The respiratory system basics – July 2015
Human body cells require oxygen and an energy source with the ability to remove waste material after. The respiratory system is half of that process being responsible for delivering oxygen from the air to the lungs then to the blood from where it is transported via circulation to the cells for use. Waste carbon dioxide produced in the cells returns back to the lungs and is exhaled to the air again.
This relies on a series of connected structures and functions. The respiratory structures include the passages from the mouth to the lungs, the muscles to make this work, built in protective devices and a means to control and regulate the whole process. At any point in this system problems or failure can occur. Some of these can then produce medical emergencies for pre-hospital responders to deal with.
The respiratory system can be divided into a series of components. They are the upper airway, the middle airway, the lower airway, the control mechanisms, the mechanical structures and the protective mechanisms.
Respiration is controlled primarily by the part of the central nervous system that controls body activities without thought or consciousness. It is usually a subconscious activity. This is in the brainstem just above where the spinal cord begins in a structure called the medulla. This is also responsible for controlling the pulse and blood pressure. The higher part of the brain that controls conscious actions can also control breathing when desired allowing for deliberate faster or slower breathing or even breathe holding.
To help subconscious breathing there is a feedback system within the body that can detect the levels of oxygen and carbon dioxide within the blood and regulate when to take the next breath. Increasing carbon dioxide in the blood stimulates the brain to trigger a breath to remove it. As a secondary backup a breath will also be triggered if oxygen levels in the blood drop too low.
When head injury or stroke occurs or a range of other medical or drug induced problems the respiratory control can be interfered with or even stopped from functioning. In this case breathing can become slower, faster or even irregular. This may be helpful in diagnosing what the problem is. It may also mean the patient will require assistance to breathe for them.
The upper airway is the exterior opening for the respiratory system. It includes the mouth (oral cavity), nose (nasal cavity), upper throat (pharynx including oropharynx and nasopharynx), tongue and the epiglottis. It ends at the larynx.
The upper airway has a number of protective structures. Since the mouth doubles as the opening for food it has the ability to ensure that food cannot usually enter the respiratory system. The tongue and the epiglottis can move during swallowing and cover the opening to the middle airway. A foreign body that tries to get past these defences and enter the middle airway will produce a cough and gag reflex as a line of last defence to forcefully eject it. As consciousness deteriorates, so too will the effectiveness of these defences until they can be lost completely when the patient becomes unconscious.
The nasal and oral cavities are very vascular and moist. This allows inhaled air to be warmed and moistened (humidified) before it reaches the lower airways. The nasal cavity contains minute projections known as cilia that move like they are hairs. They can trap larger particles within the air cleaning it before it travels to the lower airway.
The middle airway is simply a conduit between the upper and the lower airway. It commences below the larynx (containing vocal cords) and includes the trachea (windpipe) that breaks into two further pipes the left and right main bronchus. Each of these continues to break down into many smaller and smaller bronchi.
The trachea and the bronchi are made of smooth muscle. The trachea has rings made from cartilage within it to ensure that it maintains its pipe structure. They also contain more of the cilia and mucous producing structures to continue the protective functions of the upper airway.
The lower airway is the first true gas exchange part of the respiratory system. It starts with the smallest of the bronchi known as bronchioles. These are minute but also made of smooth muscle. These muscles are capable of narrowing and widening. This is necessary to allow much more air to be drawn in during deep breathing and exercise. It is also necessary to reduce air entry to the lungs when a problem is detected by the immune system. A part of the immune system response is to narrow bronchioles to limit the entry of any recognised invader.
Attached to the bronchioles at the very ends are the many alveoli. These are essentially minute ‘balloons’ made of very thin membranes. Gases can move across them by diffusion. This is the process where gas moves from areas of high concentration to areas of lower concentration. With each breath the oxygen in the alveoli is refreshed keeping it higher in concentration than the blood on the other side. Oxygen moves across entering a thin fluid layer (interstitium) then minute capillaries on the other side of the alveoli. Similarly, there is very little carbon dioxide in the air but more in the blood returning from the cells. It diffuses out of the blood and back the other way into the alveoli where it is removed with the next breath out.
The second gas exchange part of the respiratory system is at the cellular level. Oxygen is transported attached to haemoglobin and carried in the blood to the cells. It also crosses into the cells via diffusion. Similarly, carbon dioxide diffuses back out of the cells and is carried back in the blood to the lungs.
The mechanical structures of breathing include the physical chest wall of the rib cage and sternum. It also includes intercostal muscles that help make it move. These allow the chest wall to expand to accommodate lung inflation. Inhalation is caused by muscle action whilst exhalation is usually passive relaxation. Chest muscles do have some small ability to force exhalation.
Most importantly there is the diaphragm, the principle muscle of breathing. When it contracts it pulls the lungs downward increasing their size. These bigger lungs have a greater space with the same amount of air in them meaning it is now at lower pressure compared to surrounding air. This causes air to be sucked in until that pressure equalises. As the diaphragm relaxes the lungs return back to their normal resting size and exhaled air escapes.
The pleura is the lining that attaches to the outside of the lung and to the chest wall itself. Essentially this is responsible for keeping the lung attached to the chest wall as it moves in and out with breathing.
Jeff Kenneally www.prehemt.com
Jeff is the author of the 2014 – 2016 Ambulance Victoria First Responder clinical practice protocols and accompanying education program.