Gas Exchange Structure

The respiratory system is a vital factor of gas exchange due to its structures. The structure provides an efficiency to the process. Gas exchange requires an efficient system because it is a process through which blood circulates around the body, collects oxygen, delivers the oxygen to the organs that require it and releases carbon dioxide. oxygen and carbon dioxide enter and exit the body in opposite directions across a respiratory surface area that supplies enough oxygen to allow the process to occur efficiently. The process begins when gases from the atmosphere, including oxygen, enter the body through either the nasal cavity, or the oral cavity. Afterwards, the oxygen enters the pharynx, a path that connects the nasal cavity and the trachea.
Once the oxygen has entered the pharynx, it passes through the larynx, which is an area in the throat made up of cartilage, typically known as the voice box. Next, the oxygen enters a strong, hollow tube encircled by tough and flexible cartilage rings. This hollow tube is called the trachea; the trachea leads the oxygen to the lungs.
As the oxygen enters the lungs, the trachea divides into two smaller tubes known as the bronchi. These further divide into smaller and smaller tubes called bronchioles. The bronchioles, at their smallest point of division, bring air to clusters of small, circular air sacks called alveoli. Though alveoli are of a small size, they are the main sites of gas exchange. This is due to the abundance of the alveoli; at average, humans have nearly 500 million alveoli, which combined, form a surface area that is 40 times the surface area of human skin. This provides sufficient surface area for gas exchange to occur. Gas exchange occurs here with the help of a dense network of capillaries surrounding the alveoli. Blood arriving in the alveoli is deoxygenated, produced during respiration, however, the blood already contained in the alveoli is oxygenated. Therefore, the oxygen enters the deoxygenated blood by diffusion. As the oxygen diffuses through the cell membrane and into the blood, each oxygen molecule merges with a haemoglobin molecule -red protein found in red blood cells that helps transport oxygen- to form
oxyhaemoglobin. This blood, which is now oxygenated, travels to the heart while the carbon dioxide molecules in the alveoli exit the body with the next exhalation. Even so, the alveoli has several adaptations that increase gas exchange efficiency; the alveoli, thus, providing sufficient surface area for gas exchange to occur.
Additionally, the walls of the alveoli have a very thin layer, this shortens the time and distance of the process. Furthermore, each alveolus is ventilated and surrounded by capillaries. The capillaries ensure a good blood supply, which is essential due to the blood repeating the process of taking oxygen and delivering carbon dioxide, which ensures that the oxygen continues its flow from the alveoli into the blood. The ventilation on the other hand removes excess carbon dioxide that is not required and restores oxygen levels in the atmosphere of the alveolus. This also helps maximise the concentration gradient between the alveolar air and the blood entering and exiting through the capillaries. Pulmonary ventilation is the act of breathing comprised of two phases; inhalation and exhalation. In inhalation, air from the environment is drawn into the lungs and in exhalation internal air is drawn out of the lungs. The main mechanisms responsible for pulmonary ventilation are atmospheric pressure(Patm), alveolar pressure(Palv) and the intrapleural
pressure(Pip). As breathing occurs in various different modes, they are processed slightly differently than each other. For instance, tidal breathing also known as eupnea is a passive process that occurs at rest and processed automatically without requiring a cognitive thought of the individual. In this type of breathing, inhalation takes place due to a difference in pressure gradient, an increase In a volume of thoracic cavity and lungs causes intrapulmonary pressure to drop, forcing air to rush into the lungs largely while in the passive process of exhalation the elastic tissues of lungs recoil causing a descend in a rib cage which makes inspiratory muscles, the diaphragm and external intercostal muscles, to relax. As a result the volume of the lungs will decrease and alveolar pressure will increase and becomes greater than the atmospheric pressure . This decreased intrapulmonary volume will lead to a compression of alveoli and deoxygenated air will be forced to flow out of the lungs to equalise both pressures. On the other hand, forced breathing, also known as hypernea, is an active process that take place during strenuous activities such as exercise and involves the contraction of accessory muscles along with diaphragm and intercostal muscles. Forced inspiration involves the contractions of several additional muscles such as pectoralis minors and sternocleidomastoids( neck muscle) to enlarge the thoracic cavity decreasing the intra-alveolar pressure to great extent. While forced expiration is accomplished by contracting the accessory muscles of abdomen, including oblique to force the diaphragm upward and compress the rib cage downward resulting in an increase in abdominal cavity pressure to empty the lungs more rapidly. Breathing is usually an automatic process that occurs subconsciously without any thoughts. The total numbers of respiratory cycles that a person completes during a one minute period of time is described as his respiratory rate. This fundamental pattern of ventilation is regulated by the neurones innervating the muscles of respiratory system. The main control centres of pulmonary ventilation are located in the medulla and pons in brain stem which constantly monitor the changes in the concentrations of carbon dioxide, oxygen and pH levels in the bloodstream. This information is then transmitted down to spine, and other muscles involving in breathing, in order to regulate breathing rate and keep it within normal limits. The upper centre located in pons is called pontine respiratory centre which interacts with the medullary respiratory centre to generate smooth and rhythmic breathing pattern according to the information received from cerebral cortex, limbic system and hypothalamus. Whereas the medullary centre is further subdivide into two group of neurones; dorsal or inspiratory group and ventral or expiratory group. When a body is at rest, the ventilation rate remain steady as no movement of extra muscles is involved. In this type of breathing only one group of medulla oblongata, the dorsal group or inspiratory centre, comes into action. Inhalation occurs by sending the nerve impulses, along phrenic nerve, to diaphragm and external intercostal muscles to contract. As a result a right amount of air flows into the lungs due to a drop in their volume. When the stimulation of nerve impulse ceases, it causes the muscles to relax and expel a right amount of deoxygenated air out of the body. Whereas the ventral respiratory group involves in when breathing demand increase such as in strenuous activity or exercising as it involves various other body muscles to contribute too. This result in more cellular respiration ,requiring more supply of oxygen,in order to generate more energy(ATP) to fullfill their metabolic need and consequently more carbon dioxide is being produced as a by product. Carbon dioxide circulates in the blood in the form of bicarbonate ions(HCO3-), converted from Carbonic acid(H2CO3) to into hydrogen ion and Carbonate ion. As the concentration of carbon dioxide gets higher, known as herpercapnia, in the blood , it causes the pH level of blood to drop. This change in the concentration of chemicals can be sensed by the peripheral chemoreceptors located in carotid and aortic bodies, and central chemoreceptors located in the brain and they will start firing more action potential toward the respiratory centre to initiate the contraction of the diaphragm and intercostal muscles at more faster rate as a result more oxygen will be inhaled and more carbon dioxide will be exhaled to neutralise everything. Blood is the main avenue by which respiratory gases are transported throughout the body. Red blood cells or erythrocytes, production of bone marrow, can be found abundantly, approximately 20 trillion, in the circulation fluid, blood, throughout the body. Inside the red blood cells there are red pigments called haemoglobin, an oxygen binding protein. Since red blood cell has no nucleus, all of the space is packed with entirely of the pigment protein Haemoglobin, each with approximately 270 millions molecules of Haemoglobin, enzymes and chemicals that contribute in to deliver an efficient and a higher volume of oxygen and carbon dioxide. Red blood cells have regular biconcave disk like shape maintained by internal scaffolding of protein fibres called cytoskeleton. This particular structure of cells provides a large enough volume and surface area to load and unload oxygen more efficiently and rapidly. Owing to the flexible and smooth elastic membrane, red blood cells can distort and squeeze through the narrow vascular lumens more easily. Oxygen is transported to the body tissues in two forms: either combined with molecule of haemoglobin and travels in the form of oxyhemoglobin or in dissolved from in the plasma. Every 100 ml of oxygen rich blood contained with about 20 ml of oxygen from which 19.7 ml is transported to the body tissues by haemoglobin and remaining 0.3 is dissolved in plasma. Haemoglobin is of vital importance in replenishing the all body tissues with a rich supply of oxygen; its main responsibility is to deliver oxygen from the lungs to the all body tissues and carbon dioxide from muscles to the lungs. Haemoglobin is a large, with molecular mass of about 64500 kilodaltons, conjugated and globular protein in red blood cells. Each molecule of haemoglobin is composed of four polypeptide chains, 2 alpha and 2 beta, each tightly associated with a prosthetic group( non-protein) of haem in its centre. Each central haem group contains a iron ion(Fe2+) capable of combining one oxygen molecule therefore each haemoglobin can be loaded with 4 oxygen molecules to deliver. Once this oxyhemoglobin reaches the oxygen consuming tissues, the oxygen is switched over to another oxygen-binding protein, myoglobin, mainly responsible for storing and replenishing active muscles with oxygen during aerobic respiration. Myoglobin is a monomeric muscle protein which shares a similar molecular constituent, like haemoglobin, called heam/ prosthetic group which favours reversible binding of oxygen. Unlike haemoglobin, myoglobin consists of one haem group and eight alpha helices connected together by short non-helical regions...... ...Gas exchange is the biological and passive process of transferring vital respiratory gases, oxygen and carbon dioxide, across a specialised respiratory surfaces by simple diffusion. In this process the gas molecule naturally move from a region of high concentration to a region of low concentration. Gaseous exchange between air and blood takes place in the tiny grape like structures called alveoli. As we take a breath, the air entering the nostrils/ external nares travels through the larynx, trachea, bronchi, bronchioles and into the alveoli. From the alveoli, where is partial pressure of oxygen is relatively high(100 mmHg), oxygen diffuses across 2 cells thick membrane into the deoxygenated blood, where the oxygen level is fairly low, flowing through the pulmonary capillaries surrounding the alveoli. In order to reach and bind to haemoglobin, oxygen needs to be dissolved in the plasma first and then diffuse into the red blood cell. As four polypeptide of haemoglobin can interact with each other, this protein exhibit cooperativity, which means after the first delivery of oxygen to its heam group, it will undergo conformational change, increasing the affinity of other heam groups to allow additional oxygen molecules to attach to haemoglobin even more easier . This process will continue until the red blood cells are highly saturated(95%) with oxygen before leaving the alveolar capillaries. This oxygen rich blood is then return to the left atrium, and into the left ventricle from where it is pumped into systemic arteries to be taken to the body tissues for metabolic use. No gas exchange of blood will take place through out its journey to muscle tissues as diffusion is inhibited by impermeable walls of arteries. When this oxygenated blood reaches to its destination which are active muscle, oxygen will dissolve in the plasma and then diffuse to the muscle tissues, oxygen depleted area. The oxyhemoglobin, oxygen bound with oxygen, will dissociate into haemoglobin and oxygen by the process of allosteric inhibition and transfer oxygen into the body tissues. Haemoglobin becomes allosterically inhibited when carbon dioxide and hydrogen ions bind to certain parts of it, other than the active site, and reduce its affinity for oxygen resulting in dissociation of oxygen more easily. As muscles are constantly utilising oxygen to generate ATP, they are producing a large amount of carbon dioxide as a waste product of cellular respiration. Carbon dioxide is then transported into the blood, where its concentration is low, in three different ways. A little amount of it , about 5%, will simply dissolve in the plasma and travel as free carbon dioxide. However a large amount of carbon dioxide , about 85% , will enter the red blood cell and react with water, in the presence of carbonic anhydrase, to form a weak carbonic acid which later dissociates into carbonic ion and hydrogen ion, whereas remaining 10% will bind with protein haemoglobin to form carbaminohaeglobin and transported in the blood. This oxygen poor blood, high in carbon dioxide, is then carried back to the lungs where soluble bicarbonate, in a reverse sequence in the red blood cells, reacts with hydrogen ion to form, again with the help of carbonic anhydrase, carbon dioxide and water. This disassociated carbon dioxide diffuses out of the bloodstream into alveoli, where carbon dioxide level is low, and breathed out during expiration.