Mechanisms of Respiration
Mechanisms of Respiration
Mechanisms to achieve the goal of respiration are pulmonary ventilation; the inflow and outflow of air between the atmosphere and the lung alveoli, diffusion of oxygen and carbon dioxide between the alveoli and the blood, transport of oxygen and carbon dioxide in the blood and body fluid to and from the body's tissue cells; and regulation of ventilation and other facets of respiration by the respiratory centre.
The thoracic cavity is made up of 12 pairs of ribs that connect in the posterior thorax to the vertebral bodies of the spinal column. The lungs lie within the thoracic cavity on either side of the heart, extending from the diaphragm to just above the clavicles or collarbones. Light, spongy and elastic structures, of the lungs inflate with inspiration and deflate, but do not completely collapse, with expiration. The right lung is shorter and wider than the left lung. Each lung is divided into lobes - the right lung has three lobes; the left lung has two lobes. The lung's lobes are further divided into segments. The pleura membranes that cover each lung and line the thoracic cavity.
Mechanics of Pulmonary Ventilation
Normal quiet breathing is accomplished almost entirely by movement of the diaphragm. The diaphragm is principal muscle of respiration which is attached to the inferior margin of the rib cage and to the bodies of the lumbar vertebrae behind (Figure 3.6).
Inspiration
An active process to move air in to the lungs by enlarging chest cavity and contracting the muscles of inspiration (Figure 3.6). The enlargement of the chest is in three directions:
- Antero-posterior, i.e. from behind forwards- This is produced by the intercostal muscles drawing the ribs and the sternum forwards.
- Laterally, or from side to side- This is produced by the intercostal muscles drawing the lower ribs upwards and outwards.
- Vertically, or from above downwards- This is produced by the contraction of the diaphragm which pulls the lower surfaces of the lungs downward. The diaphragm accounts for 75% of the tidal volume. The distance the diaphragm moves varies from 1.5 cm to 7 cm during deep inspiration.
The lungs can be expanded and contracted in two ways: by downward and upward movement of the diaphragm to lengthen or shorten the chest cavity, and by elevation and depression of the ribs to increase and decrease the anteroposterior diameter of the chest cavity.
When the chest cavity enlarges, the pressure in the intrapleural space becomes more negative. This expands the lungs and the gases are drawn into the lungs from the atmosphere. For air to enter the lungs at inspiration the pressure gradient must be great enough to overcome the elastic resistance to expansion of the lung and chest wall (total compliance).
The most important muscles that raise the rib cage are the external intercostals, but others that help are the sternocleidomastoid muscles, which lift upward on the sternum; anterior serrati, which lift many of the ribs; and scaleni, which lift the first two ribs.
Expiration
Expiration is the movement of air out of the lungs to the atmosphere is a passive process. It is not produced by the active contraction of any muscles but by the relaxation of the muscles that were contracting during inspiration. The diaphragm simply relaxes, and the elastic recoil of the lungs, chest wall, and abdominal structures compresses the lungs and expels the air. During heavy breathing, however, the elastic forces are not powerful enough to cause the necessary rapid expiration, so that extra force is achieved mainly by contraction of the abdominal muscles
Exchange of Oxygen and Carbon Dioxide Between the Lungs and the Atmosphere
The amount of oxygen in inspired atmospheric air is 21%. The pressure or tension of this oxygen is 158 mmHg (atmospheric total air pressure is 760 mmHg). The inspired air is drawn into the alveoli where it mixes with the air left from the previous respiration. Therefore, the oxygen is diluted, the CO2 raised and water vapor is added. The pressure of O2 in the alveolar air becomes 103 mmHg.
Exchange Between the Blood and the Lungs
The venous blood arrives in the lung (from the tissues) with an O2 pressure of 40 mmHg. Because of the difference in pressure (103 - 40 mmHg), O2 passes from the alveoli into the blood stream. This transfer occurs by the process of diffusion. The O2 in solution determines the oxygen tension. It moves from the alveoli where the tension is high to the capillary blood where the tension is low. The capillary blood leaves the lungs after picking up O2 and giving up CO2. It is high in O2 (both dissolved O2 and O2 in combination with hemoglobin). The arterial blood leaves with an oxygen tension of 100 mmHg (Figure3.7).
Exchange of Carbon Dioxide
In addition to the exchange of oxygen, an exchange of CO2 takes place. The venous blood arriving in the lung is rich in CO2. The pressure of CO2 in the venous blood is 46 mmHg. The alveolar pressure is 40 mmHg. The CO2 diffuses from the venous blood to the alveoli. The arterial blood leaves the lung with an oxygen tension of 100 mmHg and a CO2 tension of 40 mmHg (Figure3.7).
Diffusion of Gases Through the Respiratory Unit and Respiratory Membrane
The alveolar walls are extremely thin, and between the alveoli is an almost solid network of interconnecting capillaries (Figure 3.8 and 3.7). Because of the extensiveness of the capillary plexus, the flow of blood in the alveolar wall has been described as a "sheet" of flowing blood. Thus, it is obvious that the alveolar gases are in very close proximity to the blood of the pulmonary capillaries and the pulmonary blood occurs through the membranes of all the terminal portions of the lungs, not merely in the alveoli themselves. All these membranes are collectively known as the respiratory membrane, also called the pulmonary membrane. If any of these layers is thickened then diffusion is impeded.
Diffusion: Is the passage of fluid or gas from an area of high pressure across a membrane. The gases in the alveoli pass into the blood in the capillaries by the process of diffusion. The respiratory unit is composed of a respiratory bronchiole, alveolar ducts, atria, and alveoli (Figure 3.7). There are about 300 million alveoli in the two lungs, and each alveolus has an average diameter of about 0.2 millimeter. The alveolar lining may be thickened by disease of the lung. The interstitial fluid may be increased in heart disease.
Respiratory Membrane
Respiratory Membrane diffusion of oxygen from the alveolus into the red blood cell and diffusion of carbon dioxide in the opposite direction occur in the respiratory membrane (Figure 3.8). The overall thickness of the respiratory membrane in some areas is as little as 0.2 micrometer, and it averages about 0.6 micrometer. It has been estimated that the total surface area of the respiratory membrane is about 70 square meters in the normal adult human male. The total quantity of blood in the capillaries of the lungs at any given instant is 60 to 140 milliliters.
Factors that determine how rapidly a gas will pass through the respiratory membrane are the thickness of the membrane, the surface area of the membrane, the diffusion coefficient of the gas in the substance of the membrane, and the partial pressure difference of the gas between the two sides of the membrane.
- Layers of the respiratory membrane:
- A layer of fluid lining the alveolus and containing surfactant that reduces the surface tension of the alveolar fluid
- The alveolar epithelium composed of thin epithelial cells
- An epithelial basement membrane
- A thin interstitial space between the alveolar epithelium and the capillary membrane
- A capillary basement membrane that in many places fuses with the alveolar epithelial basement membrane
- The capillary endothelial membrane
Carriage of Oxygen in the Blood
Oxygen is carried in combination with the hemoglobin in the red blood cells (1 gm of hemoglobin can carry 1.34 ml of O2) and carried in solution (0.3 ml of oxygen per 100ml of blood). When the Hgb combines with O2 it is termed oxyhemoglobin (oxyHb). When it gives up the O2 in the tissues it is termed reduced hemoglobin.
The Passage of Oxygen From the Blood to the Tissues
In the tissues the capillary blood is exposed to a low O2 tension. The oxygen diffuses out of the blood into the tissues. Initially the dissolved O2 in the plasma diffuses out first. When this happens the O2 in combination with the hemoglobin is no longer in equilibrium with the dissolved O2. The O2 passes from the red blood cells (hemoglobin) into the plasma and thence into the tissues. The O2 is thus unloaded into the tissues. There is a relationship between the O2 tension in the plasma and the O2 in combination with the hemoglobin. If the O2 tension is low the O2 content of the hemoglobin will be low. If the O2 tension is high the O2 content of the hemoglobin will be high.