the respiratory system
why do we breathe?
Did you know that the average sneeze travels about 100 miles per hour? Or that over 20 million people in the United States have asthma? Or that, in 2012, a German diver named Tom Sietas set the breath-holding record at 22 minutes, 22 seconds?
We breathe over 20,000 times a day, but most of us don’t even think about it: Breathing is automatic. When we breathe in, our body takes in oxygen, a natural gas floating around in the atmosphere. Oxygen is necessary to maintain body function and keep us alive. As you’ve learned, oxygen is essential because of its role in cellular respiration. When we breathe out, we release the carbon dioxide produced in cellular respiration, which, in high amounts, is toxic to our body (because it affects our blood pH).
Do not confuse cellular respiration, which is the energy-converting process that happens in the mitochondria of cells, with external respiration, which is the exchange of respiratory gases—oxygen and carbon dioxide—in the lungs when we breathe.
Because we need oxygen to stay alive and because we breathe to take in oxygen, in order to stay alive, we must breathe. We breathe using the respiratory system.
We breathe over 20,000 times a day, but most of us don’t even think about it: Breathing is automatic. When we breathe in, our body takes in oxygen, a natural gas floating around in the atmosphere. Oxygen is necessary to maintain body function and keep us alive. As you’ve learned, oxygen is essential because of its role in cellular respiration. When we breathe out, we release the carbon dioxide produced in cellular respiration, which, in high amounts, is toxic to our body (because it affects our blood pH).
Do not confuse cellular respiration, which is the energy-converting process that happens in the mitochondria of cells, with external respiration, which is the exchange of respiratory gases—oxygen and carbon dioxide—in the lungs when we breathe.
Because we need oxygen to stay alive and because we breathe to take in oxygen, in order to stay alive, we must breathe. We breathe using the respiratory system.
the structure of the respiratory system
The respiratory system is composed of the nose, pharynx (throat), larynx, trachea (windpipe), bronchi, lungs, and diaphragm.
Much like with your circulatory system, the respiratory system has a “main organ”—in this case, the lungs—that carries out the main function, and it also has a bunch of tubes, plus a muscle (the diaphragm) that is responsible for the mechanics of breathing. As important as these tubes are, we won’t worry too much about them for this class because we’re much more interested in what happens in the lungs and how our breathing pattern is controlled.
gas exchange at alveoli
The lungs are the site of gas exchange, or the swap of oxygen and carbon dioxide, between the air and your blood. Every time you breathe, oxygen from the outside world moves into the capillaries in your lungs, oxygenating your blood. At the same time, carbon dioxide moves from the capillaries in your lungs to the outside world, getting rid of this waste product. In other words you breathe in oxygen and breathe out carbon dioxide.
Gas exchange is just a specific example of passive diffusion. Like all other types of passive diffusion, it involves small, non-polar molecules (in this case, oxygen and carbon dioxide) directly crossing the phospholipid bilayer down their concentration gradients. This video gives a great overview:
Gas exchange is just a specific example of passive diffusion. Like all other types of passive diffusion, it involves small, non-polar molecules (in this case, oxygen and carbon dioxide) directly crossing the phospholipid bilayer down their concentration gradients. This video gives a great overview:
For this special function of gas exchange, lungs have a special structure. They need to have an interface between the air and your blood that is thin enough for diffusion to occur. Tiny air sacs called alveoli (singular: alveolus) are the functional unit of the lung. By functional unit, we mean that it is the microscopic feature where all of the really important functions are happening.
Between your two lungs, you have over 1 billion alveoli working to exchange oxygen and carbon dioxide with every breath. The wall of each alveolus is only 1 cell thick, making it a super-thin barrier for diffusion. Each alveolus is wrapped with a net of capillaries, providing a super-close interface with the blood. Alveoli look like this.
Between your two lungs, you have over 1 billion alveoli working to exchange oxygen and carbon dioxide with every breath. The wall of each alveolus is only 1 cell thick, making it a super-thin barrier for diffusion. Each alveolus is wrapped with a net of capillaries, providing a super-close interface with the blood. Alveoli look like this.
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You'll note an interesting grape-like structure. This gives the capillary a lot of surface area to interact with the outside air, to give diffusion more space to occur. We’ve mentioned surface area being important before, when we talked about the special structure of mitochondria and chloroplasts.
The alveoli are the only part of your lungs that are filled with air. Everything else is capillaries and tissue. But, since you have so many alveoli, your lungs are mostly filled with air. Your lungs can hold a total volume of more than a gallon of air! (They can hold 4-6 L of air on average). |
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the diaphragm and negative pressure breathing
This air doesn’t just get into the lungs on its own, though. For this, you need a special muscle called the diaphragm. When the diaphragm contracts, you breathe in. When the diaphragm relaxes, you breathe out.
This can be easily explained based on what you know about gas laws: when volume increases (when the diaphragm contracts and pulls downwards), pressure decreases. Like a vacuum, this low pressure allows the lungs to suck in air. The opposite occurs when the diaphragm relaxes: volume decreases, pressure increases, and air is forced out to maintain a pressure equilibrium between your lungs and the outside air.
This video gives a great overview of negative pressure breathing:
This can be easily explained based on what you know about gas laws: when volume increases (when the diaphragm contracts and pulls downwards), pressure decreases. Like a vacuum, this low pressure allows the lungs to suck in air. The opposite occurs when the diaphragm relaxes: volume decreases, pressure increases, and air is forced out to maintain a pressure equilibrium between your lungs and the outside air.
This video gives a great overview of negative pressure breathing:
You’ll note from the video that the curved, flat structure of the diaphragm is responsible for its function (in that the chest volume increases when the diaphragm contracts). Structure gives rise to function!
Summary
Now, hopefully you understand the incredible importance of the respiratory system in keeping us alive. Every part of it works together carefully to ensure that oxygen reaches our cells like it should. Think of it like a machine—a squishy, wet, complicated machine that makes sure our oxygen levels are normal and proper at all times. So, when you have the time, take a moment to thank your respiratory system for keeping you alive and healthy!
This video gives a great overview of the most important concepts discussed in this lesson:
This video gives a great overview of the most important concepts discussed in this lesson:
You should understand:
- That the respiratory system is made up of the lungs and all of the tubes that connect the lungs to the outside air.
- How carbon dioxide and oxygen are exchanged at the alveolus by passive diffusion.
- How the structure of alveoli contributes to its function, including the role of the thin membrane, the surrounding net of capillaries, and the high surface area.
- The mechanics of negative pressure breathing, and how the structure of the diaphragm contributes to this.
Learning Activity
Contributors: Megha Kori, Emma Moulton
Some images made using biorender.com
Some images made using biorender.com