The respiratory system is a key section of the MCAT syllabus and forms part of the Biological and Biochemical Foundations of Living Systems section of the exam. If you’re applying to medical school in the US, you’ll be aware that you need to achieve a good score on the MCAT exam. We’ve written this overview of the respiratory system to support your MCAT prep.
The respiratory system is one of the most important organ systems in the human body, consisting of the airways from the nose and mouth down to the base of the lungs. Its main role is in the gaseous exchange of oxygen and carbon dioxide, however, there are several functions of this system with regards to acting as a reserve for blood, maintaining acidity of the blood, aiding in immune function, and assisting in speaking. The normal respiratory rate, consisting of full cycles of inspiration and expiration, varies between 12-20 breaths per minute.
The basic divisions of the respiratory system are the upper respiratory tract; nose/mouth down to the larynx and lower respiratory tract; trachea down to the alveoli.
Air starts its journey through the respiratory tract through either the nose or the mouth. In the nasal cavity, there are ‘conchae’ which are shelves of bone that assist in warming and humidifying the air in order to assist in conduction and diffusion through the alveoli. After passing through the conchae, the pharynx, and comes across the epiglottis. This is a flap of cartilage that separates the respiratory system from the digestive system. During the process of swallowing, muscles connected to the epiglottis contract so that it moves down over the entrance to the larynx. This prevents food from entering the respiratory system. In the larynx lie the vocal folds, which assist in phonation.
After passing through the larynx, the air passes through the trachea. This is a singular tube that projects straight down and is formed of several C-shaped cartilages. The purpose of this cartilage is to prevent the airway from collapsing due to changes in air pressure. At the bottom of the trachea, it bifurcates at a point called the carina. This results in a right and left bronchus, which travels to the right and left lungs respectively. It is worth remembering here that ‘right’ and ‘left’ in anatomy refer to the patient’s right and left, not what is right when a physician is looking at them face-on. The bronchi divide further into bronchioles, one for each lobe of the lungs and then further and further until they terminate as alveoli. Alveoli are the air sacs within the lungs that are responsible for gas exchange and have an epithelium that is one cell thick. They are lined by pneumocytes, of which there are 2 types. Type I pneumocytes are the most common and are very narrow. They are responsible for promoting gas exchange across the epithelial membrane from the alveoli to the bloodstream. Type II pneumocytes are less common but make up more of the surface area of the alveoli. They are responsible for making surfactant, a substance made of phospholipids that maintains surface tension and prevents the collapse of the alveoli due to changes in air pressure within the alveoli.
Image source: https://commons.wikimedia.org/wiki/File:Human-respiratory-system-structure.jpeg
The most well-known function of the respiratory system is gas exchange, with oxygen entering the bloodstream through the respiratory system, and carbon dioxide leaving the bloodstream. Oxygen is required for aerobic respiration throughout the body, and carbon dioxide is a waste product of this process, therefore as oxygenated blood travels through the body, it loses oxygen and picks up carbon dioxide, which must be expelled. The process of gaseous exchange is able to adapt to different physiological situations, for instance during exercise, in order to maintain homeostasis. This fails in different forms of respiratory failure; Type I occurs when diffusion of oxygen is impaired, whereas Type II occurs when diffusion of both carbon dioxide and oxygen are impaired.
The process of thermoregulation describes the maintenance of the optimal body temperature, which is ~98°F (~37°C). The main way that heat is removed from most bodily tissues is through the blood. Thermal energy is transferred to the cooler blood, which is then carried either in capillaries to the surface of the skin to be expelled due to the evaporation of sweat, or back to the lungs where the energy is expelled into the surrounding air. This is a continuous process because there is a constant blood flow, which means that as soon as thermal energy from one area is transferred to the blood, it is taken away and that area has fresh blood that can accept more thermal energy again. In this way, the thermal homeostatic state of the body is maintained.
The respiratory system itself acts as an important physical barrier against infections. It is not directly in contact with the blood, oxygen, and carbon dioxide both pass from the blood to the lungs and vice versa through diffusion across epithelial membranes. Therefore, organisms and particles of a certain size are not able to get into the bloodstream and spread to different parts of the body. Furthermore, the action of breathing itself assists in preventing infections, because the constant flow of air reduces the grouping and propagation of organisms, as well as expelling them from the body during expiration. The lungs also produce mucus, which allows for particles to be trapped instead of ending up in the alveoli, and there are several phagocytes and mast cells within the respiratory system that further assist in breaking down pathogens.
The lungs are surrounded by two layers of pleura: parietal pleura which covers the chest wall, and visceral pleura which covers the lungs. Between these pleurae is the ‘intrapleural space’, which contains intrapleural fluid to prevent friction of these two layers of pleura against each other. It is through changes in pressure gradients in the pleura that the process of breathing occurs.
The process of breathing involves a series of pressure changes through which air is forced in and out of the chest cavity. Air moves along a pressure gradient, therefore when there is a negative pressure within the chest cavity, air will be drawn into the lungs, and when the pressure within the chest cavity exceeds that outside of the chest cavity, air is then forced out of the lungs.
Inhalation begins with the expansion of the chest wall. This is done through two mechanisms, the first of which causes the ribs to expand through contraction of the external intercostal muscles. This contraction allows for the ribs to move upwards and outwards. The second mechanism is contraction of the diaphragm, which makes it flatten from its previous domed shape. As a result of both of these, the chest wall has expanded but the lungs have remained the same size. This decreases the pressure within the intrapleural space, meaning that lung pressure exceeds the intrapleural pressure. The lungs then expand, reducing the volume of the intrapleural space to equalize the pressure between the two, however this, in turn, reduces the pressure of the lungs in comparison to the outside atmosphere. Air is drawn into the lungs, and pressure between the lungs and atmosphere equalizes.
Expiration is initiated due to relaxation of the external intercostal muscles and the diaphragm. As a result, the chest wall moves downward and internally, and the diaphragm returns to its domed, relaxed state. Pressure within the intrapleural space is increased, so to equalize the pressure between the intrapleural space and the lungs, the pleura pushes against the lungs. The pressure on the lungs, therefore, increases too, causing it to surpass the pressure of the outside environment, and therefore forcing the air out into the atmosphere to once again equalize the pressure. It is worth noting that while expiration is normally not an active process (as it is caused by relaxation of muscles), during periods of stress, for instance exercise, muscles such as the internal and innermost intercostal muscles can cause forced expiration to further increase respiratory rate.
The different parts of the respiratory system must be able to cope with the rapid pressure changes that occur in breathing, and it is modified to do so. The trachea is the first area in which these adaptations occur, through a number of C shaped cartilages that work to hold the trachea open regardless of the pressure changes. The bronchi and bronchioles still have areas of cartilage in the wall, although the concentrations progressively decrease further down the respiratory tract.
At the alveoli, there is no cartilage, therefore there must be a different mechanism in place. This is through surfactant, which is a lipid-based substance that works to decrease the surface tension within the alveoli. Compliance is a concept relating to the elasticity of an alveolus, and high compliance decreases the ability of the lungs to expand. A decrease in surface tension also decreases the compliance of an alveolus, therefore increasing its elasticity, assisting in keeping it open and preventing collapse.
Thermal energy is transferred from tissues around the body to the blood when the temperature of the body tissues exceeds that of the blood. The constant flow of the blood means that heat can continuously be taken away from tissues until they cool down. However, this heat energy cannot remain in the blood and needs to be transferred to the environment. This is down through the skin (using sweat) and through the lungs. Both of these mechanisms use the same theories though, which are outlined below.
Blood high in thermal energy travels around the body to capillary beds, which are very close to the surfaces of the nose, trachea and skin. These surfaces are capable of secreting water or sweat. Capillaries have an endothelium that is one cell thick, and as they are very close to the surface of the aforementioned areas, thermal energy can easily be transferred to the water. Once the thermal energy of the surface liquid is high enough, it evaporates, either directly into the air in the case of skin, or into the airways to then be exhaled in the case of the nose and trachea.This process can be exploited during exercise by the way of panting, which is commonly seen in other animals such as dogs. Panting increases the frequency of exhales, therefore increasing the loss of thermal energy from the body into the surrounding environment.
The initial mechanism that prevents particles from getting to the lower airways is through vibrissae, which are small nasal hairs that move via protraction and retraction caused by muscles that are attached to them. This movement reduces the likelihood that different particles will be inhaled, as it wafts them out of the nose.
The upper respiratory system contains several goblet cells which secrete mucus. This mucus works to trap microorganisms that may make their way into the body through the airways. They contain an enzyme called lysozyme, which works to kill pathogens. The respiratory tract is lined by cilia, which are tiny hair cells that work to ‘waft’ the mucus up the respiratory tract in an elliptical motion, similar to how water movement forms a wave. This process is called the ’muco-ciliary escalator’ and means that the mucus can move high enough in the tract to either be coughed out, sneezed out, or swallowed out. Finally, the alveoli of the lungs contain several macrophages and mast cells, which assist in the phagocytosis of pathogens and promote an immune response respectively.
Partial pressure can be defined as the pressure one gas would exert on a solution if it was the only gas in that solution. It is an important concept to understand when considering the movement of different gasses across the alveoli from the bloodstream. Blood enters the pulmonary system through the pulmonary artery, straight from the right ventricle. This blood is deoxygenated and contains waste products of cell respiration, and the partial pressures of both are about 6kPa. The partial pressures at the alveoli are vastly different though, with the partial pressure of oxygen being at about 13kPa (as it has been inhaled), whereas the carbon dioxide partial pressure is comparatively low (5.3kPa). The differences in the partial pressures between the alveoli and the blood causes a pressure gradient, across which the particles diffuse. This means that oxygen readily diffuses across to the bloodstream, whereas carbon dioxide is removed from the bloodstream and diffuses into the alveolus.
Image source: https://commons.wikimedia.org/wiki/File:Gas_exchange.jpg
Henry’s Law is a theory whereby the amount of gas that dissolves in a liquid ∝ partial pressure of that gas in a liquid. In practice, this means that the pressure of the outside environment increases (such as when underwater), the partial pressures of gasses in the alveoli also increase, and therefore more oxygen will dissolve into the blood. This is a mechanism that must be understood for divers, as the deeper they go, the less oxygen they will require to breathe in for the same amount to diffuse into the bloodstream and therefore be delivered into tissues.
The respiratory system is very important in maintaining the acidity of the blood, predominantly through its role in removing carbon dioxide from the circulatory system. This is important because carbon dioxide, when in the body, reacts with water to form carbonic acid. This is a reversible equilibrium reaction, however, if carbon dioxide is not expired in the air and water is not removed through the kidneys or through thermoregulatory processes in the body, more carbonic acid gets formed. This decreases the overall pH of the blood, which can have detrimental effects on different enzymes, membranes, and cells throughout the body. Therefore, it is necessary for the body to be able to expel this carbon dioxide to maintain the correct acidity. In periods of exercise, panting is especially important. Not only does it assist in thermoregulation and provide the cells of the body with more oxygen, but it also reduces acid build-up from carbon dioxide and water that are released from aerobic respiration.
The rate of breathing is monitored by the regulatory centers in the medulla oblongata of the brain. This area contains central chemoreceptors, which measure changes in pH and levels of carbon dioxide in the interstitial fluid of the brain. If the pH is too low, the medulla increases the rate of respiration, whereas if it is too high, it decreases the rate of respiration. This is done through the dorsal and ventral respiratory groups. The dorsal respiratory group is required for inspiration and is located in the dorsal medial medulla. As inspiration is an active process whereas expiration is a passive process, the DRG in most instances can work independently The VRG, however, is located ventrally in the medulla. It is responsible for ‘forced’ breathing, that is, forced inspiration and forced expiration. This is utilized in times of high stress where either intentional or increased breathing is required. All of these systems work together to regulate the rate of breathing and therefore allow the human body to work as efficiently as possible.
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