BIOA02H3 Chapter Notes -Pulmonary Alveolus
Chapter 48 – Gas Exchanges in Animals
48.1 What Physical Factors Govern Respiratory Gas Exchange? (1025)
- CO2 and O2 are the respiratory gases that animals must exchange.
- Diffusion (random movement of molecules or other particles, resulting in even distribution of the particles when no
barriers are present) is the only means of gas exchange between the internal body fluids of an animal and the outside
medium (water or air). Diffusion is a physical process. Diffusion is faster in higher temperatures and faster in air than
Diffusion is Driven by Concentration Differences (1025)
- Net movement of molecules via diffusion is always down its concentration gradient.
- Partial pressure of the gases is one way biologists express the concentrations of different gases in a mixture.
- Solubility of a gas in liquid is a factor that makes it more difficult to describe of respiratory gases in a liquid such as
- Actual amount of a gas in a liquid depends on the partial pressure of that gas in the gas phase in contact with the liquid
as well as on the solubility of that gas in that liquid.
Fick’s Law Applies to All Systems of Gas Exchange (1026)
- Fick’s law of diffusion describes diffusion quantitatively with an equation (all environmental variables that limit
respiratory gas exchange and all adaptations that maximize respiratory has exchange are included):
Q = DA-
- Q is the rate at which a gas such as O2 diffuses between two locations.
- D is the diffusion coefficient (i.e. perfume has a higher D than motor oil vapour).
- A is the cross-sectional area through which the gas is diffusion.
- P1 and P2 are the partial pressures of the gas at the two locations.
- L is the path length, or distance, between the two locations.
- (P1-P2)/L is a partial pressure gradient.
Air is a Better Respiratory Medium than Water (1026)
- Oxygen can be obtained easier from air than from water because:
- O2 content of air is much higher than in water.
- O2 diffuses about 8000 times more rapidly in air than in water.
- More energy is done to move water than to move air, because water is 800x more dense.
- Eukaryotic cells carry out cellular respiration in the mitochondrion which is in the cytoplasm – an aqueous medium – as
well as they are bathed in extracellular fluids which is also an aqueous medium.
- Animals in liquid mediums (i.e. fish) have gills which are very efficient in gas exchange – provide large surface area for
High Temperatures Create Respiratory Problems for Aquatic Animals (1026)
- Because most water breathers are ectotherms, their body temperature and metabolic rate increases as the environment’s
temperature increases; they need more O2 as the water gets warmer and warm water hold less dissolved gas than cold
O2 Availability Decreases with Altitude (1026)
- Rise in altitude reduces O2 availability. The P02 (partial pressure of oxygen) decreases as well, and since diffusion (gas
exchange) relies on this, gas exchange is less efficient and O2 uptake is constrained.
C O2 is Lost by Diffusion (1027)
- CO2 diffuses out of the body as O2 diffuses in. Direction and rate of diffusion across the respiratory exchange surfaces
depend on the partial pressure gradients of the gases.
- Partial pressure of CO2 does not change with altitude.
- Getting rid of CO2 is not a problem for water breathers because if is more soluble in water than O2.
48.2 What Adaptations Maximize Gas Exchange? (1028)
Respiratory Organs Have Large Surface Areas (1028)
- Larger surface areas = greater rate of gas exchange/diffusion.
- External gills provide a large surface area for gas exchange with water. They minimize the path length (L) (i.e. larval
- Internal gills are similar to external gills but have protective body cavities (i.e. fish).
- Lungs are the internal cavities for respiratory has exchange with air. They have a large surface area because they are
highly divided, and they are elastic so that they can be inflated with air and deflated.
- Most abundant air-breathing invertebrate are insects which have a gas exchange system consisting of a network of air-
filled tubes called tracheae that branch through all tissues of the insect’s body.
*Look at Lecture notes for ventilation and perfusion*
- Minimizing the path length, higher surface area, and low volume is good for gas exchange/diffusion.
- An animal’s gas exchange system is made up of its gas exchange surfaces and the mechanisms it uses to ventilate and
perfuse those surfaces.
Insects Have Airways Throughout their Bodies (1028)
- Respiratory gases diffuse through air most of the way to and from every cell in insects.
- Spiracles are gated opening in which the insect repiratiory system communicates with the outside environment. Spiracles
can open to allow gas exchange and close to decrease water loss.
- Spiracles extend and become smaller and smaller – from spiracles to tracheae, tracheoles, and then air capillaries.
Fish Gills Use Countercurrent Flow to Maximize Gas Exchange (1029)
- In fish, water flow unidirectionally into the fish’s mouth, over the gills, and out forom under the opercular flaps. The
constant, one-way flow of water moving over the gills maximizes P02 on the external gills surfaces. Gills have a large
surface area for gas exchange because they are so highly divided.
- The lamellae which cover the gill filaments are the actual gas exchange surfaces, and they minimize L.
- Flow of blood perfusing the inner surfaces of the lamellae is unidirectional as well.
- Affarent blood vessels bring blood to the gills, and efferent blood vessels do the opposite.
- The blood flow of the lamellae is opposite to the flow of water over the gills – countercurrent flow – optimizes PO2
gradient between water and blood.
Birds Use Unidirectional Ventilation to maximize Gas Exchange (1030)
- Air flows through the lungs in the parabronchi and diffuse into the air capillaries, which are the gas exchange surfaces –
provide a large surface area because they are so numerous. Birds take two breaths instead of one and their non-ventilated
volume (dead space) is less; they do not as much extra air left after breathing unlike mammals.
Tidal Ventilation Produces Dead Space that Limits Gas Exchange Efficiency (1031)
- Lungs’ structures have evolved, but still remain dead-end sacs in all air-breathing vertebrates except birds. Ventilation
cannot be constant and unidirectional because lungs are dead-end sacs, but must be tidal: air flows in and exhaled gases
flow out by the same route.
- The residual air in the lungs after exhalation represents dead space.
- Spirometer is a device that measures the volumes of air that a person breathes in or breathes out.
- Tidal volume: Amount of air that moves in and out per breath when we are at rest.
- Inspiratory Reserve Volume: the additional volume of air we can take in above normal tidal volume.
- Expiratory Reserve Volume: The extra air that we can forcefully breathe out after normal exhalation.
- Vital Capacity: Tidal volume + inspiratory reserve volume + expiratory reserve volume. Vital capacity decreases with
age and is greater in an athlete than in a non-athlete.
- Even after the deepest exhalation, there is still some air in the dead space; vital capacity is not the entire lung volume.
The total lung capacity is the residual volume + vital capacity.
- Tidal breathing limits the partial pressure gradient available to drive the diffusion of oxygen from air into the blood.
- Fresh air is not moving into the lungs during part of the breathing cycle; therefore, the average PO2 of air in the lungs is
considerably less than it is outside the lungs.
48.3 How Do Humans Lungs Work? (1032)
- Air enters the lungs through the oral cavity or nasal passage, which join together in the pharynx. Below the pharynx, the
esophagus conducts food to the stomach, and the trachea leads to the larynx (“voice box”; has vocal cords and is the
- The trachea branches into two bronchi, one leading to each lung. After four branching of the bronchi, the cartilage
supports disappear, marking the transition to bronchioles.
- After 16 branches of the bronchioles, they are now air sacs called alveoli – sites of gas exchange.
- Conducting bronchioles are the bronchioles before the appearance of alveoli and respiratory bronchioles are ones that
- Because airways conduct air only to and from the alveoli and do not themselves participate in gas exchange, their volume
is dead space.
- Network of capillaries surrounds the alveoli.
Lungs are Ventilated by Pressure Changes in the Thoracic Cavity (1034)
- Human lungs are suspended in the thoracic cavity, a closed body compartment bounded on the bottom by a sheet of
muscle called the diaphragm.
- The right and left lungs are covered by a pleural membrane that also lines the thoracic cavity adjacent to the lung.
- Space between the pleura lining the lung and that lining the thoracic cavity is called the pleural space.
- In a functional lung, there is no real space between the pleural membranes; there is only a thin film of fluid that
lubricated the inner surfaces of the pleura so they can slide against each other.
- Breathing involves changes in the volume of the thoracic cavity.
- Because the pleural membranes are attached to the walls of the thoracic cavity, and because the pleural space is a closed
compartment, any attempt to increase the volume of the thoracic cavity creates a subatmospheric pressure (referred to as
negative pressure) inside the pleural space.
- Even between breaths, there is normally a slight negative pressure in the pleural space because the rib cage is pulling
outward and the elasticity of the lung tissue is pulling inward.
- If the thoracic cavity is punctured (i.e. knife wound) air leaks into the pleural space, the negative pressure in that closed
space is lost, and the lung collapses. If the wound is not sealed, breathing movements pull air into the pleural space rather
than into the lung, and there is no ventilation of the alveoli in that lung.
- In inhalation, the diaphragm contracts, it pulls down, expanding the thoracic cavity and pulling on the pleural
membranes. Air rushes through the trachea from the outside and lungs expand.
- Exhalation occurs when the contraction of the diaphragm ceases; the diaphragm relaxes, the elastic recoil of the lung
tissues pull the diaphragm up and pushes air out through the airways. When a person is at rest, inhalation is an active
process and exhalation is a passive process.
- Between the ribs are two sets of intercostals muscles. The external intercostals muscles expand the thoracic cavity by
lifting the ribs up and outward. The internal intercostals muscles decrease the volume of the thoracic cavity by pulling the
ribs down and inward. During strenuous exercise, the external intercostals muscles increase the volume of air inhaled,
making use of the inspiratory reserve volume, and the internal intercostals muscles increase the amount of air exhaled,
making use of the expiratory reserve volume.
48.4 How Does Blood Transport Respiratory Gases? (1036)
- Perfusion of the lungs is one of the functions of the circulatory system. The circulatory system uses a pump (the heart)
and a network of vessels to transport extracellular fluids and associated cells (blood) around the body.
Hemoglobin Combines Reversibly with Oxygen (1036)
- Red blood cells contain a lot of hemoglobin molecules – a protein consisting of four polypeptide subunits.
- Each polypeptide surrounds a heme group – an iron-containing ring structure that can reversibly bind to a molecule of
- As oxygen diffuses into the red blood cells, it binds to hemoglobin; it cannot diffuse back across red cell plasma
membrane once oxygen is bound.
- Ability of hemoglobin to pick up or release oxygen depends on the partial pressure of oxygen of its environment (the
higher it is, a molecule of hemoglobin can carry more oxygen (max. load is four oxygen molecules).
- Positive Cooperativity is the influence of the binding of O2 by one subunit on the oxygen affinity of the other subunit.