MadSci Network: Zoology
Query:

Re: Which types of animals do not have hearts? I can't quite remember.

Date: Fri Dec 8 17:40:23 2000
Posted By: June M. Wingert , RM(NRM),Associate Scientist
Area of science: Zoology
ID: 975970719.Zo
Message:

Greetings 

I don’t know of any animals that do not have hearts , unless you are 
referring to single celled animals which diffuse nutrients across a cell 
membrane into its cytoplasm and thus circulation is complete.  At any rate 
the following information should help sort out the different types of 
circulation.

Invertebrates have circulatory systems that range from complex to
simple. Some invertebrates, such as earthworms and octopuses, have
a closed circulatory system. Other invertebrates have an open
circulatory system, in which the blood is only partially confined to the
vessels. It fills the hollow spaces of the body as well. Animals with an
open circulatory system include insects, spiders, and most shellfish. 

In many invertebrates, the blood is pumped by contracting vessels or
by pumping centers (contracting portions of vessels), or by both.
Among insects, for example, the "heart" consists of an internal
contracting vessel that extends almost the length of the back. 

The simplest animals with a true circulatory system include certain
Worms have contracting vessels that pump the blood. A group of simpler
worms, called ribbon worms or proboscis worms, have a
circulatory system with no pumping centers and no contracting
vessels. The movements of the animal keep the blood flowing through
the body. http://school.discovery.com/homeworkhelp/worldbook/atozscience/c/115905.htm
l


The distribution of nutrients and gases from the environment is one of the 
essential tasks of all living organisms. It is a job which takes
place at the most basic levels in bacteria and single celled organisms all 
the way to the intricately complex circulatory systems of
mammals and humans. Though the circulatory system begins with simple 
diffusion and the gastrovascular cavity and seems to end with
the four chambered mammalian heart, the development of different hearts in 
organisms is actually convergent evolution, the frog, worm,
and human heart all developed separately and independently of each other. 
It is the demands of each different type of animal that brings
about the very different types of heart in each animal.

INVERTBRATES

Microorganisms and single-celled animals need only to diffuse nutrients 
across a cell membrane into its cytoplasm, and thus circulation is
complete. However as multicellular animals become larger, with layers of 
cells stacked on one another, their more interior cells
experienced greater difficulty in exchanging materials with the 
environment by simple diffusion. This problem is solved with the advent of
the body cavity seen in such organisms as the jellyfish. It is here that 
true circulation takes place, the transport of material from one place
to another within an organism by passage through an internal fluid. The 
different types of circulatory systems that then develop are
specific to the different phyla of animals. Open and closed circulatory 
systems then serve organisms whether vessels are needed to
transport materials (closed) or merely circulation through the whole body.

VERTEBRATES

Like invertebrates, vertebrates have also needed different types of 
systems to suit the many different animals. Any closed circulatory
system requires both a system of passageways though which fluid can 
circulate and a pump to force the fluid through them. The three
different types of vertebrate hearts reflect the vastly different needs of 
each of these animals.

Fish

The development of gills by fish necessitated a more efficient pump to 
force blood through the fine capillary network and in fish there is
a true chamber-pump heart, beyond the simple closed circulatory system. 
The fish heartbeat is the peristaltic sequence, starting at the
rear and moving to the front. The first of the four chambers to contract 
is the sinus venosus, then the atrium, then the ventricle, and
finally the conus arteriosus. Despite shifts in the relative positions 
that evolved later heartbeat sequence is maintained and unchanged in
all vertebrates.

Fish utilize the one-cycle chamber pump, which is ideally suited to its 
gill respiratory apparatus and represents one of the major
evolutionary innovations of the vertebrates. Its greatest advantage is 
that the blood it delivers to the tissues of the body is fully
oxygenated. Thus blood is pumped directly to and through the gills, where 
it becomes fully aerated; from the gills it flows through a
network of arteries to the rest of the body and then returns to the heart 
through the veins. Thus, in a fish, blood must pass through two
capillary beds during each circuit, one in the gills and a second one in 
another organ. When blood flows through a capillary bed, blood
pressure, the hydrostatic pressure that pushes blood through vessels, 
drops substantially. This means the flow of blood has lost much of
the force contributed by the contraction of the heart, so the circulation 
to the rest of the body is quite slow. However the process is aided
by the whole-body movements produced by the fish during swimming.

Amphibian 

The advent of gaseous respiration in the lungs involved a major change in 
the pattern of circulation. Ultimately this evolutionary change
allowed vertebrates to overcome the limitations of the one-chambered heart 
of fish, which requires aid by swimming. The change
consisted of the development of an additional pair of veins. After blood 
is pumped through a fine network of capillaries in lungs, it is not
dispersed to the tissues of the body but is instead returned to the heart 
through large veins called pulmonary veins for re-pumping. The
development of these new veins lead to a great improvement in the 
performance of the circulatory system, since the blood being pumped
to other tissues of the body could be pumped at a much higher pressure 
than if it were not returned to the heart at this stage. The
disadvantage of pulmonary veins is that the aerated blood from the lungs 
is mixed in the heart with non-oxygenated blood that is
constantly being returned to the heart from the rest of the body. 
Consequently the heart pumps out a mixture of oxygenated and
non-oxygenated blood rather than fully oxygenated blood.

Mammal and Birds

Only a relatively slight alteration is seen in the heart of mammals, 
birds, and crocodiles. Though the changes occurred separately for
these three types of animals, they created the same basic anatomical 
changes with slight variations.

The closure of the ventricular septum (a wall between two cavities) 
created the double circulatory system toward, the last step in heart
evolution. Four-chambered hearts have the advantage re-pumping blood after 
its passage through the lungs, without mixing oxygenated
and non-oxygenated blood. The blood that is pumped by the heart of a 
mammal or bird into the systemic arterial system if fully
oxygenated.

The great increase in efficiency that the double circulatory system 
provides is believed to have been important in the evolution of
endothermy in mammals. More efficient circulation is necessary to support 
the great increase in metabolic rate that is required to generate
body heat internally. Also, blood is the carrier of heat within the body, 
and an efficient circulatory system is required to distribute heat
evenly throughout the body.

The separation of the blood flow into two circuits also has a second 
favorable result. Because overall circulatory system is closed, in each
full passage through the system, the same volume of blood has to move 
through the circulation path as through the much more extensive
body circulation path. This means that the blood must move through the 
lungs much faster than through the rest of the body. This more
rapid circulation is not accomplished by higher pressure in the pulmonary 
circuit. Instead the blood vessels in the lung are larger in
diameter than those in the rest of the body and offer less resistance to 
flow. The favorable result is that the rapid flow of blood through
the lungs greatly increases the efficiency with which oxygen is captured 
by the bloodstream. http://www.bhs.berkeley.k12.ca.us/departments/science/anatomy/anatomy98/Hea
rts/html/evolution.html


Thanks for taking the time to send in a question to the Mad Scientist 
Network.

June Wingert
Mad Scientist



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