Download Gas Exchange Circulation - University of Toronto Mississauga

CIRCULATION GAS EXCHANGE
Source readings
Chapman, R.F. (1969) The Insects, Structure and Function, Chapters 23, 24. English Universities Press, London.
Liem, K.F. (1985) Ventilation, pp.185-209. In Hildebrand, M. et al. (Eds) Functional Vertebrate Morphology.
Belknap, Harvard Univ. Press, Cambridge, Mass.
Milsom, W.K. (1989) Mechanisms of ventilation in lower vertebrates: adaptations to respiratory and
non-respiratory constraints. Can. J. Zool. 67: 2943-.
Perry, S.F. & McDonald, G. (1993) Gas exchange, pp. 251-278. In Evans, D.H. (Ed.) The Physiology of Fishes.
CRC Press, Boca Raton, Fla.
Ramsay, J.A. (1 957) Physiological approach to the lower animals, Chapters 2,3. Cambridge Univ. Press,
Cambridge.
Gas exchange and transport: closed and open systems
Sufficiently inactive, small or thin-bodied animals (e.g., coral polyps, flatworms, Phylum Platyhelminthes)
can exchange respiratory gases (oxygen and carbon dioxide) directly to/from their tissues out of their
surroundings, because diffusion paths are always adequately short. Such animals have no need of a
high-pressure vascular circulatory system. But for active animals of substantial body thickness, the exchange of
oxygen and carbon dioxide (respiratory gases) usually involves such a system. Oxygen is taken up by blood
pigments which circulate under pressure within vessels driven by the contractions of one or more pumps (hearts).
As they move toward a gill or lung, afferent blood vessels branch repeatedly to form a plexus of fine capillaries,
so creating a larger surface area for more efficient gas exchange. Leaving this respiratory surface, the
capillaries anastomose (join together) to become a few large efferent vessels, conducting oxygenated blood
away to the body tissues.
Insect respiration and circulation systems: a separate gas exchange system
Most insects are active creatures whose tissues require that oxygen be supplied at a high rate; but their
circulatory system plays no role in the transport of respiratory gases. They have an open circulatory svstem in
which the blood moves mostly within large open spaces (sinuses) rather than vessels. A single muscular tube
(heart) lies within the haemolymph (insect blood) it pumps. This heart is located dorsad in the body, just above
a muscular, fenestrated, diaphragm which separates off a blood space called the pericardial sinus. As its name
indicates it is associated with the heart. Blood enters the heart via lateral (metameric) valved openings called
ostia. It is pumped fowvard discharging into a sinus in the head, then circulates rearward in perivisceral and
perineural blood sinuses.
Insects can be exceptions to the rule that active animals have closed circulation systems, because they
use a special system of interconnected branching tubes called tracheae to conduct air directly to their tissues.
In terrestrial insects these tubes open at the side of the animal through spiracles which can be closed or opened.
The tubes are strengthened by helical cuticular thickenings called taenidia. These thickenings are absent in
certain regions of the tracheal tubes, giving rise to tracheal sacs that can play a role in ventilation.
Immature stages of many insects (mosquitoes, dragonflies, mayflies, stoneflies, caddisflies) live in fresh
water. Some species have an open tracheal system: their spiracles are associated with a siphon which
protrudes through the water surface (e.g., mosquito larvae and pupae). Others have a closed tracheal system
(without spiracles), become secondarily adapted to function with cuticular tracheal gills: evaginations of
double-layered thin cuticle sandwiching finely branched tracheae (tracheoles) that absorb oxygen from water
while dissolving away Carbon Dioxide. Oxygen from the water diffuses through the gill cuticle thence into the
tracheal system, then moves around the body to the tissues. In the Anisoptera (a suborder of dragonflies)
tracheal gill plates project into the lumen of the final chamber of the gut, the rectum, and are known as rectal gills.
Their irrigation by water is associated with jet propulsion as mentioned previously.
Ventilation: pumping fluids
Ventilation is the name given to the process of moving water or air to, by and from respiratory (gas
exchange) surfaces: e.g. moving water over gills, moving air over lungs or air through tracheae. A pump is a
structure that alters the pressure in fluid and produces fluid flow. There are two possible pumping actions: 1)
force and 2) suction. A Force Pump is one that pushes or forces fluid ahead by reducing container volume and
increasing local pressure: a heart is an example of this sort of pump. (Heart structure in animals is diverse and
different hearts can occur at multiple locations within the same animal where fluid pressure needs enhancing.)
When sucking up liquid with a straw your mouth cavity acts as a Suction Pump: air is drawn from the
mouth by the expansion of your lungs, which drops the pressure in the mouth cavity and straw relative to the
ambient air pressure. So the fluid is drawn ahead by an increased volume and reduced local pressure. The
cibarial pump of a cicada is an example of a suction pump: this insect draws fluid (sap) up through its stylets
(mouthparts modified into a tube) by pulling farther apart the walls of an intemal space (cibarium) and so creating
a drop in relative pressure. It is much easier to force water than air, because of water’s relative
incompressibility. A suction pump can become a force pump simply by reversing the process. (Ultimately jet
propulsion is based upon force and suction pumping.)
The abdomen of a grasshopper functions in ventilation of the insect tracheal system by pumping air.
Expiration: intersegmental muscles contract and cause each segment to telescope into the one ahead,
increasing intemal blood pressure and collapsing the abdominal air sacs within. Telescoping abdominal
segments reduces the net volume of the tracheal system. During expiration all but the most posterior pair of
spiracles (10th) are closed and the air is forced out these open rear spiracles. This is a force pump stage. As
the abdomen is returned to its original volume (by other muscles or elasticity), air is drawn in through anterior
spiracles (1, 2 & 4) while the rear spiracles are kept closed. This is a suction pump stage: inspiration. The
two stages of ventilation combine with appropriate opening and closing of spiracles to create a one-way rather
than tidal, flow of air, which is more efficient in promoting gas exchange.
Locust circulatory system and tracheae dissection
Rinse a small dissection tray in Ringer's saline and also wash your dissecting instruments and a pair of
microscissors in Ringer's. (Alcohol will kill living tissue.) Obtain a live locust and remove all its legs at the
coxae with scissors. Pin it dorsum down (upside down) into the tray with 2 well-spaced small-dimension insect
pins angled through the sides of the thorax. Cut through the abdominal and thoracic pleura beginning at the rear
cutting toward the front on both left and right sides using microscissors. Add further pins as necessary, but as
few as possible. Immerse in Ringer's and keep constantly wet. Discard the ventral body wall and remove the
gut and reproductive viscera. In the process of removing the viscera you will see many silver air-filled tracheae.
Try to recognize some of the typical branching tracheal arrangement occurring in the abdominal segments and
the lateral connections to abdominal spiracles: there is a pattern.
The dorsal vessel should now be exposed as a semi-transparent tube in the mid-dorsal line. It runs
from the 9th abdominal segment through the thorax into the head. The portion in the abdomen is called the
heart. It has a slightly larger diameter than the part that runs through the thorax, which is sometimes termed the
aorta. Slight constrictions shape the heart into segmental 'chambers'. A pair of right and left lateral openings
controlled by valves are associated with each chamber and are called ostia. These openings intake
haemolymph (insect blood) from the space (pericardial sinus) in which the heart sits. You may be able to
observe the beating action of the heart.
Also visible and occurring metamerically are alary muscles. These are triangular in shape with their
bases inserting on the ventral side of the heart and originating near the posterolateral edges of the terga. These
muscles are embedded in a thin semitransparent membrane, the dorsal diaphragm, which separates the
pericardial sinus, within which the heart lies, from the perivisceral sinus that surrounds the gut.
Make a drawing in dorsal view of the locust heart. Label: dorsal vessel, heart, aorta, ostia, alary muscles,
dorsal diaphragm, pericardial sinus.
Dragonfly rectal gills (optional dissection)
The tracheal gills of a dragonfly nymphal stage are intemal: gill plates are arranged densely in series
upon the inner walls of the rectum, functioning here as a gill chamber. (The typical functions of the rectum in an
insect are perhaps performed elsewhere in dragonfly nymphs?) Extemally the anus is surrounded by tapered
sclerites that function collectively like a nozzle to direct water expelled from the gill chamber as a jet. When
disturbed, a dragonfly nymph will expel the water from its rectum to jet swiftly off into cover. The following is
written as instructions for dissection. So if you wish to do this yourself please do so. However a dissected
specimen is on display.
Cut off the abdomen at its base. Using microscissors, trim off the sharply triangular lateral margins of
the segments over the length of the abdomen on both sides. Pin the abdomen into a dissection tray ventral side
down. Then make a transverse cut just anterior to the last abdominal segment. Carefully raise and remove the
entire dorsal exoskeletal wall with additional cutting to separate it from underlying tissue. Avoid shifting the
underlying viscera. Above the gut tube at the anterior of the abdomen you should find two large right and left
dorsal tracheal trunks; they run parallel to each other, then posteriorly each breaks up into a great brush of lesser
tracheal branches that penetrate the walls of the final gut chamber (gill chamber: rectum). This chamber takes
up more than half of the length of the abdomen. Lift and tum the gut so as to expose another pair of longitudinal
tracheal trunks, the ventral tracheal trunks, breaking up in the same way into many finer branches that enter the
wall of the rectum from below. Cut open the gill/rectum chamber longitudinally and separate the sides of the cut
to examine the chamber's intemal face. Gill plates are lined up in rows with a black pigment at their base.
Circular muscles in the wall of the rectum will often tum the whole organ inside out as a result of this cut. The
feathery gill plates are purple-tinted in life. Remove a gill plate, put it in a drop of water on a slide and examine it
with transmitted light (use a medical microscope).
Make a drawing showing the rectum of the dragonfly nymph and the tracheae associated with its external
surface. Label: dorsal and ventral tracheal trunks, gill plates, rectum = gill chamber, anus, nozzle sclerites. Make
a second drawing showing the structure of a gill plate.
Fish gill ventilation
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 nearly
continuous flow of water. (Recall the three muscular fans of Chaetopterus which operate out of phase to
achieve the same thing.) The water enters the fishes' 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 edge of the operculum and the body.
Force pump
Follow the cycle: at outset mouth is closed (via the oral valves) and a high pressure is created inside the
constricting buccal cavity. The pressure in the branchial (opercular) cavity also increases but is always less than
that in the buccal cavity. The opercular flap (valve) is opened. Water flows from the buccal cavity across the
gills, into the branchial cavity and to the outside.
Suction pump
The mouth is opened, the opercular flap is closed, and a low pressure is created inside the buccal cavity
by its expansion. This draws water from the outside into the buccal cavity. At the same time the branchial
cavity with its closed operculum, is also expanding, but its pressure is kept lower (drops faster) than that in the
buccal cavity. So water continues its one-way flow from the buccal into the branchial cavity. “...the differential
pressure between the buccal and opercular cavities is induced by the resistance created by the sieve formed by
gill filaments on the four gill arches." (There are also muscles intrinsic to the gill arches and filaments.) "The
pores of the gill sieve can be adjusted by changing the angles of the gill filaments using adductor and abductor
muscles. In this way the gill resistance, and thus the differential pressure, can be regulated" (Liem, p.185).
Perca dissection
Examine the perch from a ventral aspect. Open the operculum and note the gills within the branchial
cavity. There are four curved, bony structures called aill arches which bear the gills. Between these gill arches
are spaces, the gill slits, which open into the cavity of the pharynx. Each arch supports a gill made up of a
double row of filaments. Open the mouth and make cuts on each side running posteriorly from the angle of the
jaw to expose the buccal cavity. Note the teeth on the upper and lower jaws and the tongue. Behind the teeth
on both jaws are the oral flaps. On the inner edge of each gill arch is a row of tooth-like gill rakers. Viewed on
the inner surface of the buccal cavity, the rakers of adjacent arches interdigitate with each other, making the gill
slit a zig-zag opening. Being careful not to go too deep, make an incision in the mid-ventral line between the
branchial region and the pelvic fins. (Pelvic fins are quite anteriorly placed near the pectorals in this species and
in many teleosts.) Just anterior to the liver a cavity is exposed: the pericardial cavity, delimited by mesentery.
At its posterior end you should see the muscular ventricle of the heart. Tipping the distal end of the ventricle up
you will find the thin-walled sinus venosus beneath. This collects venous blood retuming from the body and
directs it into the thin-walled saddle-shaped atrium which arches over one end of the ventricle. Blood moves
from the sinus venosus into the atrium (auricle) and then into the ventricle. The conus arteriosus takes the blood
out of the ventricle into the ventral aorta which branches into four pairs of afferent branchial arteries (afferent:
going to the gill) which convey the blood into the filament capillaries of the gills. Capillaries leaving the gills
coalesce into efferent (from the gill) branchial arteries which ultimately join with the dorsal aorta.
Draw and Label: heart of Perca showing the four major chambers and an internal view of two adjacent
gill arches showing gill rakers, gill slits, filaments.
Demonstrations
1. Hornworm spiracle
2. Mayfly nymph tracheal gill
3. Stonefly nymph tracheal gill
4. Squid circulatory system
5. Cibarial pump of cicada