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2. Let’s Learn
The circulatory system is also known as the cardiovascular
system. The term cardio refers to anything that is related to the heart,
while vascular refers to the channel or vessel through which blood
flows. The circulatory or cardiovascular system consists of the heart,
the blood, and the blood vessels.

3. Parts of the Circulatory System and their Functions
A. Heart
The heart is a hollow muscular organ.
Its wall encloses four chambers. The whole
heart enclosed in a sac called pericardium.
The heart is the main organ of the circulatory
system. This system carries substances to
and from all parts of the body through the
bloodstream.
The heart has four compartment
called the chambers.

4. The interior of the heart has a left side and a right side separated
by a thick muscular wall called septum. Each side has an upper chamber
called atrium and a lower chamber called ventricle.
The atria (sing. atrium) are smaller ventricles, and their walls are
thinner. Their job is to hold blood before it goes down the ventricles.
The ventricles are bigger and have thicker muscular walls. Their job
is to pump blood. The right ventricle pumps blood to the lungs, while the
ventricle pumps blood to all parts of the body. For this reason, the left
ventricle has a thicker muscular wall.

5. Blood moves through the heart in just one direction. This is made
possible by the one-way valves found between the atria and ventricles and
blood vessels attached to them. All the heart valves prevent blood from
flowing backwards.

6. B. Blood Vessels
A blood vessels is a tube that carries blood in the circulatory
system. Your body is able to control blood pressure by expanding and
constricting blood vessels.
Smooth muscle cells line the blood vessels throughout the body.
These cells are able to respond to the chemical signals even though they
cannot be controlled voluntarily.
The blood vessels consist of arteries, arterioles, capillaries,
venules, and veins. This blood vessels differ in structure and function.

7. Arteries and Arterioles
Arteries are the blood vessels with the function of carrying blood away
from the heart to other tissues. Arteries split and branch many times to form
tiny capillaries, which then join to the veins that carry the blood to the heart.
From the large arteries, blood travels to the smaller arteries and moves
into the arterioles around the body. As compared to larger arteries, arterioles
are elastic. They also have more smooth muscles. When blood enters the
arterioles, they do not expand as much since they are smaller muscular
arteries.
The arteries and arterioles serve as the transportation vessels of
blood in the body. Therefore, they can be considered as the highways and
back roads of the circulatory system.

9. Capillaries
It is the tiniest blood vessels. The microscopic capillaries serve as
the location for the exchange of gases and nutrients between blood and
tissue cells. The capillary walls are very thin and composed of a single
layer of endothelial cells, which are the very thin flattened cells that line
the inner walls of all of the blood vessels.
Veins and Venules
From the capillary beds, the blood makes its journey back to the
heart through the venules. These are group of capillaries that unite to
become small vessels that drain the capillary beds. Veins are blood
vessels with the function of carrying blood toward the heart from the
different body tissues.

10. Superior vena cava – brings blood from the head and arms
Inferior vena cava – brings blood from the legs and trunk

11. C. Blood
Blood travels throughout the body. This thick, red, sticky fluid
essential to life. Without it, you would die. It does many vital jobs,
including carrying oxygen around the body, removing wastes, and fighting
diseases.

12. Blood consists of billions of microscopic cells floating in a liquid called
plasma.
Three main kinds of cells in the blood:
a. red blood cells (erythrocytes) – a numerous shaped like
doughnuts without the holes and their main tasks are to take in oxygen
from the lungs and release it to the organs and tissues.
b. white blood cells (leukocytes) – These cells fight germs and
diseases and remove wastes from the blood and the body.
c. platelets (thrombocytes) – These are more like cell fragments,
and their major tasks is to help blood clot in cuts and wounds to form a
scab.

14. Blood Circulation
Circulation is the process by which blood goes to all parts of the
body. The blood circulates or flows around the body in a system of
blood vessels called vascular network. Arteries take blood away from
the heart, veins bring it back again.
Two parts of circulation:
1. systematic circulation (arteries and veins that lead to
from the left side of the heart) – The systemic arteries deliver oxygen,
energy, nutrients, and other important substances to the body’s billions
of microscopic cells. The systematic veins then collect products.

15. 2. pulmonary circulation (arteries and veins takes blood
from the right side of the heart to the lungs for oxygen). As the blood
leaves the heart, it is low in oxygen and is dark reddish-blue in color. In
the lungs, the blood absorbs oxygen, turns bright red, and flows back to
the left side of the heart.

16. Heartbeat – is a sequence of actions that makes blood flow through the
heart’s chambers and valves, into the body, and back to the heart
again. An adult’s heart beats about 75 times in one minute. Children
have a faster heart rate.
Adrenaline or epinephrine – hormone sent by adrenaline glands that
speed up the beating of the heart, as well as the breathing rate.
Blood pressure – is the force exerted by blood against the walls of the
main arteries when it is pumped by the ventricles.
As blood leaves the heart with each beat, it surges into the
arteries and makes them bulge or pulsate. This pulsation is called the
pulse.
Sphygmomanometer or blood pressure (BP) apparatus – a machine or
device that measures blood pressure.

18. Healthful Habits That Promote the Proper Functioning of the
Circulatory System
 1. Observe a balanced diet. Eating foods that contain a lot of the
mineral iron helps to keep your red blood cells healthy.
 2. Exercise daily. Activities like swimming, jogging, and cycling are
aerobic exercises that are good for the heart.
 3. Have a regular physical checkup. Have a physical checkup at least
once a year even if you have no symptoms of any illness.