Respiratory System Lecture Notes PDF

 Lecture in PHYSIOLOGY The Respiratory System (B) By Prof. Dr. Mohamed Fathy Farag Bayomy  In the lungs, each bronchus branches into smaller and smaller tubes termed bronchioles that end in clusters of small grape-like structures called alveoli which represent the functional...

 Lecture in PHYSIOLOGY The Respiratory System (B) By Prof. Dr. Mohamed Fathy Farag Bayomy  In the lungs, each bronchus branches into smaller and smaller tubes termed bronchioles that end in clusters of small grape-like structures called alveoli which represent the functional respiratory units in which gaseous exchange takes place. The alveoli are lined with a layer of flat epithelial cells and are surrounded by a network of blood capillaries. Alveoli are surrounded by a network of thin-walled capillaries. Only about 0.2 μm separate the alveoli from the capillaries due to the extremely thin walls of both structures. The walls of the chest region contain the ribs and the intercostal muscles. Below, the chest region is separated from the abdominal cavity by the muscular diaphragm. Above the diaphragm there are 3 closed cavities; these are 2 lateral pleural cavities surrounding the lungs, and the median pericardial cavity that contains the heart. Breathing: The functional parts of the lung for gaseous exchange, the alveoli, are connected with the outside air at all times. That is, the nares, the nasal chambers, pharynx, glottis, larynx, trachea, bronchi, and bronchioles form a continuous system with the outside.  In the trachea, there are horseshoe-shaped cartilages in its wall which keep it always open. In the bronchi and larger bronchioles, rings of cartilages function in the same way. So, in such a system, when cavities surrounding the lungs are enlarged, air rush in. Because of the pressure of the outside air and the elastic nature of the lungs: lungs will expand to fill the pleural cavities. That is, the human breathing mechanism is essentially a suction pump mechanism. During inspiration, the muscle of the diaphragm contract, changing it from a dome-like structure to a more flattened straight sheet that press down on the viscera. During expiration, the muscles relax, and the diaphragm resumes its dome-shaped position. Moreover, contraction of the intercostal muscles and the rise of the ribs during inspiration increases the diameters of the pleural cavities. In expiration, the ribs return to their former position, decreasing these diameters of the cavities and allowing the lungs to force the air out. External respiration Oxygen taken in during inhalation diffuses from the alveoli into the blood, and CO2 in the blood diffuses into the alveoli, to be given off in exhalation. This process is purely diffusion. The concentration of O2 in the alveoli is greater than the concentration of O2 in the blood. The thin membranes separating the alveoli and the blood are permeable. Oxygen goes into solution on the moist lining the alveoli, and diffuses from a region of higher to a region of lower concentration. The reverse is true for CO2, and each gas behaves independently. Transport of O2 and CO2 by the blood When O2 diffuses into the blood in external respiration, most of it enters the red blood cells (erythrocytes), and units with hemoglobin (Hb) in these cells, forming a compound called oxyhemoglobin (HbO2). The great affinity of Hb for O2 enables the blood to carry about 50 times more O2 than the plasma alone can carry. Therefore, as the blood passes through the alveolar capillaries, the Hb becomes saturated with O2. The reaction may be represented by the following equation: Hb + O2 HbO2 Hemoglobin Oxygen Oxyhemoglobin  Oxyhemoglobin (HbO2) is very unstable compound, and when the blood reaches the capillaries in the tissues where the tension is low, HbO2 compound breaks down into Hb and O2 and O2 diffuses into the cells. The difference in color between oxygenated and non- oxygenated blood is due to HbO2 which is bright scarlet color whereas Hb (without O2) is dull purple. The property of Hb to combine reversibly with O2 may be represented by the oxygen dissociation curve (ODC) which indicates that Hb is almost completely saturated at an O2 pressure of 100 mmHg. When the O2 pressure falls below 60 mmHg, the O2 saturation of Hb falls rapidly. Thus, at low O2 pressure, HbO2 releases its O2 very rapidly. Carbon dioxide affects the ability of Hb to combine with O2: as CO2 increases the curve shifts to the right hand side, i.e. % HbO2 decreases. Figure indicates: The shift of the O2 dissociation curve (ODC) by PCO2. Effect of pH on O2 transport: It was found that, HbO2 dissociates more readily in a more acidic media. Thus, the affinity of O2 for Hb decreases when the pH is lowered (see the Fig.). This effect helps in providing more O2 to tissues when CO2 output is increased. Effect of Temperature (T): At constant pH values and different T, the affinity of Hb for O2 decreases by a rise in temperature. An increase in T weakens the bond between Hb and O2, and the HbO2 dissociation curve is shifted to the right, thus, at higher T, more O2 is delivered to the tissues as in case of muscles that are warmed during exercise. CO2 Transport: Most of the CO2 is transported in the plasma in the form of bicarbonates, whereas some of it is carried in the red blood cells in combination with amino groups of the Hb molecules as carbaminohemoglobin. In the alveolar capillaries, due to the low CO2 tension in the alveoli, the bicarbonates and carbaminohemoglobin liberate CO2. Hemoglobin combines with gases other than O2 and CO2. Hydrogen sulphide is a poison as a result of its affinity for Hb. Carbon monoxide (CO) formed by incomplete combustion of carbon, combines with Hb to give a stable compound called carboxyhemoglobin. It diminishes the amount of O2 that can be carried by the blood, and therefore, victims of CO poisoning suffer from a lack of O2. Internal respiration This process takes place throughout the body. It is the exchange of gases between the blood and the cells, with O2 passing from the blood into cells and CO2 going from the cells into the blood. This is again a process of diffusion, with each gas diffusing from a region of its higher concentration into a region of lower concentration. Actually, O2 does not diffuse directly into the cells, and also CO2 does not diffuse directly into the blood. The cells are separated from the capillaries by a film of lymph, and the gases diffuse through this fluid. Cellular respiration As it has been discussed, before, that this is a complex process including many steps. The energy stored in the glucose molecule, for example, is released in the step-by- step process, and this energy is trapped in the formation of high energy molecules called adenosine triphosphates (ATP). The ATP is then available for the various requiring processes in the cell.  Diseases of the respiratory system The condition of the airways and the pressure differences between the lungs and atmosphere are important factors in the flow of air in and out of lungs. Many diseases affect the condition of these airways, among these are: Asthma: It narrows the airways by causing an allergy-induced spasms of the surrounding muscles or by clogging the airways with mucus. Bronchitis: Is an inflammatory response that reduces airflow and is caused by long-term exposure to irritants such as cigarette smoke and air pollutants. Cystic fibrosis: Is a genetic defect that causes excessive mucus production that clogs the airways.