Circulatory System Basics: Blood, Heart Flow, and ASD

Name: Circulatory System and Pathway of Blood Through the Heart
Uploaded: 2026-03-26T14:32:27.267726+00:00
Duration: 8 min 14 s
Channel: Amoeba Sisters
Description: Summary and key takeaways on Circulatory System and Pathway of Blood Through the Heart — Summary, covering to the Circulatory System The circulatory system

Amoeba Sisters

Mar 26, 2026

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8 min video

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2 min read

YouTube video ID: _vZ0lefPg_0

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The circulatory system moves glucose, oxygen, and carbon dioxide throughout the body. This overview provides a basic introduction to how the system functions and how blood travels through the heart.

Blood: The Transport Medium

Human blood is always red, although the shade varies with oxygen concentration. Diagrams often color arteries red and veins blue to indicate oxygen levels, but vessels are not actually those colors. Blood maintains pH, temperature, and osmotic pressure, thereby supporting homeostasis. It also transports hormones, nutrients, and gases.

Blood consists of plasma—the liquid portion containing water, proteins, salts, and lipids—and cellular components. The cellular portion includes red blood cells (which carry gases), white blood cells (which fight infection), and platelets (which help clot blood). Red blood cells contain hemoglobin, an iron‑bearing protein that gives blood its red color.

Heart Anatomy and Blood Pathway

The heart is divided into deoxygenated and oxygenated partitions. Arteries generally carry blood away from the heart and are usually oxygen‑rich, while veins generally carry blood toward the heart and are usually oxygen‑poor, with a few exceptions. Capillaries are the sites where oxygen is delivered to tissues and carbon dioxide is collected.

The right side of the heart pumps deoxygenated blood; the left side pumps oxygenated blood. The four chambers are the right atrium, right ventricle, left atrium, and left ventricle. Atria sit at the top and have thinner walls than the ventricles at the bottom. One‑way heart valves prevent backflow between chambers.

Deoxygenated blood from the body enters the right atrium via the vena cava, passes through the tricuspid valve into the right ventricle, and is pumped through the pulmonary valve into the pulmonary artery toward the lungs. In the lungs, blood picks up oxygen and releases carbon dioxide. Oxygenated blood returns via the pulmonary vein to the left atrium, moves through the mitral (bicuspid) valve into the left ventricle, and is expelled through the aortic valve into the aorta, the major artery that distributes oxygenated blood to the body. The heart receives its own blood supply from coronary arteries branching off the aorta.

Heartbeat and Cardiac Cycle

A human heart beats over 100,000 times per day, requiring coordinated contractions. The cardiac cycle describes the orderly sequence of heart muscle contractions and relaxations that produce each beat.

Cardiovascular Conditions

Some congenital heart conditions cause mixing of oxygenated and deoxygenated blood. An atrial septal defect (ASD) is an opening in the septum—the muscular wall separating the right and left sides of the heart—that allows such mixing. Potential consequences of ASD include abnormal heartbeat, stroke, or heart failure. Treatments may involve medication or surgery.

Takeaways

The circulatory system transports glucose, oxygen, and carbon dioxide while maintaining pH, temperature, and osmotic pressure.
Blood is always red; its shade changes with oxygen levels, and it consists of plasma and cellular components including red cells, white cells, and platelets.
The heart's four chambers and one‑way valves direct deoxygenated blood from the body to the lungs and oxygenated blood from the lungs to the rest of the body.
A human heart beats more than 100,000 times each day, relying on the coordinated cardiac cycle of contraction and relaxation.
An atrial septal defect creates a hole in the heart's septum, allowing blood mixing that can lead to abnormal rhythm, stroke, or heart failure, and may be treated with medication or surgery.

Frequently Asked Questions

What is an atrial septal defect and how does it affect blood flow?

An atrial septal defect (ASD) is an opening in the septum that separates the heart's right and left sides, allowing oxygenated and deoxygenated blood to mix. This mixing can cause abnormal heartbeats, increase the risk of stroke, and potentially lead to heart failure if untreated.

Why are arteries shown as red and veins as blue in many diagrams?

Diagrams use red for arteries and blue for veins to visually indicate typical oxygen content—arteries usually carry oxygen‑rich blood and veins usually carry oxygen‑poor blood. In reality, both types of vessels are not colored this way; the colors are symbolic.

Who is Amoeba Sisters on YouTube?

Amoeba Sisters is a YouTube channel that publishes videos on a range of topics. Browse more summaries from this channel below.

Does this page include the full transcript of the video?

Yes, the full transcript for this video is available on this page. Click 'Show transcript' in the sidebar to read it.

Helpful resources related to this video

If you want to practice or explore the concepts discussed in the video, these commonly used tools may help.

Human Circulatory System Model Recommended

A model of the human circulatory system can help visualize the complex pathways of blood flow and the structure of the heart, reinforcing the concepts explained in the video.

Amazon →

Heart Anatomy Model

A detailed model of the heart's anatomy, showing chambers, valves, and major vessels, can aid in understanding the specific structures and how blood moves through them as described.

Amazon →

Blood Composition Chart Poster

A poster illustrating the components of blood (plasma, RBCs, WBCs, platelets) and their functions can serve as a quick reference and aid in memorization of this information.

Amazon →

Cardiovascular Health Book For Beginners

A book on cardiovascular health can provide further context and detail on topics like congenital heart conditions and maintaining a healthy circulatory system, expanding on the video's brief mention.

Amazon →

Links may be affiliate links. We only include resources that are genuinely relevant to the topic.

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Full Transcript YouTube

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Did you have breakfast this morning?
Did you ever wonder after you eat breakfast
and your food is digested, how is that glucose
getting transported around your body?
Or as you take a breath in, how does that
oxygen get transported around?
Or when you exhale, how does that carbon dioxide
get transferred out?
Are these only questions I ponder?
Maybe.
But your circulatory system is absolutely
fascinating and highly involved in this.
In this short intro video of the circulatory
system, we will mention some basics about
its functions and trace the pathway of how
blood travels through your heart, but please
know before we get started, there are gigantic
textbooks on the circulatory system itself.
So, obviously, this video is just an intro.
We’re going to first talk about blood: the
medium of how we transport glucose and gases.
As we mention in our body systems intro video
from many years ago- there are some misconceptions.
Human blood is red and always red although
the shade of red can vary based on how much
oxygen is in the blood.
Veins and arteries are often drawn in diagrams
as blue or red to show whether they have lower
or higher concentrations of oxygen, but that’s
just how it is used in most diagrams.
It doesn’t mean the blood, or the veins,
or arteries are actually that color.
Veins that you see under your skin may look
blue or green by the way, but that involves
the way they appear under the skin and the
reason for this would make a great physics
topic ----but I digress.
Human blood has a lot of functions.
It maintains a certain pH, temperature, osmotic
pressure – all of this is very important
for homeostasis.
It transports things like hormones, nutrients,
and gases.
And it’s made up of different components.
One component includes plasma, the liquid
portion.
Water, proteins, salts, lipids- you’ll find
them in this liquid portion of blood known
as plasma.
Another component includes cellular components.
This means red blood cells, which do the transporting
of gases.
White blood cells which can fight infections.
And platelets, which are actually cellular
fragments, and they’re involved with helping
your blood clot.
Very important when there is damage to the
body.
Red blood cells have an iron-containing protein
called hemoglobin, and that is where that
red coloring of blood comes from.
So, when we’re talking about blood, and
we’re just introing the circulatory system,
we’re going to focus on how this blood moves
around in the human body.
Human heart anatomy observes the heart divided
into two distinct and separated partitions;
a deoxygenated, or low-oxygen partition, and
an oxygenated partition.
There are some human congenital heart conditions
that can result in this oxygenated and deoxygenated
blood mixing, however.
More on that at the end.
Arteries generally carry blood “away”
from the heart.
Think “A” for away.
Arteries are typically oxygen-rich but there
are exceptions.
Veins generally carry blood “to” the heart.
Veins typically are oxygen-poor but there
are exceptions.
Capillaries are small blood vessels and it
is at the capillary level where oxygen is
delivered to organs and tissues and where
carbon dioxide will also be picked up to travel
back to the lungs.
So, looking at this heart, the right side
(and that’s the person’s right, so for
you it will look opposite) pumps deoxygenated
blood and the left side pumps oxygenated blood.
We can also see 4 chambers: the right atrium
and right ventricle and the left atrium and
left ventricle.
I like to remember that A comes before V in
the alphabet so that helps me remember the
A’s – for atria- are at the top of the
heart.
V for ventricles- are at the bottom of the
heart.
Atria also have thinner walls than the thicker
walled ventricles.
The heart also contains valves which we’ll
see when we get to tracing the pathway of
blood.
The valves are one-way structures that help
separate the chambers and also prevent backflow
of blood.
Ready to take the adventure of a lifetime?
An adventure tracing the pathway of blood
through the heart?
We’re going to start with blood that is
in a human toe.
This blood is deoxygenated.
It needs to get to the heart so that it can
be pumped to the lungs to pick up oxygen and
then be spread throughout the body.
It’s going to get there through the vena
cava.
Inferior vena cava to be specific as superior
vena cava is above the heart.
The blood enters the right atrium.
The right atrium contracts, pushing the blood
through the tricuspid valve into the right
ventricle.
The right ventricle contracts, pumping the
blood through the pulmonary valve to the pulmonary
artery.
By the way, when you see the word “pulmonary,”
it likely involves the lungs.
The pulmonary artery takes blood to the lungs
where the red blood cells in the blood will
take on oxygen and release carbon dioxide.
Now this blood is oxygenated!
It needs to return to the heart so that the
heart can pump it throughout the body.
The oxygenated blood travels through a pulmonary
vein to the left atrium.
The left atrium contracts and the blood travels
through the mitral valve, also known as the
bicuspid valve, into the left ventricle.
The left ventricle contracts and pumps the
blood through the aortic valve and out a major
artery known as the aorta.
The aorta is a major artery that carries oxygenated
blood throughout the body.
Now I don’t want to neglect the fact that
the heart needs its own blood supply to deliver
oxygen and glucose.
The heart can receive this blood supply through
coronary arteries.
Coronary arteries branch off the aorta and
eventually deliver blood into capillaries.
These capillaries deliver oxygen and glucose
to the heart.
Coronary veins will take the deoxygenated
blood to the right atrium where the blood
will eventually travel the pathway to become
oxygenated.
In fact to quiz yourself, can you pause the
video and trace the pathway of blood again?
Ok, all together.
Right atrium, tricuspid valve, right ventricle,
pulmonary valve, pulmonary artery, lungs,
back through the pulmonary vein, left atrium,
mitral valve (bicuspid valve), left ventricle,
aortic valve, aorta…takes it to the body
and then it will eventually return through
the vena cava back to the right atrium again.
*phew* It almost makes you want to turn it
into a song!
But we won’t.
The significance of the pathways, how they
interact, the coordination of contraction,
and many more elements are part of every beat
of your heart.
A human heart beats over 100,000 times per
day so it’s significant that every beat
is coordinated and blood is directed where
it should go.
The complexity of the cardiac cycle – which
is the coordinated sequence of the heart’s
contractions and relaxations – isn’t something
this short video can go into; hopefully a
separate video on that soon.
One last thing: there are many conditions
in which the heart doesn’t function correctly.
Anatomically, some heart conditions change
the pathway flow of blood.
One example that we had mentioned before is
an atrial septal defect.
The septum is the muscular wall that separates
the right and left side of the heart.
So, a septal defect could mean an opening
and oxygen-rich blood could mix with oxygen-poor
blood.
Depending on the size, this can cause future
problems such as an abnormal heartbeat, stroke,
or potentially heart failure in severe cases.
Some medications may help the symptoms or
surgery can be an option.
There continues to be more advancements for
treating cardiovascular conditions.
If you have interest in the amazing field
of cardiology, take a look at the suggested
further reading links in the video details!
Well, that’s it for the Amoeba Sisters,
and we remind you to stay curious.