Gasses are able to move in and out of the lungs through muscular energy exerted on the thorax and changes between intrathoracic and atmospheric pressures. The pressure within the lungs and thorax must be less than atmospheric pressure for inspiration to occur. Air then flows from an area of higher pressure to one of lower pressure. As the diaphragm and intercostal muscles work to increase the size of the thorax, intrathoracic pressure decreases below atmospheric pressure and air moves into the lungs. During exhalation, the inspiratory muscles relax, and the elastic recoil of the lung tissues, combined with a rise in intrathoracic pressure, causes air to move out of the lungs
The diaphragm, a dome shaped structure that separates the thoracic and abdominal cavities, is the major muscle of respiration. The phrenic nerve innervates the diaphragm. The external and internal intercostal muscles elevate the ribs, increasing the anterior-posterior diameter of the thoracic cavity. Breathing may need to be assisted by other muscles, known as secondary or accessory muscles of respiration. These muscles may include the parasternal, scalene, sternocleidomastoid, trapezius, and pectoralis muscles. Accessory respiratory muscles do not function during normal ventilation, but may be needed in some respiratory disorders.
The illustration below shows the movement of the diaphragm during the respiratory cycle.
Watch as the diaphragm contracts, forcing the abdominal viscera downwards as the chest expands from top to bottom.
The diaphragm is relaxed during exhalation, curving up towards the deflating lungs, as the elastic tissue passively recoils.
The diaphragm is the major muscle of respiration and is innervated by the phrenic nerve.
Structures in the respiratory conduction system help conduct air into the lungs where the exchange of oxygen and carbon dioxide takes place. The respiratory conduction system is divided into the upper and lower airways.
Carbon dioxide must be
eliminated on a continuous basis to maintain the body's acid-base balance.
Acid-base balance is controlled by chemoreceptors located near the respiratory
center that are sensitive to changes in the pH of cerobrospinal fluid. When
ventilation is inadequate, the pH drops and the carbon dioxide level rises.
The rise in carbon dioxide stimulates the respiratory center to increase
the rate and depth of respirations to remove excess carbon dioxide.
If hypoventilation becomes chronic, as in patients with chronic obstructive pulmonary disease (COPD), chemoreceptors lose their sensitivity and respond to increases in carbon dioxide levels inadequately. When central chemoreceptors fail, peripheral chemoreceptors attempt to regulate respiratory function and restore acid-base balance. Peripheral chemoreceptors are sensitive to the amount of oxygen in peripheral blood. Therefore, the patient's stimulus to breathe is no longer an increase in carbon dioxide levels, but from a low oxygen level sensed by peripheral chemoreceptors. If the blood oxygen level is increased significantly by giving supplemental oxygen, the peripheral chemoreceptors will not stimulate breathing, resulting in apnea. This alteration in physiologic function is the reason that patients with COPD are given supplemental oxygen at very low levels.