15 Fascinating Facts About The Respiratory System

Respiratory system complete en.svg Photo by LadyofHats, Jmarchn – Wikimedia Commons

15 Fascinating Facts About The Respiratory System

The respiratory system also known as the respiratory apparatus is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen vary greatly, depending on the organism’s size. The environment in which it lives and its evolutionary history. 

Land animals have a respiratory surface that is internalized as linings of the lungs. Gas exchange in the lungs occurs in millions of tiny air sacs. They are rich in blood supply thus bringing the air into close contact with the blood. The air sacs communicate with the external environment via airways, the branched in the middle of the chest into the two main bronchi.

In birds, the bronchioles are termed parabronchi, they generally open into the microscopic alveoli in mammals and atria in birds. In most fish, the respiratory system consists of gills, either partially or completely external organs bathed in the watery environment. This water flows over the gills through various active or passive means. Gas exchange in fish takes place in the gills which consist of thin or very flat filaments and lamellae which expose a vast surface area of highly vascularized tissue to the water.

As for insects, they have respiratory systems with straightforward anatomical features, in amphibians, even the skin plays a vital role in gas exchange. Plants also have respiratory systems, but the directionality of gas exchange can be opposite anatomical features such as stomata found in various parts of the plant.

1. The Respiratory Anatomy Of Mammals

In humans and other mammals, the anatomy of a typical respiratory system is the respiratory tract. The tract is divided into an upper and a lower respiratory tract. The upper tract includes the nose, nasal cavities, sinuses, pharynx, and the part of the larynx above the vocal folds. The lower tract consists of the lower part of the larynx, the trachea, the bronchi, the bronchioles, and the alveoli.                 

The branching airways of the lower tract are often described as the respiratory tree or tracheobronchial tree. The intervals between successive branch points along the various branches of the ‘tree’ are often referred to as branching generations.                                                                                                     

The first bronchi to branch from the trachea are the right and left main bronchi, the second in diameter to the trachea. These bronchi enter the lungs at each hilum where they branch into narrower secondary bronchi which are known as segmental bronchi. The alveoli are dead-end terminals of the ‘tree’ this means that any air that enters them has to exit via the same route.

This system creates dead space which is a volume of air that fills the airways after exhalation and is breathed back into the alveoli before environmental air reaches them. At the end of inhalation, the airways are filled with environmental air which is exhaled without coming in contact with the gas exchanger.             

2. The Ventilatory Volumes 

Respiratory System (Illustration).png Photo by BruceBlaus – Wikimedia Commons

When the lungs expand and contract during the breathing cycle, this causes the volume of air to move in or out of the lungs under normal resting circumstances. Volumes are moved during maximally forced inhalation and maximally forced exhalation is measured in humans by spirometry. 

Not all the air in the lungs can be expelled during maximally forced exhalation, this is the residual volume of about 1.0-1.5 liters which cannot be measured by spirometry. Volumes that include the residual volume can therefore also not be measured by spirometry. The functional residual capacity of about 2.5-3.0 liters, and the total lung capacity of about 6 liters.             

The number of breath cycles per minute is known as the respiratory rate, an average healthy human breathes 12-16 times a minute.

3. The Measurement of Each Ventilation Cycle     

 Minute Ventilation is the total volume of air entering or leaving the nose or mouth per minute or normal respiration. The equation of this measurement is tidal volume * respiratory rate. Alveolar ventilation is the volume of air entering or leaving the alveoli per minute, the equation is tidal volume – dead space * respiratory rate.

Dead space ventilation is the volume of air that doesn’t reach the alveoli during inhalation but instead, remains in the airways per minute. The equation for this cycle is dead space * respiratory rate.                                     

4. The Mechanics of Breathing  

Human respiratory system-NIH.PNG Photo by United States National Institute of Health: National Heart, Lung and Blood Institute – Wikimedia Commonshttps://upload.wikimedia.org/wikipedia/commons/thumb/9/99/Human_respiratory_system-NIH.PNG/640px-Human_respiratory_system-NIH.PNG

Mammals inhalation at rest is primarily due to the contraction of the diaphragm, an upwardly domed sheet of muscle that separated the thoracic cavity from the abdominal cavity. When the diaphragm contracts, the sheet flattens which increases the volume of the thoracic cavity in the anteroposterior axis.

The contracting diaphragm pushes the abdominal organs downwards. The pliable abdominal contents cause the belly to bulge outwards to the front and sides, this is because the pelvic floor prevents the lowermost abdominal organs from moving in that direction.

5. The Basics of Inhalation and Exhalation

Inhalation diagram.svg Photo by LadyofHats – Wikimedia Commons

As the diaphragm contracts, the rib cage is simultaneously enlarged by the ribs being pulled upwards by the intercostal muscles. All the ribs slant downwards from the near to the front but the lowermost ribs also slant downwards from the midline outwards. This causes the rib cage’s transverse diameter can be increased in the same way as the anteroposterior diameter is increased by the pump handle movement.

During exhalation, the diaphragm and intercostal muscles relax. This returns the chest and abdomen to a position determined by their anatomical elasticity. This is the resting mid-position of the thorax and abdomen when the lungs contain their functional residual capacity of air. Resting exhalation lasts about twice as long as inhalation, this is because the diaphragm relaxes passively more gently than it contracts actively during inhalation.

6. The Mechanism of Gas Exchange

The primary purpose of the respiratory system is the equalizing of the partial pressures of the respiratory gases in the alveolar air. This process occurs by simple diffusion across a very thin membrane, it forms the walls of the pulmonary alveoli. This consists of the alveolar epithelial cells, their basement membranes, and the endothelial cells of the alveolar capillaries. 

The blood gas barrier folded into about 300 million small air sacs called alveoli, they branch off from the respiratory bronchioles in the lungs. This provides an extremely large surface area for gas exchange to occur. The air that is contained within the alveoli has a semi-permanent volume of about 2.5-3.0 liters which completely surrounds the alveolar-capillary blood. 

7. The Alveolar Air and Ambient Air

Ambient (14518768785).jpg Photo by michael dornbierer from St.Gallen, switzerland – Wikimedia Commons

Alveolar air is tiny air at the end of the bronchioles. The alveoli are where the lungs and the blood exchange oxygen and carbon dioxide in the process of breathing in and breathing out. As for ambient air, it is the atmospheric air in its natural state and is not contaminated by air-borne pollutants. Ambient air is typically 78% nitrogen and 21% oxygen.

The marked difference between the composition of the alveolar air and that of the ambient air can be maintained because of the functional residual capacity. It is contained in dead-end sacs that are connected to the outside air by fairly narrow and relatively long tubes. Through these tubes, the air has to be breathed both in and out.

Typical mammalian anatomy combined with the fact that the lungs are not emptied and re-inflated with each breath ensures that the composition of the alveolar air is only minimally disturbed. The animal is then provided with a very special whose composition differs significantly from the present-day ambient air. It is this portable atmosphere to which the blood and therefore the body tissues are exposed but not to the outside air.

8. Partial Pressure of Air 

The alveolar capillaries have a partial pressure of oxygen on average 6 kPa, while the pressure in the alveolar air is 13-14 kPa, there will be a net diffusion of oxygen into the capillary blood. This changes the composition of the 3 liters of alveolar air slightly. The blood arriving in the alveolar capillaries had a partial pressure of CO2 of also about 6 kPa. 

The changes brought about by these net flows of individual gases into and out of the alveolar air necessitate the replacement of about 15% of the alveolar air with ambient air every 5 seconds. This is very tightly controlled by the monitoring of the arterial blood gases by the aortic and carotid bodies, as well as by the blood gas and pH sensor on the anterior surface of the medulla oblongata in the brain. 

9. The Control of Ventilation

Ventilation of the lungs in mammals occurs via the respiratory centers in the medulla oblongata and the pons of the brainstem. These areas form a series of neural pathways which receive information about the partial pressures of oxygen and carbon dioxide in the arterial blood. This information determines the average rate of ventilation of the alveoli of the lungs to keep these pressures constant.

The breathing rate increases when the partial pressure of carbon dioxide in the blood increases. This is detected by central blood gas chemoreceptors on the anterior surface of the medulla oblongata. The aortic and carotid bodies are the peripheral blood gas chemoreceptors that are particularly sensitive to the arterial partial pressure of O2. 

Under normal circumstances, the breathing rate and depth, it is determined primarily by the arterial partial pressure of carbon dioxide rather than by the arterial partial pressure of oxygen it is allowed to vary within a fairly wide range before the respiratory centers in the medulla oblongata and pons respond to it change the rate and depth of breathing.

10. The Effect of Breathing During Excercise

US Army 52844 Soldiers learn to connect mind, body, soul through breathing.jpg Photo by Sgt. 1st Class Rodney Jackson – Wikimedia Commons

Exercise increases the breathing rate due to the extra carbon dioxide produced by the enhanced metabolism of the exercising muscles. To cope with the extra demand for carbon dioxide, the breathing rate has to increase from 15 times a minute which is 12 liters of air. While at rest, up to 40-60 times a minute which is 100 liters of air during exercise.

Passive movements of the limbs also reflexively produce an increase in the breathing rate. Information received from stretch receptors in the lungs limits tidal volume which is the depth of inhalation and exhalation.

11. The Local Defenses Function of The Lungs

Irritation od nerve endings within the nasal passages or airways can induce a cough reflex and sneezing. These responses cause air to be expelled forcefully from the trachea or nose respectively. Irritants caught in the mucus which lines are respiratory tract are expelled or moved to the mouth where they can be swallowed. During coughing, contraction of the smooth muscle in the airway walls narrows the trachea by pulling the ends of the cartilage plates together and pushing soft tissue into the lumen. 

Respiratory epithelium can secrete a variety of molecules that aid in the defense of the lungs, these include secretory immunoglobulins, collectins, defensins, and other peptides. These secretions can act directly as antimicrobials to help keep the airway free of infection. A variety of chemokines and cytokines have also been secreted that recruit the traditional immune cells and others to the site of infections.

Most of the respiratory system is lined with mucous membranes that contain mucosa-associated lymphoid tissue which produces white blood cells such as lymphocytes.

12. Contributions to Whole Body Functions

Lung vessels contain a fibrinolytic system that dissolves clots that may have arrived in the pulmonary circulation by embolism this is often from the deep veins in the legs. They also release a variety of substances that enter the systemic arterial blood, and they remove other substances from the systemic venous blood that reach them via the pulmonary artery. 

Some prostaglandins are removed from circulation while others are synthesized in the lungs and released into the blood when lung tissue is stretched. Lungs activate one hormone which is the physiologically inactive decapeptide angiotensin I and is converted to the aldosterone-releasing octapeptide, angiotensin II in the pulmonary circulation.

The reaction occurs in other tissues as well but it is particularly prominent in the lungs.  Angiotensin II also has a direct effect on arteriolar walls causing arteriolar vasoconstriction and consequently a rise in arterial blood pressure. Large amounts of the angiotensin-converting enzyme responsible for this activation are located on the surfaces of the endothelial cells of the alveolar capillaries.

Circulation time through the alveolar capillaries is less than one second yet 70% of the angiotensin I reaching the lungs of converted the angiotensin in a single trip through the capillaries. Four other peptidases have been identified on the surface of the pulmonary endothelial cells,

13. Vocalization and Temperature Control

The movement of gas through the larynx, pharynx, and mouth allows humans to speak or phonate. Vocalization or singing in birds occurs via the syrinx, it is an organ located at the base of the trachea. The vibration of air flowing across the larynx in humans and the syrinx in birds results in sound. This causes gas movement to be vital for communication purposes.

Panting in dogs, cats, birds, and some other animals provide a means of reducing body temperature by evaporating saliva in the mouth instead of evaporating sweat on the skin.

14. Respiratory System on Exceptional Mammals

Horses are obligate nasal breathers which means they are different from many other mammals because they don’t have the option of breathing through their mouths and must take in air through their noses. Elephants are the only mammals known to have ni pleural space. The parietal and visceral pleura are both composed of dense connective tissue and joined to each other via loose connective tissue.

The lack of a pleural space along with an unusually thick diaphragm are thought to be evolutionary adaptions allowing the elephant to remain underwater for long periods of time while breathing through its trunk which emerges as a snorkel. In the elephant, the lungs are attached to the diaphragm and breathing relies mainly on the diaphragm rather than the expansion of the ribcage.

15. Respiratory System in Reptiles and Amphibians

The anatomical structure of the lungs is less complex in reptiles than in mammals with reptiles lacking the very extensive airway tree structure found in mammalian lungs. Gas exchange in reptiles still occurs in alveoli, however. Reptiles do not possess a diaphragm this means that breathing occurs via a change in the volume of the body cavity which is controlled by the contraction of intercostal muscles in all reptiles except turtles. In turtles, the contraction of specific pairs of flank muscles governs inhalation and exhalation.

As for Amphibians, both the lungs and the skin serve as respiratory organs in amphibians. The ventilation of the lungs in amphibians relies on positive pressure ventilation. Muscles lower the floor of the oral cavity, enlarging it and drawing in air through the nostrils into the oral cavity. With the nostrils and mouth closed, the floor of the oral cavity is then pushed up, this forces air down the trachea into the lungs.

The skin of amphibians is highly vascularized and moist, with moisture maintained via the secretion of mucus from specialized cells, and is involved in cutaneous respiration. The lungs are of primary organs for gas exchange between the blood and the environmental air. Amphibians have unique skin properties that aid rapid gas exchange when they are submerged in oxygen-rich water. Some amphibians have gills in the early stages of their development while others retain them into adulthood.