The Respiratory System

I've spoken before about the circulatory system, which carries nutrients and oxygen to the cells that need it, as well as the digestive system, which carries nutrients to the blood in the first place. Now I'm going to talk about the respiratory system, which carries oxygen to the blood, and removes carbon dioxide from the body.

The respiratory system, like every other system, consists of several organs (which is, incidentally, pretty much the very definition of a "system" IIRC). The air you breathe in goes through the nose or mouth, then goes through the pharynx and larynx, into the trachea and then into the bronchi which enter the lungs. From there, the bronchi branch out into smaller bronchi, which branch out into bronchioles, and then into small air sacs called alveoli, where the membrane walls are thin enough for gases to diffuse through into the blood.

tl;dr: basically the air just goes through a whole bunch of tubes until they reach a point where gases can diffuse into and out of the blood.

Long version:

The Nose

Yes, I know that sometimes people breathe through their mouths, especially during exercise (because it's pretty difficult to comfortably breathe in lots of air through your nose), but today we're going to talk about the nose as it's the starting point, okay? The reason I'm doing this is because the nose- or rather the nasal cavity, which is the internal portion of the nose inside the skull- has more relevant stuff to talk about.

This aforementioned nasal cavity has a central partition in the middle, which divides the cavity into a left and right chamber. Each chamber has three "shelves" known as conchae. Conchae divide up the passages and increase the overall surface area.

So why is surface area actually important? After all, what is it that the nasal cavity does in the first place, and why are its functions important enough to require a larger surface area for the nasal cavity to work at optimal capacity? Here are some of the functions of the nasal cavity:

1. Filtering the air: The nose contains coarse hairs which filter out large dust particles. Additionally, the nasal cavity contains a mucous membrane, which traps dust particles that make it through the first set of hairs.
2. Warming the air: Capillaries in the nasal cavity contain warm blood, warming the air as well.
3. Moistens the air: The mucous membrane takes care of this part.
4. Smell: The upper part of the nasal cavity contains olfactory receptors, which are nerve endings that are sensitive enough to distinguish different smells.
5. Pushing down mucus etc.: The mucous membrane in the lower part of the nasal cavity has hair-like projections called cilia, which push mucus and any trapped dust towards the throat. They do this by rhythmically beating back and forth.

The next part of the respiratory system is...

The Pharynx

I briefly mentioned the pharynx in my post about the digestive system, and I'm going to make another quick mention here. And by quick mention I literally mean that all I have to say about it this time is that it's a tube that's roughly 13 centimetres long, extending from the nasal cavity downwards. Oh and there's also a tube called the Eustachian tube that leads to the middle ear. Just a random fact that popped up in my book. I have no idea why it's relevant in this chapter, but maybe you might find a linkage somewhere.

Okay now that's done. Onto...

The Larynx

The larynx is also called "the voice box." It connects the pharynx with the trachea, and thus it is yet another organ that the air has to pass through to get to the lungs. It's made up of a few different pieces of cartilage (including the "Adam's apple," the large one at the front of the neck), with some mucous membranes stretched between them. These membranes are called the vocal folds, and the edges, otherwise known as the vocal cords, have elastic ligaments that can vibrate. The opening between the vocal cords is called the glottis. The muscles that move the cartilages can move the vocal folds, which then changes the size of the glottis.

When the muscles are relaxed, air simply passes through; when the muscles are contracted, the vocal cords vibrate, which in turn makes the air in the pharynx, nose and mouth vibrate, producing sound. More air produces a louder sound, while the pitch is controlled by the tension on the vocal cords. Thus the larynx is able to fulfil one of its important functions- allowing us to talk! (It also allows us to shut up after talking, but that bit's hard. I'm not good at that yet. Which is why I ramble a lot on this blog- hey wait, writing doesn't require my vocal cords! Scrap that, then.)

The larynx has yet another important function for such a humble small organ. When swallowing, it moves upwards, and meets a flap of cartilage called the epiglottis, which projects from the rear wall of the larynx. The epiglottis closes the glottis, which stops food from passing through the larynx and into the trachea and the lungs. And speaking of the trachea...

The Trachea

The trachea is about 12cm long and about 2.5cm in diameter. It contains C-shaped bands of cartilage, which allow the trachea to be flexible without being at risk of collapsing. Just like the other parts of the respiratory system covered so far, the inside of the trachea also has a mucous membrane. The membrane in the trachea contain cilia, like the lower part of the nasal cavity. The cilia in the trachea beat mucus and other solid stuff upwards, towards the pharynx, where they can be swallowed. Sounds gross, but preferable to entering the lungs.

At the bottom of the trachea, the tube divides into two smaller ones, otherwise known as...

The Bronchi

The bronchi (singular bronchus) bring air in and out of the lungs as well as through the lungs. The first set of bronchi, the pair that first enter the lungs, are known as primary bronchi. They then divide into several secondary bronchi, which then divide into tertiary bronchi, and so on. The bronchi have cartilage like the trachea, and a ciliated mucous membrane like pretty much every other part we've seen so far. Soon the bronchi divide out into...

Bronchioles

Bronchioles are basically even smaller bronchi, but with some big differences: they have no cartilage (only walls of smooth muscle), and they have no cilia. The smallest bronchioles then end in...

Alveoli

Alveoli (singular alveolus) are tiny air sacs that occur in clusters throughout most of the area of the lung. Just like the walls of villi and capillaries, alveoli walls only have one layer of cells, allowing gases to diffuse through easily. Also like villi and capillaries, alveoli come in large numbers in order to maximise surface area, thereby making them more efficient at their job, which in this case is to exchange gases between the inside of the alveoli and the numerous blood capillaries surrounding the alveoli. The inside of the alveoli have thin layers of moisture, which is prevented from evaporating completely due to the lungs' placement deep inside the body. This moisture is essential for dissolving gases, which is essential for allowing them to diffuse into the blood.

The blood in the capillaries surrounding the alveoli is the blood that's come through the pulmonary arteries after going through the rest of the body and the right side of the heart (see my post on the circulatory system). Hence, the blood has a low concentration of oxygen, much lower than that in the alveoli, allowing oxygen to dissolve into the moisture on the inside of the alveoli and diffuse through into the blood.

Once inside the blood, only around 3% of oxygen dissolves in the plasma, as it's not very soluble in water. The other 97% is combined with haemoglobin to form a compound called oxyhaemoglobin. Oxygenated blood is red since oxyhaemoglobin is bright red. As the concentration of oxygen in the blood is reduced, particularly around cells that use up oxygen, oxyhaemoglobin breaks down to release haemoglobin and oxygen. Haemoglobin is dark red, so deoxygenated blood is also dark red.

Aside from allowing oxygen to be absorbed by the blood, the alveoli also absorb carbon dioxide, which gets exhaled later. The concentration of carbon dioxide in the deoxygenated blood that arrives at the lungs has a relatively high concentration of carbon dioxide, while the alveoli have a relatively low concentration. This also provides great conditions for diffusion.

There are several ways in which carbon dioxide is transported to the alveoli for the all-important diffusion stage. Around 7-8% is dissolved in the plasma, just like the 3% of oxygen mentioned above. This carbon dioxide simply diffuses into the alveoli. Another 22% of carbon dioxide combines with the globin part (i.e. the protein part) of the haemoglobin molecule, which forms a compound called carbaminohaemoglobin, which later breaks down, allowing the carbon dioxide released to diffuse into the alveoli. The remaining 70% or so of carbon dioxide reacts with the water to produce carbonic acid, which then breaks down to produce hydrogen and bicarbonate (HCO3-) ions (there's a bit about this reaction on one of my posts about reactions and equations). These ions are carried in the plasma. Later on, the ions recombine to form carbonic acid and then, with the aid of enzymes, decompose into water and carbon dioxide, the latter of which is able to diffuse into the alveoli.

Now that I've rambled on for a bit about the mechanics of breathing, let's look at the lungs in general, as well as some other muscles that aid in breathing.

The Lungs

The lungs are located in the thoracic cavity, which is basically the area bounded by the ribs and diaphragm. Aside from the lungs, the thoracic cavity also contains the heart, aorta (the main artery), venae cavae (the two main veins leading into the heart), pulmonary veins and arteries, oesophagus, thymus gland and part of the trachea and bronchi, inside a space called the mediastinum, which is located between the two lungs.

The lungs have a two-layered membrane called the pleural membrane or pleura. Between the two, there is a narrow space called the pleural cavity, which is full of pleural fluid, which provides some lubrication, allowing the two layers to slide against each other. The fluid also holds the lungs in place. The outer layer of the pleural membrane adheres to the inner wall of the chest cavity (i.e. the ribs. Or at least I'm pretty sure it's the ribs). Meanwhile, the inner layer covers the inner surface of the lungs.

The Diaphragm

The diaphragm is the muscle separating the thoracic and abdominal cavities. It can contract and relax to change the volume of the thoracic cavity.

Intercostal Muscles

Intercostal muscles are the muscles between the ribs. They come in two varieties: external and internal. The fibres of the internal intercostal muscles are at right angles to those of the external intercostal muscles. The external intercostal muscles can contract to move the ribcage upwards and outwards in order to increase the volume of the thoracic cavity; the internal intercostal muscles contract to pull the ribs closer together and decrease the volume of the thoracic cavity.

Now that we've familiarised ourselves with the tools, let's put it all together to get a better picture of ventilation, or breathing:

Inspiration

The word "inspiration" here doesn't mean "the thing that gives you a good idea for some creative work," but "inhalation," or "taking in air." Inspiration works by increasing the volume of the thoracic cavity, thus making the air pressure inside the thoracic cavity lower than the pressure outside (my post on the Kinetic Theory explains why volume and pressure are inversely proportional). Air then rushes into the lungs from outside in order to make the pressure equal (pretty much the same principle as that of diffusion).

There are several processes in place that make the thoracic cavity bigger. The diaphragm and external intercostal muscles contract, flattening the diaphragm and moving the ribs upwards and outwards. The outer layer of pleural membrane adheres to the inner wall of the thoracic cavity, so as the thoracic cavity expands, the lungs expand too. During normal breathing, the diaphragm does most of the work; during heavier breathing, the rib cage becomes more important.

Expiration

"Expiration," which refers to "exhalation" here and not expiry dates or whatever, occurs as a result of the thoracic cavity's volume being reduced, which increases the pressure in the lungs, thus forcing the air outside in order to make the pressure equal again. The process is basically the opposite of inhalation: the diaphragm and external intercostal muscles relax, making the diaphragm bulge into the cavity and moving the rib cage downwards. More forceful expiration also requires the contraction of the intercostal muscles to lower the rib cage more actively.

Now for some more random bits and pieces about the respiratory system before I get off and get back to having a life doing other nerdy stuff because I have no life:

Respiratory Volumes

Respiratory volumes are basically different measures of lung capacity. They can be measured with different instruments, such as spirometers and vitalographs. Here's some terms for you:

• Residual volume- the volume of air remaining after maximum expiration (since you can't completely empty the lungs). This is normally around 1 200 mL for men, and 1 000 mL for women.
• Tidal volume- the volume of air that moves in and out with each regular breath. This is about 500mL for both men and women. About 150mL of tidal air doesn't reach the alveoli, but stays in dead space instead- the interior of the other organs of the respiratory system that are not involved in the actual exchange of gases.
• Vital capacity- The maximum amount of air that can be exhaled after inhaling as much air as possible. Roughly 4 800 mL for men and 3 400 mL for women.

Disorders of the Respiratory System

I'm just going to list and add in a few key points because I really should get off the computer at some point...

• Asthma- An allergic response which results in the muscles surrounding the bronchioles going into spasm (sudden involuntary contractions). As the bronchioles have no cartilage to keep them open, this can cause difficulty in breathing. Sometimes, the irritation of the membranes lining the air passages results in excessive mucus being secreted, which then also restricts air movement.
• Emphysema- Caused by long-term exposure to irritating particles. Air in general contains irritating particles, but certain groups, such as smokers and people who live in highly polluted cities, are more at risk. The particles damage the alveoli, which lose their elasticity, are replaced with fibrous tissue, and may break down. The loss of elasticity means that the lungs are constantly inflated, which then means that exhalation requires voluntary effort.
• Lung cancer- Involves the development of a tumour, just like other cancers. Risk factors include exposure to certain irritants like asbestos and tobacco. Normally begins in the walls of the bronchi- excessive production of mucus is caused by irritation of the mucous membrane lining. Trapped mucus causes alveoli to rupture, resulting in emphysema. Eventually a cancerous growth develops in a bronchus and may spread to other parts of the body.
• Laryngitis- Swelling of the mucous membrane covering the vocal cords, making it difficult for them to vibrate.
• Bronchitis- An irritation that causes an increase in mucus production in the bronchi and bronchioles, which can result in an accumulation of mucus which can be cleared by coughing.
• Pneumonia- An infection caused by some bacteria, viruses or fungi, most notably the pneumococcus bacterium. The inflammation causes fluid to accumulate in the alveoli.
• Carbon monoxide poisoning- Carbon monoxide (CO) can combine with haemoglobin 250 times more readily than oxygen can. When CO combines with haemoglobin, the oxygen-carrying capacity of the blood is reduced.
• Altitude sickness (or mountain sickness)- Higher altitudes contain lower pressure air (i.e. fewer gas molecules in the same amount of space as compared to lower altitudes). People who aren't used to the lower pressures may feel sick as their bodies are unable to absorb enough oxygen. The body can, however, adapt by firstly increasing the rate of breathing while more red blood cells (and therefore more haemoglobin) are produced to increase the oxygen-carrying capacity of the blood. People who have lived at high altitudes for a very long time may also have more alveoli and more blood vessels than those at lower altitudes, and their haemoglobin can combine with oxygen at the lower concentrations experienced at high altitudes.

Phew! I am done. I am so done. Good night!

(Mind you, I guess I didn't really have to do all this in the first place. Heck, I'm not even taking courses on human bio. Not at the moment, anyway.)