THE CIRCULATORY SYSTEM
The circulatory system is composed of the heart and blood vessels,
including arteries, veins, and capillaries.
The circulatory system is the transportation system for your body. It touches
every organ and system in your body, and is connected to all of your body’s
cells. The circulatory system carries chemicals, oxygen and nutrients in the
blood cells to all points in your body. It also takes away waste products and
dissolved carbon dioxide from your body’s cells.
Your heart is the most hard-working muscle in your body. It works day and
night, never stopping, even when you are resting. Try working non-stop for
24 hours, 7 days, 12 months, every year of your life. Can you do it? Your
heart can, and does!
How important is your circulatory system?
Every cell that needs oxygen from the respiratory system needs access to
the fluids in your circulatory system. These fluids are super important to your
digestive system that has absorbed nutrients from your food, and needs to
get that energy out to all parts of your body. Guess what? Hormones created
by your endocrine system are sent through the body by the circulatory
system. That’s how your body regulates your growth, among other things; and
how you know when you’re hungry, or sleepy, or hot.
Structure of the Heart
The main organ of the circulatory system is the heart. It is a hollow, coneshaped muscle located between the lungs, behind the sternum. A child’s
heart, such as yours, is about the size of a fist. (An adult heart is about the
size of two fists.) The heart’s job is to pump blood to every living cell in the
body.
The heart is divided into four chambers. The two upper chambers are the
atria (singular = atrium), and the two lower chambers are the ventricles. A
partition, called the septum, separates the right side of the heart from the
left side. This prevents the blood on the left and right chambers from
mixing. This is important because the left side of the heart has blood rich in
oxygen, but the right side has blood with very little oxygen.
How the Heart Works
When the heart contracts, it squeezes — try squeezing your hand into a
fist. That's sort of like what your heart does so it can squirt out the blood.
The heart operates like two pumps joined together. The right side of your
heart receives blood from the body and pumps it to the lungs. The left side
of the heart does the exact opposite: It receives blood from the lungs and
pumps it out to the body.
How the Heart Beats: The heart muscle contracts in two stages to
squeeze blood out of the heart.
First stage:
Before each beat, your heart fills with blood. Then the
upper chambers contract (squeeze) at the same time to squirt the blood
into the lower chambers. Blood is pumped from the right atrium down into
the right ventricle and from the left atrium down into the left ventricle.
Second stage: The lower chambers (ventricles) contract at the same
time to push this blood out of the heart. The right ventricle pushes the
blood to the lungs to pick up oxygen, while the left ventricle pushes the
blood to the aorta, to be sent around the body.
Blood that has been sent to the lungs to receive oxygen, returns to the
heart through the pulmonary veins, to be sent to the body via the aorta.
Diagram of the Heart
The Valves of the Heart
Each chamber of the heart has a sort of one-way valve at its exit that
prevents blood from flowing backwards. When each chamber contracts
(squeezes), the valve at its exit opens. When it is finished contracting, the
valve closes so that blood does not flow backwards.
Blood Vessels
Blood is carried around the body in blood vessels. The vascular system
(blood vessels) contains three types of vessels--arteries, veins and
capillaries. Of these three, arteries are the thickest. Veins are a little
thinner, and capillaries are so thin nutrients can pass directly through them.
Arteries
The arteries carry blood full of oxygen from the heart to other organs in the
body. Arteries are red in color because the blood they transport has oxygen
in it. Blood flows through the arteries with tremendous force; It is under high
pressure generated by the strong contractions of the heart. The arteries
therefore have thick outer walls, thick layers of muscle and elastic fibres.
The aorta is the largest artery in the body. It is attached to the left ventricle
of the heart, and is where all other arteries start. As blood travels away
from the heart, the arteries become smaller and smaller until eventually, the
blood enters the smallest vessels – the capillaries.
Veins
Veins carry blood low in oxygen back to the heart from other organs. Veins
are blue because the blood they carry is not oxygen rich. The Vena Cava is
the largest vein in the body. It is made up of the inferior vena cava on the
lower end, and the superior vena cava, on the upper end. The inferior vena
cava pushes blood into the heart's right atrium, and the superior vena cava
pushes blood from the right ventricle to the lungs.
The blood in veins is under lower pressure than the blood in arteries
because blood flows through them at a slower rate. Veins therefore have
thin walls and thin layers of muscle and elastic fibres. Unlike arteries, veins
have one-way valves in them to keep the blood moving in the correct
direction.
Capillaries
Capillaries connect arteries and veins. They carry blood to and from the
trillions of cells in the body. It is within the capillaries that the transfer of
oxygen, nutrients, carbon dioxide and waste products takes place.
Capillaries are very small and have thin walls to allow an easier transfer of
materials to and from the tissues of the body. Capillaries are so tiny, in fact,
that blood usually can only get through one cell at a time. As the blood
travels towards the heart on the return journey, the capillaries get bigger
and bigger, until the blood enters into veins.
How Blood Circulates
The heart squeezes (contracts) and sends the blood into the first and
largest artery - the aorta. Now it is on its way. Arteries carry blood full of
oxygen, picked up in the lungs after you inhale, away from the heart to
every tissue of the body. As the blood travels further from the heart, the
arteries get smaller and smaller. Soon they are so tiny, they are just one
cell thick. Now they are called capillaries. There are capillaries lying right
next to all of the tissues in the body. They will give off their oxygen and
nutrients, and take on carbon dioxide and waste products. Now the blood is
"oxygen poor" and will start its trip back to the heart. The blood travels back
to the heart through the veins. The veins get larger and larger as they get
closer to the heart. The last and largest veins, that dump blood into the
heart, are called the vena cava. That's it, the system is complete (and
closed). Then the cycle starts again.
Caring for your Circulatory System – Love your heart
If your heart is healthy it will be able to work for a very long time. It is up to
you to keep your heart healthy. Your heart will like it if you:
1.
Exercise every day to keep your heart strong. Exercise strengthens
your heart by making it beat harder, which makes the heart muscles
larger and able to push more blood with each “squeeze” or “beat”.
You need to exercise your heart by making it work harder for at least
30 minutes a day.
2.
Eat healthy foods to avoid diseases such as hypertension, diabetes
and heart disease. Too much fatty food will block up the arteries
with fat and your heart will have to work harder and harder just to do
its regular job of pumping blood round your body.
3.
Drink lots of water every day rather than sodas or sugary drinks.
4.
Keep germs out of your blood. Wash cuts with soap and water.
5.
Never touch someone else’s blood.
6.
Do not ever smoke. Smoking harrows blood vessels and can cause
high blood pressure.
7.
Choose NOT to take illegal drugs, and NOT to drink too much
alcohol.
Your heart will love you for it!