The circulatory system consists of the heart, blood vessels, and blood. The heart pumps blood through arteries, veins, and capillaries to deliver oxygen and nutrients to tissues and organs throughout the body while also transporting waste products for removal. Deoxygenated blood returns to the heart through veins and is pumped to the lungs to receive oxygen before repeating the cycle. The circulatory system works with other body systems to regulate temperature, transport hormones and nutrients, and fight disease and infection.

3. Circulatory System, or cardiovascular system, in
humans, the combined function of the heart, blood, and
blood vessels to transport oxygen and nutrients to
organs and tissues throughout the body and carry away
waste products. Among its vital functions, the circulatory
system increases the flow of blood to meet increased
energy demands during exercise and regulates body
temperature. In addition, when foreign substances or
organisms invade the body, the circulatory system
swiftly conveys disease-fighting elements of the immune
system, such as white blood cells and antibodies, to
regions under attack. Also, in the case of injury or
bleeding, the circulatory system sends clotting cells and
proteins to the affected site, which quickly stop bleeding
and promote healing.

4. The heart, blood, and blood vessels are the three structural
elements that make up the circulatory system. The heart is the
engine of the circulatory system. It is divided into four chambers: the
right atrium, the right ventricle, the left atrium, and the left ventricle.
The walls of these chambers are made of a special muscle called
myocardium, which contracts continuously and rhythmically to pump
blood. The pumping action of the heart occurs in two stages for each
heart beat: diastole, when the heart is at rest; and systole, when the
heart contracts to pump deoxygenated blood toward the lungs and
oxygenated blood to the body. During each heartbeat, typically about
60 to 90 ml (about 2 to 3 oz) of blood are pumped out of the heart. If
the heart stops pumping, death usually occurs within four to five
minutes.
Blood consists of three types of cells: oxygen-bearing red blood
cells, disease-fighting white blood cells, and blood-clotting
platelets, all of which are carried through blood vessels in a liquid
called plasma. Plasma is yellowish and consists of
water, salts, proteins, vitamins, minerals, hormones, dissolved
gases, and fats.

5. Three types of blood vessels form a complex
network of tubes throughout the body. Arteries carry
blood away from the heart, and veins carry it toward
the heart. Capillaries are the tiny links between the
arteries and the veins where oxygen and nutrients
diffuse to body tissues. The inner layer of blood
vessels is lined with endothelial cells that create a
smooth passage for the transit of blood. This inner
layer is surrounded by connective tissue and smooth
muscle that enable the blood vessel to expand or
contract. Blood vessels expand during exercise to
meet the increased demand for blood and to cool the
body. Blood vessels contract after an injury to reduce
bleeding and also to conserve body heat.

6. Arteries have thicker walls than veins to
withstand the pressure of blood being pumped
from the heart. Blood in the veins is at a lower
pressure, so veins have one-way valves to
prevent blood from flowing backwards away
from the heart. Capillaries, the smallest of blood
vessels, are only visible by microscope—ten
capillaries lying side by side are barely as thick as
a human hair. If all the arteries, veins, and
capillaries in the human body were placed end to
end, the total length would equal more than
100,000 km (more than 60,000 mi)—they could
stretch around the earth nearly two and a half
times.

7. The arteries, veins, and capillaries are
divided into two systems of circulation:
systemic and pulmonary. The systemic
circulation carries oxygenated blood from the
heart to all the tissues in the body except the
lungs and returns deoxygenated blood
carrying waste products, such as carbon
dioxide, back to the heart. The pulmonary
circulation carries this spent blood from the
heart to the lungs. In the lungs, the blood
releases its carbon dioxide and absorbs
oxygen. The oxygenated blood then returns to
the heart before transferring to the systemic
circulation.

8. Only in the past 400 years have scientists recognized
that blood moves in a cycle through the heart and body.
Before the 17th century, scientists believed that the liver
creates new blood, and then the blood passes through the
heart to gain warmth and finally is soaked up and consumed
in the tissues. In 1628 English physician William Harvey first
proposed that blood circulates continuously. Using modern
methods of observation and experimentation, Harvey noted
that veins have one-way valves that lead blood back to the
heart from all parts of the body. He noted that the heart
works as a pump, and he estimated correctly that the daily
output of fresh blood is more than seven tons. He pointed
out the absurdity of the old doctrine, which would require
the liver to produce this much fresh blood daily. Harvey’s
theory was soon proven correct and became the cornerstone
of modern medical science.

9. The heart ejects oxygen-rich blood under high
pressure out of the heart’s main pumping chamber, the
left ventricle, through the largest artery, the aorta.
Smaller arteries branch off from the aorta, leading to
various parts of the body. These smaller arteries in turn
branch out into even smaller arteries, called arterioles.
Branches of arterioles become progressively smaller in
diameter, eventually forming the capillaries. Once blood
reaches the capillary level, blood pressure is greatly
reduced.

10. Capillaries have extremely thin walls that
permit dissolved oxygen and nutrients from the
blood to diffuse across to a fluid, known as
interstitial fluid, that fills the gaps between the cells
of tissues or organs. The dissolved oxygen and
nutrients then enter the cells from the interstitial
fluid by diffusion across the cell membranes.
Meanwhile, carbon dioxide and other wastes leave
the cell, diffuse through the interstitial fluid, cross
the capillary walls, and enter the blood. In this
way, the blood delivers nutrients and removes
wastes without leaving the capillary tube.

11. After delivering oxygen to tissues and
absorbing wastes, the deoxygenated blood in
the capillaries then starts the return trip to the
heart. The capillaries merge to form tiny
veins, called venules. These veins in turn join
together to form progressively larger veins.
Ultimately, the veins converge into two large
veins: the inferior vena cava, bringing blood from
the lower half of the body; and the superior vena
cava, bringing blood from the upper half. Both of
these two large veins join at the right atrium of
the heart.

12. Because the pressure is dissipated in the
arterioles and capillaries, blood in veins flows
back to the heart at very low pressure, often
running uphill when a person is standing. Flow
against gravity is made possible by the one-
way valves, located several centimeters
apart, in the veins. When surrounding muscles
contract, for example in the calf or arm, the
muscles squeeze blood back toward the heart.
If the one-way valves work properly, blood
travels only toward the heart and cannot lapse
backward. Veins with defective valves, which
allow the blood to flow backward, become
enlarged or dilated to form varicose veins.

13. C-2 . PULMONARY CIRCULATION
In pulmonary circulation, deoxygenated blood returning from
the organs and tissues of the body travels from the right atrium of the
heart to the right ventricle. From there it is pushed through the
pulmonary artery to the lung. In the lung, the pulmonary artery
divides, forming the pulmonary capillary region of the lung. At this
site, microscopic vessels pass adjacent to the alveoli, or air sacs of
the lung, and gases are exchanged across a thin membrane: oxygen
crosses the membrane into the blood while carbon dioxide leaves
the blood through this same membrane. Newly oxygenated blood
then flows into the pulmonary veins, where it is collected by the left
atrium of the heart, a chamber that serves as collecting pool for the
left ventricle. The contraction of the left ventricle sends blood into the
aorta, completing the circulatory loop. On average, a single blood
cell takes roughly 30 seconds to complete a full circuit through both
the pulmonary and systemic circulation.

14. In addition to oxygen, the circulatory system also
transports nutrients derived from digested food to the
body. These nutrients enter the bloodstream by
passing through the walls of the intestine. The
nutrients are absorbed through a network of
capillaries and veins that drain the intestines, called
the hepatic portal circulation. The hepatic portal
circulation carries the nutrients to the liver for further
metabolic processing. The liver stores a variety of
substances, such as sugars, fats, and vitamins, and
releases these to the blood as needed. The liver also
cleans the blood by removing waste products and
toxins. After hepatic portal blood has crossed the liver
cells, veins converge to form the large hepatic vein
that joins the vena cava near the right atrium.

15. The circulatory system plays an important role in
regulating body temperature. During exercise, working
muscles generate heat. The blood supplying the muscles
with oxygen and nutrients absorbs much of this heat and
carries it away to other parts of the body. If the body gets too
warm, blood vessels near the skin enlarge to disperse excess
heat outward through the skin. In cold environments, these
blood vessels constrict to retain heat.
The circulatory system works in tandem with the
endocrine system, a collection of hormone-producing
glands. These glands release chemical messengers, called
hormones, directly into the bloodstream to be transported to
specific organs and tissues. Once they reach their target
destination, hormones regulate the body’s rate of
metabolism, growth, sexual development, and other
functions.

16. The circulatory system also works with the immune
system and the coagulation system. The immune system
is a complex system of many types of cells that work
together to combat diseases and infections. Disease-
fighting white blood cells and antibodies circulate in the
blood and are transported to sites of infection by the
circulatory system. The coagulation system is composed
of special blood cells, called platelets, and special
proteins, called clotting factors, that circulate in the
blood. Whenever blood vessels are cut or torn, the
coagulation system works rapidly to stop the bleeding by
forming clots.

17. Other organs support the circulatory system. The
brain and other parts of the nervous system constantly
monitor blood circulation, sending signals to the heart
or blood vessels to maintain constant blood pressure.
New blood cells are manufactured in the bone marrow.
Old blood cells are broken down in the spleen, where
valuable constituents, such as iron, are recycled.
Metabolic waste products are removed from the blood
by the kidneys, which also screen the blood for excess
salt and maintain blood pressure and the body’s
balance of minerals and fluids.

18. The pressure generated by the pumping action
of the heart propels the blood to the arteries. In
order to maintain an adequate flow of blood to all
parts of the body, a certain level of blood pressure
is needed. Blood pressure, for instance, enables a
person to rise quickly from a horizontal position
without blood pooling in the legs, which would
cause fainting from deprivation of blood to the
brain. Normal blood pressure is regulated by a
number of factors, such as the contraction of the
heart, the elasticity of arterial walls, blood
volume, and resistance of blood vessels to the
passage of blood.

19. Blood pressure is measured using an inflatable device
with a gauge called a that is wrapped around the upper arm.
Blood pressure is measured during systole, the active
pumping phase of the heart, and diastole, the resting phase
between heartbeats. Systolic and diastolic pressures are
measured in units of millimeters of mercury (abbreviated mm
Hg) and displayed as a ratio. Blood pressure varies between
individuals and even during the normal course of a day in
response to emotion, exertion, sleep, and other physical and
mental changes. Normal blood pressure is less than 120/80
mm Hg, in which 120 describes systolic pressure and 80
describes diastolic pressure. Higher blood pressures that are
sustained over a long period of time may indicate
hypertension, a damaging circulatory condition. Lower blood
pressures could signal shock from heart
failure, dehydration, internal bleeding, or blood loss.

20. Disorders of the circulatory system include any injury or
disease that damages the heart, the blood, or the blood vessels.
The three most important circulatory diseases are
hypertension, arteriosclerosis, and atherosclerosis.
Hypertension, or elevated blood pressure, develops when the
body’s blood vessels narrow, causing the heart to pump harder
than normal to push blood through the narrowed openings.
Hypertension that remains untreated may cause heart
enlargement and thickening of the heart muscle. Eventually the
heart needs more oxygen to function, which can lead to heart
failure, brain stroke, or kidney impairment. Some cases of
hypertension can be treated by lifestyle changes such as a low-salt
diet, maintenance of ideal weight, aerobic exercise, and a diet rich
in fruits, vegetables, plant fiber, and the mineral potassium. If
blood pressure remains high despite these lifestyle
adjustments, medications may be effective in lowering the
pressure by relaxing blood vessels and reducing the output of
blood.

21. In arteriosclerosis, commonly known as hardening of the
arteries, the walls of the arteries thicken, harden, and lose their
elasticity. The heart must work harder than normal to deliver
blood, and in advanced cases, it becomes impossible for the heart to
supply sufficient blood to all parts of the body. Nobody knows what
causes arteriosclerosis, but heredity, obesity, smoking, and a high-fat
diet all appear to play roles.
Atherosclerosis, a form of arteriosclerosis, is the reduction in
blood flow through the arteries caused by greasy deposits called
plaque that form on the insides of arteries and partially restrict the
flow of blood. Plaque deposits are associated with high
concentrations of cholesterol in the blood. Recent studies have also
shown an association between inflammation and plaque deposits.
Blood flow is often further reduced by the formation of blood clots
(see Thrombosis), which are most likely to form where the artery
walls have been roughened by plaque. These blood clots can also
break free and travel through the circulatory system until they
become lodged somewhere else and reduce blood flow there (see
Embolism). Reduction in blood flow can cause organ damage. When
brain arteries become blocked and brain function is impaired, the
result is a stroke. A heart attack occurs when a coronary artery
becomes blocked and heart muscle is destroyed.

22. Risk factors that contribute to
atherosclerosis include physical
inactivity, smoking, a diet high in fat, high
blood pressure, and diabetes. Some cases of
atherosclerosis can be corrected with healthy
lifestyle changes, aspirin to reduce blood
clotting or inflammation, or drugs to lower the
blood cholesterol concentration. For more
serious cases, surgery to dilate narrowed
blood vessels with a balloon, known as
angioplasty, or to remove plaque with a high-
speed cutting drill, known as
atherectomy, may be effective. Surgical
bypass, in which spare arteries are used to
construct a new path for blood flow, is also an
option.

23. One-celled organisms and many simple multicelled
animals, such as sponges, jellyfishes, sea
anemones, flatworms, and roundworms, do not have a
circulatory system. All of their cells are able to absorb
nutrients, exchange gases, and expel wastes through direct
contact with either the outside or with a central cavity that
serves as a digestive tract.
More complex invertebrates have a wide range of
circulatory system designs. These invertebrate circulatory
systems are classified as either open or closed. Open systems—
found in
starfishes, clams, oysters, snails, crabs, insects, spiders, and
centipedes—lack capillaries, and the blood bathes the tissues
directly. In closed systems, the blood is confined to a system of
blood vessels. Invertebrates with closed systems include
segmented worms, squids, and octopuses.

24. All vertebrate animals have closed circulatory
systems. These systems are classified by the
number of chambers in the heart, which determines
the basic configuration of blood flow. Fish have two-
chambered hearts with one atrium and one
ventricle. Blood pumped from the ventricle travels
through arteries to the gills, where it diverges into
capillaries and exchanges gases. Leaving the
gills, the capillaries reconvene into blood vessels
that carry the oxygenated blood to the rest of the
body, where the vessels again diverge into
capillaries before reconvening into veins that return
to the heart. In this way, the blood passes through
first the respiratory organs (the gills) and then the
systemic circulation between each pass through the
heart.

25. Frogs and amphibians have three-chambered
hearts, with two atriums and one ventricle. Blood
pumped from the ventricle enters a forked artery.
One fork, the pulmonary circulation, leads to the
lung. The other fork, the systemic
circulation, leads to the rest of the body. Blood
returning from the pulmonary circulation enters
the left atrium, while blood from the systemic
circulation enters the right atrium. Although there
is some mixing of oxygenated and deoxygenated
blood in the ventricle, a ridge within the ventricle
assures that most of the oxygenated blood is
diverted to the systemic circulation and most of
the deoxygenated blood goes to the pulmonary
circulation. In reptiles, this ridge is more
developed, forming a partial wall. In
crocodiles, the wall is complete, forming a four-
chambered heart like that found in mammals and
birds.