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Circulatory Systems
(Chpt 46, 5th Ed; Chpt 49, 6th Ed)
Gastrovascular Cavities:
• In primitive animals, where each cell is close
enough to its environment to exchange gases by
diffusion, a circulatory system is unnecessary.
• In animals like the hydra (Fig. 49.1), the entire
middle of the animal is a large sac that serves for
digestion and transport.
• Planarians have a slightly more elaborate
gastrovascular cavity, but the principle remains the
same.
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Open Circulatory Systems:
• These are more common in animals like insects,
where the blood does not serve a major gas transport
function.
• The blood and interstitial
fluid are collectively know as
hemolymph and are
circulated by a heart.
• Fluid enters the heart
through pores called ostia
and then is pumped back to
the hemocoel (Fig. 49.2).
Closed Circulatory Systems:
• In closed circulatory systems, the blood remains
within blood vessels at all times, separating the blood
from the interstitial (lymphatic) fluid.
The advantages of a closed system are:
1) delivery of gasses and nutrients to the tissues is
more rapid.
2) blood flow can be directed to specific tissues as
needed.
3) blood cells and large molecules are confined to a
restricted compartment.
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• In vertebrates the flow of blood is from the heart =>
arteries => arterioles => capillaries => venules =>
veins => heart.
Circulatory Systems of Fishes:
• Fishes have a two-chambered heart. The atrium
receives systemic blood and pumps it into the more
muscular ventricle.
• Blood is pumped through the gills for gas exchange
and then through the large aorta which distributes it
to the tissues after which it returns to the heart (page
868).
• The high resistance of the gills dissipates most of the
blood pressure, so the systemic blood pressure is
very low.
• Lung fish have modified this system somewhat in
that part of the gill circulation goes through the air
bladder (rather than the aorta) and returns directly to
the heart, which has two atria.
• This may be how
two separate
circulatory
systems evolved
(page 869).
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Circulatory Systems of Amphibians:
• Amphibians have a three-chambered heart and
separation of the pulmonary (lung) and systemic
(body) circulation.
• The left atrium receives
oxygenated blood from the lungs
and the right atrium receives
deoxygenated blood from the
tissues. Both pump into the single
ventricle which pumps blood to
both the lungs and the tissues
(page 869).
• This mixing of the oxygenated and deoxygenated
blood sounds inefficient, but in fact the spiral valve
inside the aorta effectively separates pulmonary and
systemic blood under most conditions.
• Keeping pulmonary separate from systemic
circulation permits higher blood pressures and better
circulation.
Circulatory Systems of Reptiles:
•Reptiles have gone a little farther in that while they
have three-chambered hearts, there is a partial
septum, or wall, (complete in the crocodilians) that
separates pulmonary and systemic blood flow (page
870).
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Circulatory Systems of Birds and Mammals:
• Birds and mammals have complete four-chambered
hearts.
• In essence there are two separate hearts joined
together. The right heart pumps blood to the lungs,
the left heart takes the oxygenated blood and pumps
it out to the body (page 870).
Fig. 46.4
• The left heart is generally larger because it has a
larger work load.
• Pulmonary resistance is high, but high blood
pressures would tend to make the lungs leaky, which
is not desirable.
• The two circulations operate at different pressures
and keeping them separate means that fully
oxygenated blood is available at high pressures to
the body. This is important for sustained rapid
activity.
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The Human Heart: Two Pumps in One:
• The human heart is really two pumps, one for
pulmonary circulation, one for systemic.
• A series of one-way valves prevent backflow and
keep the circulation unidirectional.
• The atrioventricular
valves lie between the atria
and the ventricles, the
pulmonary valve sits
between the right ventricle
and the pulmonary artery
and the aortic valve seals
the aorta from the left
ventricle (Fig. 49.4).
• Normally the two hearts contract together in the
cardiac cycle, which consists of contraction
(systole) followed by relaxation (diastole, Fig. 49.5).
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• Normal blood pressures (systolic/diastolic,
measured as mm Hg) are 120/80 in %%, lower in &&.
• Medically, the major concern is the diastolic
pressure; values above 95 cause long-term cardiac
damage, values above 105 pose immediate risk for
heart failure or stroke.
Cardiac Muscle and the Heartbeat:
• Cardiac muscle is myogenic, it can beat without
nervous input.
• Near the junction of the superior vena cava with the
right atrium lies the sinoatrial node, the pacemaker
for the heart.
• At regular intervals the node fires (initiates an action
potential).
• Because the cells in the two atria are linked by
intercalated disks, a wave of contraction spreads
through the atria and they contract, forcing blood into
their respective ventricles.
• Electrical resistance
between atria and ventricles
is high, so only the atria
contract.
• However, another specialized
region, the atrioventricular
node, picks up the AP, and
after a slight delay sends an
AP to the bundle of His and
the Purkinje fibers.
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• The Purkinje fibers are arranged so that the
ventricles contract from the tip upwards, producing the
maximum force into the pulmonary artery and the
aorta (Fig. 49.8).
• This intense
electrical activity can
be detected even at
the skin surface to
produce a recording
called an electrocardiogram, or EKG,
as shown in Fig.
49.8.
The Vascular System:
Arteries and arterioles:
• Blood pressure is highest in those vessels that carry
blood away from the heart.
• The walls of these vessels have elastic fibers and
smooth muscle.
• The elastic fibers allow them to withstand high
pressures, as well as to expand during systole, and
contract during diastole.
• This dampens (smooths out) the pulses in the blood
flow and maintains arterial pressure high.
• Arteries also have smooth muscle, which can
contract or relax to direct blood flow.
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Capillaries:
• Blood moves from arteries to veins through a series
of ever smaller vessels until the level of the capillary
bed, where the blood vessels are so small that the
red cells must move through them in single file (Fig.
49.11).
• There are so many branches that the total crosssectional area of the capillaries is greater than any
other class of blood vessel and no cell in the body is
more than two cells from a capillary (Fig. 49.12).
Fig.
46.11
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Exchange in Capillary Beds:
• Capillary walls are permeable to water and small
molecules, so hydrostatic pressure (blood pressure)
tends to force these out of the vessels into the
interstitial spaces (interstitial fluid is called lymph).
• The hydrostatic pressure is opposed by the
osmotic pressure caused by the large molecules in
the blood.
• The high resistance of the capillary beds causes a large
drop in blood pressure across the capillary bed, so that there
is in fact a negative pressure on the venous side (Fig.
46.13).
• This drains the interstitial space, preventing the
accumulation of fluid.
• If the blood vessels
become more leaky,
perhaps in response
to histamine, the
balance shifts and
the tissue swells, a
condition called
edema. Edema can
also be caused by
high blood pressure
or poor venous
return.
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Veins and venules:
• The venous system operates at very low pressure
and blood flow is slower, so that the blood tends to
accumulate in the veins.
• Veins have expandable walls and as much as 80% of
the blood may be in the veins at any one time.
Because of their capacity to store blood, veins are
known as capacitance vessels.
• The blood pressure in
the veins is insufficient to
overcome the force of
gravity, and so veins have
pocket valves to ensure
the one-way flow of blood
(Fig. 49.13).
• Venous flow is also
assisted by the
contraction of the
skeletal muscles which
squeeze the veins,
pushing the blood
towards the heart.
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• Absence of muscular activity is why your feet swell
on a plane trip; the prolonged inactivity results in the
pooling of blood in the lower extremities.
Blood: A Fluid Tissue:
• Blood has two primary components -- cellular and
plasma.
• The cellular component, called the packed cell
volume or hematocrit, consists of erythrocytes
(red blood cells; RBCs), leukocytes (white blood
cells) and platelets.
• Hematocrit runs about 40% under normal
conditions in humans.
• The plasma components are primarily water with
some salts and plasma proteins (Fig. 49.15).
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Red Blood Cells:
• There are about 5 million (5 x 106) erythrocytes per
ml of blood, whose main function is to transport
respiratory gasses between tissues and lungs.
• Erythrocytes are made in the bone marrow by stem
cells and survive only about 120 days once they
enter the circulation.
• The rate of erythrocyte formation is controlled by
the hormone, erythropoietin.
White Blood Cells:
•There are only 5-10,000 leukocytes, or white cell, per
ml of blood.
• Leukocytes are capable of independent movement
using ameboid motion, and will squeeze through the
walls of blood vessels in response to chemical signals
in order to attack bacteria or other pathogenic
organisms.
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