Lecture 19
Gas exchange and circulation in fishes
Open and closed circulatory systems: fish and insect
Gas exchange: acquiring oxygen and getting rid of carbon dioxide
Smaller animals can exchange these gases at the body surface by diffusion and
convection because of a favourable surface to volume ratio.
For larger multicellular animals there are special regions that serve for gas
exchange: gills or lungs.
Particulate feeders (e.g., amphioxus, sabellid worms) already expose a large
surface area to oxygen-rich water; for gas exchange they need only deploy the
circulatory system: capillaries, networks.
Distinction between lung and a gill
Lung: any specially adapted respiratory chamber whose walls are highly vascularized
and form the primary region for exchange of respiratory gases (uptake of oxygen,
release of carbon dioxide). A collection of invaginations providing a large surface area
for gas exchange with a circulatory system fluid (blood).
Gill: the same but ‘outpocketed’, an outgrowth of the body wall. The parapodia of
Nereis are gills as well as paddles. And they are serviced by efferent and afferent
vessels. Efferent: away from parapodium; afferent: toward parapodium. Fish a
recessed outpocketing.
 There must for larger
animals, be a close
association with the
circulatory system: fine
capillaries forming a
plexus or network --increasing the surface
area to allow for pickup
of oxygen from water
(or air) and for the loss
of carbon dioxide.
Tracheal gills of a dragonfly
Structural designs for gas exchange
1. Keep diffusion paths short.
2. Circulate the fluid (more important if the fluid is water than air because oxygen has a
relatively low solubility in water compared to air) and maintain the steepest possible
diffusion gradient
3. Maximize surface area (trade-off with dessication: water loss: important in terrestrial
4. Utilize counter-current flow of the external medium that is the oxygen /carbon
dioxide source and the internal medium that is the oxygen/carbon dioxide transport.
Gill ventilation in fishes
Oxford Illustrated
Science Encyclopedia
The gills of bony fishes are located in the
pharyngeal cavity in a branchial
chamber, covered on the outside by the
operculum. Inside the branchial chamber
thin epidermis is folded into plates called
lamellae grouped on filaments. The
filaments are arranged along a gill arch
(four arches on each side).
Unidirectional flow of water in fishes is generated
by a buccal force pump combined with an
opercular suction pump. These two pumping
actions operate out of phase, to achieve a near
continuous flow of water. (Like the phase
differences in the cycles of the three muscular
fans of Chaetopterus that also improve flow
efficiency within its burrow.) The water enters the
fish’s mouth, passes through the buccal cavity,
then is forced out through gill slits in the gut wall
into a branchial cavity containing gills. The
branchial cavity is covered with an operculum and
the water exits posteriorly between the edges of
the operculum and the body. This unidirectional
flow of water over the gills is maintained by both
types of pump.
There are two ways of
passing water by blood:
same direction parallel
current or in opposite
directions; the latter is
counter-current . It is far
more efficient to use
counter-current because
diffusion gradients are
maintained. Parallel flows
will very soon reach
saturation and diffusion
will stop. The same
principle applies to heat
Circulation systems
Circulatory systems are closed or open.
An example of a closed circulatory system is that of a fish: a two-chambered heart
creates high pressure and relatively high flow rate within vessels, arteries and veins,
through which blood circulates.
An example of an open circulatory system is that of an insect: a muscular dorsally
situated tube (heart) pumps fluid anteriorly and sits in the blood it pumps which
circulates at low pressure within large body spaces.
Open systems function to transport gases. In the case of the insect gas exchange is
accomplished by a tracheal system.
Highly muscular regions form
chambers: hearts

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