VPB 121: Veterinary Physiology - II (2+1)

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.