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Lesson 3: The Circulatory System
Assignment:
 Read Chapter 4 in the Textbook
 Read and Study the lesson discussion
 Complete the Lesson 3 worksheet
Objectives: After you have completed this lesson, you will be able to List blood components and explain the functions of blood.
 Identify the basic structures of mammalian and reptilian hearts.
 Trace the flow of blood through the heart and body, identifying the parts of
blood vessels and their structural significance.
 Use knowledge of the heart function and control to explain the clinical
significance of the electrocardiogram; heart sounds, including heart murmurs;
and blood pressure.
 Discuss the clinical significance of the academic material in this lesson.
 Identify common animal circulatory system disorders.
The Circulatory System
In an article titled, "Cardiovascular System: The
Heart and Vessels," found at PetEducation.com,
doctors Foster and Smith write:
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 the animal's body, but the mystery of what
function it actually performs has been solved.
The heart is the pump that drives the
cardiovascular (circulatory) 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, 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.
Heart Anatomy
In the article titled "Cardiovascular System: The Heart and Vessels," found at
PetEducation.com, doctors Foster and Smith write:
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.
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.
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 then 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.
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 backflow
of blood. The characteristic "lub-dub, lub-dub" heart sounds heard through the
stethoscope are the result of vibrations caused by the closing of the respective
valves.
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 nodes 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.
A general rule of thumb is the smaller the animal, the more elevated the heart
(pulse) rate.
A vessel is a hollow tube fortransporting something, like a garden hose transports
water. A blood vessel is a hollow tube for transporting blood. There are three main
types of blood vessels: arteries, capillaries, and 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.
Mammalian and avian (bird) 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.)
Reptiles and amphibians are occasional visitors to the veterinary office. These two
types of animals have a three-chambered heart (which consists of two atria and one
ventricle.)
Drs. Foster and Smith comment on these unique three-chambered hearts:
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 that 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 …
the higher the temperature, the faster the heart rate.
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.
Watch this video which identifies the parts of a
sheep’s heart.
Watch this video which begins with dissection
of a sheep’s heart and also contains very good
explanations of the parts of the heart and their
functions.
Now try to identify the parts of a sheep's heart.
Common Circulatory System Disorders
Another article obtained from PetEducation.com, titled "Mitral Valve Insufficiency: A
Cause of Heart Failure" by Race Foster, DVM, and Holly Nash, DVM, MS makes some
interesting comparisons between heart disease in people and in dogs:
In people, heart disease usually involves the arteries that supply blood to the heart
muscle. In some cases they harden, losing their elasticity and the ability to respond
to blood pressure differences as they distribute blood to the cardiac muscle. In other
cases, possibly due to diet or genetics, the arteries may become obstructed with a
plaque that builds up internally on the artery walls. This causes the muscles of the
heart to receive less than adequate amounts of blood. Starved for oxygen, the result
is usually a heart attack. In dogs, arteriosclerosis (the hardening of the arteries),
plaque formation, and heart attacks are all very rare. However, heart disease is very,
very common.
Canine heart failure simply means that the muscles 'give out.' This is usually caused
by one chamber or side of the heart being required to do more than it is physically
able to do. It may be that excessive force is required to pump the blood through an
area, and, over time, the muscles fail. Also, in some cases, the volume of blood that
must be pumped to keep up with other areas of the heart is more than the particular
chamber can adapt itself to move, again leading to muscle failure. Unlike a heart
attack in humans, cardiac failure in the dog is a slow, insidious process that occurs
over months or years. In these cases, once symptoms are noted, they will worsen
over time until the animal is placed on treatment, dies, or is 'put to sleep’.
Heart failure in older dogs is usually due to problems with the mitral valve of the
heart, and occurs most commonly in smaller breeds, such as Poodles, Yorkies, Pugs,
Pomeranians, Lhasas, etc. As the dog ages, the mitral valve between the left atrium
and left ventricle starts to fail. A heart valve is designed to prevent back flow. Blood
can be easily pumped through the valve, but once the more forward chamber is
filled, the valve closes to prevent blood from flowing backwards into the atrium.
These valves greatly increase the efficiency of the heart as a pump, since the blood
only has to be pumped once to keep it moving forward from one chamber to another
chamber. No blood is spilling back, which
would require additional effort to move it
forward. In the case of a mitral valve
disorder (referred to as mitral
insufficiency), the valve ages and shrinks
and thereby fails to completely close off the
area on the left side of the heart between
the two chambers. The left ventricle is very
strong, and with mitral insufficiency, it
easily forces a portion of the blood
backward into the left atrium with each
heartbeat. The pressure of blood within a
normally functioning heart is highest in the
left ventricle, as it is required to move the
blood throughout the entire body. When
blood flows backwards into the atrium, it
elevates the blood pressure in that chamber
and even further back into the lung field.
There are additional complications, which
increase pressures in the right side of the
heart as it attempts to correctly pump
blood forward into the lungs for
oxygenation. Confronted with this elevated
pressure within the lung field, the right side
of the heart must work harder in moving
the blood into the area.
Symptoms: The result of mitral
insufficiency is elevated blood pressure within both the lung field and right side of
the heart. The symptoms caused by this condition are very predictable. When
hypertension (higher than normal blood pressure) occurs within the lungs, fluid
actually leaves the blood vessels and leaks into the tissues. The medical term used to
describe 'fluid in the lungs' is 'pulmonary edema.' This causes the affected individual
to cough in an attempt to clear the lungs of the fluid, just as you would do with a
chest cold. The cough that typically occurs after exercise, excitement, or when the
animal has first gotten up after sleeping is the first clinical symptom noted with
mitral insufficiency. In most cases, this is controlled with diuretics that cause the
animal to urinate more and thereby remove excess fluids from his lungs.
As the condition progresses, the right side of the heart starts to fail because of its
increased work and elevated pressures. At first, the muscles strengthen, thereby
increasing the mass and thickness of its walls. Over time, however, even these
'athletic' muscles cannot keep up with the ever-increasing pressures, and they start
to fail. During this latter stage of the disorder, the animal will be weaker and tire
more easily, [and may] even faint.
Treatment: Medications are used to strengthen and coordinate the muscles'
contractions and they will help (and possibly eliminate) the clinical signs… .
Diuretics (water pills)… can help remove the excess fluid that can accumulate in the
lungs. Diets lower in sodium may assist in decreasing the fluid build-up. Limited
exercise may be beneficial—however, consult with your veterinarian before starting
your dog on an exercise program. Regardless of treatment, anatomical changes
continue to occur internally so the medications will only slow down the progression
of the disease. Even with therapy, the condition usually worsens over time, and in
many cases, finally results in the death of the animal.
Prevention: There are few things that can be done to prevent mitral valve
insufficiency. Diagnosing the disease earlier in its course is very helpful in slowing
the progression. Notify your veterinarian of any signs of heart failure in your dog.
Keeping your dog healthy and at his ideal weight can lessen the severity of
symptoms if mitral insufficiency does occur. Valves of the heart can be injured by
infection; this infection can result from severe dental problems, which allow
bacteria from the mouth to enter the bloodstream. Good oral health, then, is also
important.
Summary
Based on the information from the textbook and this discussion, it is obvious that
veterinarians must have a wealth of knowledge about blood and the circulatory
system in order to properly care for the animals that they see every day. If
veterinarians didn't know the specifics about blood and the two circulatory
pathways, they simply would not be able to care for our pets effectively.
Sources Cited:
Kahn, Cynthia, ed. ":Musculoskeletal System Introduction." The Merck Veterinary Manual. Ninth Edition.
New Jersey: Merck & Co. Inc. 2006
<http://merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/90100.htm>.
Veterinary and Aquatic Services Department, Drs. Foster and Smith, Inc. "Cardiovascular System: The
Heart and Vessels." PetEducation.com. 1997—2007. 28 Dec. 2006
<http://www.peteducation.com/article.cfm?cls=17&cat=1848&articleid=2951>.