Download Lesson 31 - Zoology, UBC

Lesson 31
Lesson Outline:
• The Circulatory System
Phylogenetic Trends - Heart and Aortic Arches
• Chondrichthyes
• Actinopterygians
• Birds and Mammals
Evolution of the Completely Divided Ventricle
• Air-breathing Actinopterygians
• Lungfish
• Amphibians
• Reptiles
Objectives:
At the end of this lesson you should be able to:
Describe the phylogenetic trends that are seen in the design of vertebrate hearts
and the aortic arches that arise from them.
Describe the evolution of the completely divided ventricle
References:
Chapter 14:
314-350
Reading for Next Lesson:
Chapter 14: 314-350
The Circulatory System
Phylogenetic Trends - Heart and Aortic Arches
(In most of the diagrams that accompany this lecture, the arterioles, venules and
capillaries have been omitted for simplicity. Also note that the chambers of the heart are
drawn in a straight line from posterior to anterior, again for simplicity. Finally, note that
all figures are viewed looking into the body cavity with the animal lying on its back and
so its right hand side is on your left, and vice versa)
The hypothetical protochordate had a primitive heart and six gill
arches.
The heart only consists of three chambers in hagfish and early
vertebrates. It has one-way valves and a simple tubular structure.
The ventral aorta gives rise to the afferent branchial arteries, which
enter the gills and divide down into the capillary beds. The capillaries
then reanastomose into the efferent branchial arteries that join together
dorsally to form the dorsal aorta.
Chondrichthyes
In sharks and rays, the heart now consists of four chambers, with one way valves
and is twisted into an "S" shape.
- the ventral portion of the first aortic arch is lost. The spiracle
remains but is supplied with blood from a branch of the efferent
branchial artery of the second gill arch.
I
II
VI
CA – cardiac muscle
- with valves
- the remaining aortic arches give rise to efferent branchial
arteries that initially form paired dorsal aortae which fuse into one
common dorsal aorta to supply oxygenated blood to the body.
- Thus the major change is the loss of one half of the first
aortic arch. This reflects the fact that the spiracle no longer contributes
to gas exchange but appears to become a sensory organ (sensing water
composition) - and requires oxygenated blood.
Actinopterygians
The hearts of bony fishes are almost identical to that of the shark.
III
VI
The most notable difference is that the conus arteriosus is now a
bulbus arteriosus (smooth muscle and not cardiac muscle and no
valves).
BA (smooth muscle)
- no valves
In most Actinopterygian fishes, the spiracle is lost and so too are both of the first two
efferent branchial arteries (i.e. the first two aortic arches).
Birds and Mammals
The hearts of birds and mammals also have four chambers but a different four chambers than the fish heart:
EC
SC
The sinus venosus is reduced to a small patch of tissue where
the vena cavae enter the right atrium - the sinoatrial node. It
retains the fastest rhythm of all the myogenic tissue and hence
still serves the role as the pacemaker for the heart.
IV
IC
III
VI
PA
Aorta (Systemic Arch)
Bird
RV LV
RA LA
SA Node
The conus or bulbus arteriosus disappears during embryonic
development. It splits during development giving rise to two
major arteries and these are the pulmonary artery and the aortic
trunk.
The two remaining chambers, the atrium and the ventricle each become divided into
paired chambers. Thus there is a right and left atrium and a right and left ventricle.
The venous blood from the body returns to the right atrium, passes on to the right
ventricle, which pumps it to the lungs. The blood returning from the lungs in the
pulmonary veins returns to the left atrium, passes to the left ventricle, which pumps it to
the body. This gives rise to a dual circulation.
In both birds and mammals, up to six aortic arches develop in the embryo but only three
remain in the newborn animal. The VIth aortic arch develops into the pulmonary artery,
the IVth aortic arch develops into the systemic arch (aorta) and the IIIrd aortic arch
develops into the major vessels supplying the head.
There are two major differences in this development between birds and mammals.
In birds it is the right aortic arch (IV) that gives rise to the
systemic arch while in mammals it is the left arch.
In birds, the blood vessels to the forelimb (subclavian arteries)
arise from the anterior arteries supplying the head
(brachiocephalic, common carotid, internal carotid arteries).
SC
Mammal
SC
In mammals, the blood vessels to the forelimbs (subclavian
arteries) arise from the dorsal aorta and left systemic arch.
All of the difference in the anatomy of the heart and the aortic arches that we have
described so far should make logical sense. In fishes we have a single circulation that
makes sure that blood is oxygenated first and then distributed to all of the tissues.
In the birds and mammals, gills are no longer present and we now have a dual system
where blood goes to the lungs to be oxygenated and then returns to the other side of the
heart to be pumped to the tissues. We have in essence two pumps now (that work
synchronously) but contained within the same structure.
There are two questions that we need to consider next:
How did we get from the fish system to the bird/mammalian system?
In the process (i.e. in the lungfish, amphibians and reptiles) we see a variety of partially
divided hearts. Why have these evolved? Why aren't all of these other hearts completely
divided too?
Evolution of the Completely Divided Ventricle
Air-Breathing Actinopterygians
In those fish with accessory air-breathing organs, blood supply to the
organ arises from either the dorsal aorta, or from the efferent branch of the
VIth branchial artery.
III
either/or
VI
Oxygenated blood leaving these organs enters the general venous
circulation and mixes with deoxygenated venous blood returning to the
heart.
Lungfish
With lungfish, we begin to see the start of major change.
II – reduced, has gill
As with the teleosts, the first afferent branchial artery fails to develop
and the second aortic arch is reduced (but present).
Aortic arches III to VI do develop. In the Australian lungfish
(Neoceratodus) they all deliver blood to fully functional gills. In the
African lungfish (Protopterus), however, the III and IVth arches are
intact but there are no gills associated with these arches.
ductus
arteriosus
VI
Finally, in all lungfish, the VIth arch gives rise to a branch that services the lungs. It also
maintains a connection to the dorsal aorta via the ductus arteriosus.
This should give rise to a very inefficient system.
While blood entering the dorsal aorta from aortic
arches II, V and VI will carry oxygenated blood
from the gills, it appears as though blood from
aortic arches III and IV can also enter the dorsal
aorta carrying deoxygenated blood straight from the
heart. This is misleading, however as we will see
when we look at the heart.
There are several changes to the lungfish heart:
- In all three genera of lungfish, the single atrium is partially divided by an
interatrial septum into a larger right and smaller left atrial chamber.
- In the African and South American lungfish, the blood returning from the lungs
enters into this smaller left atrial chamber while blood returning from the body, via the
sinus venosus, enters into the larger right atrial chamber.
- In place of the atrioventricular valve is an atrioventricular plug which functions
very much like the atrioventricular valve in other vertebrates.
- The ventricle is also partially divided by an interventricular septum and the
conus arteriosus contains a spiral valve. The spiral valve divides the aorta into two
separate channels.
The net result of these partial divisions is to create internal channels within the heart.
Oxygenated blood returning from the lungs is kept relatively separate from deoxygenated
blood returning from the body. It passes through the left atrium, the left side of the
ventricle and enters one side of the spiral valve in the conus arteriosus and emerges to
enter into the anterior gill arches. Thus the blood that passes through arches 3 and 4,
which have no functional gills, is already oxygenated.
Deoxygenated blood enters the sinus venosus, the right atrium, the right side of the
ventricle and the opposite side of the spiral valve that delivers it to the posterior two gill
arches.
This blood then passes through the functional gills and can then either enter the dorsal
aorta or be diverted to the lungs. When oxygen levels in water are high, most of the blood
passes from the gills into the dorsal aorta but when the levels are low, much of this blood
bypasses the capillaries in the gills and is then shunted to the lungs and not into the dorsal
aorta, as a result of constriction of the ductus arteriosus.
While this explains why it is not deleterious to not have gills on arches 3 and 4, it does
not really explain why this is an advantage. It is an advantage because it prevents the loss
of oxygen from the body into the water under conditions when O2 levels in water are low.
At this time, the blood returning from the lungs contains more O2 than the water and if
these arches had gills with a good exchange surface, O2 obtained from the lungs would be
lost to the water before it could be delivered to the body. (The reasons for the retention
of the 2nd aortic arch are not clear but again, this arch may serve a chemosensory
function).
This story had been a generalized one and primarily fits the situation in the African
lungfish. The Australian lungfish is not as highly evolved, while the South American
lungfish is even more highly evolved.
Amphibians
The situation we find in amphibians is slightly different between
groups (anurans versus urodeles) as well as within groups. For
simplicity we will consider only the general adult anuran scheme.
(Remember that the larvae (i.e. tadpoles) are aquatic and have
gills).
Note: no
connection
III
IV
cutaneous
VI
As with the teleosts and lungfishes, the first afferent branchial
artery fails to develop.
In the Adult anurans, the second and fifth aortic arches also fail to
develop.
Aortic arches III, IV and VI do develop.
The IIIrd aortic arch now only supplies the head (it is not
connected to the systemic arch) and the IVth aortic arch
becomes the systemic arch.
Finally, the VIth arch primarily services the lungs although it
does have a branch that still supplies part of the systemic
circulation - the skin (the pulmocutaneous artery). It does not
maintain a connection to the dorsal aorta (the ductus arteriosus
disappears during metamorphosis).
All arches are bilateral.
The heart is also different from that of the lungfish.
The atrium is now completely divided into a right and left
atrium.
The ventricle is completely undivided with no septum. It is,
however, very trabeculate (spongy).
The conus arteriosus has a spiral valve just like the lungfish.
bilateral systemic
arches
Now, oxygenated blood returning from the lungs is kept completely separate from
deoxygenated blood returning from the body in the atria. While there is the opportunity
for blood to mix within the ventricle, this is largely prevented by the spongy nature of the
cavity, which limits (but does not prevent) mixing.
The blood on the two sides of the ventricle then enters different sides of the spiral valve
in the conus arteriosus. The oxygenated blood emerges to enter into the anterior aortic
arches and be distributed to the body, while the deoxygenated blood enters the VIth aortic
arch to go to either the lungs and/or skin to be oxygenated.
Note that blood returning from the two sites will not return to the same place. To allow
this to occur, it is essential that the ventricle not be divided - otherwise, not enough blood
would return to the left side of the heart to be pumped to the body.
Reptiles
The primary difference seen in the aortic arches of reptiles is that the
ventral aorta divides into three channels embryonically.
III
LIV
RIV
As a result, the base of the VIth aortic arch retreats and now arises
separately from the ventricle.
VI
The base of the IVth aortic arch also retreats and now the left branch of the IVth arch
arises from the ventricle independent of the right branch of the IVth arch, which arises
along with the remainder of the ventral aorta, and the IIIrd arches.
Thus there is one pulmonary circuit and two systemic circuits (one of which gives rise to
all the anterior arteries) that arise independently from the ventricle.
(Note that this is much akin to the situation seen in birds (right arch
predominates - and unlike that seen in mammals (left arch predominates)).
The hearts in reptiles all possess both right and left atria but retain varying
degrees of incomplete separation of the ventricles.
We don't have time to go into these in detail other than to point out that the
hearts of the chelonia, as well as those of the snakes and lizards
(squamates) are incompletely divided within the ventricle while the hearts
of the crocodilians have a completely divided ventricle but, the left aorta
arises from the right ventricle along with the pulmonary artery and has a connection just
outside the heart to the right aorta.
All reptile hearts are capable of functionally separating oxygenated from deoxygenated
blood but retain the ability to mix blood from the two circuits.
The functional separation is critically important for it allows the two sides of the heart to
develop differently. The left side provides blood to the entire body. As animals increase
their levels of activity, they also increase the blood supply to their tissues. The rise in the
number of capillary beds leads to a rise in resistance to blood flow and requires that the
heart pump harder to push the blood through all the tubes. It needs to be large and strong.
The right side of the heart, on the other hand, only supplies the lungs and the resistance of
this circuit is low. Indeed, if blood pressure in this circuit rises, fluid exudes out of the
capillaries into the tissues (pulmonary edema). The pressure to this circuit must be kept
low. Thus there is a conflict and the only solution is to separate the two sides of the heart.
So why not separate them anatomically as well as functionally? This is a question that
has tantalized anatomists and physiologists for over a century and the answer is still not
clear.
Almost all textbooks tell us that the reason for retaining the ability to shunt blood
between the two circuits has to do with diving and the ability to redistribute blood when
animals are unable to breathe. This is not a very satisfying explanation since most of the
squamate reptiles do not dive. It also does not conserve O2 or reduce the work of the heart
- two common explanations given for the presence of these shunts.
It is clear that the amount of shunt (mixing between the two sides) is reduced whenever
the demand for O2 for the tissues increases. It is not clear why mixing occurs when the
demand is reduced. One current theory is that it allows some oxygenated blood returning
from the lungs to reach the inside of the ventricle, which would otherwise only receive
deoxygenated blood, which is a good thing. When the animals become active, however,
and the heart must generate high levels of flow to the tissues, the shunt must be reduced.