Gas Exchange: Animals
Cellular Respiration
• All living things obtain the energy they
need by metabolizing energy-rich
compounds, such as carbohydrates and
fats
• In most organisms, this metabolism takes
place by respiration, a process that
requires oxygen (and produces carbon
dioxide, which must be removed from the
body in animals)
Cellular Respiration
• Cellular respiration is the process by which
animals and other organisms obtain the energy
available in carbohydrates
• Cells take the carbohydrates into their cytoplasm
where, through a series of metabolic reactions, it
is broken down into ATP
– O2 is the oxidizing agent (electron acceptor) in plants
and animals (aerobic)
– Bacteria and Archaea use inorganic molecules such
as sulfur, methane, iron, and metal ions (anaerobic)
Gas Exchange Across Respiratory
Surfaces
• Gas exchange involves diffusion across
membranes
• The external environment in gas exchange
is always aqeuous
• Diffusion is passive; driven by the
difference in O2 and CO2 on the two sides
of the membrane
• The rate of diffusion is governed by Fick’s
law of diffusion
Fick’s Law of Diffusion
D A Dp
R=
d
R = rate of diffusion
D = diffusion constant (molecule specific)
A = area over which diffusion occurs
Dp = pressure difference between two sides
d = distance over which diffusion occurs
Gas Exchange Across Respiratory
Surfaces
• The rate of diffusion can be optimized by
– Increasing surface area
– Decreasing the distance over which diffusion
occurs
– Increasing the concentration difference
D A Dp
R=
d
The evolution of respiratory systems has involved
changes in all of these factors
Maximization of Gas Diffusion
• The levels of O2 needed for cellular
respiration cannot be obtained by diffusion
alone over distances >0.5mm
• Multicellular organisms require structural
adaptations to enhance gas exchange
– Increasing pressure difference
– Increasing area and decreasing distance
Maximization of Gas Diffusion
• Increasing pressure difference, Δp – many
organisms create a water current that
continuously replaces the water over the
respiratory surfaces (the part of an
organism over which gases are
exchanged with the environment)
– Cilia often used to produce this current
– Because of the continuous replenishment of
water, the external oxygen concentration does
not decrease along the diffusion pathway
Maximization of Gas Diffusion
• Increasing area and decreasing distance –
Vertebrates (and more complex
invertebrates) possess respiratory organs
that increase the surface area available for
diffusion
– Gills, tracheae, and lungs
– These adaptations bring the external
environment (air or water) close to the internal
fluid such as blood or hemolymph, which is
circulated throughout the body
Maximization of Gas Diffusion
Maximization of Gas Diffusion
Maximization of Gas Diffusion
H20
http://life.bio.sunysb.edu/marinebio/o2countercurrent.jpg
• Large surface area, high blood flow
• Countercurrent exchange – deoxygenated blood flows in
one direction, while oxygenated blood flows in the other;
maintains a concentration gradient
Countercurrent gas exchange
Gills
• Gills are specialized extensions of tissue
that project into water
– External gills are not enclosed within body
structures; many fish and amphibian larvae
– External gills require constant movement to
ensure contact with fresh (high O2) water
axolotl
Gills
• Other types of aquatic animals evolved
branchial chambers, which provide a
means of pumping water past stationary
(internal) gills
– Mantle cavity of mollusks – contraction of
muscular walls of cavity draws water in
towards gills (and expels it)
– Branchial chamber of crustaceans –
located between body and exoskeleton,
with an opening at a limb; movement of
limb draws water through chamber and
over gills
Gills
• In bony fishes, the gills are located
between the oral cavity and the opercular
cavities
• These two cavities operate as pumps that
alternately expand
– Water is moved into the mouth, through the
gills and out of the fish through the open
operculum, or gill cover
Gills
Gas exchange in fish
• Mobile fish (such as tuna) swim with their
mouth open to continuously move water
passed the gills (Ram ventilation)
• Most bony fish use pumping action to
ventilate; some can alternate
Gas exchange in fish
• Bony fish have four gill arches on each
side of their heads
• Each gill arch is composed of two rows of
gill filaments, which consist of lamellae,
thin membranous plates that project out
into the flow of water
– Water flows past the lamellae in one
direction only; blood flows opposite to this
direction (countercurrent gas exchange)
High
O2
Low
O2
Gas exchange in fish
• Most cartilaginous fish swim constantly
• Others must pump H2O across gills
• Sand tiger sharks
and nurse sharks
alternate between
pumping and RAM
- spiracles
- 5 gills; 6-7 in
more primitive sp.
http://www.shark-info.com/images/respiration.jpg
Cutaneous Respiration
• O2 and CO2 are able to diffuse across
cutaneous (skin) membranes in some
vertebrates (amphibians and turtles)
• Requires constant moisture
• Supplementary to lung respiration; only a
few species rely on cutaneous respiration
exclusively
• Many turtles can respire underwater in this
fashion, while some are capable of cloacal
respiration, especially during hibernation
Tracheal systems
• Tracheal systems are found in arthropods
• No single respiratory organ
• Respiratory system consists of small,
branched trachae, or air ducts, which
branch into tracheoles, a series of tubes
which transfer gases directly across
cellular membranes
• Air enters into trachea through spiracles
– In most species, can be open and closed
Lungs
• Lungs replace gills in terrestrial animals
– Air is less (structurally) supportive than water
• Unlike gills, internal air passages such as trachea
and lungs can remain open because the body
provides the necessary structural support
– Water evaporates
• Terrestrial organisms constantly lose water to the
atmosphere; gills would provide an enormous
surface area for water loss
– The lung minimizes evaporation by moving air
through an internal tubular passage
Lungs
• Air drawn into the respiratory passages
becomes saturated with water vapor prior
to reaching the inner region of the lungs
• A thin, wet membrane permits gas
exchange
• A two-way flow system (gases move into
and out of lungs through same airway
passages)
Lungs
• Air contains a constant proportion of gases
– 78.09% nitrogen
– 20.95% oxygen
– 0.93% argon and other inert gases
– 0.03% carbon dioxide
• Because of gravity, air exerts a downward
pressure (atmospheric pressure; 760mm Hg)
• The pressure contributed by each gas is
called its partial pressure
Lungs of Amphibians
• The lungs of amphibians are saclike
outpouchings of the gut
• Surface area increased by folds
• Amphibians breathe by forcing air into
their lungs: positive pressure breathing
– They fill their oral cavity with air, close their
mouth and nostrils, and then elevate the floor
of their oral cavity; this pushes air into their
lungs in the same way that a pressurized tank
is used to fill balloons
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Nostrils
open
External
nostril
Air
Buccal cavity
Esophagus
Lungs
a.
Nostrils
closed
Air
b.
Lungs of Reptiles
• Terrestrial reptiles have dry, scaly skins
which prevent cutaneous respiration
• Reptiles expand their rib cages by
muscular contraction; this creates a lower
pressure inside the lungs compared to the
atmosphere, which moves air into the
lungs: negative pressure breathing
• Reptilian lungs have more surface area
than amphibians
Lungs of Mammals
• Endothermic (“warm-blooded”) animals,
such as birds and mammals, require more
efficient respiratory systems than
ectothermic (“cold-blooded”) animals due
to their increased metabolic demands
• The lungs of mammals are packed with
millions of alveoli, sites of gas exchange
– Provides lung with enormous surface area for
gas exchange
Lungs of Mammals
• The alveolus (singular) is composed of
epithelium only 1 cell thick, and is
surrounded by capillaries with walls that
are also only 1 cell layer thick
• The distance across which gas must
diffuse is very small, only 0.5-1.5μm
D A Dp
R=
d
Maximization of Gas Diffusion
Lungs of Mammals
• Inhaled air is taken in through the mouth
or nose, past the pharynx to the larynx
(the voice box), where it passes through
an opening in the vocal cords, the glottis,
into the trachea, a tube supported by Cshaped rings of cartilage
• The trachea bifurcates into right and left
bronchi, which each enter into their
respective lung, and further divide into
bronchioles that deliver air to the alveoli
Lungs of Mammals
Lungs of Mammals
• Alveoli are surrounded by an extensive
capillary network
• Gas exchange between the air and blood
occurs across the thin walls of the alveoli
• Red blood cells pass through capillaries in
single file; O2 from alveoli enters the red
blood cells and binds to hemoglobin
• Surface area of respiratory system is
greatly enhanced; much more than
amphibians and reptiles
Lungs of Birds
• Most efficient respiration of all terrestrial
vertebrates
• Gas exchange occurs in parabronchi
• Air flows through the parabronchi in one
direction only
• In other terrestrial vertebrates, inhaled
(fresh) air is mixed with O2-depleted air
from the previous breath
• In birds, the unidirectional flow allows only
fresh air to enter the site of gas exchange
Lungs of Birds
• Respiration in birds
occurs in 2 cycles:
– 1. Inhaled air is drawn
in from the trachea into
posterior air sacs, and
exhaled into lungs
– 2. Air is drawn in from
the lungs into anterior
air sacs, and exhaled
through the trachea
http://people.eku.edu/ritchisong/birdrespiration.html
Lungs of Birds
Red =
inhaled air
Lungs of Birds
• Blood runs 90° to the air flow
– Crosscurrent flow
– Not as efficient as countercurrent, but greater
capacity to extract O2 from the air than a
mammalian lung
– Enables birds (which fly, by the way) to
respire efficiently at altitudes of 6000 meters
• Bar-headed geese can fly over Mt. Everest (29,028
feet)
Mechanisms of Gas Exchange
• Gas exchange is driven by differences in
partial pressures
• Blood returning from circulation is depleted
in O2 and has a partial O2 pressure (PO2)
of ~40 mm Hg
• The gas mixture in the alveoli is ~105 mm
Hg
• Because of the difference in pressures
(Δp), oxygen moves into the blood
Mechanisms of Gas Exchange
• The diaphragm is a sheet of muscle
extending across the bottom of the ribcage
• The diaphragm separates the thoracic
cavity from the abdominal cavity
• During inhalation, the diaphragm
contracts, causing the diaphragm to lower
and assume a more flattened shape
– This expands the volume of the thorax and
lungs, produces negative pressure which
draws air into the lungs
Mechanisms of Gas Exchange
• If breathing is insufficient to maintain
normal blood gas measurements (PCO2 &
PO2), hypoventilation occurs
• If breathing is excessive, PCO2 is
abnormally low, and hyperventilation
occurs (why you should blow into a brown
bag to stop hyperventilating)
• The maximum amount of air that can be
exhaled forcefully is the vital capacity
– 4.6L in men; 3.1L in women
Mechanisms of Gas Exchange
• Breathing is involuntary and is under
nervous system control
• Neurons stimulate the diaphragm and
external intercostal muscles to contract,
causing inhalation
• O2 is transported by respiratory pigments
– Bound to hemoglobin inside red blood cells
(all vertebrates, most inverts), or hemocyanin
in the plasma (arthropods and some
molluscs)
Mechanisms of Gas Exchange
• Oxygen has a low solubility; blood plasma
can only contain a maximum of 3mL O2
per liter
• However, whole blood contains ~200mL
O2 per liter since most of the O2 in the
blood is bound to hemoglobin
• Hemoglobin is a protein composed of 4
polypeptide chains and 4 heme groups
– In the center of each heme group is an atom
of iron, which can bind to the O2 molecule
Hemoglobin
Hemoglobin
• Hemoglobin acquires O2 in the alveolar
capillaries
– O2-bound hemoglobin (oxyhemoglobin)
appears bright red
– Hemoglobin without O2 (deoxyhemoglobin)
appears dark red, but has a bluish hue in
tissues
– Hemoglobin provides an oxygen reserve
• Only 1/5 of oxygen is released to muscles by
oxyhemoglobin; the reminder serves as a reserve
during physical exertion, and ensures enough O2
to maintain life for 4-5 minutes if breathing is
interrupted or the heart stops
Mechanisms of Gas Exchange
• CO2 is transported by hemoglobin (bound
to protein portion) and dissolved in plasma
and red blood cells as bicarbonate, HCO3+
• Because CO2 binds to the protein portion
and not to the heme group, it does not
compete with O2 molecules, but it does,
however, change the shape of
hemoglobin, reducing its affinity for O2
Mechanisms of Gas Exchange
• Removal of CO2 into the alveoli occurs
because of the lower PCO2 of the gas
mixture inside the alveoli
• Hemoglobin transports other dissolved
gases, including carbon monoxide, CO
• Carbon monoxide binds strongly to the
iron atom in the heme group preventing
oxygen from binding with hemoglobin
Thank you for not smoking
Cancer smoking lung cancer correlation from NIH.svg
• Lung cancer is
the #1 cancer
killer
• Caused
mainly by
cigarette
smoking
1900 1920 1940 1960 1980
Gas Exchange in Plants
• More than 90% of the water taken up by
the roots of a plant are lost to evaporation
• However, photosynthesis requires large
amounts of CO2 from the atmosphere
• Plants must therefore balance their need
to minimize water losses and the need to
admit CO2
• The stomata and cuticle have evolved in
response to one or both of these
requirements
Gas Exchange in Plants
• Transpiration (evaporation of water from
the leaves) decreases at night, when the
vapor pressure gradient between the leaf
and the atmosphere is less
• Closing the stomata will reduce water loss;
but limit CO2 uptake
Gas Exchange in Plants
• The stomata of plants are surrounded by
two sausage-shaped guard cells
• Guard cells are distinctive because of their
cell wall construction: thicker on the inside
and thinner elsewhere
– This results in bulging out and bowing when
they become turgid
• Guard cells regulate the opening and
closing of stomata
Guard Cells & Stomata
• Turgor in guard cells results from the
active uptake (requires ATP) of K+, Cl-,
and malic acid (organic compound)
– As solute concentration increases, water
potential decreases in the guard cells, and
water enters osmotically
– Guard cells accumulate water, becoming
turgid
• Opens stomata
Stomata open (turgid guard cells)
Stomata closed (flaccid guard cells)
Stomata & Guard Cells
Stomata and Guard Cells
• Guard cells of most plants regularly
become turgid in the day (stomata open),
when photosynthesis occurs, and become
flaccid at night (stomata closed),
regardless of the availability of water
• Guard cells are also unique in that they
possess chloroplasts (the only epidermal
cells to do so)
– The active pumping of sucrose out of guard
cells in the evening leads to a loss of turgor
and stomata closing
Gas Exchange in Plants
• Transpiration rates increase with temperature
and wind velocity because both conditions
cause water molecules to evaporate more
readily
• CO2 concentration, light and temperature can
influence stomatal opening
– When CO2 concentrations are high, guard cells are
triggered to decrease the opening (conserves H2O)
– Blue light triggers H+ transport, opening K+ channels
and stimulating the opening of stomata (facilitates
evaporative cooling)
Gas Exchange in Plants
• Stomata frequently close when the
temperature exceeds 30-34°C (~90-95°F)
• Stomata also close when water conditions
are unfavorable
– Under intense heat, stomata open when it is
dark and temperature has dropped
– Some plants collect CO2 at night and utilize it
in photosynthesis during the day
• Cacti take in CO2 at night, and store it in organic
compounds, which are converted back to CO2
during the day (when stomata are closed to
prevent evaporation)