Getting to the Heart (and Lungs) of the Matter: An Intro to the Cardiorespiratory System

Every cell in our body uses oxygen for fuel; hence, we need to breathe air to survive. But have you ever wondered how our bodies turn the air that surrounds us into a molecular energy source?

Cue up the cardiorespiratory system! The cardiorespiratory system is composed of two main systems: first is the cardiovascular system, which pumps oxygenated blood to every cell in your body. The other part is your respiratory (or pulmonary) system, which pulls air into your body, cleans it, and gets the oxygen into your bloodstream. The two systems work together to not only get oxygen to your cells but also clear out carbon dioxide and put it back into the air.

Okay, ready to dive in? Let’s get started!

Here to pump you up: The Cardiovascular System

The cardiovascular system consists of your heart, your blood, and blood vessels. The heart and blood vessels form a closed-loop circuit flowing in one direction: oxygenated blood flows out of the heart, and deoxygenated blood flows in. Your heart pumps faster or slower depending on your activity level; this is because as you are more active, you need more oxygen to fuel your cells, and vice versa.

 

A diagram of the cardiovascular system, taken from (1). The heart pumps deoxygenated blood (blue) through the lungs and then pumps the oxygenated blood (red) to the arteries and capillaries to provide oxygen to individual cells. Deoxygenated blood (blue) is then shipped back to the heart via the capillaries and veins. Lather, rinse, repeat.

 

The heart is roughly the size of your fist and has four chambers spread over two sides. For either side, blood collects up top, in the smaller chambers called atria. The blood gets pumped out of the lower, larger chambers called the ventricles. The right ventricle has it easy, as it only has to pump blood to the lungs. The left ventricle, however, has a lot more powerful pump, since it’s pumping to the whole body. The right side (or pulmonic side) collects blood low in oxygen then pumps it to the lungs to get—you guessed it—oxygen. From the lungs, the freshly pumped-up blood collects in the left side of the heart (the systemic side) where it is then pumped out to the body through the blood vessels.

 
Simplified diagram of the human heart showing the atrium and ventricles

Diagram of the human heart, taken from (2). Note: Left and right are “backwards” because you’re looking at someone from the front. The smaller upper chambers are where blood collects; they are called atria. The larger, lower chambers are the ventricles; they store blood. The right side of the heart works with the lungs; the left side works with the rest of the body.

 

You can think of blood vessels as a sort of road system, consisting of highways, roads and streets, with all roads ultimately leading to the heart. (Kinda sounds like song lyrics, doesn’t it?) If we start with the freshly oxygenated blood in the left ventricle, it gets pumped out through large highways called arteries. Just like a highway, blood exits through the offramps onto smaller roads, or arterioles. (These are kinda like your main city roads.) Finally, from the larger arterioles you break off onto a bunch of tiny side streets, aka your capillaries. Capillaries are where the magic happens! And by “magic,” I mean oxygen exchange.

 
simplified diagram of blood vessels, showing arteries, veins, aterioles, venules and capillaries

A simplified diagram of blood vessels, taken from (3). Arteries and arterioles carry oxygenated blood from the heart; venules and veins carry deoxygenated blood back to the heart. Your capillaries are where oxygen is delivered at the cellular level.

 

There are estimated to be 40 BILLION capillaries in the human body. Like most city streets, a home (ie, a cell) is always close by; a human cell is usually within 100um of a capillary. Capillaries are tiny, only 5-10um in diameter. If you were to stretch out all of them and lay them end to end, they would stretch for over 60,000 miles! At any rate, your oxygenated blood passes through your capillaries. Oxygen in the blood passes through the capillary walls to your cells. At the same time, your cells pass off their waste—carbon dioxide—back into the blood to be removed from your body. It’s kinda like if Amazon delivered you a package and took your empty boxes back at the same time!

As the blood passes through the capillaries, it goes from containing oxygen to containing carbon dioxide, which, like all those empty boxes, need to be taken to the recycling center. Your capillaries meet back up with the city streets, this time referred to as venules because they’re carrying blood back to the heart. Venules combine into your larger veins, which then go back directly to the right side of the heart. Blood collects at the city dump (okay, that’s mean; it’s your right atrium) and waits to be recycled, that is, passed to the lungs. Your right ventricle pumps the blood through small capillaries in your lungs (more on that in a sec), the blood drops off its CO2, gets more O2, then the cycle begins anew.

Every breath you take: The Respiratory System

Okay, so while the heart and blood vessels are doing their thing, what’s going on with your respiratory system? With each breath, you are taking in air (ie, inhaling) and releasing carbon dioxide (ie, exhaling).

It all starts with a few of your muscles. For “regular” breathing, that includes your diaphragm and intercostals, but if you put a little more effort in (heavy breathing or while exercising), your scalenes, sternocleidomastoid (SCM for short), and pectoralis minor can pitch in too. Anyhoo, those muscles contract to create a low pressure system in your lungs. Since the air pressure in your lungs is now lower than the pressure in the atmosphere, air rushes in to fill them. As your muscles relax, the air pressure rises above the atmosphere, so all of your CO2 leaves the body.

Breathing (4)! As your diaphragm contracts (gets smaller), the lungs get larger, and the atmospheric pressure inside the lungs becomes lower than the atmosphere. Thus, air (blue) flows IN. When you exhale, the diaphragm expands, making the lungs smaller, the air pressure inside higher than the atmosphere, and now CO2 flows OUT into the atmosphere.

Just like the blood, air needs a pathway into and out of the body. The main entrances are your nose and mouth, these are connected to conducting airways such as your pharynx, larynx and trachea. As they pass through, the incoming air is filtered, humidified and heated or cooled to more closely match your body temperature.

 

Diagram of respiratory system (5).

 

These passages continue to your left and right pulmonary bronchi, located in your lungs. Similar to capillaries, you get smaller roads—first your bronchioles, then your respiratory airways—next the alveoli, and finally your proverbial cul-de-sacs, named your alveolar sacs. Air is collected in these sacs, and it is here where O2 is taken in and CO2 is released. And how does that happen? Just like the capillaries in the rest of your body, there are capillaries around all of the air sacs with blood pumping through and ready to trade off waste product for new oxygen.

 

Lung detail (6). Inside the lungs are tens of thousands of bronchioles. At the end of each bronchiole are alveoli where oxygen and CO2 are exchanged. Again, this is facilitated by capillaries (the red and blue in the image).

 

Overview (ie, WRAP IT UP!)

So your lungs pull air in from the world around you, clean it out and pass the oxygen off to your bloodstream. From there, tag, your heart is “it”—it pumps that blood to the rest of your body. Your heart beats roughly 100,000 times a day, working to pump 2,000 gallons of blood every day. And your lungs are no slouch, either— while the heart is doing its thing, your lungs are breathing in 2,000 gallons of air to oxygenate all that blood. How long does it take for blood to pump all the way through your system? Roughly one minute. (Isn’t that crazy?)

If your brain is, well, the brain of the system, the cardiorespiratory system is the brawn—and it’s a powerhouse duo at that! In my next blog, we’ll look at a metric used to characterize the performance of this system—VO2max, and what this number means for your health, longevity, and athletic peformance.


References



Image Credits

(1) Modified from "Human circulatory system" by Tomáš Kebert & umimeto.org is licensed under CC BY-SA 4.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-sa/4.0/?ref=openverse.

(2) Modified from "Diagram of the human heart (cropped)" by Wapcaplet is licensed under CC BY-SA 3.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-sa/3.0/?ref=openverse.

(3) Modified from "Blood vessels-en" by Kelvinsong is licensed under CC BY-SA 3.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-sa/3.0/?ref=openverse.

(4) Brbbl at nl.wikipedia, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons. Image located at: https://commons.wikimedia.org/wiki/File:Diafragma_ademhaling.gif

(5) Modified from "Respiratory system" by Theresa knott. Is licensed under CC BY-SA 3.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-sa/3.0/?ref=openverse.

(6) Modified from "File:An alveolus, is an anatomical structure that has the form of a hollow cavity. Mainly found in the lung, the pulmonary alveoli are spherical outcroppings of the respiratory bronchioles and are the.png" by LadyOfHats is marked with CC0 1.0. To view the terms, visit https://creativecommons.org/publicdomain/zero/1.0/deed.en?ref=openverse.

 
 



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