Psst. If you’re running out of time and need a crash course in the Cardiovascular System, then you should read: The Anatomy and Physiology of the Cardiovascular System. If you’ve got the time, then hop on and let’s get started 🙂
Heart Anatomy Part 1: Introduction
The heart is a muscular hollow organ, typically weighing around 300g. However factors such as age, gender, body weight, illnesses, and exercise can increase or shrink its size.
Normally, the heart is found in the middle of the thorax, squeezed between the lungs and resting on the diaphragm, in a little cosy spot known as The Mediastinum. The heart forms the most important part of The Cardiovascular System, and its role is to pump blood to the body tissues and supply them with oxygen and nutrients.
Our hearts have three layers:
The Endocardium is the inner layer that lines the inside walls of the heart and valves with endothelial tissue.
The Myocardium is the middle layer that carries out the pumping action thanks to its muscle fibers.
The Epicardium is the outer layer and it’s made up of a visceral layer.
Then all of these layers are wrapped up nicely in a thin fibrous sac called The Pericardium which again is composed of two layers. The layer touching to the epicardium is known as The Visceral Pericardium, while the layer on top is called The Parietal Pericardium.
The little space in between the Visceral and Parietal Pericardium is not wasted, instead it is filled with around 20mL of fluid to reduce the friction created when the heart contracts.
Heart Anatomy Part 2: The Heart Chambers
As I mentioned above, the heart is a hollow organ meaning that it’s “empty” inside. But in reality, the heart is not just one big pocket, it’s actually made up of four different pockets known as chambers. There are two small chambers at the top known as Atria and two larger chambers at the bottom called Ventricles.
When the heart relaxes, all four chambers soften and this is known as Diastole, the part where blood rushes into the ventricles. The second part is known as Systole, and it refers to the contraction of the heart chambers. But unless you want your heart to explode, your body does not contract all chambers at the same time. Instead, it first squeezes the atria sending blood into the ventricles and filling them completely, and then once the ventricles are full they contract to push out the blood into the rest of the body.
So far we’ve said that we have two atria and two ventricles, first everything relaxes and fills up with blood. Then the atria contract, push blood into the ventricles and from there the ventricles contract, sending blood to the body. But exactly where does this blood come from in the first place, and where does it go?
Well, the blood entering the Right Atrium comes from three veins:
The Superior Vena Cava which drains the head, neck and upper extremities
The Inferior Vena Cava which drains the trunk and lower extremities
The Coronary Sinus that flushes blood from the coronary circulation
What this means is that the right atrium is taking blood that has already been used by the body. So this blood does NOT have any oxygen left because the head, neck, trunk and extremities have already used it. So we refer to it as Deoxygenated blood or Venous Blood.
The Deoxygenated (venous) blood is then squeezed into the Right Ventricle and squeezed again through the Pulmonary Artery to carry it into the lungs and mix it with oxygen.
So pause right here, and say it with me:
The right side of the heart takes Deoxygenated/ Venous Blood from the body and sends it to the lungs for oxygen.
Got it? Repeat it one more time.
Venous blood leaves the body tissues, enters the right side of the heart and then travels to the lungs for oxygen.
Good now let’s see what happens on the left side of the heart. The left atrium receives Oxygenated blood from four pulmonary veins, so this blood came from the lungs and its carrying oxygen. From there it goes into the left ventricle, through the Aorta and runs all over the body distributing oxygen.
Let’s pause again and say it together:
The left side of the heart takes Oxygenated blood from the lungs and sends it to the rest of the body.
If you look at the image below, you’ll notice that the walls of the atria and ventricles are not equally thick.
This is because their thickness is relative to their ‘strength’ or workload. When blood flows out of the atria and into the ventricles, the heart doesn’t use much force. So there’s little resistance, which means that the myocardial layer of both atria is thin compared to that of the ventricles. On the other hand, during ventricular systole (contraction) the heart has to withstand a great resistance to blood flowing from the pulmonary and systemic circulatory systems. So they are thicker than the Atrial walls. Plus, because the left ventricle has to push the blood through the aorta (which has super high pressures), it is actually thicker than the right ventricle.
Heart Anatomy Part 3: The Heart Valves
Have you ever done a handstand? Or if that’s too much, maybe you’ve tried touching your toes before. Did you notice that even though you basically flip your heart upside down it still functions well? And if we had to look at it the blood would still pump out of the atria into the ventricles and not the other way round. Well all of that can be credited to our heart valves.
Our heart has four valves made up of thin leaflets of fibrous tissue which keep the blood flowing in the right direction. Unless born with a birth defect, each of us have two types of heart valves:
Atrioventricular valves
Semilunar valves
The Atrioventricular valves are between the atria and the ventricles. The one separating the right atrium from the right ventricle has three flaps and so it’s called the Tricuspid valve. While the one separating the left atrium from the left ventricle has only two flaps and is called the Bicuspid valve (or sometimes referred to the Mitral valve).
Both valves open during Diastole (relaxation) to allow the blood in the atria to flow freely into the ventricles. Once filled, the ventricles start Ventricular Systole (contraction) and the pressure created forces both the Tricuspid and Bicuspid valve to close. This action prevents Regurgitation (aka. backflow)
The Semilunar valves are made up of three flaps all shaped like a semi-circle. There is one valve between the right ventricle and the pulmonary artery known as the Pulmonic valve and another in between the left ventricle and the aorta known as the Aortic valve.
Both valves close during Diastole to allow the ventricles to fill up with blood, and then are forced open during Systole to push the blood into the pulmonary artery and aorta.
Heart Anatomy Part 4: The Coronary Arteries
Just like every other organ, the heart requires oxygen to function properly. So the aorta has two branches that give arterial blood (aka. Oxygenated blood) back to the heart. These branches are known as the Left and Right Coronary Arteries.
These arteries are perfused with blood during diastole, and in individuals with a normal heart rate of 60 – 80 bpm, there is more than enough time for the arteries to take up Oxygenated blood. However, in patients with tachycardia that timeframe is shortened and as consequence, the myocardial cells (heart lining cells) do not get enough oxygen. This can lead to Myocardial Ischaemia especially in patients with Coronary Artery Disease.
Heart Anatomy Part 5: The Myocardium
If you go back up to the top, I mentioned that the middle layer of the heart is the muscular layer and it’s referred to as the Myocardium.
This layer is made up of specialised cells called Myocytes which together form an interconnected network of muscle fibers that surround the heart in a figure of eight pattern. When you observe these Myocytes you would see a spiral from the top part of the heart called The Base all the way to the bottom point of the heart known as The Apex.
When the heart contracts, the muscular pattern provides a twisted and compressed movement that starts off at the Atria and moves down to the Ventricles. This movement continues to repeat itself rhythmically and as a result it maximises the volume of blood ejected with each contraction.
Now the last question that you should ask is, What starts this whole process though? What makes the heart want to contract and relax?
And the answer to that is The Conduction System, but we’ll discuss that on another page.
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