Respiratory cycle | Anatomy2Medicine
Respiratory cycle,NEET PG ,Physiology

Respiratory cycle

  • Forces Acting on the Lungs
    • Lung recoil
      • refers to forces that develop in the lung wall during expansion.
      • Recoil increases as the lung enlarges.
      • Recoil always acts to collapse the lung.
    • Intrapleural pressure (also called pleural pressure, or PPL)
      • the pressure in the thin film of fluid between the lung and chest wall
      • PPL is generally subatmospheric (~ −5 cm H20). (MCQ)
      • Negative subatmospheric pressures act to expand the lung
      • positive pressures act to collapse the lung. (MCQ)
      • When PPL exceeds recoil ,forces the lungs expand.
      • When recoil forces exceed PPL the lungs decrease in volume.
    • Alveolar pressure (PA)
      • It is the pressure of the alveolar air
      • PA drives airflow into and out of the lungs.
      • If PA equals 0 (ie, no airflow), then PA is the same as atmospheric pressure. (MCQ)
      • PA is less than 0 during inspiration(MCQ)
      • PA is greater than 0 during expiration. (MCQ)

.

 

  • Alveolar and intrapleural pressures during normal breathing.
    • Intrapleural pressure remains negative during inspiration and expiration. (MCQ)
    • Alveolar pressure is negative during inspiration and positive during expiration. (MCQ)

  • Transpulmonary pressure (PTP)
    • difference between the pressure inside the lung (alveolar pressure) and the pressure outside the lung (intrapleural pressure).
    • PTP determines the degree of inflation of the lung.
    • PTP is greater in the upper regions of the lung, (MCQ)
    • PPL is more negative and holds the lungs in a more expanded position. (MCQ)
    • The upper regions of the lungs also have greater volumes than the lower regions. Further increases in volume per unit increase in PTP are smaller in the upper than lower regions of the lungs because the upper expanded lung is stiffer (ie, less compliant).

 

  • Lung Compliance
    • Compliance (CL) is the stretching of the lungs and is calculated as follows
  • Compliance (CL) = change in lung volume

transpulmonary pressure

  • Compliance is the change in lung volume per unit change in airway pressure

 

  • High CL means more air will flow for a given change in pressure. (MCQ)
  • Low CL means less air will flow for a given change in pressure(MCQ)

 

  • If PTP becomes more negative, more air will flow into the system, (MCQ)
  • if PTP becomes more positive more air will flow out of the system. (MCQ)
  • CL is an indicator of the effort required to expand the lungs to overcome re- coil.
  • Compliant lungs have low recoil, whereas stiff lungs have a large recoil force (MCQ)

Airway Resistance

  • The rate of airflow for a given driving pressure depends on airway resistance:

 

  • where
    • V = flow rate (L/s)
    • PA = alveolar pressure (mm Hg)
    • R = airway resistance (R units)
  • The more negative the intrapleural pressure (eg, during inspiration), the lower the airway resistance. (MCQ)
  • According to Poiseuille’s equation,
  • Where r = radius of the airway
    • strong relationship exists between resistance and the radius of the airway.
    • The following factors influence airway resistance:
    • Stimulation of parasympathetic nerves produces bronchoconstriction. (MCQ)
    • Stimulation of sympathetic nerves or circulating catecholamine produces bronchodilation. (MCQ)
    • Low lung volumes are associated with increased airway resistance(MCQ)
    • High lung volumes are associated with decreased resistance. (MCQ)
    • Breathing a high-density gas increases resistance to airflow
    • Breathing a low-density gas decreases resistance to airflow.
    • The first and second (ie, medium-sized) bronchi represent most of the airway resistance. (MCQ)

 

  • The pressure-volume curve

 

      • not the same for inspiration and expiration

 

  • this difference is called hysteresis, which is due primarily to the effects of airway resistance

 

 

  • Static compliance curves
    • In fibrosis (lower curve) the lungs are stiff and less compliant and have increased alveolar elastic recoil force
    • Emphysema (upper curve) increases the compliance of the lungs and decreases alveolar elastic recoil forces because the alveolar septal tissue that op- poses lung expansion is destroyed

Breathing cycle

  • the breathing cycle is divided into phases: rest (the period between breaths), inspiration, and expiration.
  • Transmural pressure
    • transmural pressure is calculated as alveolar pressure minus intrapleural pressure.
    • If transmural pressure is positive, it is an expanding pressure on the lung,
    • if alveolar pressure is zero and intrapleural pressure is -5 cm H2O, there is an expanding pressure on the lungs of +5 cm H2O (0 – [-5 cm H2O] = +5 cm H2O).  (MCQ)
    • If transmural pressure is negative, it is a collapsing pressure on the lung
    • For all phases of the normal breathing cycle, despite changes in alveolar and intrapleural pressures, transmural pressures across the lungs are such that they always remain open.

 

Volumes and pressures during the normal breathing cycle.

    • Rest
      • At rest, no air is moving into or out of the lungs.
      • Alveolar pressure equals atmospheric pressure, and because lung pressures are always referred to atmospheric pressure, alveolar pressure is said to be zero. (MCQ)
      • There is no airflow at rest because there is no pressure difference between the atmosphere (the mouth or nose) and the alveoli.
      • At rest, intrapleural pressure is negative, or approximately -5 cm H2O. (MCQ)
      • Why is the intrapleural pressure is negative (MCQ)
        • The opposing forces of the lungs trying to collapse and the chest wall trying to expand create a negative pressure in the intrapleural space between them.
      • The transmural pressure across the lungs at rest is+ 5 cm H2O (MCQ) (alveolar pressure minus intrapleural pressure), which means that these structures will be open.
      • The volume present in the lungs at rest is the equilibrium volume or FRC, which, by definition, is the volume remaining in the lungs after a normal expiration. (MCQ)
    • Inspiration
      • During inspiration, the diaphragm contracts, causing the volume of the thorax to increase
      • As lung volume increases, the pressure in the lungs must decrease. (Boyle’s law states that P V is constant at a given temperature.)
      • Halfway through inspiration
        • alveolar pressure falls below atmospheric pressure (-1 cm H2O).
        • The pressure gradient between the atmosphere and the alveoli drives airflow into the lung.
      • Air flows into the lungs until, at the end of inspiration
        • alveolar pressure is once again equal to atmospheric pressure
        • the pressure gradient between the atmosphere and the alveoli has dissipated
        • airflow into the lungs ceases
      • The volume of air inspired in one breath is the tidal volume (VT), which is approximately 0.5 L (MCQ)
      • the volume present in the lungs at the end of normal inspiration is the functional residual capacity plus one tidal volume (FRC + VT). (MCQ)
      • During inspiration, intrapleural pressure becomes even more negative than at rest. (MCQ)

 

  • intrapleural pressure become more negative, or approximately -8 cm H2O at the end of inspiration (MCQ)

 

      • Explanations : As lung volume increases
        • the elastic recoil of the lungs also increases and pulls more forcefully against the intrapleural space
        • airway and alveolar pressures become negative.
    • The extent to which intrapleural pressure changes during inspiration can be used to estimate the dynamic compliance of the lungs.
  • Expiration
    • Normally, expiration is a passive process.
    • Alveolar pressure becomes positive (higher than atmospheric pressure) because the elastic forces of the lungs compress the greater volume of air in the alveoli.
    • When alveolar pressure increases above atmospheric pressure air flows out of the lungs, and the volume in the lungs returns to FRC.
    • The volume expired is the tidal volume (MCQ)
    • At the end of expiration , all volumes and pressures return to their values at rest and the sys- tem is ready to begin the next breathing cycle.
  • Forced Expiration
    • In a forced expiration, a person deliberately and forcibly breathes out.
      • The expiratory muscles are used to make lung and airway pressures even more positive than those seen in a normal, passive expiration.
      • In a person with normal lungs, the forced expiration makes the pressures in the lungs and airways very positive.
      • Both airway and alveolar pressures are raised to much higher values than those occurring during passive expiration.
    • in forced expiration (MCQ)
      • airway pressure is +25 cm H2O
      • alveolar pressure is +35 cm H2O
      • intrapleural pressure-is +20 cm H2O
    • Expiration will be rapid and forceful because the pressure gradient between the alveoli (+35cmH2O) and the atmosphere (0) is much greater than normal whereas during a normal passive expiration, alveolar pressure is +1 cm H2O
    • Why the lungs and airways do not collapse under these conditions of positive intrapleural pressure?
      • As long as the transmural pressure is positive, the airways and lungs will remain open
      • During a normal forced expiration (MCQ)
        • transmural pressure across the airways = airway pressure minus intrapleural pressure is +5 cm H2O
          • (+25 minus + 20] =  +5 cm H2O
        • transmural pressure across the lungs= alveolar pressure minus intrapleural pressure is  +15 cm H2O
          • (+35 minus +20) = +15 cm H2O
        • Therefore, both the airways and the alveoli will remain open because transmural pressures are positive.
  • Why forced expiration may cause the airways to collapse in a person with COPD.  (MCQ)
    • In COPD, lung compliance increases because of loss of elastic fibers
    • During forced expiration,
      • intrapleural pressure is raised to the same value as in the normal person, +20 cm H2O.
      • However, because the structures have diminished elastic recoil, alveolar pressure and airway pressure are lower than in a normal person.
      • The transmural pressure gradient across the lungs remains a positive expanding pressure, +5 cm H2O, and the alveoli remain open
      • However, the large airways collapse because the transmural pressure gradient across them reverses, becoming a negative (collapsing) transmural pressure of -5 cm H2O.
    • Obviously, if the large airways collapse, resistance to airflow increases and expiration is more difficult.
    • Persons with COPD learn to expire slowly with pursed lips, which raises airway pressure, prevents the reversal of the transmural pressure gradient across the large airways, and, thus, prevents their collapse. (MCQ)