This Is Your Circulatory System
Your body's plumbing network and how it feeds and sustains every single cell in your body.
Hello!
Welcome to month two of our Basics Matter series. In January we got the basics right and then we learned about the nervous system, covering everything from how it looks, how it works, what happens when it gets sick, and how to keep it in good health in the long run.
This month we will explore a new system–or should I say navigate…
Say hello to The Circulatory System!
After having learned about the control tower of your body and how all its functions are organically orchestrated by the incredible nervous network, we now move into what feeds and fuels every single cell, tissue and organ in the body to be able to perform what the nervous system controls. Beautiful!
In today’s chapter you will learn:
What is the circulatory system?
What parts make the circulatory system?
How does each part work?
The lymphatic system and how do these two work together?
A movie suggestion for you
Get ready for a fascinating ride!
The Circulatory System is the system in charge of pumping blood from the heart to the entire body, and bringing it back to the heart (and lungs for oxygenation).
The circulatory system is basically the pipe system of the human body. Through it, the main fluids are able to flow all over the place and back, transporting the most important nutrients, oxygen, hormones, and other vital elements for the body’s healthy functioning. It’s also known as the Vascular system.
Nerdy Fact: The image above is a screenshot from the movie ‘Fantastic Voyage’—a 1966 American science fiction adventure film about a submarine crew who is shrunk to microscopic size and venture into the body of an injured scientist to repair damage to his brain. I highly recommend watching it, especially after today’s newsletter. Apparently, James Cameron has announced a remake of this incredible movie. I have to thank for making me aware of these nerdy facts that I love!
Let’s continue. Here’s a great metaphor to explain the circulatory system:
“Your body has a "highway" system that sends blood to and from your body parts. It's called the circulatory system and the roads are called arteries and veins. Arteries, which usually look red, carry blood away from the heart. Veins, which usually look blue, return blood to the heart.” ~ Kids Health
As you can see in this general illustration, the circulatory system reaches every corner of the body:
The circulatory system is made of four main parts:
The heart
The blood
The blood vessels: arteries, veins and capillaries
The lymphatic system*—Even though this is a separate system, it’s highly connected to circulation as it helps drain excess fluids and waste from the circulatory system. This system is formed by lymphatic vessels, nodes, and the lymph. We’ll expand on this system later into the series.
The way the circulatory system works is simple:
The heart pumps blood that leaves through the arteries, carrying oxygenated blood from the heart to the entire body. As arteries spread out and thin up, capillaries are formed in the most distant ends. In here, oxygen is taken through capillaries by cells to be used in their biological processes.
After this, blood is now left without oxygen. It proceeds to leave the capillaries, into the veins, taking the deoxygenated blood back to the heart (right side), then the lungs where they load on oxygen, back to the heart (left side) and off to the body it goes again!
Here’s a short video for a clear visualisation of this circulatory and oxygenation process:
As you might know, every cell in the body needs oxygen to live. The oxygen enters the body through our breathing. We breathe fresh air filled with oxygen into the lungs, the fresh air reaches the capillary vessels in there, where oxygen will be caught by a specific type of blood cell (erythrocytes or red blood cells) and taken to be transported to every cell in the body. The only way in which oxygen can be carried throughout the body is through the blood, hence why blood and the circulatory system are key to our survival. Even while we sleep, the circulation keeps flowing and working hard overnight.
Did you know that…
“Your heart circulates about 2,000 gallons (more than 7,500 litres) of your blood every day. During physical activity, your heart pumps even more than usual. Your circulatory system adapts to meet your body’s needs for blood and oxygen.
Your circulatory system makes it a high priority to supply blood to your heart and brain. If your brain doesn’t get the blood it needs, you can lose consciousness within seconds. You can have brain damage after four minutes without blood flow. Similarly, your heart quickly starts to have trouble pumping if it’s not getting enough blood.” ~ Cleveland Clinic
That is how vital and connected these organs and systems are. The human body is without doubt a most fascinating machine!
Besides transporting blood and oxygen, the circulatory system has many other functions:
It transports nutrients, hormones and other important substances to every cell in the body.
It removes waste products from the metabolism and takes it to the organs in charge of excreting waste outside the body (kidneys, bowels, skin).
It protects the body against disease and infection, thanks to the white blood cells.
It makes the blood clot after an injury to prevent excessive bleeding and loss of pressure in the system.
It helps regulate the body’s temperature.
It secretes hormones that help in salt-water balance and blood pressure regulation.
Now, let’s dive into each part of the circulatory system.
The Heart
A beautiful and magical pump that beats every second to its own rhythm, pushing blood through arteries towards the entire body. Call me a romantic but I’m deeply in love with the heart. I wanted to be a heart surgeon during med school. However, I feared for my future wellbeing, so I eventually decided to let such dream to rest and chose another equally exciting path. Life can be curious and wise like that.
Back to the heart! This organ is about the size of your fist, and it’s made of different layers of tissue, being muscle the most prominent one (called myocardium), and in charge of doing the hard pumping work. The heart is located right in the middle of your chest, behind and slightly to the right side of your sternum bone, also known as the breastbone, and fully protected by your ribs. An average adult heart can weight between 230-350 grams and it beats between 60-100 times per minute at rest under normal conditions.
In rare occasions, the heart can be located on the right side of the chest. This situation is called Dextrocardia, and it’s a rare congenital (present at birth) condition that happens in less than 1 % of the population. People with dextrocardia can lead normal lives. However, it’s not uncommon to find other conditions associated with it.
Anatomically speaking, the heart has:
Three layers: called endocardium, myocardium (muscle), and epicardium
Four chambers: two called atrium (plural: atria) and two called ventricles
Four valves: Aortic, Pulmonary, Mitral, and Tricuspid
One thick, badass, protective bag called Pericardium
Just for anatomical learning purposes, let’s have a closer look at these all (ignore the white arrows). Focus on atrium, ventricle and valves in the image below:
The atriums and ventricles are spaces or chambers where the blood flows within the heart. In between these chambers, we have the valves. Their function is to work like a gate that can only open in one direction (forward), preventing blood from flowing backwards into the previous chamber. Blood must always flow in one direction only. When it flows backwards for whatever anomalous reason, we have a situation called regurgitation (basically, a leak in the heart’s flow) and it needs to be studied and controlled by a heart specialist (Cardiologist).
The most common heart regurgitation problem happens in the Mitral valve. It can go without symptoms for years. If it evolves, it could give some symptoms such as shortness of breathe when lying flat or with exercise, fatigue, palpitations, swelling in legs, neck or abdomen. If unattended, it can later become an acute and risky medical emergency. These symptoms won’t automatically translate into a heart valve problem, but it’s important to always consult your doctor about any symptoms or sensations out of the usual.
The Heart’s Electrical System
Now, how does the heart beat?
Turns out that the heart has its own tiny electrical system or engine to make it beat, called the sinus node. This sinus node is the heart’s own pacemaker, made by modified cardiac muscle fibres, specialised to carry nervous impulses. (I find this particularly fascinating.) This node, also known as the conductive system of the heart, is made of five parts, named according to the direction in which the impulse flows:
Sino-Atrial (SA) Node
Atrio-Ventricular (AV) Node
Bundle of HIS
Right and left bundle branches
Purkinje fibres
Under normal conditions, the one factor which makes the heart beat non-stop continuously, is the triggering of neural impulses in the SA Node (Sino-Atrial Node). This is where the heartbeat begins. The SA Node is located right in the right atrium at the junction of the superior vena cava, and is controlled by the Vagus Nerve (a vast nerve in the body that controls endless functions in different organs. We briefly mentioned it in Nervous System Part 1.
The sequence of how the Sinus Node flows looks like this:
An electric impulse is released at the SA Node and travels towards → AV Node → Bundle of HIS → Left bundle branches and right bundle branches→ Purkinje Fibres
To better understand, watch this short video showing exactly how the heart beat flows:
The normal rhythm of the heart is called sinus rhythm—after the Sinus Node.
A normal adult heartbeat (or pulse) goes between 60-100 beats per minute (bpm). If it goes lower than 60 bpm, we call it bradycardia—’bradys’ from the Greek word meaning slow, and ‘kardia’ meaning heart. If it goes higher than 100 bpm, we call it tachycardia—’tachy’ from the Greek word meaning rapid or fast. A slow or fast heart rate isn’t always a concern, unless there’s an associated symptom. In this case, always make sure to consult your doctor.
In newborns, a normal heart rate can be as slow as 90-160 when they’re resting or sleeping, and as high as 200 bpm when they’re crying or agitated. Kids have a faster heart beat due to their continued growth, until it normalises to more adult parameters after the age of 8.
There are many ways in which us doctors check, measure and study the heart. The most common ways are via our beloved stethoscope, and also the test called electrcardiogram. There are also ultrasounds (echocardiogram), stress tests (the one were you run on a treadmill), X rays, scans, blood tests, and non-invasive procedures. Always an option to perform a good heart check up!
Now, let’s jump onto a super nerdy heart-fact:
The Heart’s Endocrine System
Did you know that there is something called the ‘endocrine system of the heart’? Meaning, that the heart has its own little hormone system that helps it communicate with other organs in the body. One of the main heart hormones is called Atrial Natriuretic Peptide (APN or NP), which is produced in the heart’s right atria, specifically in its muscle cells (myocytes)—in response to high blood pressure and excess salt and water in the body.
“Maintaining salt-water balance is of fundamental importance for all animals. Under high blood volume and pressure, heart muscle cells release ANP into the circulation. In the kidney, ANP enhances salt and water excretion. In the blood vessel, ANP promotes vasodilation. As such, ANP acts as a cardiac hormone to regulate blood volume and pressure. Defects in the ANP pathway now are known to contribute to major diseases such as hypertension, cardiac hypertrophy and heart failure (HF).”
In simple words: whenever the heart detects high blood pressure, an increased amount of fluids, or high sodium in the blood, or whenever the vessels stretch a bit too much, the heart releases APN into the bloodstream. This APN tells the cells in blood vessels to dilate and relax, lowering the blood pressure; and in the kidneys to loose extra water and salt (making you pee). These two mechanisms—relaxing blood vessels and releasing salt and water—are the main responses towards fixing high blood pressure peaks in the body.
As you can see, the heart is a powerful, smart and resilient machine with an incredible nature of its own, yet impacting your entire body, mind, emotions and life. Seriously amazing.
Let’s keep travelling forward onto the next part.
The Blood Vessels
As the heart pumps, it pushes blood out of your heart to be distributed throughout the entire body and back. This happens via the blood vessels of the circulatory system: arteries, veins, and capillaries.
Have you ever wondered about those purple lines on your skin, or that pulsating sensation on your wrist or the side of your neck? It all has to do with your blood vessels! The purple lines are your veins; the pulsating sensations are your arteries.
Let’s have a quick overview:
Arteries transport blood with oxygen from the heart to the rest of the body.
Veins transport blood without oxygen from the body back to the heart, to be oxygenated again in the lungs.
Capillaries are the bridge section between arteries and veins, and where oxygen and nutrients are taken by cells to be used.
In diagrams and illustrations, blood is always depicted in red when travelling through arteries (carrying oxygen), and in blue when travelling through veins (without oxygen). It’s just an easy way to study, remember, and understand the whole vascular system.
Let’s learn about them in more detail.
Arteries
Arteries are thick, strong tubes that transport blood with oxygen from the heart to the entire body. Their wall is highly elastic, with a muscle layer that pulsates, allowing arteries to dilate or stretch according to the body’s needs. Arteries branch and re-branch, until they become smaller units called arterioles. As they get closer to the surface or into smaller spaces, blood is transferred into microscopic vessels called capillaries.
The most important and largest artery in the body is called Aorta. It travels from the heart all the way down to your lower abdomen, where it splits into the two main arteries that will continue further down to your legs and feet. The aorta has on average a 2,5 cm diameter—imagine that! A tube almost as wide as a banana running down your chest and abdomen. Mind-blowing, isn’t it?
Veins
Veins are the tubes transporting blood without oxygen (except in the lungs—more on this later) from all corners of the body and back into the heart. Veins have thin, non-elastic walls and they have no pulse. Instead, they have valves to prevent blood from travelling backwards. In the capillaries, microscopic venules merge giving way to veins.
The most important and largest vein in your body is called Cava, which divides in two sections: superior and inferior. It goes from your lower abdomen all the way into your heart.
Capillaries
Capillaries are the thinnest network of vessels between arterioles and venules—thin as a hair! Capillaries are so small that blood cells can only move through them one at a time, in a queue. Oxygen and nutrients coming through arteries and arterioles pass from these capillary section to the cells. Capillaries then become venules and veins, where waste from the cells can be transferred to the blood to be removed through waste excreting organs, like the kidneys.
It’s thanks to this amazing pipe system that every single cell in your body gets all the nutrients, oxygen, messages and more, to function normally and keep you healthy. Without this pipe system, there wouldn’t be life in any organ of your body.
It’s also in the blood vessels where so many of the most common health problems happen, such as atherosclerosis, high blood pressure, clots, and more. We will cover these in our next email, part two of the circulatory system.
Now, let’s circulate—like the blood (pun intended!) and continue with the third element of the circulatory system.
The Blood
The master fluid of the human body. The one that flows nonstop in one direction your entire life, feeding every single cell, tissue and organ in your body.
The blood is the main fluid running through your body’s arteries and veins, transporting all vital elements for cells to function—yes, every single cell in the body. Elements such as oxygen, hormones, nutrients, and proteins, as well as waste products to be removed.
One adult person has around 5 litres of blood in their body. This blood consists of approximately 45% solids (cells) and 55% fluids (plasma). Plasma is mostly water and some elements like oxygen, hormones, proteins and electrolytes. Cells in the blood are plenty and of different types—we’ll talk about them next.
There are three main groups of cells in the blood: the Red blood cells, (RBC), the white blood cells (WBC), and the platelets. Let’s have a closer look at them:
Red Blood Cells: also called Erythrocytes, they are the ones in charge to carry the oxygen. They are made in the bone marrow and live in the blood. They have a special protein within them called haemoglobin which is the oxygen carrier. To put it simply: oxygen is the passenger, the red blood cell is the car, and haemoglobin is the seat where oxygen seats while being transported. Red blood cells have a cute, soft look (1, 2), and when there’s a bunch of them it looks like a bowl of Fruit Loops (3):
Images 1, 2 & 3 In order to understand red blood cell oxygenation, we need to have a quick look into the lungs (which we will study in depth next month):
White Blood Cells: they are in charge of defence and immunity, and they’re called Leukocytes. There are many subtypes, which you can normally read on an average blood test (haemogram), such as: lymphocytes, monocytes, neutrophils, monocytes, eosinophils, basophils, T-cells, B- cells, and mast cells. They all have specific functions, but basically, they all help fight bacterias and infections.
Platelets: also called Thrombocytes, are in charge of blood clotting. This clotting process happens to make sure that no blood is lost from its main course, allowing oxygen and vital elements to reach cells so they can continue working and keeping you alive. Platelets have a funny look, almost like a deranged octopus:
When seen through a microscope (with a special dye called H&E Stain that turns cells pink and purple), a drop of blood with all its cells looks like this:
Fun Fact! Doctors who study and specialise in all things blood are called Haematologists. Scientists who study all tissues of the body through a microscope are called Histologists. Medical doctors who specialise in all things heart are Cardiologists, and if it involves surgery, they’re called Cardiovascular surgeon (peripheral circulation specialist) and Cardiac surgeon (heart surgery specialist).
Blood Types
Now, you might have heard or know about blood types. Let’s talk about them.
Here’s an interesting fact:
“All blood does the same thing, but not all blood is the same.”
There are different blood types according to specific particles in the blood called antigens—substances that can make your body’s immune system react. The presence or absence of an antigen, makes the different blood types.
As Cleveland Clinic explains:
“Think of an antigen as a marker (like a nametag) that identifies a substance in your body as belonging or not belonging. Your blood type is compatible with someone else’s if your immune system recognizes the antigens in donated blood as belonging.”
This matters because is the way healthcare providers have to know if your blood is compatible with another, useful in blood transfusions, donations, or organ transplants. This can also save a person’s life during a medical emergency.
There a several blood classification systems, but the most common ones are:
The ABO System
The Rhesus (RH) System
The ABO system is based on A and B antigens on your red blood cells, resulting in the blood types: A, B, AB or O. Like this:
Type A: Red blood cells have the A antigen.
Type B: Red blood cells have the B antigen.
Type AB: Red blood cells have both A and B antigens.
Type O: Red blood cells have neither A nor B antigens.
The RH system determines through the RH factor if a blood type is (+) Positive or (-) Negative.
Positive (+): Red blood cells have the RhD antigen.
Negative (-): Red blood cells don’t have the RhD antigen.
All these make the eight common blood types as we know them:
A positive (A+)
A negative (A-)
B positive (B+)
B negative (B-)
AB positive (AB+)
AB negative (AB-)
O positive (O+)
O negative (O-)
There are other existing blood groups but they are quite rare. If you want to learn more about blood types, here’s an interactive guide by the Red Cross showing you the most important things about blood types, transfusions, and rare types.
Do you know your blood type? I always advice my patients, friends, and relatives to know their blood type. In some countries they even include it in the driving license document (smart!). If you don’t know yours, I highly recommend you ask your healthcare provider. Mine is AB+ and I’ve been told I should donate blood for myself because it’s among the least common types—eeeek! Thankfully, AB blood type can receive blood from all the other types, so it’s unlikely that I would die from lack of blood availability. Whew!
The Lymphatic System
As I mentioned earlier, the lymphatic pathways are a separate system that work intricately with the circulatory system, so let me give you a brief overview of this important network.
In short, the lymphatic system is a parallel circulatory system made up of lymph vessels, which are much like blood vessels. It drains extra fluid (called lymph) that has passed from the blood into tissues, and returns it back to the blood.
This group of organs, vessels and tissues protects you from infections and keeps a healthy balance of fluids throughout your body. Lymph organs include tonsils, spleen, bone marrow (where blood cells are made), thymus, lymph nodes and vessels. Physiotherapists study this network in depth as it is vital when performing therapeutic and draining massages to aid the circulatory system.
Here’s a good and easy explanation of the lymphatic system by Cleveland Clinic:
“Every day, about 20 liters of plasma (the liquid part of your blood) flow out of tiny pores in the thin walls of your capillaries. Imagine water seeping out of a sponge. Where does this liquid go? It delivers oxygen and nutrients to the tissues surrounding each capillary. The tissues hungrily soak up all the nutrients while leaving behind waste. The plasma doesn’t mind cleaning up the mess — it picks up the waste and then returns to your bloodstream via their tiny lymphatic capillaries. Each day, about 17 liters of plasma return to your bloodstream in this way.”
Here’s a diagram of the lymphatic system. The green lines are the lymphatic vessels, which include tiny (lymph) nodes for immune defence and drainage, and lymphatic organs that help fight infections and beyond:
How do the circulatory and lymphatic systems work together?
If we put both circulatory and lymphatic system next to each other, you’ll easily realise why they work together. The waste and excess fluids from the circulatory system are picked up by the lymphatic system and drained back into circulation before entering the heart again. They help each other move all fluids in the body as needed, making sure nothing gets left behind, and that no waste is left unremoved.
As you can see, the circulatory system is a passionate topic. I personally LOVE this system for many reasons. It’s more like a feeling I suppose! After all, some may say feelings live in the heart.
If you enjoyed today’s newsletter—and if you’re nerdy like me—don’t forget to watch the Fantastic Voyage movie. After everything you’ve learned today, you’d be mind-blown by how much you will be able to understand from the movie. Considering the visual are effects from 1966, it’s amazing how real it seems.
Here’s a short clip showing the exact moment when the miniaturised crew enters the bloodstream. Let’s geek out together! 🤓
Now, Let’s Summarise
As we come to an end, it’s time to summarise—since we’ve covered a good lot today! Let me give you the main points as a reminder for future reference:
The circulatory system is the system of the body in charge of blood circulation to feed oxygen, nutrients and all vital elements for cells to work and thrive.
The circulatory system is made of the heart, the blood, and the blood vessels.
The blood vessels are arteries, arterioles, veins, venules, and capillaries.
The blood is 54% plasma, 45% red blood cells, and 1% white blood cells and platelets. It is classified in blood types according to two different antigen-based systems: ABO and RH systems.
The circulatory system works in close connection with the lymphatic system, which helps drain all excess fluids and waste from the tissues back into the bloodstream to be removed.
The heart has a hormonal system that responds to high blood pressure peaks, releasing a special substance (APN) that communicates with kidneys and blood vessels to lower blood pressure to safe levels.
Next Time
In part two of the circulatory system you will learn everything about cardiovascular disease (cholesterol, high blood pressure, heart attacks, just to mention a few), how heart, arteries and veins get sick, how they can be affected by your daily habits and lifestyle. And most importantly, how to prevent (as possible) affections of our beloved circulatory system. If there’s any particular disease that you’d like me to cover, please leave it in the comments and I’ll include it as best as possible!
Stay tuned and see you next time. 😉
Love,
Dr. Mariana
Ps. If you got any doubts or questions, please share them in the comment section. I’ll clear them there for everyone to learn from.
If you found this article helpful, please ❤️ it so that other readers can find it. Also share it with your friends and relatives. This is (vital) free learning for everyone. Thank you!
Image Credits: Cover
Ay ayyyyy this is AWEsomeeeee😭😭😭😭 so excited to read!!!! It’s 2am here lol i should go to bed but OH just wait for me and my milky earl gray tomorrow, dear circulatory system!!! Can’t wait to meet you.
( I just really want to thank you even before reading it bc i know it’ll be a great guide, Thank you for all your effort.❤️)
Fascinating and easy to understand! Thank you, Dr. M. I would like to know about cholesterol. How does the body make it naturally (or too much of it) as opposed to taking in too much in one’s diet.