The Respiratory System: An environmental interface.

The body is critically reliant on oxygen that enters the body via the respiratory system.  Every day we inhale and exhale nearly 20,000 litres of air which every cell in the body depends on to function and live.  The brain can only survive three or four minutes without oxygen before brain cells begin to die.  Oxygen is also needed to oxidize food to provide energy to every cell.

Breathing is so vital to life that it is under autonomic control– this is the division of the brain that functions without any conscious effort, so that we can ‘forget’ about breathing.  While breathing continues and seems simple, breathing processes and their controls are far from simple and to some degree not even fully understood.

Let’s look more closely at this system and see how it enables these functions.

Structures of the Respiratory System from the top down

The structures of the respiratory system are divided into two zones: the conducting zone and the respiratory zone; The conducting zone provides conduits for air to travel through.  They filter, cleanse, humidify and warm incoming air.  The conducting zone consists of the nose, nasal cavities, pharynx, larynx and trachea, bronchi and bronchial tree.  The respiratory zone is the actual site of gas exchange and is composed of the bronchioles, alveolar ducts and alveoli.

In addition to these organs are the respiratory muscles – the diaphragm, intercostal (between each rib) muscles and accessory breathing muscles - that bring about volume changes of the thorax to allow for the lungs to fill.

Anatomy of the Conducting Zone

The nose is the only externally visible part of the respiratory system.  On inhalation air enters through the nostrils and goes through the internal nasal cavity where it is filtered, cleaned, moistened, and warmed.

The nasal cavity is lined with respiratory mucosa, a specified type of skin which contains sebaceous and sweat glands and hair follicles.  These glands secrete mucous, an antibacterial enzyme called lysozyme and natural antibiotic substances called defensins.  The sticky mucous traps debris while the lysozyme and defensins attack and destroy pathogens.  Hair-projections called cilia then move this contaminated mucous backwards towards the throat to be swallowed and digested by stomach juices.

The Sense of Smell

Smell is a remarkable sense and differs from other senses in that we actually smell microscopic bits of a substance that have evaporated into the air.  Substances that do not evaporate, for example steel, have no smell. 

The roof of the nasal cavity is lined with olfactory mucosa which contains a patch of olfactory nerve cells about the size of a postage stamp that are the smell receptors.  Airborne particles come into contact with the olfactory mucosa and stimulate olfactory hairs which in turn stimulate olfactory nerve cells.  A chemical reaction occurs within these cells and a nerve signal is sent to a part of the brain we call the olfactory bulb and it is interpreted as an odour.  Some of the neurons of the olfactory bulb lead directly to the limbic system of the brain, the part of the brain where emotion is experienced.  This explains why smell has such a powerful effect on our mood and triggers emotional memories.

While taste is a chemical sense perceived only by the tongue, flavour is a fusion of multiple senses including taste and smell. 

The nasal mucosa is also richly supplied with sensory nerve endings which, when in contact with irritating particles, triggers cause the sneeze reflex to forcefully expel air outward.

The nasal epithelium is a thin layer of tissue under which lie rich plexuses of capillaries (fine blood vessels).  When the air temperature drops or body temperature rises the capillaries can become engorged with blood leading to epistaxis, or nosebleeds.

The Sinuses

The nasal cavity is surrounded by a ring of paranasal sinuses in the frontal, sphenoid, ethmoid and maxillary bones of the skull.  Sinuses serve to lighten the skull, warm and moisten air and also as reverberating chambers for the production of sound.

The next structure down the line is the funnel-shaped pharynx.  Commonly called the throat, the pharynx links the nasal cavity and the mouth to the larynx.

The larynx is an intricate framework of cartilage, its roles are to hold the airway open; provide a switching mechanism to route air and food into the proper channels; and house the vocal cords for voice production. One of these cartilage plates is externally visible, especially in males, and is commonly referred to as the Adam’s apple.

Voice Production

The vocal cords are actually ligaments that stretch across the larynx on either side of the opening it creates.  The vocal cords coordinate with the breath, opening and closing to create pitch.  Vocal cords come in different sizes and the larger they are, the lower the pitch of the voice.  Men’s vocal cords are typically 17mm – 25m long, women’s vocal cords are typically 12.5mm – 17.5mm long. 

The trachea, commonly called the windpipe, is also lined with the hair-like cilia that propel mucous and debris upwards and away from the lungs.  The trachea descends into the chest and divides into the left bronchus and right bronchus (collectively bronchi) which plunge into the left and right lungs respectively.

By the time air reaches the bronchi it is cleansed of most impurities and saturated with water vapour by the various mechanisms of previous airway passages.

The bronchi divide into smaller and smaller pathways, the smallest of which are called bronchioles.  Due to the pattern this branching network forms it is often called the respiratory tree. 

The bronchioles lead to alveolar ducts which lead to clusters that look like a bunch of grapes called alveolar sacs.  Each ‘grape’ in an alveolar sac is a single alveolus; the alveoli are the sites of gas exchange.  Each alveolus is connected to the next by a shared alveolar pore.  Having this pore means that the air pressure in the lungs is equalized and provides an alternative passage for air to reach any alveoli whose bronchiole has collapsed through disease.

There are approximately 300 million or so gas-filled alveoli in the lungs and they make up most of the lung volume, providing tremendous surface area for the exchange of gases.

Ventilation and Respiration

Ventilation is a mechanical process whereby gases enter and leave the lungs.  Respiration is the metabolic process of the exchange of oxygen and carbon dioxide both within the lungs and within body tissues. This exchange of gases intimately involves the cardiovascular system as the gases must travel to and from the body tissues, via the blood stream, to the lungs; the roles of these two systems are so intimately related they cannot be talked about independently when referring to gas exchange.

Ventilation

Whilst respiratory muscles and accessory breathing muscles can be under our conscious control so we can override the breathing mechanism if we choose to, autonomic (unconscious)breathing is controlled by the level of carbon dioxide (CO₂) in the blood, not a lack of oxygen (O₂).Specialist chemoreceptors in the body monitor and measure the acidity (pH level) of the blood.  Too much CO₂ acidifies the blood (lowers the pH), which, when detected, triggers these chemoreceptors to send nerve impulses to our brain that stimulate us to breathe in an effort to get rid of the CO₂.

On inhalation the respiratory muscles contract: the diaphragm moves downwards and the intercostal muscles expand separating the ribs so that the sternum lifts.  These movements increase the space within the thoracic cavity and stretch the lungs so that the volume within the lungs increases.  This action causes a suction effect, literally drawing air into the lungs.  Just to be clear, we don’t expand the lungs by breathing in, we breathe in because the lungs have expanded.  This expansion is triggered by rising CO₂ levels.  This is a contradictory, yet very important concept for people with breathing disorders such as hyperventilation and asthma to understand.

Air is a gas and follows the laws of all gases, which are that they flow from areas of high pressure into areas of lower pressure and always fill their container.  As the volume within the lungs increases, air flows into the lungs ‘down the pressure gradient’ until the pressure within the lungs and in the atmosphere is equal.

When we exhale these same muscles relax, the rib cage descends and the lungs recoil due to their elasticity.  As lung volume decreases the pressure within the lungs rises, expelling gases out of the lungs.

The lungs are able to expand smoothly because they are enclosed in a double layered skin called the pleurae.  The pleurae is a continuous skin, one side of which covers the lungs, the other side of which attaches to adjacent body structures inside the thoracic cavity.

Between these two layers is a very narrow space (the pleural cavity) that is lubricated by pleural fluid which creates great surface tension the way a drop of water does between two plates of glass.  The pleurae allow the lungs to glide easily over the adjacent surfaces and ensure the lung’s movement closely follows volume changes of the thoracic cavity.

The smooth muscle within the walls of the bronchi and bronchioles contracts and dilates in response to various stimuli.  Inhaled irritants, inflammation in the airways and allergic reactions can cause these muscles to contract, constricting the airways such as during an asthma attack.  Conversely the release of adrenalin can dilate the bronchioles, reducing airway resistance and promoting respiration.

Respiration – Gas Exchange

Gas exchanges takes place in the alveoli, the tiny sacs at the end of the alveolar ducts. In most places the walls of the alveoli are just one cell thick.  The external surface of the alveoli is covered with a ‘cobweb’ of tiny blood vessels called pulmonary capillaries.  Between the alveoli and the capillaries is an extremely thin (200–800 nanometer) semi-permeable membrane called the alveolar-capillary barrier or respiratory membrane, across which oxygen and carbon dioxide pass during gas exchange.

Two blood supplies enter and leave the lungs - the pulmonary circulation serves the whole body by returning deoxygenated blood to the lungs and sending the oxygenated blood from the lungs to the heart for distribution around the body.  The bronchial circulation supplies the lung tissues themselves with oxygenated blood.

Oxygen is mainly transported in the blood stream bound to the haemoglobin in red blood cells.  Carbon dioxide is primarily transported in the blood plasma (70%) with only small amounts bound to the haemoglobin.

The actual diffusion of the gases from lung to blood stream to tissue is complex and is subject to the laws that govern gases, pressure, pH and temperature, as well as the physical human anatomy, so it is too detailed to describe in this article.  However we will mention neural control of respiration.

While breathing seems simple, its control is fairly complex.  Breathing rhythms are governed by a group of nerve cells called the dorsal regulatory group (DRG).  When the DRG fire, nerve impulses stimulate the diaphragm and intercostal muscles to contract which expands the thorax so that air will rush into the lungs.  The DRG then falls dormant and expiration occurs passively as the inspiratory muscles relax and the lungs recoil. When the brain senses an excessive rise in pH in the blood stream due to a build up of carbon dioxide, for any reason, the DRG generates gasping in an attempt to restore balance and to get oxygen to the brain.  These regulatory mechanisms can be suppressed by sleeping pills, morphine and alcohol.

In a nut shell summary:

1. Air enters our respiratory system through the nose and mouth, travels through passageways known collectively as the
2. The conducting zones have specialised linings and structures that warm, moisten and clean the inhaled air and function as a barrier to infection.
3. The respiratory pathway or conducting zone terminates in the respiratory zone which is the site of gas exchange or respiration in the lungs.
4. Ventilation is the mechanical process of the respiratory muscles contracting and relaxing to increase and decrease the volume of the thoracic cavity.
5. Respiration is under neural control and breathing is regulated by brain cells that perceive carbon dioxide levels in the blood.
6. Two blood supplies enter and leave the lungs - the pulmonary circulation supplies the body tissues with oxygenated blood and returns deoxygenated blood to the lungs. The bronchial circulation supplies the lung tissues with oxygenated blood directly from the heart.
7. Structures of the respiratory system are also responsible for the sense of smell and vocal function. They also act as barriers to infection and house components of the immune system.