The term "respiration" can be interpreted in two different ways. Strictly speaking, it means the gaseous exchanges between a cell and its surrounding environment, or the consumption of oxygen and elimination of carbon dioxide. In a broader, more general sense, respiration is the back-and-forth movement of air through the respiratory system. In this latter sense, "respiration" is synonymous with "ventilation" the preferred term. Ventilation is closely linked to the circulatory function, which it complements. One of the parameters used to describe the combined efficiency of the two systems is the ventilation/perfusion ratio, whether applied to the entire respiratory network or to a more limited portion.
The cycles of ventilation (inhalation, then exhalation), which are the source of respiratory volume, enable the renewal of air in the alveoli (alveolar air), which has a different composition from atmospheric air. This composition is maintained constant during rest and under conditions of thermal neutrality.
Air penetrates the respiratory passages through the nostrils. As needed, and especially when it is hot, the cat can also breathe through its mouth. After the air passes through the nasal fossae and pharynx, it reaches the trachea and bronchi before being distributed into the pulmonary alveoli. The airways are not only simple passages for air movement. As air moves through the airways, it is also heated and takes on water vapor until it is saturated (approximately 6% water vapor at 38°C). This process is used maximally in combating heat: The animal breathes quickly to eliminate a large amount of heat. This mode of ventilation is called thermal polypnea (panting).
As air travels through the airways, it is also filtered by numerous cilia on the cells of the mucosa and by the secretion of mucus that forms a sort of conveyor belt to carry away dust and dirt particles to the pharynx, where they are swallowed. Finally, the nasal fossae are the center of olfaction, and the vocal cords cause air to vibrate in the larynx, enabling the cat to meow.
In cats, as in humans, the trachea and bronchi are equipped with muscles that contract to limit the diameter of these passageways and significantly increase the resistance to air flow. This bronchoconstriction remains moderate in healthy animals. It is a process justified only by its ability to be inhibited during effort, in order to increase air flow through the respiratory system. In some respiratory disorders, such as bronchial asthma, bronchospasms can greatly limit breathing and have serious consequences, leading even to death from asphyxia.
Lung mechanics is the study of the motor forces enabling air movement through the use of muscles in the respiratory system. During inhalation, the diaphragm, or muscle separating the thorax from the abdomen, contracts and increases the volume of the thoracic cage toward the back. It is aided by the external intercostal muscles.
This increase in thoracic volume creates a depression in the space between the lungs and thoracic wall (pleural cavity), enabling air penetration. Under normal conditions, during rest and at thermal neutrality, exhalation then follows passively: The elasticity of the lungs and thorax returns them to their resting position, forcing air out. Still, some muscles, particularly in the abdominal wall, can contract as needed to speed exhalation.
The cat's resting respiratory rate varies from 20 to 40 cycles per minute. The amount of air mobilized during each cycle, or tidal volume, is approximately 30 ml. The product of the respiratory rate and the tidal volume gives the respiratory volume, which varies from 0.5 to 1 liter per minute during rest. During effort, this volume increases, thanks to an increase in the tidal volume and especially the respiratory rate.
Hematosis means all gaseous exchanges between the alveolar air and the blood. It is produced by purely passive phenomena involving the partial pressure gradients of gases between the air and the blood. Because oxygen is more concentrated in the alveoli than in the blood arriving in the lungs, it passes from the air to the blood. Carbon dioxide follows the opposite route. The animal's only means of acting on these exchanges is to vary these partial pressures by modifying respiratory volume. Any increase in respiratory volume (hyperventilation) speeds the renewal of alveolar air and increases gaseous exchanges. Receptors located in the carotid arteries and aorta constantly analyze the composition of arterial blood and adapt ventilation to any change, in order to always maintain the same partial pressure values for oxygen and carbon dioxide.
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