Functional Components of Respiration
Respiration is to replenish tissues and each body cell with oxygen and carry away carbon dioxide. Respiration serves several important functions. These are (1) the swapping (exchange) of gases in the alveoli, also called as pulmonary ventilation that comprises inspiration and expiration; (2) transfer of gases, i.e.
oxygen by diffusion from the alveoli to blood and carbon dioxide from blood to alveoli; (3) transport of oxygen and carbon dioxide, respectively, to and from the tissues via blood; and (4) controlling the overall process of ventilation.7.2.1 Pulmonary Ventilation
The process of breathing air in and out of the lungs is known as pulmonary ventilation. The process comprises two distinct phases: (1) inspiration (or inhalation), which enables air to enter the lung, and (2) expiration (exhalation), which enables air to move out of the lungs. One cycle of inspiration followed by expiration is termed as respiratory cycle.
Two major groups of muscle take part in regular inspiration: (1) diaphragm and (2) external intercostals. Apart from these, other muscles are also engaged depending on the requirements during a deeper breath. The diaphragmatic contraction generates a large space within the thorax as it is pushed down the abdomen, creating more room for the lung. Similarly, contraction of external intercostals increases the size/volume of the thoracic cavity by causing outward and upward rib movement. Since the lung is surrounded by pleural fluid, the force of expansion in thoracic cavity expands the lungs too. Expansion of lungs causes intra-alveolar pressure to fall below atmospheric pressure; the resultant pressure gradient draws air into the pulmonary system from the ambient conditions. As the inspiratory muscles relax after completion of inspiration process, the elastic recoil tendency of the lung tissues contributes towards evacuation of the lungs albeit not completely.
The expiration is a passive process in mammals during which both the lung and thoracic volume decrease causing an increase in inter-pulmonary pressure. The rise in inter-pulmonary pressure beyond Patm generates a gradient of pressure that propels air out of lungs.As described above, two distinct kinds of movements are associated with breathing. When the visible abdominal movements predominate in breathing, it is called abdominal breathing, and if the breathing is predominated by rib movements, it is called costal breathing. In dogs and cats, both diaphragm and respiratory muscles coordinate respiratory movements and the respiration is termed as costo- abdominal respiration. In cases involving loss of diaphragmatic function (e.g. rupture or other causes), bulging of the abdomen alone does not support inspiration. Under such situations, the abdominal circumference usually decreases during inspiration and the resulting respiration pattern is called as pendulous respiration.
In severe diseases involving the lungs, its airways or the heart, animals or humans will show shortness of breath; this is known as dyspnoea or difficulty breathing. In contrast, normal breathing is termed eupnoea in a healthy state.
7.2.2 Pressures Driving Ventilation
The mechanical process of ventilation is dependent upon three factors: (1) the atmospheric pressure (Patm), (2) the alveolar pressure (Paιv) and (3) the intrapleural pressure (Pip).
7.2.2.1 Atmospheric Pressure
According to Dalton’s law, in a mixture that contains two or more gases that do not react with each other, the combined resultant pressure is same as the sum of the individual partial pressures of each gas present in that mixture. Hence, the barometric pressure (BP) and fractional concentration in the gaseous mixture play a key role in determining the partial pressure of oxygen (PO2), which implies that the altitude of a place in comparison to mean sea level is critical in determining the barometric pressure.
With rise in altitude, there is a corresponding decrease in the number of gas molecules per unit volume, so the air density is lesser than that at sea level. Human life depends on oxygen, this gas being acquired from the atmosphere where the partial pressure of oxygen (PAtmO2) within the troposphere depends on BP according to the Dalton’s law:PAtmO2 = 0.21 ■ 760mmHg = 159mmHg
7.2.2.2 Alveolar Pressure
Alveolar pressure (Paιv) is the pressure exerted by air inside the lung alveoli. If we assume the value of atmospheric pressure as zero, then the alveolar pressure (cm H2O) can be depicted as positive or negative with respect to Patm. During the process of inspiration, the respiratory muscles cause the Palv to drop below Patm (-1 cm of water). This causes air to move inside lungs along pressure gradient, thus storing potential energy in the elastic structures. At the end of inspiration, the respiratory muscles relax leading to elastic recoil of the respiratory system. This causes the Palv to be positive (+1 cm of water) relative to atmospheric pressure, and hence, expiration occurs.
7.2.2.3 Pleural Pressure
Pleural pressure (also called intrathoracic pressure), or Ppl, is the pressure surrounding the lung within the pleural space. Similar to intra-alveolar pressure, intrapleural pressure also varies during the different phases of breathing. During rest or quiet breathing, the pleural pressure assumes a negative value. The intrapleural pressure always assumes a negative value with respect to the atmospheric pressure and the intra- alveolar pressure (approx. —4 mmHg), with minor fluctuations during both phases of respiration,
i. e. inspiration and expiration.
The negativity of the intrapleural pressure arises out of opposing forces within the thoracic cavity. The centripetal forces that pull the lungs inwards, tending to collapse the alveoli, are elastic forces of the lung tissues plus the alveolar fluid surface tension.
The thoracic wall and fluid contribute to the forces acting in the opposite direction within the pleural space. The pleural layer lining the inner thoracic wall, i.e. the parietal pleura, is strongly adhered to this wall which counters the inward-acting forces of the lungs. Therefore, the intrapleural pressure always remains lesser or negative (—4 mmHg), whereas the outward forces remain greater than the centripetal pull though only to a smaller extent. The difference between the intra-alveolar and intrapleural pressure is called transpulmonary pressure, and this pressure determines lung size.Pneumothorax refers to air or gas in the pleural cavities, resulting in atelectasis because negative intrapleural pressure cannot be maintained.
7.2.3 Ventilatory Asynchronism
In horses, intrapleural pressure values vary in upper and lower thoracic cavity. In a standing horse, the pressure in the uppermost part of thorax is more sub-atmospheric than in the lowermost parts. Consequently, dorsal part of equine lung is highly distended and less compliant than ventral part. Air preferentially moves to more compliant regions, resulting in a vertical gradient of air movement in a standing horse. However, when viewed in terms of factors affecting the distribution of ventilation, this phenomenon of relative lung distention of different regions in horses is just one among many factors. The air distribution within the lung is also influenced by local lung compliance and airway resistance. This altered distribution of air in lungs of horses is called ventilator asynchronism.
7.2.4 Respiration Rate
The respiration rate, also called the respiratory frequency, is defined as the number of breaths per minute and is often used as a measure of health status of any animal or individual. The respiration rate varies widely among different species of animals. In addition, it also depends on several parameters like age, health conditions, ambient temperature, body size, pregnancy, excitement and exercise. As the ambient temperature rises, the respiration rate increases in animals to aid in thermoregulation. An overview of the respiration rate of several animals under varied conditions is presented in Table 7.1.
7.3