Mechanism of Respiration
Boyle’s law states that at constant temperature and pressure, there is an inverse relationship between volume and pressure of a gas-
P1V1 = P2V2
where P is the pressure of the gas (mm of Hg), and V is the volume of gas (ml).
Therefore, the pressure inside a closed container will decrease as the volume of the container increases, and vice versa.
When this law is applied to the lungs, the pressure inside the lungs decreases as the volume of the lungs increases.
Air moves into the lungs as a result of a decrease in air pressure within the lungs.
In most animals, inspiration is an active process.
The main muscles responsible for quiet inhalation are the diaphragm and external intercoastal muscles. The diaphragm is a dome-shaped muscle innervated by the phrenic nerves.
As the diaphragm contracts, it increases the horizontal dimensions of the thoracic cavity, thus increasing the volume of the thorax.
Diaphragmatic contraction enlarges the thorax in a caudal direction, and intercoastals muscle contraction enlarges the thorax in a craniad and outward direction. This causes the volume of the lungs to expand and thereby decreases the pressure inside the lungs. This results in inspiration.
Contraction of the external intercoastals muscles causes the ribs to move cranially and ventrally, thus increasing the diameter of the thorax. This accounts for about 25% of the entry of air into the lungs.
In a horse at rest, the intrapleural pressure is negative, about 754 mmHg. As inspiration begins, this pressure drops to approximately 744 mmHg.
Atmospheric pressure is 760 mmHg at sea level. As the volume of the thoracic cavity increases during inspiration, the parietal pleura is pulled outward, and the visceral pleura is pulled with it. As a result, the pressure inside the lungs, (the alveolar pressure) decreases. Air then flows from an area of high pressure (the atmosphere) to an area of lower pressure (the alveoli).
As an animal increases the force of inspiration, additional muscles are engaged. These include the sternocleidomastoid muscles that move the sternum rostrally, the scalene muscles that pull the first two ribs forward, and the pectoralis minor muscles that pull several other ribs forward.
Expiration
Normal expiration is a passive process involving no active muscle contraction. Like inspiration, it is due to pressure gradients, but in an opposite direction. Because of the elastic recoil of the lungs and chest wall, there are two inwardly directed forces resulting 1) the elastic fibers that were stretched during inhalation, and 2) the inwardly directed force due to the surface tension arising from the alveolar fluid.
As the neural signals to the diaphragm cease, it relaxes and this dome-shaped muscle moves rostrally, thus decreasing the volume of the thoracic cavity.
The external intercostals also relax, and allow the ribs to move dorsally and caudally, thus further decreasing the volume of the thoracic cavity. This decreases lung volume and causes alveolar pressure to increase approximately 2 mmHg above atmospheric pressure. As a result, air flows out of the lungs to an area of lower pressure.
During forceful exhalation, the abdominal and internal intercostals muscles contract. This causes the ribs to move caudally and dorsally compressing the abdominal viscera and decreasing the thoracic volume. This increases pressure inside the thoracic cavity and forces air outward.
Surface tension of alveolar fluid aids in expiration
Alveolar fluid coats the inside surface of the alveoli. The surface tension produces an inwardly directed force causing the alveoli to assume the smallest possible diameter. In order to expand the lungs, this surface tension must be exceeded. This surface tension accounts for approximately two thirds of the lung’s elastic recoil.
Surfactant, produced by type II alveolar cells, is a complex of lipids and proteins that reduces the surface tension in much the same way the soap allows lipids to dissolve in aqueous solutions. Surfactant decreases surface tension which increases pulmonary compliance (reducing the effort needed to expand the lungs) reduces tendency for alveoli to collapse. Since surfactant is one of the last compounds produced during embryonic development, premature animals often have respiratory distress as a result of the underdeveloped respiratory system.
In the case of sheep, surfactant is released into the alveolar spaces near the beginning of the fourth month of gestation. Its release correlates with a rise in plasma cortisol levels.
Inspiration
Normal expiration is a active process.
Types of breathing
There are two types of breathing- abdominal and coastal breathing
Abdominal breathing is characterized by visible movements of the abdomen, Normally in animals the abdominal type of breathing predominates.
The coastal breathing is characterized by pronounced rib movements. During painful conditions of the abdomen, such as peritonitis, in which movement of the viscera would aggravate the pain, coastal breathing can predominate. Similarly, during painful conditions of the thorax, such as pleuritis, abdominal breathing will be more apparent.
Eupnea is the normal quiet breathing, with no deviation in frequency or depth.
Dyspnea is difficult breathing, in which visible effort is required to breathe. The animal is conscious of this state.
Hyperpnea refers to breathing characterized by increased depth, frequency, or both after severe exercise. The animal is not actually conscious of this state.
Polypnea is rapid, shallow breathing, similar to panting. Polypnea is similar to hyperpnea in frequency, but not in depth. Apnea refers to a cessation of breathing.
Compliance of the lungs
The distensibility of the lungs is referred to as lung compliance. High lung compliance means that the lungs will expand easily.
- Compliance is seen decreased in these two conditions-
- Pulmonary edema resulting from accumulation of fluid in the lungs
- Insufficient of surfactant.