Gas exchange requirements vary with metabolism and may increase up to 30 times during strenuous exercise (Figure 45-1). Surprisingly, these variations are normally accomplished with only a small energy cost. In animals with respiratory disease, the energy cost of breathing can increase. This results in less energy available for exercise or weight gain, and the owner notices the animal’s poor performance. The respiratory system also is involved in communication by sound and pheromones and is important in thermoregulation; metabolism of endogenous and exogenous substances; and protection of the animal against inhaled dusts, toxic gases, and infectious agents. Additionally, the increase in abdominal pressure that facilitates urination, defecation, and parturition requires active participation of the respiratory muscles.

FIGURE 45-1 Effect of exercise on oxygen consumption in the horse. Oxygen consumption increases in a linear manner as the horse increases speed; the total increase is approximately thirtyfold. (Modified from Hörnicke H, Meixner R, Pollman U: *Equine exercise physiology,* Cambridge, UK, 1983, Granta Editions.)

Figure 45-2 shows the processes involved in gas exchange, including ventilation; distribution of gas within the lung; diffusion at the alveolocapillary membrane; transport of O₂ in the blood from the lungs to the tissue capillaries and of CO₂ in the reverse direction; and diffusion of gases between blood and tissues.

FIGURE 45-2 Diagrammatic representation of the processes involved in gas exchange. The lung is shown on the left, the heart in the center, and tissues on the right. The brain is shown at the top.

Ventilation

Ventilation Is the Movement of Gas Into and Out of the Lung

The oxygen needs of metabolism require that an animal take a certain volume of air into its lungs, especially its alveoli, each minute. The total volume of air breathed per minute, also known as minute ventilation (), is determined by the volume of each breath, known as the tidal volume (VT), and the number of breaths per minute, known as respiratory frequency (f), as clarified next from the following equation:

The increase in , which must occur when an increase in metabolic rate demands more oxygen, can be brought about through an increase in VT, f, or both.

Air flows into the alveoli through the nares, nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles. These structures constitute the conducting airways. Because gas exchange does not occur in these pathways, they are also known as the anatomic dead-space (Figure 45-3). Dead-space can also occur within the alveoli. This alveolar dead-space is caused by alveoli that are poorly perfused with blood, so that gas exchange cannot occur optimally (see Chapter 47). Physiologic dead-space is the sum of the anatomic and the alveolar dead-space. Let us define the portion of each VT that enters the alveoli as VA and the part that enters the dead-space as VD. Then:

FIGURE 45-3 Respiratory dead-space includes the ventilated parts of the respiratory system where gas exchange does not occur. Three schematic alveoli are shown attached to the conducting airways and perfused by differing amounts of blood. The volume of the trachea and bronchi constitutes the anatomic dead-space; the part of the endotracheal tube extending beyond the respiratory system constitutes equipment dead-space; and the volume of air ventilating poorly perfused alveoli is alveolar dead-space. *Top,* an unperfused alveolus is dead-space because there is no blood flow to allow gas exchange; *bottom,* an ideally perfused alveolus contributes no dead-space because all of the air participates in gas exchange; *middle,* when an alveolus has less-than-sufficient perfusion for the amount of ventilation received [high ventilation/perfusion () ratio)], some of the air entering that alveolus is not involved in gas exchange and contributes to dead-space.

If each side of this equation is multiplied by respiratory frequency (f) as follows:

The result is:

Therefore, minute ventilation () is the sum of alveolar ventilation (), which is essential for gas exchange, and dead-space ventilation (), which is wasted ventilation.

Alveolar ventilation is regulated by control mechanisms to match the O₂ uptake and CO₂ elimination necessitated by metabolism. Thus, when an animal exercises, its alveolar ventilation increases to take in more O₂ and eliminate more CO₂.

The fraction of each breath ventilating the dead-space is known as the dead-space/tidal volume ratio (VD/VT). The VD/VT varies considerably among species. In smaller species, such as dogs, it approximates 33%, whereas in some larger species, such as cattle and horses, it approximates 50% to 75%. Because the volume of the anatomic dead-space is relatively constant, changes in VT, f, or both can alter the relative amounts of air that ventilate the alveoli and the dead-space. These changes in VT and f occur in animals during exercise and thermoregulation.

The anatomic dead-space is important in thermoregulation. Air entering the respiratory system is usually cooler than body temperature and not saturated with water vapor. As air passes through the dead-space into the lung, it is warmed by transfer of heat from respiratory mucosal capillaries and humidified by evaporation of water from the dead-space mucosal surface. When the animal exhales, heat is lost because the warmed and humidified air leaves the body. When some species such as the dog are heat stressed, they pant. The small VT and high f characteristic of panting in dogs cause more air to ventilate the dead-space in order to increase water evaporation and heat loss. Cattle, pigs, and mules subjected to heat stress also increase respiratory rate and dead-space ventilation when trying to lose heat. In contrast to the effects of heat stress, cold-stressed animals have a higher metabolic rate, which is necessary to maintain body temperature in cold conditions. This leads to an increase in both O₂ consumption and CO₂ production, making it necessary for the animal to increase alveolar ventilation and decrease dead-space ventilation. Reducing the f and increasing VT accomplishes the latter adaptations.

The veterinarian needs to ensure that equipment used for anesthesia or respiratory therapy does not increase the dead-space. Excessively long endotracheal tubes or overly large face masks create a large amount of equipment dead-space. The consequence of this is that the animal must take in a large VT to obtain adequate alveolar ventilation.

Ventilation Requires Muscular Energy

Inhalation occurs when the respiratory muscles contract to expand the thorax, stretch the lung, and create the subatmospheric alveolar pressure that causes air to enter the respiratory system. During exhalation, the elastic energy stored in the stretched lung and thorax causes them to decrease in volume, leading to an increase in alveolar pressure that drives air out of the respiratory system. Therefore, in most resting mammals, exhalation does not require muscular effort. Horses are an exception because they have an active phase to exhalation even at rest. During exercise or in the presence of respiratory disease by contrast, exhalation is assisted by muscle contraction in most mammals.

The most important inspiratory muscle is the diaphragm, which is a domed musculotendinous sheet separating the abdomen from the thorax and innervated by the phrenic nerve. The diaphragm consists of a costal portion, arising from the xiphoid process and the costochondral junctions of the eighth to twelfth ribs (eighth to fourteenth ribs in Equidae), and a crural portion, arising from the ventral surface of the first three to four lumbar vertebrae and extending toward the tendinous center of the diaphragm. The apex of the dome of the diaphragm extends rostrally to the seventh or eighth intercostal space at the level of the base of the heart. During contraction of the diaphragm the dome is pulled caudally and thereby enlarges the thoracic cavity. The tendinous center pushes against the abdominal contents, elevating intra-abdominal pressure, which displaces the abdominal wall and caudal ribs outward, thus also tending to enlarge the thorax. It is the enlargement of the thorax that creates the negative (subatmospheric) pressure necessary to make air flow into the lungs during inhalation.

The external intercostal muscles also are active during inhalation. The fibers of these muscles are directed caudoventrally, from the caudal border of one rib to the cranial border of the next, so that muscle contraction moves the ribs rostrally and outward. The relative contributions of diaphragmatic and costal movement to ventilation under different metabolic demands are not well defined in animals. Because the cranial ribs support the forelimbs in quadrupeds, they participate less in ventilation than do the more caudal ribs. Other inspiratory muscles, including those connecting the sternum to the head, contract during strenuous breathing and move the sternum rostrally and assist in thoracic enlargement.

The subatmospheric pressure generated within the respiratory tract during inhalation tends to collapse the external nares, pharynx, and larynx. Contraction of abductor muscles attached to these structures is essential for preventing collapse. Abductor muscle contraction during inhalation can be observed as dilation of the external nares. Laryngeal hemiplegia (also known as recurrent laryngeal neuropathy) in horses is a condition in which the muscles on the left side of the larynx undergo atrophy as a consequence of an axonopathy of the left recurrent laryngeal nerve. The left dorsal cricoarytenoid muscle, which is the most important laryngeal abductor, fails to contract during inhalation. Consequently, during exercise, the left vocal fold is not abducted and creates an abnormal breathing sound that is sometimes called roaring.

The principal expiratory muscles are the abdominal and internal intercostal muscles. Contraction of the abdominal muscles increases abdominal pressure, which forces the relaxed diaphragm forward and reduces the size of the thorax. The fibers of the internal intercostal muscles are directed cranioventrally, from the cranial border of one rib to the caudal border of the next cranial rib, so that their contraction decreases the size of the thorax by moving the ribs caudally and ventrally. As the thorax becomes smaller, the intrathoracic pressure increases and forces air out of the lungs.

During exercise, respiratory muscle activity increases in order to generate the increase in . In cursorial (running) mammals, ventilation is synchronized with gait in the canter and gallop, but not in the walk or trot (Figure 45-4). Inhalation occurs as the forelimbs are extended and the hind limbs are accelerating the animal forward. Exhalation occurs when the forelimbs are in contact with the ground. In the galloping horse and perhaps in other galloping quadrupeds, much of the increase in size of the thorax during inhalation is a consequence of elongation of the trunk as the spine extends rather than an increase in the diameter of the thorax.