RESPIRATORY SYSTEM: FUNCTIONS, PARTS, FUNCTIONING - BIOLOGY - 2025

The respiratory system or respiratory system comprises a series of specialized organs mediating gas exchange, which involves the uptake of oxygen and the removal of carbon dioxide.

There are a series of steps that allow the arrival of oxygen to the cell and the elimination of carbon dioxide, including the exchange of air between the atmosphere and the lungs (ventilation), followed by the diffusion and exchange of gases on the lung surface, oxygen transport and gas exchange at the cellular level.

By LadyofHats, Jmarchn, via Wikimedia Commons

It is a varied system in the animal kingdom, composed of various structures depending on the lineage of study. For example, fish have functional structures in an aquatic environment such as gills, mammals have lungs, and most invertebrates have tracheae.

Unicellular animals, like protozoa, do not require special structures for respiration and gas exchange occurs by simple diffusion.

In humans the system is made up of the nasal passages, pharynx, larynx, trachea, and lungs. The latter are successively branched into bronchi, bronchioles, and alveoli. Passive exchange of oxygen and carbon dioxide molecules occurs in the alveoli.

Definition of respiration

The term "respiration" can be defined in two ways. In a colloquial way, when we use the word breathe, we are describing the action of taking oxygen and eliminating carbon dioxide to the external environment.

However, the concept of breathing encompasses a broader process than the simple entry and exit of air in the rib cage. All the mechanisms involved with oxygen utilization, blood transport, and carbon dioxide production occur at the cellular level.

A second way of defining the word respiration is at the cellular level and this process is called cellular respiration, where the reaction of oxygen occurs with inorganic molecules that produce energy in the form of ATP (adenosine triphosphate), water and carbon dioxide.

Therefore, a more precise way to refer to the process of taking in and expelling air through thoracic movements is the term "ventilation."

Features

The main function of the respiratory system is to orchestrate the processes of oxygen uptake from the outside through ventilation and cellular respiration mechanisms. One of the wastes from the process is carbon dioxide that reaches the bloodstream, passes into the lungs, and is removed from the body into the atmosphere.

The respiratory system is in charge of mediating all these functions. Specifically, it is responsible for filtering and humidifying the air that will enter the body, in addition to filtering unwanted molecules.

It is also responsible for regulating the pH of body fluids - indirectly - controlling the concentration of CO ₂, either retaining it or eliminating it. On the other hand, it is involved in the regulation of temperature, secretion of hormones in the lung and assists the olfactory system in detecting odors.

In addition, each element of the system performs a specific function: the nostrils heat the air and provide protection to germs, the pharynx, larynx and trachea mediate the passage of air.

In addition, the pharynx is involved in the passage of food and the larynx in the phonation process. Finally, in the alveoli the process of gas exchange occurs.

Respiratory organs in the animal kingdom

In small animals, less than 1 mm, gas exchange can occur through the skin. In fact, certain animal lineages, such as protozoa, sponges, cnidarians, and some worms carry out the process of gas exchange by means of simple diffusion.

In larger animals, such as fish and amphibians, skin respiration is also present, in order to supplement the respiration performed by the gills or lungs.

For example, frogs can carry out the entire process of gas exchange through the skin in the hibernation stages, since they are totally submerged in ponds. In the case of salamanders, there are specimens that completely lack lungs and breathe through the skin.

However, with the increase in animal complexity, the presence of specialized organs for gas exchange is necessary in order to meet the high energy demands of multicellular animals.

The anatomy of the organs that mediate gas exchange in different animal groups will be described in detail below:

Tracheas

By BruceBlaus. When using this image in external sources it can be cited as: Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI: 10.15347 / wjm / 2014.010. ISSN 2002-4436., from Wikimedia Commons

Insects and some arthropods have a very efficient and direct respiratory system. It consists of a system of tubes, called tracheae, that extend throughout the animal's body.

The tracheae branch into narrower tubes (approximately 1 µm in diameter) called trachealas. They are occupied by fluid and end in direct association with the cell membranes.

By Indolences (File: Throat Diagram.svg), via Wikimedia Commons

Air enters the system through a series of valve-like openings, called blowholes. These have the ability to close in response to loss of water to prevent desiccation. Likewise, it has filters to prevent the entry of unwanted substances.

Certain insects, such as bees, can perform body movements that are aimed at ventilation of the trachea system.

Gills

The gills, also called gills, allow effective respiration in aquatic environments. In echinoderms they consist of an extension of the surface of their bodies, while in marine worms and amphibians they are tufts or tufts.

The most efficient are in fish and consists of a system of internal gills. They are filamentous structures with adequate blood supply that goes against the current of water. With this "counter-current" system, the maximum extraction of oxygen from the water can be ensured.

The ventilation of the gills is associated with the movements of the animal and the opening of the mouth. In terrestrial environments, the gills lose the floating support of the water, they dry out and the filaments coalesce, leading to the collapse of the entire system.

For this reason, fish suffocate when out of the water, even though they have large amounts of oxygen around them.

Lungs

The lungs of vertebrates are internal cavities, provided with abundant vessels whose function is to mediate gas exchange with blood. In some invertebrates we speak of "lungs", although these structures are not homologous to each other and are much less efficient.

In amphibians, the lungs are very simple, similar to a bag that in some frogs is subdivided. The surface area available for exchange increases in the lungs of non-avian reptiles, which are subdivided into numerous interconnected sacs.

In the lineage of birds, the efficiency of the lungs increases thanks to the presence of air sacs, which serve as a reserve space for air in the ventilation process.

The lungs reach their maximum complexity in mammals (see next section). The lungs are rich in connective tissue and are surrounded by a thin layer of epithelium called the visceral pleura, which continues in the visceral pleura, aligned with the walls of the chest.

Amphibians use positive pressure for air entry into the lungs, while non-avian reptiles, birds, and mammals use negative pressure, where air is pushed into the lungs by the expansion of the rib cage.

Parts (organs) of the respiratory system in humans

In humans, and in the rest of mammals, the respiratory system is made up of the upper portion, composed of the mouth, nasal cavity, pharynx and larynx; the lower portion made up of the trachea and bronchi and the portion of the lung tissue.

Upper portion or upper respiratory tract

Nostrils are the structures through which air enters, these are followed by a nasal chamber lined by an epithelium that secretes mucous substances. The internal nostrils connect with the pharynx (what we commonly call the throat), where the crossing of two routes occurs: the digestive and the respiratory.

Air enters through the opening of the glottis, while food makes its way through the esophagus.

The epiglottis is located on the glottis, with the aim of preventing the entry of food into the airways, establishing a boundary between the oropharynx - portion located behind the mouth - and the laryngopharynx - lowest segment -. The glottis opens into the larynx ("voice box") and this in turn gives way to the trachea.

Lower portion or lower respiratory tract

The trachea is a tube-shaped conduit, with a diameter of 15-20 mm and 11 centimeters in length. Its wall is reinforced with cartilaginous tissue, in order to avoid the collapse of the structure, thanks to this it is a semi-flexible structure.

Cartilage is located in a crescent shape in 15 or 20 rings, that is, it does not completely surround the trachea.

The tranquea branches into two bronchi, one for each lung. The right is more vertical compared to the left, as well as being shorter and bulkier. After this first division, successive subdivisions follow in the lung parenchyma.

The structure of the bronchi resembles the trachea due to the presence of cartilage, muscle and mucosa, although the cartilaginous plates diminish until they disappear, when the bronchi reach a diameter of 1mm.

Inside them, each bronchus divides into small tubes called bronchioles, which lead to the alveolar duct. The alveoli have a single, very thin cell layer that facilitates gas exchange with the capillary system.

Lung tissue

Macroscopically, the lungs are divided into lobes by fissures. The right lung consists of three lobes and the left only has two. However, the functional unit of gas exchange is not the lungs, but the alveolocapillary unit.

The alveoli are small sacs shaped like bunches of grapes that are located at the end of the bronchioles and correspond to the smallest subdivision of the airways. They are covered by two types of cells, I and II.

Alveoli

Type I cells are characterized by being thin and allow the diffusion of gases. Those of type II are more than small than the previous group, less thin and their function is to secrete a substance of the surfactant type that facilitates the expansion of the alveolus in ventilation.

The cells of the epithelium are interspersed with fibers of connective tissue, so that the lung is elastic. Similarly, there is an extensive network of pulmonary capillaries where gas exchange takes place.

The lungs are surrounded by a wall of mesothelial tissue called the pleura. This tissue is usually called virtual space, since it does not contain air inside and only has a liquid in minute quantities.

3D illustration of Larynx Trachea Bronchi Part of Respiratory System.

Disadvantages of the lungs

A disadvantage of the lungs is that gas exchange occurs only in the alveoli and the alveolar duct. The volume of air that reaches the lungs but is located in an area where gas exchange does not occur, is called dead space.

Therefore, the ventilation process in humans is highly inefficient. Normal ventilation can only replace one sixth of the air found in the lungs. In a forced breathing event, 20-30% of the air is trapped.

rib cage

rib cage

The rib cage houses the lungs and is made up of a set of muscles and bones. The bone component is made up of the cervical and dorsal spine, the rib cage, and the sternum. The diaphragm is the most important respiratory muscle, found in the back of the house.

There are additional muscles inserted into the ribs, called intercostals. Others participate in respiratory mechanics such as the sternocleidomastoid and the scalenes, which come from the head and neck. These elements are inserted in the sternum and in the first ribs.

How does it work?

The uptake of oxygen is vital for the processes of cellular respiration, where the uptake of this molecule occurs for the production of ATP based on the nutrients obtained in the feeding process through metabolic processes.

In other words, oxygen serves to oxidize (burn) molecules and thereby produce energy. One of the residues of this process is carbon dioxide, which must be expelled from the body. Respiration involves the following events:

Ventilation

The process begins with the capture of oxygen in the atmosphere through the process of inspiration. Air enters the respiratory system through the nostrils, passing through the entire set of tubes described, until it reaches the lungs.

Taking in air - breathing - is a normally involuntary process but can go from being automatic to voluntary.

In the brain, neurons in the spinal cord are responsible for the normal regulation of respiration. However, the body is able to regulate respiration depending on oxygen requirements.

An average person in a resting state breathes an average of six liters of air every minute, and this figure can increase to 75 liters during periods of intense exercise.

Gas exchange

Oxygen in the atmosphere is a mixture of gases, made up of 71% nitrogen, 20.9% oxygen, and a small fraction of other gases, such as carbon dioxide.

When air enters the respiratory tract, the composition changes immediately. The inspiration process saturates the air with water and when the air reaches the alveoli it mixes with the residual air from previous inspirations. At this point the partial pressure of oxygen decreases and that of carbon dioxide increases.

In respiratory tissues, gases move following concentration gradients. As the partial pressures of oxygen are higher in the alveoli (100 mm Hg) than in the blood of the pulmonary capillaries, (40 mm Hg) oxygen passes to the capillaries by a diffusion process.

Likewise, the concentration of carbon dioxide is higher in the pulmonary capillaries (46 mm Hg) than in the alveoli (40 mm Hg), therefore the carbon dioxide diffuses in the opposite direction: from the blood capillaries to the alveoli in the lungs.

By Fluid-filled_alveolus2_ja.svg: user: delldot (modified by Hatsukari715) derivative work: OSH FPaD (Fluid-filled_alveolus2_ja.svg), via Wikimedia Commons

Gas transport

In water, the solubility of oxygen is so low that a means of transport must exist to meet metabolic requirements. In some small invertebrates, the amount of oxygen dissolved in their fluids is sufficient to meet the demands of the individual.

However, in humans the oxygen transported in this way would only be enough to meet 1% of the requirements.

For this reason, oxygen - and a significant amount of carbon dioxide - is carried by pigments in the blood. In all vertebrates these pigments are confined to red blood cells.

In the animal kingdom, the most common pigment is hemoglobin, a protein molecule that contains iron in its structure. Each molecule consists of 5% heme, responsible for the red color of blood and reversible binding with oxygen, and 95% globin.

The amount of oxygen that can bind to hemoglobin depends on many factors, including oxygen concentration: when it is high, as in capillaries, hemoglobin binds to oxygen; when the concentration is low, the protein releases oxygen.

Other respiratory pigments

Although hemoglobin is the respiratory pigment present in all vertebrates and some invertebrates, it is not the only one.

In some decapod crustaceans, cephalopod crustaceans and mollusks there is a blue pigment called hemocyanin. Instead of iron, this molecule has two copper atoms.

In four families of polychaetes there is the pigment chlorocruorin, a protein that has iron in its structure and is green in color. It is similar to hemoglobin in structure and function, although it is not confined to any cellular structure and is free in plasma.

Finally, there is a pigment with an oxygen carrying capacity much lower than that of hemoglobin called hemeritrin. It is red in color and is present in various groups of marine invertebrates.

Common diseases

Asthma

It is a pathology that affects the respiratory tract, causing its swelling. In an asthma attack, the muscles around the airways become inflamed and the amount of air that can enter the system is drastically reduced.

The attack can be triggered by a series of substances called allergens, including pet fur, mites, cold climates, chemicals in food, mold, pollen, among others.

Pulmonary edema

A pulmonary edema consists of the accumulation of fluid in the lungs, which makes it difficult for the individual to breathe. The causes are generally associated with congestive heart failure, where the heart does not pump enough blood.

The increased pressure in the blood vessels pushes the fluid into the air spaces inside the lungs, thus reducing the normal movement of oxygen in the lungs.

Other causes of pulmonary edema are kidney failure, the presence of narrow arteries that carry blood to the kidneys, myocarditis, arrhythmias, excessively high physical activity, use of certain drugs, among others.

The most common symptoms are shortness of breath, shortness of breath, spitting up foam or blood, and increased heart rate.

Pneumonia

Pneumonia are infections of the lungs and can be caused by a variety of microorganisms, including bacteria such as Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, Mycoplasmas pneumoniae, and Chlamydias pneumoniae, viruses or fungi such as Pneumocystis jiroveci.

It presents as an inflammation of the alveolar spaces. It is a highly contagious disease, because the causative agents can be spread through the air and spread quickly through sneezing and coughing.

The people most susceptible to this pathology include individuals over 65 years of age and with health problems. Symptoms include fever, chills, coughing up phlegm, shortness of breath, shortness of breath, and chest pain.

Most cases do not require hospitalization and the disease can be treated with antibiotics (in the case of bacterial pneumonia) administered orally, rest and drinking fluids.

Bronchitis

Bronchitis occurs as an inflammatory process in the tubes that carry oxygen to the lungs, caused by infection or for other reasons. This disease is classified as acute and chronic.

Symptoms include general malaise, coughing up mucus, shortness of breath and chest pressure.

To treat bronchitis, it is recommended to take aspirin or acetaminophen to lower fever, drink large amounts of fluids and rest. If it is caused by a bacterial agent, antibiotics are taken.

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