The Respiratory System is Designed to Bring in Oxygen and Remove Carbon Dioxide

  • A person with an average ventilation rate of 7.5 L/min will breathe in and out 10,800 liters of gas each day
  • From this gas the person will take in about 420 liters of oxygen (19 moles/day) and will give out about 340 liters of carbon dioxide (15 moles/day)
  • The ratio of CO2 expired/O2 inspired is called the respiratory quotient (RQ)
    • RQ = CO2 out/O2 in = 340/420 = 0.81
    • In cellular respiration of glucose CO2 out = O2 in; RQ = 1
    • The overall RQ is less than 1 because our diet is a mixture of carbohydrates and fat; the RQ for metabolizing fat is only 0.7
  • All of the exchange of gas takes place in the lungs
  • The lungs also give off large amounts of heat and water vapor

The Thoracic Cage Encloses the Lungs and Heart

  • The thoracic cage is formed by the:
  • 12 thoracic vertebrae
  • 12 ribs (the top 10 attach directly or indirectly to the sternum; the bottom 2 floating ribs do not)
  • Sternum (manubrium, gladiolus, xiphoid)
  • Diaphragm muscle
  • Within the thoracic cage are 3 compartments:
    • The 2 pleural cavities, each with a lung
    • The mediastinum, containing the heart

Air is Brought Into the Lungs by a Set of Tubes Which Branch Repeatedly

  • Gases enter and leave lungs through:
    • Mouth and nose
    • Pharynx
    • Larynx (voice box)
    • Trachea (reinforced by cartilage rings)
    • Bronchi
    • Bronchioles
  • These tubes expand at their ends into alveoli, where gas exchange takes place
  • There are about 23 branchings, producing about 300 million alveoli

The Lungs Reside in Two Separate Pleural Cavities

  • The lungs “float” within separate pleural cavities
  • Do not attach directly to the chest wall– separated from the wall by a space containing pleural fluid
  • Pleural fluid layer allows lungs to slide sideways during expansion- prevents tearing
  • Expansion of pleural space is called a pneumothorax- caused by entrance of air (or liquid) through a wound
  • Pneumothorax causes collapse of lung on one side- person will survive if pleural pressure does not get too high

Contracting the Diaphragm or Raising the Ribs Expands the Thoracic Cavity, Producing a Tidal Flow of Air in and out of the Lungs

  • Contracting the diaphragm moves it downwards and expands the thoracic cavity, drawing air into the lungs
  • At rest contraction of the diaphragm accounts for most of inspiration
  • Diaphragm is essential for respiration. Supplied by phrenic nerve which originates from cervical spinal cord (vertebrae 3-5). If spinal cord is cut above this level respiration is inhibited -> death
  • The external intercostal muscles (between ribs) also aid inspiration- cause ribs to move upward and outward, expanding the thoracic cavity
  • At rest expiration is mostly passive- lungs contract due to elasticity
  • In exercise the internal intercostal muscles and others aid expiration- pull ribs downward and inward, reducing thoracic cavity

Respiratory Rate and Tidal Volume Determine the Pulmonary Ventilation

  • The total amount of air moved in and out of the lungs each minute (pulmonary ventilation: PV) depends upon 2 factors:
    • The size of each breath (tidal volume: TV)
    • The number of breaths/minute (respiratory frequency: BR)
    • PV = BR X TVß
    • Example: suppose your tidal volume is 500 mL (0.5 liters) and you breath 15 times/minute:
    • Pulmonary ventilation = 15 breaths/min X 0.5 L/breath = 7.5 L/min

We Have Reserve Inspiratory and Expiratory Volumes

  • During exercise we can increase tidal volume by expanding both inspiration and expiration
  • The extra inspiration available for exercise is called the inspiratory reserve volume (IRV)
    • IRV is about 2.5 liters
  • The extra expiration available for exercise is called the expiratory reserve volume (ERV)
    • ERV is about 1.5 liters
  • After maximum expiration some air is still present in the lungs- this is called the residual volume (RV)
    • The RV is about 1.5 liters- this volume is not available for respiration

The Maximum Usable Lung Volume is the Vital Capacity

  • The maximum volume available for breathing is the vital capacity (VC); it is the sum of the TV, ERV and IRV
  • VC = IRV + TV + ERV
  • An average VC is about 3.5 liters, but there is a lot of variation
  • The VC allows pulmonary ventilation to increase by a factor of 5-10
  • The vital capacity cannot be used for sustained exercise- it requires too much work for maximal inspiration and expiration

Measuring Lung Volumes

  • Lung volumes are measured with a respirometer, which makes a tracing on a graph similar to the one above. Put in numerical values for the different volumes. For example, vital capacity is measured by taking a maximal inspiration (Max In) and then breathing out maximally (Max Out).
  • Forced expiratory volume in 1 second (FEV1) is a measure of lung obstruction. The subject takes a maximum inspiration and then breaths out as fast as possible. A person with normal lungs will expire about 70-85% of his VC in one second. A person with lung obstruction (asthma, emphysema, bronchitis) will force out a much smaller percentage in 1 second.

More Information

The Duke University Center for In Vivo Microscopy has a beautiful movie of guinea pig lungs expanding and contracting during breathing. The guinea pig was breathing helium-3, which allows the lungs to be visualized by magnetic resonance. On the same page is a movie showing both the lungs and heart of a living rat. Very impressive!

When the lungs are congested they produce all sorts of interesting sounds which are useful to doctors and respiratory therapists. If you want to learn these sounds you can download a free RealAudio Player and listen to the respiratory sound library at the RALE Repository.

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