Imagine for a moment that you are a creature
the size of a speck of dust. You find yourself in a big bustling city full
of jam-packed highways, narrow alleyways, and a never-ending stream of speeding cars
and busy people. Fortunately, if you want to go from the park
to a local eatery, all you have to do is travel using the highways. Now, imagine that the city is a body and you
are an oxygen molecule that has just been inhaled into the lungs of a human. Now you have to make it from the lungs to
a cell in the toes. How does that molecule manage to do it? Just like a person navigating a bustling city,
the molecule takes the body’s highways: the circulatory system. The circulatory system is a vast, complex
network of “highways” responsible for transporting life-sustaining molecules, such
as nutrients, gasses, hormones and waste products, to the various organs of the body. The circulatory system’s highways are composed
of the heart and blood vessels, which carry the traffic of blood within them. In this video, we’ll give you a quick overview
of the vast and intricate networks of the circulatory system. We’ll start at the literal center of the
circulatory system: the heart. This is a highly specialized muscular organ
situated in the chest cavity, between the lungs. The strong heart muscles make the heart a
powerful pump, ensuring that the traffic in the system–namely, blood–keeps moving,
or rather circulating. If we peek inside the heart, we’ll see that
it is divided into four chambers: the left atria and ventricle and the right atria and
ventricle. Oxygen-rich blood, or oxygenated blood, enters
the left side of the heart, while oxygen-depleted blood, or deoxygenated blood, enters the right
side of the heart. Between the chambers of the heart are little
gates or valves to ensure that blood keeps moving in one direction. Now let’s see how and why the chambers of
the heart take in different blood and how that blood is oxygenated. Next up we have the blood vessels of the heart:
the actual highways. The blood vessels are the intricate network
of tubes that serve as roadways for blood. The blood is always contained in the vessels
and never directly interacts with the body’s tissues. This is why the human circulatory system is
called a closed system. There are three main types of blood vessels:
arteries, veins and capillaries. The arteries carry blood away from the heart
and out to the entire body. It mostly carries oxygenated blood, except
in a few cases, which we’ll explain shortly. The heart pumps blood directly into the arteries
at high pressure. In order to withstand the pressure, the arteries
have thick and highly muscular walls. The pressure is so high that we can feel it
in places where the arteries come close to the skin, such as the neck and in the wrists. Doctors use these spots to calculate a person’s
heart rate, as one pulse corresponds to one heartbeat. The veins carry blood away from the body and
back to the heart. The cells of the body have consumed all the
oxygen supplied by the arteries, so most veins just take deoxygenated blood back to the heart
to get re-oxygenated. The veins have thinner walls, as they aren’t
subject to the high pressure of the arteries. However, just like the heart, they have valves
or gates that keep the blood moving in one direction. In textbooks, veins are shown as blue, while
arteries are shown in red. This is only so textbooks can show you the
difference between the two. They aren’t actually this color. If you see real veins and arteries, they’re
white in color, and only the blood contained in them is red. Also, if you look at the backs of your hands,
you might see blue veins. This is just a cool optical trick based on
how visible light interacts with the skin! The last type of blood vessel, and the smallest,
are the capillaries. The capillaries are the tiny alleyways that
lead to individual neighborhoods and houses. These are 2 to 12 micrometers thick: thinner
than a human hair! They have tiny pores that allow for the exchange
of molecules like water, gases, nutrients and waste products between the cells. Just as any highway system would be incomplete
without vehicles, the circulatory system is similarly defined by the blood that travels
its paths. Blood is a specialized fluid tissue made of
several different types of cells. The liquid in which the cells float is called
plasma. This is the water in which all the molecules
and cells float. It contains various gasses, proteins, hormones,
and nutrients. Then, there are all the cells contained in
the blood. We have the cells that give your blood its
red color: red blood cells, RBCs, or erythrocytes. These transport oxygen throughout the body. Then there are white blood cells, WBCs, or
leukocytes, which are the body’s bodyguards against pathogens. Lastly, platelets are small cells that play
a key role in blood clotting whenever bleeding occurs. Interestingly, the blood also transports heat. The heat generated by the internal organs
is absorbed by the water in the blood and dissipated by the capillaries near the skin! We have now looked at the individual components
of the circulatory system, but how do all these parts come together to circulate blood
throughout the body? How does blood get oxygenated and then make
its way to the organs and cells that need it? The human body has a double circulation system. This means that the blood circulates through
the heart twice. Once it circulates to the lungs to pick up
oxygen; this is called pulmonary circulation. Then, oxygenated blood returns to the heart
and is pumped out to the body. This is called systemic circulation. Let’s take a look at how this happens. We’ll start with the right side of the heart,
where the deoxygenated blood enters the heart. The two big veins, the superior vena cava
and the inferior vena cava, bring in blood from the upper and lower parts of the body,
respectively, to the right atrium of the heart. From the right atrium, the blood flows into
the right ventricle. The heart contracts and the blood is pushed
out from the right ventricle into the pulmonary trunk. This is one of the few arteries in the body
that carries deoxygenated blood. The pulmonary trunk takes the deoxygenated
blood to the lungs, where gaseous exchange occurs and the blood receives oxygen. You can check out our respiratory system video
to understand that better. A quick aside here. The color of oxygenated and deoxygenated blood
is slightly different. Oxygenated blood is bright red, while deoxygenated
blood has a darker purple or maroon color. This is caused by the chemical interaction
of RBCs with oxygen. Now, the bright red, oxygen-rich blood will
go from the lung to the left atrium via the four pulmonary veins. The heart relaxes and the blood will flow
into the left ventricle. Again, the heart contracts and the blood rushes
out into the aorta, then out to the rest of the body. You might think that the “lub dub” is
the sound of the heart contracting, but it is actually caused by the vibrations of the
little valves in the heart closing. The first lub happens from the vibrations
of the valves between the atria and ventricle closing. The second and louder dub happens when the
valves between the ventricle and the arteries close. As the blood passes into the body it flows
from the aorta to the smaller arteries, which branch out several times before becoming capillaries,
where the blood exchanges its wares, and the blood flows back towards the heart as the
capillaries merge into venules, which then merge into the bigger veins. Without the circulatory system, multicellular
life would have remained very small. Every large animal has some form of a circulatory
system, even if that system doesn’t have a heart. The crazy part is that we only discovered
the mysteries of the heart and blood in the last 500 years, and there may still be much
more to learn about the secrets hidden within our blood!