In biochemistry respiration refers to the ability of the body to utilize oxygen as the final electron acceptor in the oxidative phosphorylation, where the electron transfer chain in the mitochondria generates ATP (Adenosine triphosphate); which is the major chemical that carries energy to power most processes in the body. In the process oxygen is converted to water and carbon dioxide is generated. However in human physiology respiration refers to the body’s ability to acquire oxygen and transport to cells in different tissues while at the same time getting rid of the carbon dioxide generated in the process. The definitions are related only that the biochemical definition is much more detailed and deals with respiration at the cellular level while physiology deals with respiration at the organ level. The essence of the respiratory system is to supply the basic unit of the body which are the cells with oxygen and remove carbon dioxide from the same cells. However around 15 percent to 20 percent of oxygen that is taken out is still oxygen. The major organs involved in gaseous exchange in humans are the lungs where oxygen is transferred in the circulatory system by the blood which further delivers the oxygen to cells of various tissues. The same blood carries carbon dioxide to the lungs and it is taken out via a process called exhalation. The process of breathing in air which is made up of about 20 percent oxygen is referred to as inhalation. The processes of inhalation and exhalation are actually very detailed with other organs such as the muscles being involved and various physiological conditions necessary for the body to successfully deliver oxygen to various tissues and remove carbon dioxide. Respiration involves four main steps which include:
- I. Ventilation: refers to inflating the alveoli of the lungs with air.
- II. Pulmonary gas exchange: refers to the exchange of the gas between the moist alveoli of the lungs and the blood capillaries.
- III. Gas transport: refers circulation of the gas from the capillary to the larger vessels and then to peripheral capillaries in the organs.
- IV. Peripheral gas exchange: which is the exchange of gas between tissues capillaries and cells and finally into the mitochondria.
These steps elaborate on the fact that there is a significant association between the circulatory system and the respiratory system. Furthermore it is the responsibility of the heart to pump blood to various tissues of the body thereby making gaseous exchange at these tissues possible.
The Breathing Process
Breathing refers to both inhalation and exhalation processes which are achieved via the nose or the mouth in humans and most mammals. However the nasal breathing is the most preferred due to a number of reasons. Nasal breathing involves the transport of air to the lungs via the sinuses that are able to filter the air. Due to the small diameter of the sinuses the subsequent pressure created in the lungs provides the lungs with enough time to extract oxygen. Efficiency of exchange between carbon dioxide and oxygen is very important because it helps in maintaining optimum pH in blood; carbon dioxide is transported as carbonic acid in the blood and too much of it may lower the pH of the blood to physiologically unfavorable condition. On the other hand if carbon dioxide is lost at fast rate oxygen absorption will also be decreased. Nasal breathing also prevents the drying of the throat in situations such as dehydration and cold weather. The breathing process is aided by the up and down movement of the diaphragm which is a dome shaped muscle under the rib cage that separates the abdominal cavity and the chest cavity. The muscle functions by alternating pressure and space inside the body and thereby facilitating inhalation and exhalation. When the muscle pulls down the lungs expand and the air is pushed inside the lungs due to pressure differences. When the muscle is pulled upwards, the pressure increases pushing air outside the lungs. (Standley, 2010).
Embryonically the lungs develop from the laryngotracheal groove from which the trachea and the larynx develop. In the fourth week of development of the larynx, the trachea and the bronchi of the lungs begin to develop. During this time the lung bud is ventrally positioned in relation to the caudal position of the foregut. The form bud grows to form the tracheal bud. The tracheal bud further divides to form two primary bronchial buds. In week five the bronchial buds enlarge forming the right and left bronchi.
Gaseous exchange in the Alveoli
Diffusion is the principle behind gaseous exchange between the alveoli of the lungs and the red blood cells in the surrounding blood capillaries. During inhalation the air that is taken into the lungs is rich is oxygen. When the air reaches the alveoli oxygen diffuses from the alveoli where it is highly concentrated to into the blood capillaries where its concentration is low. When 100mL of plasma is exposed to atmosphere with oxygen pressure of 100mm Hg, the only oxygen that will be absorbed is 0.3mL. On the other hand when 100mL of blood is exposed to a similar atmosphere around 19mL of oxygen will be absorbed. The difference is due to the hemoglobin in the red blood cells that optimizes the ability of blood to transport oxygen. Hemoglobin is made up of an iron porphyrin ring, heme and a protein globin. Each of the heme is attached to four pyrole groups by covalent bonds and the fifth covalent bond of the iron is attached to the globin. There are four iron atoms in each hemoglobin molecule and therefore four heme groups. Binding of oxygen to one heme groups causes a conformational change in the other groups thereby increasing their affinity to oxygen. The mechanism increases the general affinity of hemoglobin to bind to oxygen in the alveoli where oxygen concentration is high. However, when the red blood cells reach tissues where oxygen tension is low the hemoglobin quickly unloads oxygen which is taken up by these tissues.
On the other hand after carbon dioxide has been released from respiring cells 7 percent of it dissolves in the plasma while 23 percent bind to the amino groups of the hemoglobin forming carboxyhemoglobin and 70 percent is carried as bicarbonate ions. Conversion of carbon dioxide to bicarbonate ions takes place inside the red blood cells before the bicarbonate ions diffuse back to the plasma. The bicarbonate ions are then transported to the lungs where they combine to form carbon dioxide which is expelled out of the body via the process of exhalation. (Rice, D. 2010)
The lungs have adaptations that optimize its function as the site of gaseous exchange between the external environment and the internal environment which are the cells. The cells lining the alveoli are very thin to ensure that oxygen and carbon dioxide can diffuse easily. Furthermore the alveoli are supplied with a rich network of capillaries. The red blood cells are also adapted to carry oxygen because they lack nucleus like other cells and a large portion of the cells are filled with hemoglobin. Furthermore in relation to other cells of the blood red blood cells are many per volume of blood.
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Respiratory system enables the body to deliver oxygen to cells of different tissues and take up carbon dioxide which is a product of respiration from these tissues for exhalation by the lungs. Cells need oxygen as the final electron acceptor in the process of electron transfer and oxidative phosphorylation which generates ATP which is the major energy currency in the body. This process is collectively referred to as respiration and one of the products is carbon dioxide which must be expelled from the body. All this processes are achieved by the respiratory system of which the major organ is the lung which provides a site where oxygen and carbon dioxide can be exchanged between the external environment and the body. The blood also plays an important role in transportation of oxygen and carbon dioxide. Efficiency of the respiratory is important because lack of it will deprive tissues including vital organs such as the brain of energy. Furthermore gaseous exchange is crucial in maintaining the pH of the blood at optimum levels.
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