Respiration in the sense of gas exchange is the process by which an organism absorbs the gases necessary for its cellular metabolism from the environment and expels the gases that are the products of this metabolism. Cellular respiration (aerobic or anaerobic) is the chemical reaction in which organic molecules are broken down to produce ATP molecules, the main energy source for the metabolism.
Gas exchange is fundamental for cellular respiration, since the supplying of reagents (oxygen, in aerobic cellular respiration) and the expelling of products (e.g., carbon dioxide) of this chemical reaction depends on gas exchange.
The chemical equation for aerobic cellular respiration is the following:
C₆H₁₂O₆ + 6 O₂ + 36 ADP + 36 P --> 6 CO₂ + 6 H₂O + 36 ATP
Keeping in mind the chemical equation for aerobic cellular respiration, we can observe that glucose and molecular oxygen are needed as reagents and carbon dioxide and water are released. The process also consumes ADP nolecules and phosphate, turning them into ATP.
In organisms of the kingdom Animalia, gas exchange may occur either by diffusion, tracheal respiration, cutaneous respiration, branchial respiration or pulmonary respiration.
Small animals whose tissues make direct contact with or which are very close to the environment, such as cnidarians and poriferans, carry out gas exchange by diffusion.
Larger animals, whose cells are not in direct contact with the environment or are far from it, need special gas transportation systems. In these animals, respiratory and circulatory systems play this role.
The phyla of the animal kingdom whose organisms carry out gas exchange by diffusion are poriferans, cnidarians, platyhelminthes (flatworms) and nematodes (roundworms). This type of respiration is possible in these organisms because their tissues and cells are relatively close to their exterior.
Insects and arachnids are the arthropods animals that use tracheal respiration. Along the surface of the body these animals, there are many holes called spiracles, which are connected to small tubules called tracheae, through which air penetrates and carbon dioxide is expelled. The tracheae branch off into tracheoles, which reach all the tissues of the animal.
In the circulatory system of insects, blood only transports nutrients; gases are transported separately by the tracheal system.
Cutaneous respiration is not as simple as diffusion. In diffusion, gases diffuse directly between the external environment and the cells. In cutaneous respiration, molecular oxygen penetrates through the skin and is collected by blood circulation, which then distributes the gas to tissues. Carbon dioxide is also collected from the tissues by the blood and taken to the skin to be eliminated into the environment. Therefore, blood plays an important role in cutaneous respiration.
Terrestrial annelids and adult amphibians use cutaneous respiration (amphibians also use pulmonary respiration).
The thin skin and the need to live in moist environments are typical features of these animals.
Branchiae, also known as gills, are small portions of highly-vascularized tissues inside or outside the body which are in direct contact with the surrounding water. Gills are the organs that carry out gas exchange in aquatic annelids, crustaceans, fish and amphibian larvae (e.g., tadpoles).
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Gills and lungs are both highly-vascularized organs that are used for gas exchange between the environment and the circulatory system.
Lungs are different from gills in that they are saclike structures that are always inside the body and are specialized in gas exchange in terrestrial environments. Branchiae, on the other hand, are internal or external laminar structures in direct contact with water, which are specialized in gas exchange in aquatic environments.
Terrestrial molluscs and arachnid arthropods are invertebrates that present pulmonary-like respiration. Some terrestrial molluscs have a mantle cavity filled with air that is in contact with highly-vascularized tissues that function as rudimentary lungs. In addition to their tracheal respiration, some arachnids have book lungs (thin folds resembling the pages of a book), which carry out gas exchange.
The circulatory system plays an important role in cutaneous respiration, branchial respiration and pulmonary respiration. The respiratory function of blood is that of the transportation of gases for their exchange between tissues and respiratory surfaces in contact with the exterior (skin, gills, lungs).
Respiratory pigments are molecules present in the blood that bind to oxygen to transport it to tissues.
In vertebrates, the respiratory pigment is hemoglobin, which has a red color due to the iron in its composition. In crustacean and arachnid arthropods as well as some molluscs, the respiratory pigment is hemocyanin, which is blue due to the copper in its composition. Annelids use hemoglobin, hemerythrin and chlorocruorin as respiratory pigments.
The organs that form the human respiratory system can be divided into three groups: the lungs, the airway and the respiratory muscles.
The lungs are made up of right and the left lungs, which are composed of alveoli where gas exchange (the entrance of oxygen and exit of carbon dioxide) takes place. The lungs are covered by the pleura (a serous membrane). The airway is made up of the nose, the pharynx, the larynx (including the vocal cords), the trachea, the bronchi and the bronchioles. The muscles on which the breathing process depends are mainly the diaphragm and the intercostal muscles (the muscles between the ribs).
The left bronchus is more elevated than the right bronchus because of the position of the heart on the left side of the chest, anterior and inferior to the left bronchus.
Accidentally inhaled objects are frequently found in the right bronchus because the inferior angle between the trachea and this bronchus is lower than the inferior angle between the trachea and the left bronchus, as the left bronchus is more horizontal. Therefore, inhaled objects tend to fall into the right bronchus and not into the left.
The epithelium of the airway is a ciliated epithelium and contains mucus-secreting specialized cells. The secreted mucus covers the internal wall of the airway, retaining organisms and foreign particles that then are swept away by the cilia of the epithelium.
In the ciliated mucosal epithelium of the airway, there is also intense immune activity, with antibodies and leukocytes inactivating and destroying foreign agents.
Other defense mechanisms of the airway are the sneeze and the cough. They help eliminate solid and semifluid particles such as pathological residues (sputum) and accidentally inhaled objects.
In mammals, the muscles that participate in the breathing process are the diaphragm and the intercostal muscles. In respiratory insufficiency, other muscles can help in respiration, such as the muscles of the shoulders, neck, thorax and abdomen.
The diaphragm (exclusive to mammals) and the intercostal muscles can contract or relax, changing the volume of the thorax (the compartment where the lungs are located). The changes in the thorax volume force inhalation or exhalation.
When the volume of the thorax is increased, it creates a situation in which internal pressure is lower than the atmospheric pressure (external) and gases naturally enter the lungs. When the volume of the thorax is lowered, the internal pressure rises above the external pressure and the air is expelled from the lungs.
Arterial blood is the oxygen-rich and carbon dioxide-poor blood that irrigates tissues. Venous blood is the oxygen-poor and carbon dioxide-rich blood collected from the tissues.
Hematosis is the oxygenation of the blood. Venous blood (oxygen-poor) is transformed into arterial blood (oxygen-rich) through hematosis.
In humans, hematosis takes place in the lungs.
The blood vessels that carry venous blood to the heart are the inferior and the superior vena cava. The blood vessel that carries arterial blood from the heart is the aorta.
The gas exchange units of the lungs of mammals are alveoli.
Gas exchange (the entry of oxygen and exit of carbon dioxide) in pulmonary alveoli occurs via simple diffusion in favor of the partial pressure gradient.
When partial pressure of oxygen in the inhaled air is higher than the partial pressure of oxygen in the capillaries of the alveoli, the air diffuses into the circulatory system. If the partial pressure of oxygen in the air is lower (a rare situation, since the blood that reaches the alveoli is venous blood), the oxygen exits the circulatory system. The same is true for carbon dioxide.
Pulmonary respiration is controlled by the neural respiratory center located within the medulla (the lower part of the brain next to the spinal cord).
The chemical equation for the chemical equilibrium of the formation of bicarbonate from the reagents carbon dioxide and water is as follows:
CO₂ + H₂O --> H₂CO₃ --> H⁺ + HCO₃⁻
The reaction is catalyzed by the enzyme carbonic anhydrase, which is present in red blood cells.
An increase in the formation of the product of the chemical equilibrium of the formation of bicarbonate from carbon dioxide and water increases the concentration of hydrogen ions and therefore lowers the pH of the solution.
The shifting of the chemical equilibrium of the formation of bicarbonate from carbon dioxide and water into the reverse reaction (the production of water and carbon dioxide) means the consumption of hydrogen ions and therefore increases the pH of the solution.
The pulmonary ventilation frequency (number of inhalations per time unit) rises or lowers the carbon dioxide concentration in blood. If it is intense, more gas is eliminated to the exterior and, if it is reduced, the gas is retained inside the body.
Applying the principles of chemical equilibriums to the formation of bicarbonate from carbon dioxide and water, you get the following: if the carbon dioxide concentration is increased, the equilibrium shifts towards the formation of bicarbonate and the release of hydrogen ions, lowering the pH of the solution; if the carbon dioxide concentration is lowered, the equilibrium shifts reversely towards the formation of water and carbon dioxide, consuming more hydrogen ions and therefore raising the pH of the solution.
Acidosis is the condition in which blood pH is abnormally low. Alkalosis is the condition in which blood pH is abnormally high. Normal pH levels for human blood are between 7.35 and 7.45 - slightly alkaline.
If the body experiences acidosis, the respiratory center located in the medulla receives this information and increases respiratory frequency. The increase in respiratory frequency makes the body eliminate more carbon dioxide and shift the equilibrium of the formation of bicarbonate towards the spending of more hydrogen ions, thus raising the pH of the blood.
If the body undergoes alkalosis, the respiratory center located in the medulla receives this information and lowers respiratory frequency. The reduction in respiratory frequency makes the body retain more carbon dioxide and shift the equilibrium of the formation of bicarbonate towards the production of more hydrogen ions. thus lowering the pH of the blood.
Respiratory acidosis is when blood pH is low due to the increased retention of carbon dioxide caused by the lowering of respiratory frequency or by pulmonary diseases that inhibit gas exchange. Therefore, the cause of respiratory acidosis is pulmonary respiration. Metabolic acidosis is when blood pH is low not due to the pulmonary retention of carbon dioxide but due to metabolic disturbances. Some metabolic disturbances result in the release into the blood of nonvolatile acids that release hydrogen ions, lowering the pH of the blood (e.g., diabetic ketoacidosis).
Respiratory alkalosis is when the pH of the blood is high due to the increased exhaling of carbon dioxide caused by an elevated respiratory frequency. Metabolic alkalosis is caused by metabolic disturbances that increase the concentration of bases (alkalis) in the blood.
The chemoreceptors that participate in ventilation control are structures that collect information about the acidity and alkalinity of the blood. The information is then transmitted by nervous fibers to the respiratory center located within the medulla. The center then commands the respiratory muscles to compensate for the abnormal pH.
There are central and peripheral chemoreceptors. Peripheral chemoreceptors which detect pH, the partial pressure of carbon dioxide and the partial pressure of oxygen are located in the walls of the aorta and carotid arteries. Central chemoreceptors that receive pH information are located within the medulla in the respiratory center. (Pulmonary ventilation is also controlled by receptors that receive pH information from the cerebrospinal fluid.)
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