Physioooo

• Respiration


  Process by which an organism exchanges gases
• Respiration


  Oxygen is taken in and carbon dioxide is given out
• The first breath takes place only after birth. Fetal lungs are non-functional. So, during intrauterine life the exchange of gases between fetal blood and mother's blood occurs through placenta
• After the first breath, the respiratory process continues throughout the life. The permanent stoppage of respiration occurs only at death
• Types of Respiration


  • External respiration
  • Internal respiration
• External respiration


  Exchange of respiratory gases, i.e. oxygen and CO2 between lungs and blood
• Internal respiration


  Exchange of gases between blood and tissues
• Functional anatomy of airways

  • Conducting zone
  • Transitional zone
  • Respiratory zone
• Conducting zone


  • Transports gas from and to the exterior, made up of bronchi, bronchioles and terminal bronchioles
• Transitional and respiratory zones

  • Where gas exchange occurs, made up of respiratory bronchioles, alveolar ducts, and alveoli
• The multiple divisions greatly increase the total cross-sectional area of the airways, from 2.5 cm2 in the trachea to 11,800 cm2 in the alveoli. Consequently , the velocity of air flow in the small airways declines to very low values
• The alveoli are surrounded by pulmonary capillaries. In most areas, air and blood are separated only by the alveolar epithelium and the capillary endothelium so they are about 0.5 micron apart
• There are around 300 millions alveoli in humans, and the total area of the alveolar walls in contact with capillaries in both lung is about 70 m2
• Pathway of air flow

  Trachea → Bronchi → Bronchioles (Terminal → Respiratory) → Alveolar duct → Atrium → Alveoli
• Muscles of inspiration


  • Diaphragm
  • External intercostal muscles
• Diaphragm


  • Movement accounts for 75 % of the change in intrathoracic volume during quiet inspiration, attached around the bottom of the thoracic cage, arches over the liver and moves downward like a piston when it contracts, distance it moves ranges from 1.5 cm to as much as 7 cm with deep inspiration
• External intercostal muscles


  • Run obliquely downward and forward from rib to rib, ribs pivot as if hinged at the back, when they contract they elevate the lower ribs which pushes the sternum outward and increases the AP diameter of the chest
• Either the diaphragm or the external intercostal muscles alone can maintain adequate ventilation at rest
• Muscles of expiration


  • Internal intercostals
  • Rectus abdominis
• Internal intercostals


  • Depress ribs
• Rectus abdominis


  • Main muscle of expiration, compresses abdomen
• Forces acting on the lung system

  • Lung recoil
  • Intrapleural pressure
• Lung recoil


  Recoil as a force, always acts to collapse the lung
• Intrapleural pressure


  Represents the pressure in the thin film of fluid between the lung and the chest wall, subatmospheric pressure (-) act as force to expand the lung, and positive pressures (+) act as force to collapse the lung
• During normal restful breathing, intrapleural pressure is always subatmospheric or negative (-5 to -8 cm H2O) and thus acts as a force to expand the lung
• When intrapleural pressure is a greater force than lung recoil, the lungs expand
• When the recoil force is greater than that created by intrapleural pressure, lung volume will be decreasing
• When the force of recoil and intrapleural pressure are equal and opposite, a static state exists, and lung size will be constant
• Reasons for lung collapse

  • Elastic recoil caused by the elastic fibers in the alveolar walls
  • Surface tension of the film of fluid that lines the alveoli
• Factors preventing lung collapse

  • Negative intrapleural pressure
  • Surfactant
• Mechanism of Respiration - Before inspiration

  1. Glottis is open, all respiratory muscles relaxed (FRC)

  2. Functional Residual Capacity or FRC is the neutral or equilibrium point of the respiratory system

  3. Intrapleural pressure is negative at FRC, because the inward elastic recoil of the lungs is opposed by the outward- directed recoil of the chest wall

  4. The intrapleural force and recoil forces are equal and opposite, and because no air is flowing through the open glottis, alveolar pressure must be zero
• Mechanism of Respiration - During Inspiration

  1. Inspiration is induced mainly by diaphragm

  2. The net result of contracting this muscles is to decrease (make more negative) intrapleural pressure

  3. The greater the contraction, the greater the change in intrapleural pressure and the larger the force trying to expand the lung

  4. The expansion of the lung causes the gases in the alveoli to expand, creating a slightly negative alveolar pressure

  5. This causes air to flow into the lung
• Mechanism of Respiration - End of Inspiration

  1. The lung expands until the recoil force increases to equal intrapleural pressure

  2. Once the forces are again equal and opposite, the lung is at its new larger volume

  3. The inflowing air returns alveolar pressure toward zero and when it reaches zero airflow stops

  4. Under resting conditions about 500 ml of air flows into the lung system in order to return alveolar pressure back to zero
• Mechanism of Respiration - Expiration

  1. Expiration under resting conditions is produced simply by the relaxation of the muscles of inspiration

  2. The relaxation of the diaphragm and accessory muscles of inspiration increases (makes more positive) intrapleural pressure, which returns to – 5 cm H2O

  3. Lung deflation begins and continues until the recoil force decreases to again equal intrapleural pressure

  4. Once this occurs, the lung system is back to FRC

  5. Deflation of the lung compresses the gases in the alveoli, creating a slightly positive alveolar pressure

  6. This causes air to flow out of the lungs

  7. The outflowing air returns alveolar pressure toward zero, and when it reaches zero air flow stops
• Mechanism of Respiration - Applied physiology: Pneumothorax


  1. Intrapleural pressure increases from a mean at – 5 cm H2O to equal atmospheric pressure

  2. Lung recoil decreases to zero as the lung collapses

  3. Chest wall expands. At FRC, the chest wall is under a slight tension directed outward

  4. It is this tendency for the chest wall to spring out and the opposed force of recoil that creates the intrapleural pressure of - 5 cm H2O
• Positive Pressure Respiration - Assisted Control Mode Ventilation

  Respiratory cycle initiated by patient or automatically if no signal is detected within a specified time window
• Positive End Expiratory Pressure


  By not allowing intraalveolar pressure to return to zero at the end of expiration, the lung will be kept at a larger volume. This will decrease the tendency to develop regional atelectasis
• Alveolar surfactant


  Secreted by special cells called type II alveolar epithelial cells, a complex mixture of phospholipids (dipalmitoylphosphatidyl choline), proteins & ions (Ca2+)
• Type I alveolar epithelial cells

  • Flat cells with large cytoplasmic extensions and are the primary lining cells
• Functions of surfactant


  • It lowers surface tension forces in the alveoli
  • It lowers surface tension forces more in small alveoli than in large alveoli, promoting stability among alveoli of different sizes
  • It reduces capillary filtration forces and thus reduces the tendency to develop pulmonary edema