxide Transport

• Systemic capillaries deliver O2 and remove CO2
  from tissues
• Majority diffuses into red blood cells where its
  catalyzed with water to form carbonic acid
  (H2CO3)
• Disassociates into bicarbonate and H and moves
  into the plasma, exchanging a chloride ion for a
  bicarbonate (chloride shift).
• Removes large amounts of CO2 from plasma,
  facilitating diffusion of additional CO2 into
  plasma from surrounding tissues.

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• Blood carries CO2 to the lungs in this form.
• CO2 diffuses out of red blood cells, into the
  alveoli and then leaves the body with exhalation.

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Nitric Oxide Transport

• Nitric oxide acts on many cells to change their
  shape/functions.
• Causes blood vessels to expand by relaxing
  surrounding muscle cells.
• Blood flow/pressure regulated by nitric oxide in
  bloodstream.

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• One hypothesis proposes hemoglobin carries super
  nitric oxide which is able to bind to cysteine in
  hemoglobin
• Dumps CO2 and picks up O2 and NO in the lungs
• To increase blood flow, hemoglobin can release
  super NO into blood, making blood vessels expand
• Can also trap excess NO on vacant iron atoms,
  making blood vessels constrict.
• Red blood cells return to lungs, hemoglobin dumps
  CO2 and regular NO. Then ready to picl up O2 and
  super NO.

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Disease

• Emphysema - usually caused by cigarette smoking,
  the vital capacity of the lungs is reduced and
  alveoli are destroyed.
• Bronchitis - a respiratory infection affecting
  nose, sinus and throat, then moves on into the
  lungs. Cough produces an excess of mucus.
• Pneumonia - inflammation of the lungs that can be
  caused by bacteria, viruses or fungi. Causes
  coughing, fever and it will likely make it harder
  to breathe.

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