This is the first in hopefully a long series of posts about the human body. I hope for it to be an easy and comprehensive guide to help understand the wonders of our body. I’m starting with the circulatory system (my favorite!) and today we are going to focus on the heart. Since it is such a big topic, we are going to split it up into two posts. This one will focus on the structure and the functions of the different chambers.

The heart is arguably the most important muscle in the human body. It pumps our life giving fluid (blood) to all of the corners of the body. On average, an adult’s bpm (beats per minute) is 60 to 100 beats, while for a child it is between 70 – 100 ( But what really goes on behind the scenes when this is happening?

The Structure of the Heart

Diagrams are really helpful, but sometimes it is hard to understand them just from the pictures and labels themselves. That is why I’m here!

The heart is only as big as an average fist, and it only weighs around a pound. Despite this, it is one of our strongest muscles. It will be interesting to see how the human body evolved to develop such an intricate sturdy body part. Let’s get right to it!

The heart resides between the two lungs and above the diaphragm, inside the ribs. many people believe that it is placed in a straight position. However, the apex (bottom tip) of the heart, is tilted to the left direction. Hence part of the reason we say the the heart is on the ‘left’ side of our bodies.

The pericardium is the outermost part of the heart. It has two walls, and acts like a ‘bag’ for the heart. The outer fibrous pericardium protects the heart and attaches the heart to some of the other surrounding body parts (such as the previously mentioned diaphragm). The inner part of the wall is the serous pericardium, and it is further divided into two parts. The parietal layer attaches itself to the main, big arteries. After doing that, it goes back over the heart, and we call this the visceral layer/epicardium. The serous fluid is produced by pericardial membranes to reduce friction between layers.

The next layer is the myocardium. Unlike the pericardium, it is mostly composed of cardiac muscle. It is twisted into a pattern to become strong and dense. It must be, because this part of the heart is what actually contracts. Bands of collagen connective tissue further reinforce this layer. It has been nicknamed the ‘skeleton of the heart’.

Finally, the endocardium is an endothelium lining that lines the blood vessels. It is thin compared to the other layers, but it is thicker in the atria than in the ventricles.

The main heart structure inside of our heart is the interventricular septum. It separates the left and right side of the heart.

The Four Chambers and Their Functions

The heart is made of cardiac muscles which contract and expand using electric energy pulses from the body. There are mainly four parts: the left atria, the right atria, the left ventricle, and the right ventricle. The right side is responsible for pumping to the lungs (pulmonary circuit), while the left side is responsible for pumping to the body (systemic circuit). Also, as you can see, the ventricles are significantly larger than the atria. This is because that the ventricles are the ones actually pumping. When they contract, the blood rushes out of the heart into the arteries. A useful analogy would be squirting water out of a water balloon.

Now, let us move onto the functions: The right atrium receives the oxygen-poor blood of the body from the superior and inferior venae cavae and sends the blood to the right ventricle. The right ventricle then pumps the blood to the lungs for oxygenation through the pulmonary trunk, which later ranches off into the pulmonary arteries.

Quick Bits: Oxygenation As we are talking about this, it is crucial to give a slight overview on the process of oxygenation: it is a process where the blood moves through the small sacs in the lungs called alveoli. Oxygen molecules attach themselves to proteins (hemoglobin) in the red blood cells. Hemoglobin is essentially what bring the oxygen all around the body with them, keeping us alive.

Now, back to the heart. After oxygenation, the oxygen rich blood is brought back to the heart by the pulmonary vein, this time to the left atrium, which pushes the blood to the left ventricle. The left ventricle then pumps it to the aorta to distribute it to the arteries of the body. Finally, oxygen poor blood coming back from body tissues goes back to the heart through the superior or inferior venae cavae.

The Valves of the Heart

There are four valves in the heart to make sure that the blood only flows in one direction: from the atria to the ventricles. Imagine the mess if it suddenly started going the other way around!

The first set of valves are the atrioventricular (AV) valves. The left bicuspid/mitral valve has two flaps made of endocardium. The right tricuspid valve has three flaps. Both of the valves have chordae tendineae that attach the to the walls of the heart. These valves don’t do anything when the heart is filling up with blood. However, when the ventricles contract to push blood out, pressure makes the flaps close. This prevents the blood from going back up to the atria.

The second set of valves are the aortic and pulmonary semilunar valves. These are found at the base of important arteries that blood leaves through from the ventricles. Each valve has three flaps. The flaps open from the pressure of the blood when the ventricles are contracting. This pushes them against the walls of the arteries to let blood flow past. They close again when the ventricles relax.


  • The heart is placed between the lungs, above the diaphragm, inside the ribs.
  • The heart has three main layers: the pericardium, myocardium, and endocardium.
  • The heart has 4 main chambers: the left and right atria that bring blood in, and the left and right ventricles that pump blood out.
  • The left side of the heart is part of the pulmonary circuit (focused on the lungs) and the right side of the heart is part of the systemic circuit (focused on the rest of the body)
  • There are two sets of valves: the atrioventricular valves that protects the flow of blood from going from the ventricles back into the aria, and the semilunar valves that open only when blood is being pumped from the ventricles to the main arteries (when the heart contracts).

Looking ahead: NEXT — Cardiac circulation, heartbeats, and electric pulses.

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