Overview of Respiratory Function: Ventilation of the Lung

Overview of Respiratory Function

Ventilation of the Lung

Respiratory Function

The Respiratory System’s Primary Function Is the Transport of Oxygen and Carbon Dioxide Between the Environment and the Tissues

The respiratory system provides oxygen (O2) to support tissue metabolism and removes carbon dioxide (CO2). Oxygen consumption and carbon dioxide production vary with the metabolic rate, which is dependent on the animal’s level of activity. Basal metabolism, the metabolism of the resting animal, is a function of metabolic body weight (M0.75). The consequence of this relationship is that smaller species consume more oxygen per kilogram of body weight than do larger species. For example, the 20-gram mouse consumes six times more oxygen per unit body mass than does a 70-kg pig. This difference is largely due to the metabolic requirements necessary to maintain constant body temperature. Because smaller species have a greater surface area to body weight ratio, they have a greater surface for heat loss and less heat storage capacity so they need higher basal metabolism to generate more heat.

When animals exercise, their muscles need more oxygen, which leads to an increase in oxygen consumption. Maximal oxygen consumption (image) is directly related to the total mass of mitochondria within the skeletal muscles. Athletic species such as the horse and dog have greater mitochondrial density and therefore greater image than do less athletic species of similar body size such as the cow and goat.

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 45-2 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.


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 (image), 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 image, 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:


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


The result is:


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

The Respiratory Muscles Generate Work to Stretch the Lung and Overcome the Frictional Resistance to Airflow Provided by the Airways (Airway Resistance)

At the end of a normal exhalation, some air (~45 mL/kg) remains in the lung. This air volume is known as functional residual capacity (FRC). At FRC, the pressure in the pleural cavity (Ppl) that surrounds the lung is approximately 5 cm H2O below atmospheric pressure (−5 cm H2O). During inhalation, as the inspiratory muscles contract, the thorax enlarges and Ppl decreases. This decrease in Ppl stretches the elastic lung and enlarges its volume, which decreases pressure within the alveoli (Palv). The decrease in Palv causes air to flow into the lung through the tracheobronchial tree (Figure 45-5). Lung compliance is a measure of the elastic properties of the lungs, and airway resistance is a measure of the frictional resistance of the airways. The magnitude of the change in pleural pressure (ΔPpl) during each breath is determined by the tidal volume (VT), by lung compliance (C), by airflow rate (image), and by airway resistance (R), as follows:

FIGURE 45-5 Examples of pressure changes that might be expected in the respiratory system during quiet breathing. Palv, Alveolar pressure; Pb, barometric pressure; Ppl, pleural pressure; Paw, pressure within the airway; VT, tidal volume. Numbers represent the pressure difference (cm H2O) from atmospheric pressure (Pb). The gradient Palv – Ppl is the elastic pressure gradient necessary to keep the lung inflated. Pb – Palv is the pressure gradient that drives airflow through the airways. Pb – Ppl is the pressure gradient that inflates the lung. A, Before the start of inhalation, when the respiratory system is resting at functional residual capacity (FRC), there is no airflow into the lungs because Pb – Palv = 0 cm H2O and the negative pleural pressure is keeping the lung partially inflated (Pb – Ppl = 5 cm H2O). B, During inhalation, Pb – Ppl = 16 cm H2O in order to enlarge the lung (Palv – Ppl = 8 cm H2O) and make air flow through the airways (Pb – Palv = 8 cm H2O). About halfway along the airways, pressure within the lumen (Paw) is −4 cm H2O. C, At the end of a tidal inhalation, flow ceases because Pb – Palv = 0 cm H2O, but the lung contains more air (Palv – Ppl = 11 cm H2O). D, During exhalation, airflow reverses direction: Pb – Palv = −8 cm H2O, the lung volume is still greater than FRC (Palv – Ppl = 10 cm H2O), and the total pressure gradient remains slightly positive (Pb – Ppl = 2 cm H2O). About halfway along the airways, pressure within the lumen (Paw) is 4 cm H2O. It is important to remember that these pressure gradients change continually throughout a breath and with changes in tidal volume, respiratory frequency, lung compliance, and airway resistance.
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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Overview of Respiratory Function: Ventilation of the Lung
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