CHAPTER 42 -- ANIMAL CIRCULATION AND GAS EXCHANGE
I.
II.
THE FUNCTION OF THE CIRCULATORY SYSTEM
 Unicellular organisms and organisms with only a few cell layers are able to
exchange materials with their environment by diffusion.
 Most multicellular organisms require an internal transport system to carry
nutrients, oxygen to cells and waste and carbon dioxide away from cells.
 The long distance transport system needs to have structures where transport
occurs, a transporting fluid and an organ that helps the movement of the
transporting fluid
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OVERVIEW OF ANIMAL CIRCULATION AND GAS EXCHANGE:
 Porifera -- Diffusion
 Cnidaria -- Diffusion and gastrovascular cavity that extend into the tentacles
 Platyhelminthes -- Diffusion and gastrovascular cavity that is frequently
branched, flat body to accommodate diffusion.
 Nematoda -- Diffusion
 Annelida -- closed circulatory system (hearts, blood and blood vessels)
 Mollusca -- open circulatory system, hemolymph (there is no distinction
between blood and interstitial fluid). However, the fastest cephalopods have
closed circulatory systems.
 Arthropods -- open circulatory system, hemolymph.
 Echinoderms -- water vascular system
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Chordates -- mostly closed circulatory system.
III. OPEN AND CLOSED CIRCULATORY SYSTEMS:
 Both open and closed circulatory systems use the same basic design:
o fluid to transport respiratory gases (hemolymph or blood)
o vessels to transport the fluid (arteries, veins, capillaries)
o heart (to move the fluid)
A. Open circulatory system:
 The circulatory fluid (hemolymph) flows out of the vessels and directly bathes
organs in interconnected sinuses
 The contraction of one or more hearts move the fluid
 Mollusks and arthropods have this system
 Beneficial, because it requires less energy, lower hydrostatic pressures
B. Close circulatory system:
 The fluid is blood and it circulates in closed vessels.
 The materials are exchanged between the cells and the smallest blood vessels
(capillaries)
 Beneficial because the high hydrostatic pressure (blood pressure) provides
effective, fast delivery. It makes faster motion and better O2 supply possible.
The distribution of blood to different organs can be better regulated.
IV. ORGANIZATION OF THE VERTEBRATE CIRCULATORY SYSTEM
 Cardiovascular system -- a complex system of blood vessels, blood and heart
 Arteries -- blood vessels that carry blood away from the heart to organs. The smallest
arteries before they branch off to capillaries are called arterioles
 Capillaries -- microscopic vessels with very thin, porous walls. These vessels are actively
exchanging respiratory gases, hormones, nutrients and wastes with cells.
 Veins -- blood vessels that lead back to the heart. The smallest veins that directly collect
blood from the capillaries are called venules. Portal veins are special kinds of veins that
carry blood between capillary beds.
 Evolutionary trend -- the animals with higher metabolic rates have more complex
circulatory systems and more powerful hearts. The number of blood vessels in a
particular organ correlates with the organ's metabolic requirements.
 The heart of all vertebrate is made up of two different types of chambers:
a. atria -- chambers that receive blood entering the heart
b. ventricles -- chambers that are responsible for pumping the blood out of the
heart
 Evolutionary trend -- the number of chambers increases as we move from fish to
mammals as a way to adapt to life on dry land
A. Single Circulation
 Bony and cartilage fishes have only two heart chambers. The blood passes through the
heart only once.
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The blood that enters the heart is rich in CO2 and it moves to the gills where it gets
refreshed with O2. The refreshed blood moves to the capillary beds in the body where it
releases the O2 before it returns to the heart.
The blood pressure becomes very small in the second set of capillaries so the blood flow
is very slow there. However, when the fish swims, the muscle contraction helps to
increase the blood flow and makes it efficient enough.
B. Double Circulation:
 Amphibians, reptiles, birds and mammals have two distinct circuits -- double circulation
 The right side of the heart delivers oxygen-poor blood to the capillary bed of the
respiratory organs, where O2 is taken in and CO2 is released (in amphibians it also
includes the skin's capillary system) -- pulmonary circuit
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The left side of the heart delivers O2 rich blood to the capillary beds of the organs and
tissues of the body where O2 and nutrients are released and CO2 and waste materials
are picked up -- systemic circuit
The double circulation system provides higher blood pressure, more vigorous circulation
C. Adaptations of the Double Circulatory System:
 Amphibians: three chambered heart (2 atria, 1 ventricle). The blood flow to the lungs
can be shut down when amphibians are in water and they can exchange oxygen through
their skin.
 Reptiles : Three-chambered heart with partial separation of the ventricles. In alligators,
caimans and crocodilians the separation is complete but the pulmonary and systemic
circuits are connected.
 Birds, mammals: There is a complete separation of the ventricles. The four-chambered
heart is a key adaptation that support endothermic life by making the circulation a lot
more efficient
V. MAMMALIAN CIRCULATION:
The path of the blood in the mammalian circulatory system is the following:
1. The CO2 rich blood flows into from the right ventricle to the pulmonary arteries
2. From the pulmonary arteries, the blood travels into the capillary bed of the lungs
3. The blood gets loaded up with O2 and releases CO2 in the capillary beds of the left and right
lungs.
4. The blood travels back to the left atrium through the pulmonary veins
5. From the left atrium the fresh blood travels to the left ventricle that exerts enough force to
force the blood to the rest of the body.
6. From the left ventricle the blood travels to the aorta that is the largest blood vessel that
branches off to smaller arteries (first to the coronary artery that feeds the heart)
7. Branches of the aorta leads to the head and forelimbs and divides into capillary beds
8. Other branches of the aorta lead to the legs and abdominal area and divides into capillary
beds there
9. After the gas exchange the capillaries form smaller venules that larger veins. The oxygen
poor blood from the upper body travel to one of the largest veins, the superior vena cava
10. From the lower part of the body, the capillaries also collect into smaller venules than veins,
eventually to the other largest vein called the inferior vena cava
11. Both vena cava returns to the right atrium and starts the circuit over
V. THE MAMMALIAN HEART
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Located beneath the sternum, the human heart is about the size of a fist, consisting
mostly of cardiac muscle.
The two atria serve as collection chambers, while the two ventricles forward the blood
toward capillary beds by a forceful contraction.
The heart contracts and relaxes in a rhythmic cycle. When it contracts, it pumps blood,
when it relaxes, it fills up with blood.
A. Cardiac cycle: The complete cycle of contraction and relaxation of the heart. The
contraction phase is called systole, the relaxation phase is called diastole
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The volume of blood pumped by the heart in each minute depends on two factors:
o Heart rate – number of beats per minute
o Stroke volume – the amount of blood pumped by the left ventricle in each
contraction
 Valves control the blood flow into one direction in the heart: Atrioventricular valve (AV
valve) is located between each atrium and ventricle and allows the blood to flow from
the atria to the ventricles only. Semilunar valves – located in the entrance of the
pulmonary artery and aorta to allow the blood to the largest arteries
 The elastic walls of the arteries expand when they receive the blood expelled from the
ventricles. This pressure of blood and expansion of vessels cause pulse.
 Heart murmur: a defect in one or more valves and can be detected when the blood
squirts backwards through the defective valve. Can be caused by birth defect or
damage from an infectious disease
B. Maintaining the Rhythmic Heart Beat
 The cardiac muscle cells are self-excitable, meaning they contract without any signal.
However, they have to contract in unison to effectively perform heart contraction.
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The rate and timing of the contraction is set by the sinuatrial (SA) node. It is located in
the wall of the right atrium. It generates electric impulses that spread through the
intercalated disks of the cardiac muscle cells and makes the cells of the atria contract in
unison.
To contract the ventricles, another node the atrioventricular node (AV node) picks up
the impulse. The AV node is located in the wall between the right atrium and right
ventricle.
The delay between the contraction of the atria and ventricles ensures that the blood
flow from the atria to the ventricles. The impulse is forwarded to the bundle of His,
bundle branches and the Purkinje fibers to completely contract the ventricles.
The electrical currents that are produced by the impulse of the heart beat is conducted
through the body fluid onto the skin and can be detected and recorded by
electrocardiograms (EKG).
Although the pace of the heart beat is set by the SA node, the nervous system regulates
the heart beat by two nerves (long bundles of axons running from the central nervous
system to the various organs of the body). One nerve slows the heart rate down, the
other speeds it up. The SA node is also influenced by various hormones (adrenalin and
noradrenalin – hormones responsible for the fight-or-flight mechanism) of the adrenal
gland. Stress, increased temperature, exercise are some factors that increase the heart
rate – homeostasis.
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Complete animation:
http://www.argosymedical.com/Circulatory/samples/animations/Heart/index.html
 http://www.youtube.com/watch?v=AOiyjNFB0as
 http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000001
&ptid=57
 http://www.youtube.com/watch?v=yQGx4g8LUYA&feature=related
Dissection:
http://www.youtube.com/watch?v=oj1y4zHmyE8
Nervous control:
http://www.argosymedical.com/Circulatory/samples/animations/Nervous%20Influence/index.
html
VI. BLOOD VESSELS
 They form the plumbing system of the body, form a network of tubes that eventually
delivers oxygen and nutrients to every cell in the body and takes away waste materials
and CO2.
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Structural differences correlate with the different functions of arteries, veins, and
capillaries:
o Capillaries are made up of an endothelium (simple squamous epithelium) and a
basement membrane – thin wall for fast exchange of materials
o Arteries have thick muscular and elastic connective layers for strength and
elasticity to accommodate varying blood pressures and blood velocity.
o Veins have thinner walls with valves in large veins to accommodate slower
moving blood and lower blood pressure.
Blood travels fastest in the aorta and its velocity decreases as the blood moves through
the capillaries than it somewhat speeds up again in veins (consider the total diameter of
all capillaries that the other blood vessels)
B. Blood pressure
 Contraction of the heart ventricles generate blood pressure which exerts a force in all
directions. The force exerted by the ventricles pushes the blood away from the heart
and the movement is helped by the flexibility of the arteries.
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As the blood flows away from the heart and into the capillaries, the blood pressure
decreases. It further decreases as the blood moves to the veins and it is the lowest in
the superior and inferior vena cava.
Changes in the blood pressure are due to the contraction and relaxation of the
ventricles. Systolic pressure is the highest pressure which results from the contraction
of the ventricles. Diastolic pressure is the lowest pressure that is the consequence of
the relaxation of the ventricles.
HOMEOSTASIS: Blood pressure is regulated by a wide range of hormonal and nervous
controls. Stress, exercise can cause vasoconstriction that increases the blood pressure
by contracting the smooth muscles of the arteries. Rest can result in vasodilation
where the smooth muscles of the arteries relax and the blood pressure decreases.
Some other chemicals such as NO (nitrogen monoxide) can induce vasodilation in the
cardiovascular system, while a peptide called endothelin can induce vasoconstriction.
The regulation of the blood pressure ensures that the body gets the proper amount of
oxygen in every kind of activity level.
Normal blood pressure for an adult human is about 120/70 mmHg at rest. Blood
pressure is influenced by gravity. It is a real challenge to pump blood against the gravity
upwards into the head (especially in long-necked animals). Gravity also makes it
difficult to return the blood to the atria from the lower part of the body. The body
adapted to this difficulty by
o having rhythmic contractions of the smooth muscle of venules and veins
in the lower part of the body. These contractions push the blood
upwards.
o Skeletal muscle contractions also help with their contraction during
exercise
o During inhalation, as the volume of the chest cavity increases, the vena
cavae and other large veins expand and fill in with blood
o There are valves inside of the large veins. These valves open in front of
the blood flow upwards but close behind the blood flow.
C. Capillary Function
 Only about 5-10% of the body's capillaries have blood flowing through them.
 Capillaries of the brain, liver, heart and kidneys are usually filled to capacity but at other
sites the blood supply varies over time.
 The blood flow into capillaries is altered by two mechanisms:
o Contraction of the smooth muscle of the arterioles (remember, capillaries
don't have muscle) decreases the volume of blood flowing into the
capillary bed
o Precapillary sphincters -- rings of smooth muscles located at the entrance
of the capillary beds
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Blood pressure tends to drive molecules out of the capillaries but blood proteins tend to
pull them back. To be able to release nutrients, oxygen and hormones and minerals into
the cells, the blood pressure needs to overcome the osmotic pressure at the arteriolar
part of the capillary beds.
On the venous end of the capillaries, the osmotic pressure created by blood proteins
push substances into the capillaries.
VII. BLOOD
 Blood in animals with closed circulatory systems is a specialized connective tissue
 Blood is composed of several kinds of cells (45 %) suspended in a liquid matrix called
plasma (55%).
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Blood plasma is about 90% water, but it also contains inorganic salts (blood electrolytes)
– act as buffers, maintain osmotic balance, helps normal functioning of muscles and
nerves. Also contains plasma proteins – act as buffers, contribute to the viscosity of
blood, carry lipids, form immunoglobulins, act as clotting factors. The blood plasma also
carries nutrients, metabolic wastes, respiratory gases, hormones.
Red blood cells (erythrocytes) – 1 mm3 of blood contains 5-6 million red blood cells.
STRUCTURE AND FUNCTION RELATIONSHIP: They are small, biconcave disks with a fairly
large surface area. They don’t have nuclei so their cytoplasm can be filled with
hemoglobin. Hemoglobin is a complex protein that has a hem and a globin portion and
participates in carrying both O2 and CO2. One erythrocyte can transport about a billion
O2 molecules. They also lack mitochondria and get their ATP through anaerobic
metabolism (fermentation).
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White blood cells (Leukocytes) – There are five major types of white blood cells
(monocytes, lymphocytes, neutrophils, basophils and eosinophils). They fight infections
by different means. Monocytes and neutrophils are phagocytic cells that engulf and
digest bacteria and other harmful materials. Lymphocytes develop into specialized B
and T cells to produce immune response against foreign substances. White blood cells
leave the circulatory system and move around in the tissues to look for foreign
materials.
Platelets – Small fragments of cells without nuclei. They are important in blood clotting
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http://www.argosymedical.com/Circulatory/samples/animations/The%20Blood/index.html
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Replacement of cellular elements is an important process because most of these cells
have a very short life span. All of the cellular elements form in the red bone marrow
from a set of pluripotent stem cells.
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The erythrocyte production is controlled by negative feedback – when the oxygen level
decreases in the body tissues, the kidney synthesizes a hormone called erythropoietin,
which stimulates the production of erythrocytes. If the O2 concentration increases the
erythropoietin production decreases.
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Blood clotting: Because blood is not supposed to be outside of blood vessels or the
heart, a clotting process stops bleeding and closes damaged vessels. The simplified
steps of the clotting process are the following:
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o The process starts when it is initiated by the damage of the endothelium of
blood vessels
o Platelets stick to the exposed collagen fibers of the connective tissue and also
release a substance that makes nearby platelets sticky.
o The platelets form a platelet plug that can heal a very small tear but is not
sufficient to stop the bleeding of larger cuts.
o Blood clotting factors in the plasma are activated (don’t forget this is a positive
feedback) in an increasing rate until the last factor called fibrinogen is activated
to form an insoluble network of fibrin
o The insoluble fibrin filters red blood cells and platelets out of the blood and
makes a solid patch of cells.
Problems with blood clotting: hemophilia and thrombus formation (thrombosis)