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The Respiratory & Circulatory
Systems
Ch. 11-12
Introduction
• To metabolize effectively and survive, cells
within the body must replenish the oxygen
they consume and remove the carbon dioxide
they produce.
• These are the responsibility of the respiratory
and circulatory systems.
Introduction
• Most of the body’s energy needs are met by
cellular oxidation of ingested foods.
– In small animals the oxygen needed for this and the
carbon dioxide produced can be carried between the
external environment and the cells by diffusion, but
this is a slow process and efficient only over short
distances.
• Diffusion still plays a role in the movement of
gasses in larger animals, but a system is needed
for the bulk flow, or movement by a muscular
pump, of the medium containing the gases.
• Some sort of a pump must move the medium
containing the gases across a thin, moist and
vascular membrane through which the gases can
diffuse and be exchanged with those in the
blood.
– A membrane of this type is called a respiratory
membrane.
• Another pump, the heart, must move the blood
through vessels that extend between the
respiratory membrane and the vicinity of the
cells, where diffusion again takes place.
• An efficient respiratory system optimizes the
diffusion and exchange of gases between the
body and external environment, but many factors
affect the nature of the respiratory membrane
and the way it is ventilated.
• High diffusion rates require a large surface
area in the respiratory membrane and a short
diffusion distance between the external
medium and the blood.
– Flow rates of the medium across the membrane,
and of blood, must be slow enough for diffusion
to occur; but fast enough to maintain the
concentration gradients of the gases in the 2
mediums.
• Ventilation rates or the amount of membrane
in use, or both, are adjusted so that gas
exchange matches the needs of the animal as
the activity level varies.
• The quantity of gas in the water or air also
affects the size of the membrane and its rate
of ventilation.
• The density and viscosity of the medium affect
the amount of energy required to move it
across the respiratory membrane.
• The movements of other diffusible molecules,
such as water, salts, and nitrogenous wastes
across the membrane may need to be reduced
or enhanced.
• Body size is an important factor because of its
effect on surface-volume relationships.
The respiratory System of Fishes
• A fish’s respiratory system must be adapted to 2
major constraints of life in water.
• First, the amount of O2 dissolved in water is much
less than in air.
• Second, water is much denser and more viscous
than air.
– Because of these two constraints, a fish must have a
rather large respiratory surface and move large
volumes of water across it at a considerable energy
expense.
• Any design feature of the respiratory system that
reduces these energetic costs would be to the
fish’s advantage.
• Unloading of CO2 is less of a problem because it is
highly soluble in water.
– It does combine with water to form carbonic acid, but
most fishes live in a large enough body of neutral (pH
7) water that this is of no consequence.
• Several other problems arise from the close
proximity of blood and water across the
respiratory membrane.
– Heat is quickly exchanged
– Water, salts, and nitrogenous waste also diffuse across
the membrane.
Gills
• As ancestral craniates became larger and more
active gills, which provide a large surface area,
evolved where a water current could ventilate
them.
– The larvae of many fishes have external gills, which
are highly vascularized, filamentous processes with
large surface areas attached to the lateral surface of
the head between certain gill slits.
– Adult fishes have internal gills, which consist of a large
number of vascularized plates, the primary gill
lamellae, attached to the walls of the gill or brachial
pouches or to the gill arches.
The Structure and Development of
Internal Gills
• The pharyngeal pouches develop from a lateral portion
of the archenteron that will become the pharynx.
– The plates of tissues between successive pouches, to
which the primary gill lamellae attach, are called
interbranchial septa; a skeletal visceral arch lies in each
septa.
• Skeletal, supporting gill rays usually extend peripherally
from the visceral arches into the interbranchial septa
or primary gill lamellae.
– Muscles and nerves are associated with the skeletal
elements
• Each of the first 6 septa of a jawed fish contains an
embryonic artery, the aortic arch, that supplies the gill.
• Gill pouches were numerous in primitive fishes,
some had up to 15 pairs.
– Most chondrichthyans and early bony fish have 6 pairs
of pouches, the first is reduced to a pair of spiracles.
– Teleosts have lost the spiracle and now have only 5
pair of gill pouches.
• Interbranchial septa that bear gill lamellae on
both surfaces constitute a complete gill, or
holobranch.
– Jawed fishes usually have 4 of these
• Only a small, gill-like structure lies in the spiracle,
because it receives oxygenated blood from other
gills it is called the pseudobranch.
• Gill lamellae are seldom present on the posterior
surface of the last gill pouch, because no aortic
arteries exist here to supply them
Lamprey Gills
• Internal gills are arranged in different ways
among fishes, and different patterns of
ventilation exist.
• Jawless fishes have large, saccular branchial
pouches that are lined with primary gill
lamellae.
– These are called pouched gills.
• Water is drawn into the pharynx through the mouth,
enters the branchial pouches through pore shaped
internal gill slits, and exits the pouches through pore
shaped external gill slits.
Elasmobranch Gills
• The branchial pouches of chondrichthyan fishes
are narrow chambers, and the gill lamellae are
borne on the interbranchial septa, which
continue to the body surface.
– These are septal gills.
• A vertically elongated internal gill slit leads from
the pharynx into each branchial chamber.
• Gill rakers at the base of the interbranchial septa
keep food in the pharynx.
• Blood and water flow in opposite directions
through and across the secondary lamellae.
– This countercurrent flow affords a considerably
more efficient gas exchange than blood and water
moving in the same direction, concurrently.
Gills of Bony Fishes
• Bony fishes developed a branchial apparatus that
is somewhat different from that of
elasmobranchs.
– A flap of body wall supported by bones, known as the
operculum, extends from the hyoid arch region of the
head laterally and caudally over the gills.
– There is a large, common opercular cavity for all the
gills, and one valved, external gill slit.
– The interbranchial septa are reduced, so the primary
gill lamellae extend freely into the opercular cavity.
• The gills are described a aseptal.
• With the emergence of the opercular system,
teleosts have a respiratory cycle that produces
a continuous flow of water in one direction
from the oropharynx to the opercular cavity
over the gills.
– The cycle constitutes 2 phases; the suction pump
stage and force pump stage.
• This produces an uninterrupted unidirectional
flow of respiratory water that runs in the
opposite direction of the blood flow on an
optimized countercurrent system.
Accessory Respiratory Organs
• The gills of fishes are efficient respiratory organs that
remove 80-90% of the available O2 from the water.
• But in some situations the gills are not enough to meet
the respiratory demands of the fish.
– O2 levels can be very low in shallow, warm pools or in
swamps where the decay of vegetation decreases O2
levels.
• Many teleost fishes can supplement gill respiration
with accessory respiratory organs.
– Some have even become obligate air breathers
– Virtually all bimodal breathers retain gills and
simultaneously evolve a special air breathing organ.
Bimodal Breathers
• Some use vascular skin as a respiratory
surface.
• Others possess modifications of many parts of
the gut: the lining of the mouth, pharynx,
esophagus, intestine, and rectum.
• There are also those that develop an
outpocketing above the gills to form a
suprabranchial air chamber, which can be
filled with air and functions like a lung.
Lungs
• Many bony fishes have either lungs or swim bladders.
– The organ is most lung-like in the primitive members of the
group, so lungs seem to have appeared early in the
evolution of fishes.
• Actinopterygian fishes ventilate the lung with a
characteristic 4 stroke buccal pump:
– The buccal cavity expands so that spent air can pass into it.
– Once filled, it compresses to expel the air.
– The empty buccal cavity then expands again to take in
fresh air
– Finally it compresses to force the air into the lungs.
• Lungs develop embryologically in lungfishes
and amphibians as a ventral evagination of the
floor of the digestive tract just caudal to the
last pair of pharyngeal pouches
• Suggests a homology between these structures.
• All vertebrates with lungs have specialized
cells that produce a surface film of lipoprotein
known as surfactant.
– The composition of the surfactant is such that it
acts more as an “anti-glue,” preventing adhesion
of adjacent epithelial surfaces during low lung
volume.
• When breathing air lungfishes come to the
surface and inhales fresh air by expanding the
buccal cavity.
• The lungfish’s ventilation pattern is typically a
2 stroke pump, the buccal cavity is expanded
and compressed only once.
– Because mixed air is delivered to the lung, this
pattern is called the mixed-air buccal pump
system.
• Lungfishes hold air in the lungs for a
considerable time as the O2 is slowly used.
– Long periods of breath holding, or apnea,
alternate with short periods of lung ventilation.
Swim Bladders
• The lungs of actinopterygian fishes have
transformed into swim bladders in
neopterygian species and coelacanths.
– Neopterygian fishes live in O2 rich waters where
evolution would favor the conversion of lungs into
a hydrostatic organ that would endow the fishes
with neutral buoyancy.
• The swim bladder of primitive actinopterygian
fishes continues to be an important site of
respiratory exchange, but it has also acquired a
hydrostatic function.
• In a teleost it is primarily a hydrostat that can be
regulated, allowing the fish to attain neutral
buoyancy and maintain position in the water with
minimal energy expense.
• Problems arise when fish change depth.
– As it goes deeper the increased pressure will
compress the swim bladder making the fish sink
faster.
– As the fish rises the bladder would expand, so air
must be removed to maintain neutral buoyancy.
• The gas in the swim bladder is 80% O2 and it is
secreted into the swim bladder by a gas gland
on one surface of the organ.
– Often gas must be excreted against considerable
concentration gradients because pressure
increases rapidly at depth.
• Oxygen is kept in the swim bladder by a rete
mirabile
– A set of long, parallel capillaries located just
before the gas gland; together these form the red
body.
Respiration in Early Tetrapods
• Because dry air at sea level contains 210mL/L O2,
adult terrestrial vertebrates have far more oxygen
available to them than do fishes, but they need a
moist respiratory membrane because O2 must be
in solution to diffuse into the blood.
• Major problems for terrestrial verts are:
1.
how to expose and ventilate the respiratory organ
without an excessive loss of body water, and
2. how to prevent the collapse of the respiratory organ
in air.
• Lungs, which were inherited from fishes, are well adapted to
meet these problems.
• Air within the lungs contains a great deal of
water vapor.
• The respiratory surface is kept moist without a
great deal of water loss because:
– the rate of ventilation is low,
– the air is conditioned by mucous glands in the
airways prior to entering the lungs,
– and not all the air in the lungs is exchanged in
each breathing cycle.
Amphibian Respiratory Organs
• Amphibian larvae are aquatic and possess gills.
• Salamanders retain external gills throughout their
larval period and neotonic species retain them as
adults.
• The external gills of young tadpoles become
covered by an opercular fold that opens to the
body surface through a single pore, the spiracle.
– The operculum is extensive, even covering the
developing forelimbs.
• Lungs develop and begin to function late in larval
life.
– Gills are lost at metamorphosis, the forelimbs push
through the operculum, and the remains of the
operculum fuse with the body wall.
• Air passes through the buccopharyngeal cavity
and enters the glottis.
– The glottis leads to a small, triangular laryngotracheal
chamber, from which the lungs emerge.
• All of these passages are lined by cilia and by
mucous secreting cells:
– these secretions keep air moist and trap dirt particles
and carry them away from the lungs.
• Lungs in frogs are sac shaped organs, in
salamanders they are elongated.
• Ventilation of the lung in amphibians resembles
that in lungfishes, except that air enters and
leaved through the nares and nasal cavity.
• The glottis then opens and air is expelled by
elastic recoil of the lungs and by the contraction
of flank muscles.
• As in lungfish, amphibians can tolerate long
periods of apnea alternated with short periods of
ventilation.
– Because their metabolic needs are low a single breath
of air can last an amphibian for several minutes.
• In addition to lungs, the thin, moist, vascular
skin of most amphibians acts as a respiratory
membrane and considerable cutaneous gas
exchange occurs.
– In species with both lungs and skin respiration,
lungs are more important in O2 uptake and the
skin is more important in the removal of CO2.
• Cutaneous gas exchange exposes amphibians
to considerable water loss.
– Amphibians mitigate this effect by living in moist
habitats or being most active in the more humid
early morning and evening hours.
The Reptile Respiratory System
• Neck length is longer is reptiles and the
primitive laryngotracheal chamber becomes
divided into a larynx and trachea.
• Cartilaginous rings or partial rings in the
tracheal wall keep the trachea open for the
free flow of air.
• The horny skin of most reptiles makes the skin
a poor respiratory surface and most gases are
exchanged in the lungs.
• The lungs of most reptiles are more
compartmentalized and larger than those of
amphibians.
• A wide, central bronchus leads from the
trachea into each lung, secondary bronchi
branch from it, and alveolar sacs of varying
size bud off them.
• The structure of the lung varies greatly among
different species and larger reptiles have
greater compartmentalization than do the
smaller reptiles.
• Reptiles use an aspiration pump; during
inhalation:
– Contraction of intercostal muscles enlarge the
pleuroperitoneal cavity in which the lungs lie.
– Pressure within the cavity decreases to below
atmospheric pressure and, as the nares are
opened, the lung expands and air is sucked into
them.
– The glottis is then closed and the air is held until
the next respiratory cycle.
• Long periods of apnea occur.
• Expiration occurs by the contraction of the rib
muscles and smooth muscles of the lung wall.
Respiration in Birds
• Birds need an exceptionally efficient and compact
respiratory system that will sustain a high level of
metabolism and not significantly increase weight.
• Bird lungs are relatively small organs that adhere
to the dorsal wall of the pleural cavity and do not
change appreciable in size during ventilation.
• The lungs are small, but connect to a system of
air sacs that pass among the viscera and even
extend into many of the bones, this gives them a
volume 2-3 times that of a comparable mammal.
• The walls of the air sacs are not highly
vascularized, so they do not participate in gas
exchange.
• Instead, the air sacs, together with a unique
pattern of airways within the lung, make
possible a unidirectional flow of air through
the lungs.
• This one-way flow carries relatively fresh air,
with more O2 and less CO2 than in animals
with a two way air flow.
• The morphology of the avian respiratory
system is complex.
– The numerous air sacs can be grouped by function
into a anterior and posterior set.
– The trachea bifurcates into a pair of primary
bronchi, each of which passes thorough the center
of the lung in a mesobronchus, which connects
with the posterior air sacs.
– Air in the posterior sacs returns through
mediodorsal bronchi, which connect with
thousands of small parabronchi.
– Innumerable short air capillaries bud off the
parabronchi and connect with adjacent air
capillaries
– The air capillaries are interwoven into a
respiratory labyrinth in which gas exchange
occurs.
• The lungs are ventilated by rocking
movements of the sternum, which alternately
expands and contracts the bellows-like air
sack.
• The glottis is held open most of the time,
because the bird lung is continually ventilated.
• Two separate inspirations and expirations are
required to fully ventilate the lung, and no
long periods of apnea occur.
• Blood in the vascular capillaries flows
transversely to the air flow in the parabronchi
and capillaries in a pattern called cross-current
flow.
• Because air and blood flow across each other
a gradient for the transfer of O2 exists at each
crossing of the parabranchi and vascular
capillaries.
• The net result of this system, as in
countercurrent flow, is that most of the
available O2 is removed from the inspired air.
Respiration in Mammals
• The evolution of the secondary palate in
mammals made it possible to separate food
and respiratory passages.
• The paired nasal cavities, which lie dorsal to
the hard palate, are relatively larger than
found in other verts.
• Each contains 3 folds, or scrolls of bone called
turbinates which increase their surface area.
• The mucous membranes that cover the
turbinates and line other respiratory passages
are very vascular, ciliated, and contains
mucous producing cells.
• Considerable conditioning of the air occurs as
it passes across these surfaces on its way to
the lungs.
– The air is warmed and moistened, and dirt is
trapped in the mucous and carried to the throat to
be swallowed or expectorated.
• The nasal cavities connect through the choanae
with the nasopharynx, which is separated from
the oropharynx by the soft palate.
• Food and air passages cross in the
laryngopharynx
– Food normally does not enter the respiratory passage
because the larynx and hyoid apparatus are pulled
forward to the base of the tongue during swallowing.
• A trough-like fold, the epiglottis, flips back over
the glottis or deflects food around it to the
esophagus.
• Except during swallowing, the glottis is held open
because the mammalian lung is constantly
ventilated.
• The trachea continues down the neck and gives
rise to the primary bronchi, which enter the lobes
of the lungs.
• The internal passages of the mammalian lung are
more compartmentalized than those of
amphibians and reptiles.
• The airways within the lung branch, and rebranch
at least 20 times, forming a respiratory tree that
terminates in the bronchioles, alveolar sacs, and
individual alveoli.
– The walls become progressively thinner at each
branch terminating in the alveoli with walls only a few
cells thick.
• A costal ventilation pump ventilates the
mammalian lung.
• Air is moved in and out through movement of the
pleural cavities, alternately increasing and
decreasing the pressure within the lungs relative
to atmospheric pressure.
• The increase in size occurs primarily through the
contraction of the diaphragm.
• During stronger inspiration, the intercostal
muscles pull the ribs rostrally and increase the
dimensions of the thorax.
• Expiration is largely a passive process that relies
of elastic recoil of the respiratory muscles.
Vertebrate Circulatory Systems
• The principal differences in the blood-vascular
system of the vertebrates involves the
separation of the heart into two separate
pumps as vertebrates evolved from an aquatic
life to a fully terrestrial life.
– In fishes blood circulates in a single circuit,
beginning at the heart, traveling to the gills, and
then continuing to the body.
– In terrestrial vertebrates there is a distinct division
of blood flow to the lungs (pulmonary respiration)
and to the body (systemic respiration.
Pumps
• An adequate circulatory system depends on one or
more pumps (hearts) and on channels or conduits in
which the blood can flow.
• By wrapping muscle around a tube or channel it is
possible to achieve a reduction of volume.
• Two different type of pumps can be designed this way:
peristaltic pumps, found mostly in invertebrates, and
chamber pumps.
• A chamber pump may have contractile walls
(vertebrate heart) or external pressure from body parts
can cause the reduction in volume (venous system of
the human leg).
Evolution of the Circulatory System
Patterns of Circulation
Fish Heart
• A fish heart contains two main chambers in
series, an atrium and a ventricle.
– The atrium is preceded by a enlarged chamber, the
sinus venosus, which collects blood from the venous
system and assures smooth delivery of blood.
• Elasmobranchs have a fourth chamber, the conus
arteriosus, which dampens blood pressure
oscillations before they reach the delicate blood
capillaries.
– Teleosts have a bulbous arteriosus that serves the
same function.
Evolution of the Heart with the
Evolution of the Lungs
• With the evolution of the lungs vertebrates
developed a high-pressure double circulation:
– A systemic circuit that provides blood to the body
organs.
– A pulmonary circuit that serves the lungs.
• The beginning of this major evolutionary
change probably resembled the condition
seen in the lungfishes and amphibians.
Terrestrial Vertebrate Heart
• In modern amphibians the atrium is
completely separated by a partition into two
atria.
– The right atrium receives venous blood from the
body while the left atrium receives oxygenated
blood from the lungs and skin.
• The ventricle is undivided but venous and
arterial blood remain mostly separated by a
spiral fold of the conus arteriosus.
• A septum partially separates the ventricle in
most reptiles.
Amphibian Heart
Terrestrial Vertebrate Hearts Cont.
• In crocodiles, birds, and mammals the septum
completely divides the ventricle, providing a
true separation of the arterial and venous
systems of circulation.
• Birds and mammals are endothermic and
require about 10 times more energy than
similar sized ectotherm.
– This means they require 10 times more fuel, food
and oxygen.
Mammalian Heart
Arteries
• All vessels leaving the heart are called arteries.
• To withstand the pressure generated during
ventricular systole (contraction) the largest
arteries close to the heart are invested with
large layers of elastic fibers, very little smooth
muscle, and tough inelastic connective tissue.
• Arteries further away from the heart possess
more smooth muscle and less elastic fibers.
Arteries
• All vessels leaving the heart are called arteries.
• To withstand the pressure generated during
ventricular systole (contraction) the largest
arteries close to the heart are invested with
large layers of elastic fibers, very little smooth
muscle, and tough inelastic connective tissue.
• Arteries further away from the heart possess
more smooth muscle and less elastic fibers.
Veins
• Veins into which capillary blood flows for its
return to the heart are thinner walled, less
elastic, and of considerably larger diameter
than their corresponding arteries.
• Venus blood pressure is very low and
therefore it must get assistance from valves in
the veins, body skeletal muscles, suction
created during diastole (expansion), and the
rhythmic action of the lungs.
Structural Relationship of Blood
Vessels
END