Introduction
Breathing is an automatic action controlled by our medulla while asleep, unconscious, or awake. The fact that our bodies take this simple action of inhaling the air that ubiquitously surrounds us as an autonomic action suggests a significant importance to the body’s function. This is only the start of humans’ detailed, intricate, and adapted respiratory system- an undoubtedly marvellous feat of human evolution. In this article, we will explore deep into this system and question the failsafe’s- since, without oxygen in our bodies, we become ineffective, weak, and fruitless: often leading to death.
Why Do We Need Oxygen?
As multicellular organisms, humans contain an abundance of cells. These cells must carry out their specialized functions within tissues- most of which are active processes (requiring energy). This introduces the process of cellular respiration, an enzyme-controlled reaction that releases energy in the form of an activated nucleotide known as ATP (Adenine Tri-Phosphate). This enables cells to carry out DNA replication, active transport, synthetic pathways, and muscle contraction via actin-myosin interactions in muscle fibres. As a result, oxygen is necessary for cells to respire and carry out basic essential functions. Therefore the process of breathing to obtain oxygen is critical for humans.
In a fatal context- the lack of oxygen to the brain due to no inhalation of air(and hence oxygen) will lead to a deadly condition known as Cerebral Hypoxia. This is because the cells of the brain require a constant supply of oxygen to respire. Therefore a lack of oxygen means glucose is not metabolized quickly enough within brain cells, and hence there is not enough ATP being released. The lack of energy causes brain cells to die and neurons to shut down. Therefore in Cerebral Hypoxia- the brain cells can die out in 5 minutes without oxygen, leading to death.
The Mechanism and Adaptations
It is vital to understand how our cells receive oxygen via the respiratory system- but as we explore this, we shall also come across numerous organs that host multiple functions, making what seems easy, effortless. Everyday actions turn into a precise mechanical process. The respiratory system starts by a simple inhaling activity conducted manually or automatically. This first step is crucial for the function of the respiratory system. Hence it is assigned as an autonomic action by the body- controlled by the Medulla oblongata of the brain.
The air then travels through our mouths and down to the first main structural feature- The Larynx (seen in figure 1).
Figure 1. A detailed view of the larynx, including examples of Cartilage, explored further in the paragraph below. Image sourced from WFSA – Airway Masterclass 3 : The Larynx
The Larynx (also referred to as the voice box) is located just behind the tongue. It is a complex organ with nine pieces of Cartilage within it, namely: the epiglottis, thyroid, arytenoid, cuneiform, corniculate and cricoid cartilage. Cartilage is a connective tissue that has major structural significance. Referring back to the Larynx, Cartilage’s abundance ensures it is held in place without collapsing and allows it for some flexibility to change shape. The Larynx connects the throat and the trachea with four main functions: Protecting the top part of the trachea, directing food and drink away from the trachea, enabling speech, and finally allowing for unrestricted flow of air towards the trachea. We can primarily focus on the last two functions as these are the ones most concerned with the respiratory system. When we inhale air, the muscles hold the Cartilage firmly in place so that the air from the mouth or nasal passages (if inhaled through the nose) may flow through the Larynx smoothly.
The Larynx is also adapted in numerous ways to fulfill its functions as the voice box. The Larynx has a thin lining of mucous membranes and hence has several secretory and squamous epithelial cells lined on its surface. This ensures that the Larynx remains moist, so vibrations of the vocal folds are more smooth. The vocal folds are formed by narrow ligaments: the false vocal folds and true vocal folds. The false vocal folds do not make sound at all but serve to protect the true vocal folds located beneath them. Sound is produced by the muscles of the Larynx (cricothyroid and thyroarytenoid), moving the vocal folds into the stream of passing air. Pitch depends on how tight the folds have been stretched across the creek. This links back to the abundance of cartilage in the Larynx- connecting the muscles to the vocal folds and indirectly changing vocal fold length.
Figure 2. A view of the respiratory system, from Larynx to Lungs. Image sourced from Pharmacy180-Trachea
Once the air passes through the larynx, it enters the Trachea- our next prominent structural feature of the Respiratory system. The Trachea is a cylindrical tube that carries air down from the bottom of the larynx to the bronchial tubes. The Trachea is adapted to ensure that it does not restrict the mobility of the neck, enables the unrestricted flow of air, and is strong enough to prevent collapse during low internal air pressure as inspiration occurs.
The Trachea achieves this by containing alternating bands of cartilage and muscle to hold open the Trachea and allow air to flow easily. This also provides sufficient structural support to ensure the Trachea has some resistant properties to external forces, such as being crushed and not collapsing during low internal pressures due to inhalation(inspiration). Additionally, the inside and outside of the tracheal walls are lined with membranes of solid elastic fibres to provide flexibility which ensures the Trachea does not limit movement of the neck. The neck moves by a pivot joint and is vital for seeing in different directions; hence, these elastin fibres in the tracheal lining are crucial.
Figure 3. A cross section of the trachea with detail on the surface lining. Labels point to the glands on trachea surface, explored further below. Image from Yale Histology
The trachea is also lined with ciliated epithelial cells and goblet cells. The goblet cells release mucus via the Glands, seen above in Figure 3, and trap any dust or bacteria that have been inhaled before they can reach the lungs and cause infection. The cilia, hair-like projections formed from microtubules, are present on the surface of the epithelial cells lining the tracheal wall and waft the mucus up the trachea for the mucus to be expelled via coughing. The process of removing the mucus occurs by simple cilia action and coughing or sneezing (if particles/bacteria are caught in nasal passages). In the first case, the cilia move the mucus by creating a rhythmic beat and hence a current. This causes the slime to move up the trachea, through the larynx, and into the pharynx, where it shall mix with saliva from the mouth. From here, it may be swallowed back and travel down the esophagus for it to be broken down by the stomach acid. In a cough, the central airways narrow, and the phlegm/mucus globs are propelled up the trachea by a column of high-velocity air. The noticeable coughing sound is formed by the air moving past the larynx at such high speeds. The phlegm/mucus is transferred directly to the pharynx, where it may be swallowed back down into the esophagus or expectorated. The process of sneezing is a mechanism to clear the nose following the detection of foreign bodies in the nose, such as pollen, dirt, or bacteria. When we sneeze- our chest muscles contract and cause the lungs to become slightly compressed while our throat muscles relax. Then a column of air is sent through the nose at approximately 100mph to clear out the built-up phlegm.
The trachea, therefore, carries air down into the bronchi. The bronchi are transport tubes that further carry the air into the lungs. The lungs can hold about 5 liters of air, and since we have two lungs, the Bronchi must transport the air into both lungs. As a result, we see a split of the main bronchial tube into the left and right bronchus. However, this is not the end of the bronchi’s branching as the right and left bronchus now rapidly subdivide into numerous small tubes. The smallest of these are the bronchioles, with about ½ mm diameters. The bronchial tubes are attached to thousands of tiny air sacs called alveoli. Cumulatively, the millions of alveoli can create a surface of about 100 square meters. The alveolar walls are also surrounded by a network of one cell thick capillaries. As a result, the high surface area means that diffusion, where oxygen moves from the air sacs to the veins, can occur more quickly. Alveolus and Capillaries are also one cell thick- reducing the diffusion pathway significantly and allowing the gaseous exchange of CO2 and O2 to happen effortlessly. Moreover, the fact that we continuously inhale and exhale maintains a crucial diffusion gradient between the contents of O2 in the blood capillaries and the alveoli.
Once oxygen diffuses into the blood by the gaseous exchange at the alveoli, it must be transported to the cells to allow for the vital process of cellular respiration to occur. A specific protein facilitates the transportation of oxygen within the blood called hemoglobin. This quaternary globular protein contains four prosthetic Haem groups (Fe 2+). These bind with oxygen to form an Oxyhaemoglobin complex. This reaction is reversible, as when the oxyhemoglobin reaches the bodily cells requiring oxygen- it must release the oxygen.
As a result, the cells now obtain oxygen and, assuming it also receives sufficient glucose, can carry out the process of respiration to metabolize the glucose, release ATP and hence have enough energy to conduct basic, essential, and active functions.
Failsafe
Upon exploring the respiratory system, we can appreciate that it is highly detailed and relies on multiple structures to carry out their functions for oxygen to be transported to the cells. This naturally raises a few questions: has the body prepared for the case where one of these structures fail to carry out their function? What if the trachea unexpectedly closes due to the wearing of cartilage? What if the lack of cilia prevents unrestricted airflow through the trachea?
These would significantly impact the delivery of oxygen to cells and disrupt cells’ conduction of necessary actions. Therefore there must be failsafe’s in place to account for these unexpected malfunctions because this meticulous, interconnected, and adapted system is essential to sustaining life.
Focusing primarily on the lungs, the site where CO2 is removed from the bloodstream and O2 is taken up, we can see an example of a protective measure. The pleura, wall of the lungs, is composed of very soft material with two layers. The visceral pleura covers the lungs while the parietal pleura lines the diaphragm and ribs. By attaching to the inside of the rib cage- there is a prevention of collapse and damage from external pressure. This ensures that the lungs’ shape, structure, and function are maintained. However, this is only a preventative measure rather than a direct failsafe. What if a broken middle rib punctured the lungs? Are there any measures to ensure the rest of the respiratory system can continue its operations? The answer is frankly no.
Conclusion
In the prolonged absence of oxygen, the human body cannot function and shuts down and dies. The respiratory system ensures that we continuously provide our numerous cells with sufficient oxygen, enabling them to carry out essential, active bodily processes as part of tissues or organs. It’s a marvellous, meticulous and adapted bodily circuit, but it is fruitless if one part of the circuit malfunctions. The lack of failsafe makes our beautiful respiratory system a fragile network of interconnected structures on thin ice.
Aryan Bhadra, Youth Medical Journal 2022
References
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