The hardest working muscle in the human body

oxygenated blood is the same as oxygen rich blood is the same as blood that is high in oxygen saturation

deoxygenated blood is the same as oxygen poor blood is the same as blood that is low in oxygen saturation

 

The human body needs a steady supply of nutrients and gases to survive and thrive. To keep our bodies functioning properly, we all have to regularly breathe in oxygen and consume good nutrition. Thankfully, we have a few “difficult to ignore” natural signals that clearly indicate to us when we need to eat (hunger), breathe (the pain of suffocation), or drink (thirst). Without these signals, the human race would have probably gone extinct ages ago. Can you imagine a world in which none of us ever felt any hunger? We’d probably all eventually starve to death being too preoccupied with seeking out sexual encounters around every corner to remember to eat. Alright, so it is unlikely that any of us in our right state of mind would intentionally neglect eating, breathing, or drinking so it may seem that we’ve got that bit completely covered. However, it isn’t enough to just consume food and breathe in oxygen. These important life sustaining substances have to somehow get transported to all the cells, tissues, and organs in your body. When you eat for instance, the food ends up in your stomach and gets digested there for the most part, but the resulting nutrients from your digested food must somehow be distributed to the rest of your body.

Blood is the fluid medium through which nutrients, oxygen, and water are transported to all the tissues and organs in the rest of the body. Each human being has a huge network of blood vessels in their body through which ~1.5 gallons (or ~5.5 liters) of blood constantly courses through. This network of blood vessels in the average adult human body is so large that if we could somehow unwind it and lay all of the blood vessels out from end to end, the entire network would measure about 60,000 miles long. 60,000 miles will get you around the world twice! Yes, I agree with you… that’s a ridiculously long distance for blood to cover especially when it is continuously happening in both of us every minute of every hour of every day. If you take a second to digest the above facts, you will not find it hard to fathom that something very powerful must be sustaining this insane rate and volume of blood circulation in each of our bodies; and this is where the hardest working and most important muscle in your body plays a major role.

Ladies and gentlemen, the hardest working and most important muscle in the human body is the human heart. It exists between the lungs roughly in the center of your chest. Each of us naturally gets only one of these for the entirety of our natural lives. The human heart is an absolute marvel of biological engineering because it is so efficient and lasts for such a long time. It is truly remarkable that something the size of your clenched fist (yep… your heart is about the size of your clenched fist) is responsible for continuously sending blood throughout your whole body for the entirety of your life. Think about it for a second… how long do you think your current cellular phone will last before it needs to be replaced? 3-4 years maybe? In contrast, your heart will keep its current pace for 70+ years and periodically increase it as needed during that time period (like during bouts of exercise or when you get scared shitless). That to me is a mini miracle especially since the heart doesn’t require a change of batteries on any kind of regular basis. The heart serves to initiate and maintain blood flow through the complicated network of blood vessels that nourishes the cells, tissues, and organs in each and every one of our bodies. Together, the heart, the blood vessels, and the blood that flows through them, make up what is technically referred to as the cardiovascular system. The “cardio” in cardiovascular is derived from the greek word Kardia which means pertaining to the heart. While the “vascular” in cardiovascular means vessel. So the cardiovascular system in plain english is the system in the body that pertains to the workings of the heart and the blood vessels to which the heart is connected. There are a lot of important systems in the body such as the digestive system and the skeletal system, but the cardiovascular system is of especially critical importance. To put it in the proper context, if a person’s heart stops working for longer than 5 minutes, the various organs within the body will start to shut down as they’ll be starved of the nutrients and raw materials that they need to survive, killing the affected person shortly thereafter. Now that we’ve placed sufficient emphasis on the importance of the human heart to the survival of all human beings, let us delve a bit deeper to discuss its structure and function.

The human heart is tasked with continuously circulating blood throughout the body. One may ask why the continuous circulation of blood is so important? Well, recall that your organs and tissues need a near constant supply of oxygen and key nutrients to thrive and function properly. Blood happens to be the fluid medium through which these nutrients are sent to the organs and tissues of the body. As the organs and tissues of the body use up the oxygen and nutrients delivered to them via the blood, the blood becomes temporarily depleted. Since the need for oxygen and nutrients in all the organs of the human body is near constant, the blood has to somehow get “recharged” with nutrients and oxygen at “central filling stations” within the body. The thing is that these “central filling stations” such as the lungs (where blood gets oxygen) and the small intestines (where blood gets nutrients from food) are in fixed positions within the body. Organs are in fixed positions and cannot travel to the “central filling stations” to get replenished regularly. The “central filling stations” are also in fixed positions and cannot travel to the organs and tissues to replenish them regularly either. Thus the blood must serve as a delivery service which constantly shuttles nutrients and oxygen from the small intestine and lungs to the organs and tissues of the body. To make this easier to understand, you can liken all of this it in your mind’s eye to ordering your favorite food condiment from another country. In this analogous food ordering scenario, you are like a bodily organ such as the liver which needs oxygen for instance, the condiment you are ordering is like oxygen, the store in the remote country that sells the condiment is like the lungs, and FedEx or DHL that brings the package to you is like the bloodstream.

 

Structure of the human heart:

Cross section of the human heart.
Cross section of the human heart.
Thinking deeper about the responsibilities of the heart, we can sum it up as follows: the heart must pump oxygen and nutrient rich blood to the organs and tissues of the body, as well as pump oxygen and nutrient depleted blood to the lungs and small intestines for a “recharge”… rinse and repeat a ton of times for the duration of each person’s natural life. You might be wondering how the heart can be responsible for sending out both oxygen rich blood to the rest of the body and oxygen poor blood to the lungs. I mean, wouldn’t both the oxygen rich and oxygen poor blood get mixed in the heart and result in a general dilution of the nutrients and oxygen delivered to the organs? And wouldn’t that be bad? The answer to those very astute questions is that none of those things would occur because mother nature solved that problem by putting a thick muscular wall called the “septum” in the center of the heart that physically divides it into two halves. Further, this division of the heart means that there is no direct path for blood to flow between both halves of the heart. Each half of the heart is subdivided into two chambers: the atria on top, and the ventricles at the bottom. Oxygen poor blood flows back from the rest of the body into the right atrium and eventually into the right ventricle of the heart. Oxygen rich blood flows back from the lungs into the left atrium and eventually into the left ventricle of the heart. When the ventricles get filled to a certain critical volume, they contract, squeezing the oxygen rich blood from the left side of the heart to the rest of the body, and oxygen poor blood from the right side of the heart to the lungs. The astute amongst us might be wondering how the flow of blood in the heart is kept unidirectional and the answer is that the heart has valves placed in strategic locations to ensure the proper flow of blood. Below is a list of the valves of the heart and their functional purpose(s).
 

Valves of the heart

The tricuspid valve (a.k.a right atrioventricular valve) is located between the right atrium and the right ventricle

  • It makes sure that the oxygen poor blood returning from the body flows unidirectionally from the right atrium to the right ventricle
  • It also prevents backflow of blood into the right atrium when the heart flexes to pump deoxygenated blood from the right ventricle to the lungs

The pulmonary valve is located between the right ventricle and the pulmonary artery

  • It makes sure that the oxygen poor blood that is pumped from the right ventricle flows in a unidirectional manner to the lungs

The mitral valve (a.k.a left atrioventricular valve) is located between the left atrium and the left ventricle

  • It makes sure that the oxygen rich blood that returns from the lungs flows unidirectionally from the left atrium to the left ventricle
  • It also prevents backflow of oxygen rich blood into the left atrium when the heart flexes to pump oxygenated blood from the left ventricle to the rest of the body

The aortic valve is located between the left ventricle and the aorta

  • It makes sure that the oxygen rich blood that is pumped from the left ventricle flows in a unidirectional manner to the rest of the body

 
It is also important to note that there are 3 major types of blood vessels in the body: the arteries, the veins, and the capillaries. Arteries carry blood away from the heart, while veins bring blood back to the heart. Arteries (except the pulmonary artery) generally carry oxygen rich blood from the left side of the heart to the rest of the body’s tissues and organs. Veins (except the pulmonary vein) generally carry oxygen poor blood from the rest of the body back to the right side of the heart. Capillaries are tiny blood vessels that connect arteries to veins and allow for tissues and organs to absorb oxygen from the blood.
 

Tiny vessels through which gaseous exchange occurs. They are the conduits through which red blood cells deliver oxygen (O2) organs, and take carbon dioxide (CO2) away.
Tiny vessels through which gaseous exchange occurs. They are the conduits through which red blood cells deliver oxygen (O2) organs, and take carbon dioxide (CO2) away.

 

The functionality of the human heart:

Like every other muscle in the body, the heart cycles between periods of tension and relaxation. To send oxygenated blood out to the rest of the body or deoxygenated blood to the lungs, the heart must flex or contract with enough force to push blood out of its ventricles. By the way, this is why the walls of the human heart are so thick and muscular… it has to generate enough force to send blood to every last corner of the body, and it has been doing that since you and I came out of the womb. It is the same reason why bodybuilders get so big and muscular; they lift heavy loads repetitively for years to develop their physiques. The heart must also relax to fill itself with oxygenated blood back from the lungs and deoxygenated blood back from the rest of the body. So the heart has two phases of activity… when it tenses up to pump out blood, and when it relaxes to fill itself back up with blood. The technical names for these two phases of activity are called systole (pumping out blood) and diastole (filling back up with blood). In some ways, the heart is nature’s version of a pump that you might have seen in the plumbing water system of your home. It is fair to call the heart the body’s biomechanical pump but it is far from just a “dumb” pump. There is actually a sophisticated governing electrical system that synchronizes every single heartbeat. This electrical system of the heart is fairly complex, but for the purposes of this article, we will focus on just a few of its major parts: the sinoatrial node, the bundles of his, and the purkinje fibers.

The sinoatrial node is situated in the right atrium and is the natural pacemaker of the heart. A good analogy is to think of the sinoatrial node almost as you would an NFL quarterback yelling “set hut” to signal to his offense that it is time for the next offensive play. In the same sort of way, the sinoatrial node generates an electrical signal at regular intervals which “tells” the heart when to contract or flex in order to push blood out to the rest of the body. This signal generated by the sinoatrial node is sent to the bundles of his which is a collection of conductive cells that relays the signal to the purkinje fibres in the ventricles, causing them to contract and force blood out of the heart. Note that the sinoatrial node doesn’t just constantly emit a “contract” or “flex” signal at regular fixed intervals. Rather, it constantly regulates the heart rate to meet the demands of the body. The sinoatrial node will fire more rapidly when the muscles of the body need more oxygen such as during times of exercise. The sinoatrial node can also slow down the heart rate when the physical demands on the body decreases such as when you are relaxing with your significant other on your couch at home. By the way, if the sinoatrial node or the conductivity path that its electrical signal follows to the purkinje fibres is disrupted for whatever reason, it can lead to sometimes fatal heart arrhythmias which in plain english just means “a person might die because of a heart that beats out of rhythm”.

Most of us are familiar with the “lub-dub” sound that comes from the heart during every beat cycle. Let’s take a closer look to understand where this sound comes from. During every heart beat cycle, the following sequence of events occurs:

  1. De-oxygenated blood flows from the rest of the body into the right atrium and oxygenated blood flows from the lungs into the left atrium at the same time.
  2. De-oxygenated blood flows from the right atrium into the right ventricle and oxygenated blood flows from the left atrium into the left ventricle at the same time. When all the blood is in the ventricles, both the tricuspid and mitral valves snap shut. This is the first heart “lub” sound you have heard if you’ve ever stuck a stethoscope in your ear and listened to your heart beat
  3. Next, both the right and left ventricles contract at the same time to force blood out of the heart. As soon as the blood flows out of the heart, both the aortic and pulmonary valves snap shut. This is the second heart “dub” sound you have probably also heard if you’ve ever stuck a stethoscope in your ear and listened to your heart beat.

 

Coronary arteries of the heart, courtesy of ADAM
Coronary arteries of the heart, courtesy of ADAM
We should now be well aware that the heart sends oxygenated blood to all the organs and tissues of the body but let’s not forget that the heart will also need its own fair share of oxygenated blood to maintain the insane work rate it manages to keep up for the duration of each of our lives. The major source of oxygenated blood supply to the heart is through the coronary arteries which branch off the aorta. The distance that the blood has to travel from the left ventricle before it nourishes the heart is relatively small and many have speculated that the reason for this is to make sure that the heart receives the most oxygen saturated blood supply possible since it works so hard. You can probably imagine what would happen if the heart was deprived of oxygen for an extended period of time. Yep, you guessed it, it would start to shut down. And as a matter of fact, this is pretty much what happens during a heart attack. More explicitly, if you eat too much bad food, neglect exercise, and smoke a lot, you will start to develop plaque on the inner lining of your coronary arteries. These plaques will steadily get bigger and bigger until they have completely occluded the coronary arteries and blocked the supply of oxygenated blood to the heart. When this happens, some of the heart muscle cells are irreparably damaged (at least as far as the current state of medical science is concerned) making the heart weaker as a result. This is what you may commonly know as a heart attack, or in scientific terms, a myocardial infarction.

The heart is an amazing muscle and you are an amazing being. Think about it for a second… there is this muscle in your chest that continues to send blood throughout your entire body, every single day of your life. It beats anywhere from 40 – 80 times a minute depending on your level of physical fitness. It can speed up or slow down depending on what you are doing. If you’re flying down the lap pool in a dead on sprint, it will speed up to provide more oxygen to your muscles. If you are sitting in bed reading a book, it will slow down appropriately. Most amazing of all, the heart does all this without any real conscious thought on your part. This has an evolutionary purpose because it would be mentally exhausting to have to remember to consciously will your heart to send blood around your body. Seriously though, that would mean none of us would ever get to sleep! Till next time my friends, take care of yourselves, and each other.

 
 

Without Wax
Oyolu B.C. Ph.D.
chubaoyolu.org
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