Linda Crampton is a writer and teacher with a first-class honors degree in biology. She often writes about the scientific basis of disease.
A Vital System in the Human Body
The human body is a fascinating structure that can perform some very impressive feats. In order to perform these feats, the body needs input from the environment and must release the waste products that it makes. The regular input of oxygen and output of carbon dioxide via the respiratory system is vital. This system has some interesting and sometimes surprising features.
The respiratory system is a network of tubes, sacs, and muscles that obtains oxygen from the air and transports it to the bloodstream. The blood delivers the oxygen to all the cells in the body, which use it to produce energy from digested food. Carbon dioxide waste made by the cells is transported in the opposite direction, from the cells into the respiratory system to be exhaled.
We depend on our respiratory system for our survival, since all of our vital organs require oxygen in order to function. Brain cells are damaged after only a few minutes without oxygen (except under very special conditions, such as deep chilling of the body) and death may soon follow.
Respiration and Breathing: What's the Difference?
Respiration is a multi-step process involving the respiratory system, the circulatory system, and tissue cells. Unfortunately, the word "respiration" is often used instead of "breathing", which can be confusing for a biology student. When it's used in its technical sense, the term respiration refers to more than just breathing.
During respiration, oxygen is inhaled though the nose and/or mouth and then transported to the tissue cells via the bloodstream. The oxygen participates in a complex chemical reaction inside the cells. This reaction produces energy, carbon dioxide, and water. The carbon dioxide and water are transported to the lungs via the bloodstream and exhaled.
Respiration is often said to involve four processes, as described below. The respiratory system is involved in the first two steps.
- Breathing (ventilation): the inhalation of oxygen and the exhalation of carbon dioxide
- External Respiration: gas exchange between the lungs and the bloodstream; oxygen leaves the lungs and goes into the bloodstream while carbon dioxide moves in the opposite direction
- Internal Respiration: gas exchange between the bloodstream and the tissue cells; oxygen leaves the bloodstream and enters the tissue cells while carbon dioxide moves in the opposite direction
- Cellular Respiration: a chemical reaction between oxygen and carbohydrates inside the tissue cells
The airways are highly branched, as shown in the photo above. Their appearance is reminiscent of a tree. They are sometimes collectively referred to as the bronchial tree.
Facts About the Airways
1. Air enters the nose and mouth and then travels to the trachea, or windpipe. At the top of the trachea is an enlarged area called the larynx. The larynx is also called the voicebox, since it contains the vocal cords that we use to make sounds. The vocal cords are also known as the vocal folds.
2. The trachea branches into two bronchi, one going to each lung. Each bronchus divides repeatedly to form narrower bronchi and then even narrower bronchioles, producing a structure called the bronchial tree.
3. In combination, the lungs are said to contain around 2,400 kilometers of airways. As might be imagined, data like this is hard to obtain, depends on the size of the lungs, and is approximate. The total length of the airways in our lungs is almost certainly very impressive, though.
4. The bronchioles lead to tiny air sacs called alveoli, which are the site of gas exchange between the lungs and the blood. According to some researchers, a pair of adult lungs contains 300 million to 500 million alveoli in total. Some researchers say that we may have this many alveoli in a single lung. Despite the uncertainty, the number of alveoli in our lungs is very likely amazing.
5. Since they contain so many air sacs, the lungs are able to float on water.
6. If all the alveoli in both lungs were flattened out, they would have a total area of about 160 square meters—about 80% of the size of a singles tennis court and about 80 times greater than the surface area of an average-sized adult’s skin.
7. The interior lining of an alveolus is made of cells called pneumocytes and is covered by a thin layer of water. The water enables oxygen to move through the wall of the air sac and into the bloodstream efficiently.
8. Water molecules on the lining of an alveolus are attracted to each other, creating a force known as surface tension. When the alveoli become smaller during exhalation, the surface tension increases. This could cause the air sacs to collapse and prevent them from expanding again.
9. The lining of the alveoli produces a substance called a surfactant. The surfactant reduces the surface tension of water, preventing the alveoli from collapsing.
Capillaries and Blood
10. The surface of an alveolus is covered with capillaries. Capillaries are narrow blood vessels with a thin wall that is just one cell thick.
11. Like the wall of capillaries, the wall of an alveolus is also just one cell layer thick. This allows for quick absorption of oxygen from the alveoli into the capillaries and the quick release of carbon dioxide from the capillaries into the alveoli.
12. A red blood cell contains about 250 million hemoglobin molecules, which carry oxygen through the blood. Each hemoglobin molecule can carry four oxygen molecules.
13. There are 4 million to 6 million red blood cells in each microliter (cubic millimeter) of blood.
14. The lungs have several functions that are not directly related to respiration. One of them is to act as a reservoir of blood for the left ventricle of the heart. This ventricle pumps the blood around the body.
15. The right lung is larger than the left one and consists of three lobes. The left lung has only two lobes.
16. The heart is located between the lungs with its pointed tip directed towards the left side of the body. The position of the heart allows for less space for the left lung than for the right lung.
17. The bottom part of the heart fits into an indentation in the left lung called the cardiac notch.
18. An adult generally breathes between 12 and 18 times a minute when he or she is not exercising, or about 17,000 to 26,000 times in a twenty-four hour period.
19. The total lung capacity (maximum amount of air that someone’s lungs are capable of holding) is between 4 and 6 liters of air in an adult. Males usually have higher total lung capacities than females.
20. When we are relaxed, we inhale and exhale about 500 mL of air per breath. This value is called the tidal volume. We inhale and exhale greater volumes of air in certain situations, such as when we are exercising or during forced breathing.
21. About 30% of the tidal volume of air never reaches the alveoli and stays in the airways. This air is called “dead air” because it's useless for oxygen extraction as it isn’t in the alveoli.
22. Even after a very strong exhalation, about 1000 to 1200 mL of air remains in the lungs. This is known as residual volume.
23. Exhaled air contains water vapor from our bodies. Each day we lose about half a liter of water from our bodies by exhaling.
The lungs are surrounded by two membranes called the visceral pleura and the parietal pleura. The narrow space between the two membranes is called the pleural cavity and contains a small amount of lubricating fluid.
Inhalation and Exhalation
24. The diaphragm is a sheet-like muscle under the lungs. The diaphragm and the intercostal muscles between the ribs are both used for inhalation (also called inspiration), but the diaphragm plays a more important role. It’s curved upwards when relaxed and flattens as it contracts.
25. Inhaled air doesn’t push the lungs open. Instead, during inhalation the diaphragm and intercostal muscles contract, increasing the volume of the chest cavity and pulling the lungs open. Residual air inside the lungs spreads out, causing the air pressure inside the lungs to be reduced. Air outside the body, which is under a higher pressure than the air in the expanded lungs, then moves into the nose and mouth and down the airways towards the lungs.
26. During exhalation (also called expiration) the diaphragm and intercostal muscles relax, causing the lungs to decrease in volume and air to be pushed out.
27. The medulla oblongata in the brainstem stimulates us to inhale without us having to make a conscious decision to breathe.
28. A high level of carbon dioxide in the blood is more important in triggering inhalation than a low level of oxygen.
Protection of the Airways
29. The esophagus transports food to the stomach and starts at the back of the throat behind the trachea. When we swallow, a flap of tissue called the epiglottis moves downwards to cover the trachea. This prevents the entry of swallowed materials, which could block the passage of air and cause choking.
30. Mucus is a vital substance made by the air passages. Mucus traps inhaled dirt and bacteria and also moistens the airways.
31. The cells lining the airways have hair-like extensions called cilia. The cilia beat in a coordinated fashion to create a current of mucus that is swept up to the back of the throat, where it's swallowed.
32. Smoking damages cilia, allowing mucus to build up and block the airways.
Sneezing and the Photic Sneeze
33. Sneezing is technically known as sternutation. It serves to expel potentially harmful material from the airway in the nose.
34. The fastest speed at which material released by a sneeze travels is often said to be 100 miles an hour. This number became popular a long time ago. Some scientists of today say the speed is hugely exaggerated.
35. A virologist at the Alberta Provincial Laboratory for Public Health found that sneezes travel at only ten miles an hour. He did say that his subjects had a slight build and that the speed might have been higher if subjects with a bigger frame had been used in the experiment, however.
36. Sneezing can be due to other factors besides irritation in the nose. Some people sneeze when entering a bright environment after being in the dark. This type of sneeze is known as a photic sneeze, or a photic sneeze reflex. A reflex doesn't involve a conscious decision by the brain.
37. About 20% to 30% of people are thought to experience photic sneezes. A photic sneeze is also known as the ACHOO syndrome (Autosomal Dominant Compelling Helio-Ophthalmic Outbust Syndrome). Some people sneeze once when exposed to light, but most people sneeze multiple times. There have been reports of photic sneeze outbursts involving forty sneezes. The trait seems to have a genetic basis.
The Cause of Photic Sneezes
38. The nerve that carries signals from the eyes to the brain is called the optic nerve. When the pupils of the eyes are adapted to a dark environment, they are dilated. If someone moves from a dark environment to a very bright environment, the optic nerve sends an electrical signal to the brain, causing it to constrict the pupils in order to protect the inside of the eyeball from light damage.
39. The trigeminal nerve is stimulated when an irritant enters the nose. The nerve sends a message to the brain, which causes a sneeze. The trigeminal nerve lies close to the optic nerve. Scientists think that when photic sneeze sufferers enter a bright environment, some of the electrical signal traveling through the optic nerve to the brain escapes into the trigeminal nerve, causing the person to sneeze.
40. Some cases of migraines and epilepsy may be neurologically linked to photic sneezes.
A Respiratory System Quiz
For each question, choose the best answer. The answer key is below.
- The correct order of air passages in the respiratory system is:
- trachea, larynx, bronchi, bronchioles, alveoli
- trachea, larynx, bronchioles, bronchi, alveoli
- larynx, trachea, bronchi, bronchioles, alveoli
- larynx, trachea, bronchioles, bronchi, alveoli
- Approximately how many hemoglobin molecules does a red blood cell contain?
- 100 million
- 150 million
- 200 million
- 250 million
- Approximately how fast can material released in a sneeze travel (according to a recent estimate)?
- 5 miles an hour
- 10 miles an hour
- 100 miles an hour
- 200 miles an hour
- Which part of the brain triggers normal breathing?
- medulla oblongata
- What is the approximate tidal volume in normal breathing?
- According to some researchers, how many alveoli may be present in a lung?
- 100 to 300
- 200 to 400
- 300 to 500
- 400 to 600
- The scientific name for the voice box is:
- Vocal fold
- The scientific name for the windpipe is:
- larynx, trachea, bronchi, bronchioles, alveoli
- 250 million
- 10 miles an hour
- medulla oblongata
- 300 to 500
Studying the Respiratory System
The respiratory system is an impressive and essential part of our body. Avoiding activities that harm it and taking steps to keep it healthy are important for our enjoyment of life and for our survival. Understanding how the system works and learning about the factors that affect it can be an interesting pursuit for students and for the researchers who study it. New discoveries about breathing and respiration might be very helpful for us.
- Information about the respiratory system from the NIH (National Institutes of Health)
- Biology of the lungs and airways from the Merck Manual
- Lung and respiration information from the American Lung Association
- Non-respiratory functions of the lungs from Oxford Academic
- Why we sneeze in bright light from the BBC
- Speed of a sneeze from Popular Science
Questions & Answers
Question: What are the organs that work together in the respiratory system?
Answer: The respiratory system consists of organs, passageways, and structures. Air enters the respiratory system through the nose or the mouth, which are organs. The air then passes through the pharynx at the back of the nose and mouth and into the larynx, or voicebox. The air travels from the larynx into the trachea, or windpipe. The pharynx and the trachea are often considered to be passageways. The larynx is classified as an organ.
The trachea transports the air into tubes called bronchi. These lead to the lungs, which are organs. Inside the lungs, the bronchi divide into narrower passageways called bronchioles, which transport the air to the alveoli, or air sacs, within the lungs.
Question: What is pneumonia?
Answer: Pneumonia is an infection that causes the alveoli (air sacs) in the lungs to become inflamed. The alveoli may fill with fluid, making breathing difficult. Both bacteria and viruses can cause the infection. Bacterial pneumonia is generally the more serious form of the disease. Some fungi and certain organisms that resemble bacteria can also cause the disease.
Some conditions make it more likely that a susceptible person will develop pneumonia in certain circumstances. One of these conditions is the existence of chronic disorders such as asthma, COPD (chronic obstructive pulmonary disease), and heart disease.
Pneumonia often develops after someone has had a cold or the flu. Symptoms of pneumonia may resemble those of a cold or flu that doesn’t disappear when expected and becomes worse. A person may also notice chest pain while breathing, as I know from my experience with the disorder. Anyone with a respiratory problem that lasts for a long time or is severe should visit a doctor for a diagnosis and treatment.
Question: What is the structure of the respiratory system?
Answer: The first illustration shows the parts of the respiratory system, and I describe them in the article. Like other parts of the body, the respiratory system can be defined at different levels of detail. For example, the lungs are part of the system. We could go into more depth and say that the lungs contain air sacs, or alveoli. We could then go into more detail still, and mention the capillaries that cover the alveoli.
Question: When a person breathes out, do they expel cells from the respiratory system in addition to air and water?
Answer: Multiple researchers have found that exhaled air contains bacterial cells at least some of the time. Our respiratory tract contains bacteria. Some of the bacteria may be harmful, but others appear to be harmless and form part of the lung microbiome. This microbiome hasn’t been as well studied as the one in the intestine. Many unanswered questions exist in relation to the life of the microorganisms found in the respiratory tract.
© 2011 Linda Crampton
Linda Crampton (author) from British Columbia, Canada on February 12, 2019:
Hi, Allie. Bones are part of the skeletal system. Sense organs such as the eyes and ears are generally considered to be part of the nervous system.
Allie on February 12, 2019:
Hello, My name is Allie and I have a few questions and thoughts about the Respiratory System. My first question is If the Nose, Trachea, Bronchus, Larynx, Lungs, and Diaphragm are all part of the Respiratory System then what category or spot does the eyes and ears and bones come in at because I do know that it is not in the Respiratory System... Please get back to me as soon as you guys can...
Linda Crampton (author) from British Columbia, Canada on February 08, 2017:
You're welcome. I'm glad the article helped you.
someone on February 08, 2017:
thanks this is helpful
Linda Crampton (author) from British Columbia, Canada on November 08, 2015:
Thank you very much for the kind comment, fpherj48! I love learning about the human body, too. It's a fascinating topic.
Linda Crampton (author) from British Columbia, Canada on November 08, 2015:
Thank you very much for the comment and the shares, Patricia! I always appreciate your visits and the beautiful angels that you send.
Suzie from Carson City on November 08, 2015:
Alicia....A fabulous teaching hub....So thorough and fascinating to learn such detail and vital information. Thank you for this education. I love learning more about our bodies and health!
Patricia Scott from North Central Florida on November 08, 2015:
Aren't we thankful for our medulla!! ?? Alicia, I thought I knew a thing or two about these amazing respiratory organs that I possess but clearly I had no real clue.
So much information I had not even realized is true.
Thank you for taking the time to so carefully share this with us.
Angels are on the way to you this lovely Sunday afternoon (just had a gentle rain and now the sun is smiling down on us again).
Linda Crampton (author) from British Columbia, Canada on May 27, 2015:
Thank you for the interesting comment, Joe Bob!
Joe Bob on May 27, 2015:
This is like, really, like, cool ans stuff dude.
Linda Crampton (author) from British Columbia, Canada on February 17, 2015:
Thank you. I appreciate your visit.
Wftrctdctdc on February 17, 2015:
This was good
Linda Crampton (author) from British Columbia, Canada on February 21, 2014:
Thank you for the comment, sel.
sel on February 21, 2014:
Linda Crampton (author) from British Columbia, Canada on March 14, 2013:
Thank you very much, America!
Linda Crampton (author) from British Columbia, Canada on February 26, 2013:
Thank you, ramani hariharan.
ramani hariharan on February 26, 2013:
an interesting hub njoyed
Linda Crampton (author) from British Columbia, Canada on November 01, 2012:
I'm glad that the hub was useful!
... on November 01, 2012:
it helped a lot
Linda Crampton (author) from British Columbia, Canada on July 25, 2011:
Thanks a lot for the visit and the comment, Koffeeklatch Gals. I think that the human respiratory system is amazing!
Susan Hazelton from Sunny Florida on July 25, 2011:
102.5 mph is quite a powerful sneeze. That's hurricane proportions. Great information - great research. I also didn't know the lungs were pulled open, I awlays thought they opened by breathing in and filling them with air.
Linda Crampton (author) from British Columbia, Canada on April 03, 2011:
Thanks for the comment and the vote, Om Paramapoonya!
Om Paramapoonya on April 03, 2011:
Thanks for sharing these interesting facts. It was a fun read. Voted useful and up. :)
Linda Crampton (author) from British Columbia, Canada on March 16, 2011:
Thank you very much for the comment and vote, Pamela99.
Pamela Oglesby from Sunny Florida on March 16, 2011:
I also found your hub very detailed and accurate. I also studied anatomy and physiology when I was in school and became an RN. I didn't have the great video either. Voted/rated up.
Linda Crampton (author) from British Columbia, Canada on March 11, 2011:
Hi, Alternative Prime! Thank you for the comment and rating.
Thank you as well, b. Malin!
b. Malin on March 11, 2011:
Having worked for a Heart Specialist for over ten years, I found your Hub on this subject very informative and well written...Wonderful videos as well. You've made it simple for all to understand, our Respiratory Systems.
Alternative Prime from > California on March 11, 2011:
Interesting and educational at the same time. Thoroughly enjoyed it and rated accordingly.
Linda Crampton (author) from British Columbia, Canada on March 09, 2011:
Hi, Peggy W. Thank you very much for the comment and the rating!
Peggy Woods from Houston, Texas on March 09, 2011:
Back when I was studying anatomy and physiology there was no Internet with terrific videos like the one shown in this hub. You have mixed interesting details into this like #4 which I never heard previously. Makes studying the respiratory system ever so much more interesting! Rated up and useful. Thanks!
Linda Crampton (author) from British Columbia, Canada on March 07, 2011:
Thank your for the comment, A.A. Zavala!
GmaGoldie - Thank you very much for the kind comment. There’s always residual air in the lungs. A small amount of the air forming the residual volume is replaced with freshly inhaled air during each breath, and a small amount of the stale air that was part of the residual volume is exhaled with each breath, so the residual air is slowly but continually being replaced. If we increase the rate of breathing or the depth of the breaths the residual air will be replaced more quickly.
Fossillady - Thank you for commenting. I am so very sorry about your husband's lung cancer.
Kathi from Saugatuck Michigan on March 07, 2011:
This was very thorough, good job with it, Alicia! I recently studied the lung system because of my husband's lung cancer. If people paid closer attention to how the body works maybe they could prevent some of the diseases that plague us. Thank you for sharing
Kelly Kline Burnett from Madison, Wisconsin on March 07, 2011:
Fantastic Hub! Very well written and organized. Is it true that exhalation "cleans" the lungs?
Augustine A Zavala from Texas on March 07, 2011:
I always hated this ssystem on the anatomy test. But the hub made it easy to understand. Thank you for sharing.