Pressure Changes during Pulmonary Ventilation
Boyles Law : Pressure is indirectly proportional to the volume . As volume increases, pressure decreases. Therefore, inhalation and exhalation are the result of pressure differences within the lungs. Air moves into the lungs when air pressure inside the lungs is less than the air pressure in the atmosphere. Air moves out of the lungs in the opposite way.
A) Inhalation
Breathing in is called Inhalation or Inspiration . Just before this, the air pressure inside the lungs is equal to the air pressure of the atmosphere which at sea level is about 760mmHg . For air to come into the lungs, the pressure inside the lungs must be less than the atmosphere. This condition is achieved by INCREASING the size of the lungs. The first step to achieve increased volume in the lungs is contraction of the main muscles of inhalation: the diaphragm and external intercostals. Contraction of the diaphragm causes it to flatten (move caudally) increasing the diameter of the thoracic cavity. This normally produces a pressure difference of about 1-3mmHg and inhalation of air. During exercise, the diaphragm may descend 10cm producing a pressure difference of about 100mmHg and inhalation of increased amounts of air. Contraction of the diaphragm is responsible for about 75% of the air that enters the lungs. When the external intercostals contract they expand the rib cage causing an increase in lateral diameter of the chest cavity. This contraction results in 25% of the inhaled air. As lung volume increases, Alveolar (Intrapulmonic) pressure drops from 760 to 758mmHg and therefore we have established a pressure difference between the atmosphere and the alveoli and inhalation takes place. Air will continue to flow inward as long as the pressure difference is present.
B) Exhalation
Breathing out is called Exhalation or Expiration. And is also due to the pressure gradient described above however in the opposite direction. Normal exhalation during quiet breathing is a passive process and requires no muscular contraction. Instead, exhalation results from the elastic recoil of the chest wall and lungs after inhalation and relaxation of the diaphragm & external intercostals. Exhalation becomes active during forceful breathing such as playing an instrument or during exercise. During this time the muscles of exhalation: abdominals and internal intercostals contract, decreasing the volume of the thoracic cavity and thereby increasing pressure within the thoracic cavity.
NOTE: Horses always have active exhalation, even at rest.
Alveolar pressure increases to about 762mmHg. Air then flows from area of higher pressure in the alveoli to an area of lower pressure in the atmosphere. Horses are also able to synchronize the canter/gallop with respiration. When the forelimbs are extended, they inhale. When the forelimbs are on the ground the exhale. Why?
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