and Amphibians
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Throughout history, people have believed the heart plays a vital role in
the body. The ancients supposed it was the seat of the spirit, the center of
happiness, and in control of both the emotions and the intellect. Even today,
we place the heart at the root of our emotions when we speak of being
heartbroken or brave at heart. It is true that the heart plays an essential,
life-giving role in an animal's body, but the mystery of what function it
actually performs has been solved. The heart is the pump that drives the
cardiovascular system.
Function of the Cardiovascular
System
By circulating blood throughout the body, the cardiovascular system
functions to supply the tissues with oxygen and nutrients, while removing
carbon dioxide and other metabolic wastes. As oxygen-rich blood from the heart
flows to the tissues of the body, oxygen and other chemicals move out of the
blood and into the fluid surrounding the cells of the body's tissues. Waste
products and carbon dioxide move into the blood to be carried away. As blood
circulates through organs such as the liver and kidneys some of these waste
products are removed. Blood then returns to the lungs (or gills, in the case of
fish), receives a fresh dose of oxygen and gives off carbon dioxide. Then the
cycle repeats itself. This process of circulation is necessary for continued
life of the cells, tissues, and ultimately the whole organism. Up and down the
evolutionary ladder, there are different forms of cardiovascular systems with
different levels of efficiency, but they all perform this same basic function.
Mammalian Anatomy and Physiology
The cardiovascular systems of mammals, birds, amphibians, reptiles, and
fish are all slightly different. The following is an overview of the main
components of the mammalian system – the heart and blood vessels. A discussion
of the other systems will follow.
Heart
The heart is composed of cardiac muscle that differs slightly from the
skeletal and smooth muscle found elsewhere in the body. This special type of
muscle adjusts the rate of muscular contraction, allowing the heart to maintain
a regular pumping rhythm. The main parts of the heart are the chambers, the
valves, and the electrical nodes.
Heart Chambers: There are two different types of heart chambers. The
first is the atrium (plural is atria), which receives blood returning to the
heart through the veins. The right atrium pumps blood to the right ventricle,
and the left atrium pumps blood into the left ventricle. This blood is then
pumped from the atrium into the second chamber called the ventricle. The
ventricles are much larger than the atria and their thick, muscular walls are
used to forcefully pump the blood from the heart to the body and lungs (or
gills). See illustration below.
Valves:
The valves found within the heart are situated between the atria and
ventricles, and also between the ventricles and major arteries. These valves
are opened and closed by pressure changes within the chambers, and act as a
barrier to prevent the backflow of blood. The characteristic " lub-dub, lub-dub" heart sounds heard through a stethoscope are the
result of vibrations caused by the closing of the respective valves.
Electrical
Nodes: There are two different electrical nodes, or groups of specialized
cells, located in the cardiac tissue. The first is the sinoatrial (SA) node,
commonly called the pacemaker. The pacemaker is embedded in the wall of the
right atrium. This small patch of tissue experiences rhythmic excitation and
the impulse rapidly spreads throughout the atria, causing a muscular
contraction and the pumping of blood from the atria to the ventricles. The
other node, the atrioventricular (AV) node, relays the impulse of the SA node
to the ventricles. It delays the impulse to prevent the ventricles from
contracting at the same time as the atria, thus giving them time to fill with
blood. The cycle of contraction of the heart muscle is called a heartbeat, the
rate of which varies greatly between organisms. The following table gives the
average heart rates of some common mammals.
Heart Rates Comparison (beats/minute)
|
Organism
|
Average Rate
|
Normal Range
|
|
Human
|
70
|
58 -
104
|
|
Cat
|
120
|
110 -
140
|
|
Cow
|
65
|
60 - 70
|
|
Dog
|
115
|
100 -
130
|
|
Guinea Pig
|
280
|
260 -
400
|
Cardiovascular
System: The Heart and Vessels of Mammals, Birds, Fish and Amphibians - Page 1
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species.
|
450
|
300 -
600
|
|
|
|
Horse
|
44
|
23 - 70
|
|
|
Rabbit
|
205
|
123 -
304
|
|
|
Rat
|
328
|
261 -
600
|
|
|
|
|
|
|
Vessels
A vessel is a hollow tube for transporting something, like a garden hose
transporting water. A blood vessel is a hollow tube for transporting blood.
There are three main types of blood vessels:
Arteries Capillaries Veins
These main blood vessels function to transport blood through the entire
body and exchange oxygen and nutrients for carbon dioxide and wastes.
The arteries carry blood away from the heart, and are under high
pressure from the pumping of the heart. To maintain their structure under this
pressure, they have thick, elastic walls to allow stretch and recoil. The large
pulmonary artery carries unoxygenated blood from the right ventricles to the
lung, where it gives off carbon dioxide and receives oxygen. The aorta is the
largest artery. It carries oxygenated blood from the left ventricle to the
body. The arteries branch and eventually lead to capillary beds.
The capillaries make up a
network of tiny vessels with extremely thin, highly permeable walls. They are
present in all of the major tissues of the body and function in the exchange of
gases, nutrients, and fluids between the blood, body tissues, and alveoli of
the lungs.
At the opposite side of the capillary beds, the capillaries merge to
form veins, which return the blood
back to the heart. The veins are under much less pressure than the arteries and
therefore have much thinner walls. The veins also contain one-way valves in
order to prevent the blood from flowing the wrong direction in the absence of
pressure. The pulmonary vein returns oxygenated blood from the lungs to the
left atria. The vena cava returns blood from the body to the right atria. The
blood that is returned to the heart is then recycled through the cardiovascular
system.
Comparative Anatomy
Mammals and Birds
Mammalian and avian hearts have four chambers – two atria and two
ventricles. This is the most efficient system, as deoxygenated and oxygenated
bloods are not mixed. The right atrium receives deoxygenated blood from the
body through both the inferior and superior vena cava. The blood then passes to
the right ventricle to be pumped through the pulmonary arteries to the lungs,
where it becomes oxygenated. It returns to the left atrium via the pulmonary
veins, this oxygen-rich blood is then passed to the left ventricle and pumped
through the aorta to the rest of the body. The aorta is the largest artery and
has an enormous amount of stretch and elasticity to withstand the pressure
created by the pumping ventricle. The four-chambered heart ensures that the
tissues of the body are supplied with oxygen-saturated blood to facilitate
sustained muscle movement. Also, the larger oxygen supply allows these warm-blooded
organisms to achieve thermoregulation (body temperature maintenance).
Amphibians and Reptiles
Amphibians and reptiles, by contrast, have a three-chambered heart. The
three-chambered heart consists of two atria and one ventricle. (The crocodile is
sometimes said to have a four-chambered heart. The separation of the
Mammalian Heart
External
View
Cardiovascular
System: The Heart and Vessels of Mammals, Birds, Fish and Amphibians - Page 2
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Unauthorized use of any images, thumbnails, illustrations, descriptions,
article content, or registered trademarks of Foster & Smith, Inc. is strictly prohibited under copyright
law. Site content, including photography, descriptions, pricing, promotions,
and availability are subject to change without notice. These restrictions are
necessary in order to protect not only our copyrighted intellectual property,
but also the health of pets, since articles or images that are altered or
edited after download could result in misinformation that may harm companion
animals, aquatic life, or native species.
|
|
|
Amphibian Heart Internal View
|
|
|
|
Amphibian Heart External View
|
|
|
|
Mammalian
Heart
|
|
Internal View
|
ventricles is not complete, however, because a hole remains in the
septum (wall) that divides the two chambers.) Blood leaving the ventricle
passes into one of two vessels. It either travels through the pulmonary
arteries leading to the lungs or through a forked aorta leading to the rest of
the body. Oxygenated blood returning to the heart from the lungs through the
pulmonary vein passes into the left atrium, while deoxygenated blood returning
from the body through the sinus venosus passes into the right atrium. Both
atria empty into the single ventricle, mixing the oxygen-rich blood returning
from the lungs with the oxygen-depleted blood from the body tissues. While this
system assures that some blood always passes to the lungs and then back to the
heart, the mixing of blood in the single ventricle means the organs are not
getting blood saturated with oxygen. This is not as efficient as a
four-chambered
system,
which keeps the two circuits separate, but it is sufficient for these
cold-blooded organisms.
The heart
rate of amphibians and reptiles is very dependent upon temperature. For
example, the following table gives the approximate heart rate of a crocodile at
the indicated temperatures. Notice that the higher the temperature, the faster
the heart beat.
|
Temperature
|
Average Rate
|
|
(Celsius)
|
(beats/minute)
|
|
10 C
|
1 - 8
|
|
18 C
|
15 - 20
|
|
28 C
|
24 - 40
|
|
>40
C
|
Irreversible
cardiac
|
|
|
damage
|
|
|
|
Fish
Fish possess the simplest type of true heart – a two-chambered organ
composed of one atrium and one ventricle. A rudimentary valve is located
between the two chambers. Blood is pumped from the ventricle through the conus
arteriosus to the gills. The conus arteriosus is like the aorta in other
species. At the gills, the blood receives oxygen and gets rid of carbon
dioxide. Blood then moves on to the organs of the body, where nutrients, gases,
and wastes are exchanged. There is no division of the circulation between the
gills and the body. That is, the blood travels from the heart to the gills, and
then directly to the body before returning to the atrium through the sinus
venosus to be circulated again. The heart rates of fish fall within the wide
range of 60-240 beats per minute, depending upon species and water temperature.
The fish's heart rate will be slower at lower temperatures.
Conclusion
Cardiovascular
System: The Heart and Vessels of Mammals, Birds, Fish and Amphibians - Page 3
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Unauthorized use of any images,
thumbnails, illustrations, descriptions, article content, or registered
trademarks of Foster & Smith, Inc.
is strictly prohibited under copyright law. Site content, including
photography, descriptions, pricing, promotions, and availability are subject to
change without notice. These restrictions are necessary in order to protect not
only our copyrighted intellectual property, but also the health of pets, since
articles or images that are altered or edited after download could result in
misinformation that may harm companion animals, aquatic life, or native
species.
The cardiovascular system of animals consists of the heart and blood
vessels. It is responsible for providing each cell of the body with the oxygen
and nutrients it needs,
while
removing waste products.
Fish Heart
External
View
Fish
Heart
Internal
View
Mshindo media:Cadiovascular System
Website:
https://www.mshindoveterinarycentre.com
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