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How The Heart Works

By Tyler Woodward

Life begins and ends with a heartbeat. The heart is one of, if not the most important organs in the human body, so let's dive into how the heart actually works.

Contents:

Anatomy Of The Heart:

Anatomy Of The Heart

The heart is made up of four chambers;

  • The Left And Right Atria - Both of the atria receive blood from an external source outside of the heart and act as a filling station for the ventricles
  • The left And Right Ventricles - The ventricles are the larger and stronger parts of the heart and produce the large muscular contractions that we know as the heartbeat. The contractions of the ventricles are what pumps blood throughout the body. 

The right side of the heart contains deoxygenated blood, “used” blood that is returning back from the body that lacks oxygen through the body’s veins. While the left side of the heart receives freshly oxygenated blood from the lungs and pumps it out to the rest of the body through the arteries.

There are a number of different veins and arteries that connect to the heart that supply it with blood. Arteries are defined as blood vessels that carry blood away from the heart, while veins are blood vessels that carry blood toward the heart. With one except for each, arteries always carry oxygenated blood, while veins always carry deoxygenated blood. 

Veins:

  • The Superior Vena Cava - Brings blood back from the upper body
  • The Inferior Vena Cava - Brings blood back from the lower body
  • The Pulmonary Vein - Brings oxygenated blood from the lungs to the heart

Arteries:

  • The Aorta - This is the largest and hence the thickest artery in the body and is responsible for pumping blood throughout the entire body.
  • The Pulmonary Artery - This brings deoxygenated blood from the heart to the lungs.

In order to prevent blood from flowing backwards in the heart there is also a valve between each chamber of the heart. When enough blood has pooled up in the ventricles these valves will either open or close to allow blood to flow through and to prevent blood from flowing backward.

  • The Tricuspid Valve - Connects the right atria to the right ventricle and contracts squeezing as much blood into the the right ventricle before closing
  • The Pulmonary Valve - Connects the right ventricle to the pulmonary artery and opens before the right ventricle contracts allowing blood to flow through into the lungs
  • The Mitral Valve - Connects the left atria to the left ventricle and contracts pushing out as much blood as possible into the left ventricle before closing.
  • The Aortic Valve - Connects the left ventricle to the aorta and opens before the left ventricle contracts allowing blood to flow through and out to the rest of the body

The heart as a whole is surrounded by a few layers of tissue that seal and protect the heart.

  • The Fibrous Pericardium - This is the outermost layer of the heart and also the thickest. This layer serves as padding for the heart, but also prevents the heart from over expanding. The fibrous pericardium also serves a structural role connecting to the sternum and diaphragm which holds the heart in place.
  • The Serous Pericardium - The serous pericardium is a soft tissue that is composed of two layers, the Parietal Pericardium and the Visceral Pericardium
    • Between these two layers there is a small cavity known as the parietal cavity where the arteries that supply the heart itself with oxygenated blood exist

Lastly, inside this pericardium cavity exists three layers surrounding the heart:

  • The Visceral Pericardium - This is also known as the epicardium and covers the outer surface of the heart.
  • The Myocardium - This makes up the majority of the hearts wall and is composed primarily of cardiac muscle which are responsible for the contraction of the heart.
  • The Endocardium - This is the innermost layer and is a thin ayer of tissue that lines the inner surface of the heart chambers and valves.

Read More: What Does It Mean To Be Healthy

How The Heart Supplies Blood:

How The Heart Supplies Blood

The process of how the heart beats looks something like this:

  • Blood enters the right atria from the superior vena cava and inferior vena cava (the body’s two biggest veins)
  • The right atria fills up with blood which passes through into the right ventricle via the tricuspid valve (valves are door like opening that open and close to allow blood to pass through. When enough pressure builds in the ventricle it causes the tricuspid valve to close which prevents blood from flowing back into the right atria. 
  • This pressure causes the pulmonary valve to open and the right ventricle then contracts and sends this deoxygenated blood into the pulmonary artery which brings the blood to the lungs where it is reoxygenated. (The pulmonary artery is the only artery in the body that carries deoxygenated blood)
  • Once the blood has passed through lungs and been reoxygenated it enters the left atria through the pulmonary vein. (The pulmonary vein is the only vein in the body that carries oxygenated blood).
  • From here blood passes through the left atria through the mitral valve and fills up the left ventricle.
  • Once enough blood has pooled in the left ventricle it causes the mitral valve to close, preventing blood from flowing back into the left atria, and causes the aortic valve to open. 
  • The right ventricle then contracts sending blood through the aorta (the body’s biggest artery) and once the pressure of the left ventricle has dropped again the aortic valve closes and the cycle begins anew.

Both sides of the heart actually function in rhythm with one another and fill and empty at pretty much the same time. This occurs in three steps:

  1. Ventricular Filling - The heart is relaxed and allows blood to flow from the atria’s into the ventricles.
  2. Atrial Contraction - The atria contract forcing most of the remaining blood into the ventricles
  3. Ventricular Contraction - The ventricles contract pushing blood out through the aorta or through the pulmonary artery.
  4. Repeat

How The Heart Beats:

How The Heart Beats

The heartbeat is controlled by two nodes known as the Sino-atrial Node (The SA Node) and the Atrioventricular Node (AV Node). These nodes are a series of nerve that connect from the nervous system (the brain) which conduct electrical impulses and signal for the cardiac cells of the heart to contract.

  • The Sino-atrial Node - The SA Node is known as the pacemaker of the heart, as it’s responsible for maintaining the rhythm of the heart beat. The SA node is located in the right atria and are responsible for contracting the atria. 
  • The Atrioventricular Node - The AV Node is located below the SA node in the right atria and passes the electrical signal from the SA node to the Atrioventricular bundle ( a group of nerves) and the Purkinje fibers which signal for the ventricles to contract

There is a short delay between when the SA Node creates its impulse and when this electrical signal is spread to the AV Node and through the Atrioventricular bundle and the Purkinje fibers. This delay is what allows the atria to contract and empty their blood into the ventricles and subsequently the ventricles to contract to pump blood throughout the rest of the body.

This is also what creates the infamous "lub-dub" sound of the heart beating. The "lub" is the sound of the atria contracting the tricuspid and mitral valve closing, sealing off the atria from the ventricles. The "dub" is the louder, more intense sound and is the sounds of the aortic and pulmonary valve closing as the ventricles contract and pump blood through them. 

At rest the heart beats somewhere between 60 - 100 beats per minute, depending on your age and the efficiency at which your heart contracts. Extremely conditioned athletes can have a resting heart rate as low as 40 beats per minute, but during extremely intense exercise your heart can beat as many as 200 times per minute.

Read More: The Science Of Stress & How To Manage It

What Controls The Heartbeat:

What Controls The Heart Beat

The SA node is connected to both the autonomic or parasympathetic nervous system, known as the “rest and digest nervous system, and the sympathetic nervous system or the “fight or flight” nervous system. This connects the SA node to the brain which can then control the rate at which the heart contracts. Interestingly, the SA node does not require input from the nervous system and can readily contract on its own, but the heart rate is usually regulated by the nervous sytem. 

During intense exercise when a “flight or flight” state is induced the sympathetic nervous system will signal to the SA node to increase the amount of beats per minute. To do this, the body releases a group of hormones called the catecholamines which include adrenaline, noradrenaline, and cortisol. These hormones increase heart rate and as a result also increase blood pressure.

In contrast, when at rest the autonomic nervous system signals to the heart to maintain it’s regular resting the amount of beats per minute or to decrease the number of beats per minute.To decrease the amount of heart beats per minute the body releases the hormone acetylcholine, which relaxes the heart and slows it's best thereby lowering blood pressure.

An artificial pace-maker is required when the SA Node no longer functions properly and an artificial intervention is required to regulate the heart.

An electrocardiogram (ECG) is a measure of monitoring the heart beat by looking at the sequence of a heart contraction. The ECG is the graph that is seen in hospital computers to monitor the patient's heartbeat. If the heart stops beating, the graph stops “pulsing” and this is known as flatlining. Many times doctors or nurses will use electrical paddles, known as an automated external defibrillator (AED) in order to restart the heart’s beat.

Blood Pressure Explained:

Blood Pressure

When your heart increases, so does your blood pressure to allow your heart to beat faster, but also to pump more blood out of the heart. The opposite is also true at rest. When your blood pressure is measured it’s done so through diastolic blood pressure and systolic blood pressure.

  • Systolic Blood Pressure - This is the amount of blood pressure created in the heart when the ventricles contract and is the “heartbeat”. This is the first and higher number of blood pressure measurements.
  • Diastolic Blood Pressure - This is the amount of blood pressure in the heart when the heart is at rest and blood is filling into the atria and ventricles. This is the second number of a blood pressure reading and will always be the lower number.
  • Pulse Rate - Pulse is a measure of the amount of times your heart is currently beating per minute.

It’s worth noting that your blood pressure and pulse varies throughout the day depending on a variety of factors. In fact, your blood pressure will be lower if you’re lying down or even seated compared to standing, as your heart requires less work to pump enough blood throughout the body. The blood pressure test is designed to determine a baseline of your blood pressure at rest, but it’s not always indicative of your resting blood pressure. Interestingly, there's a phenomenon known as 'white-coat hypertension' in which many peoples’ blood pressure rises while in the doctor's office likely as a result of nervousness from the test.

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Conclusion:

My goal in writing this article, as always, is to provide you with logically-based principles that you can use to form your own conclusions regarding any information you may come across within this subject. I really hope you found this article interesting and if you have anything to add to this article, or any comments or criticism, feel free to reach out to me on our facebook groups or on Instagram @tylerwoodward_fit. Also, please feel free to share this article with anyone that might be interested.

Thanks for reading!

Until next time… be good

~Tyler Woodward
B.S. Physiology and Neurobiology

Citations:

Burgh, Jane De. The Human Body: An Essential Guide to How the Body Works. Thunder Bay Press, 2007.