Jul 12, 2011 / 92 notes

Respiratory System: Gas Exchange

Right & left lung:

  • The right lung has 3 lobes
  • The left lung has 2 lobes
  • The right is heavier and larger
    (but shorter and wider due to the right side of the diaphragm being higher)
  • The heart and pericardium are more prominent to the left
  • The anterior margin of the right lung is straight, where as the left lung has a deep cardiac notch

 Two sets of arteries supply the lungs with blood and oxygen:

  • Pulmonary arteries
  • Bronchial arteries

Deoxygenated blood passes the pulmonary trunk which divides into the left pulmonary artery and the right.  The pulmonary arteries are the only arteries within the body to carry deoxygenated blood. 

The return of oxygenated blood to the heart is transported via the 4 pulmonary veins, which drain into the left Atrium.  

Bronchial arteries (which branch from the aorta), deliver oxygenated blood to the lungs.  This would perfuse the walls of the bronchi and bronchioles.  Most of the blood returns to the heart via the pulmonary veins.  Some, however will return via the bronchial veins (azygos system) and into the heart via the superior vena cava. 

There are many defense mechanisms the body has to protect the airway.  Nose hair is there to stop large particles entering by trapping them.   Mucus (which is present in all of the tract above the bronchioles thanks to the epithelium is very thick and sticky.  It acts as a protective barrier against irritants as well as trapping bacteria  and foreign particles. 

Over 125mls of mucus is produced a day.  It forms a continuous sheet or mucus blanket that cover the ts of the tract.  The mucus is moved towards the pharynx on small hair-like structures called cilia.  Irritants/toxins can trigger an immediate response from the respiratory tract to produce more.

Cilia are minuscule hairs that cover the epithelium lining of the RT. Thy move 600-1000 times a minute, to move the mucus and particles trapped within it. For it to then be swallowed and digested by stomach juices/acid. 

Sneezing is a reflex activated when your nasal mucosa are irritated.  

Equally coughing occurs when the body wants to remove excess mucus or other toxins from the air passage beyond the nose.  Receptors within the mucosal lining are activated and cause stimulation of the respiratory control center of the brain (within the medulla oblongata). 

Phagocytes are present in the epithelium.  They endeavor to “suck up” debris, dust and bacteria, filtering incoming breaths.  The final line of defense of the airways is found within the alveoli.  Macrophages are able to pick up carbon particles, bacteria and other foreign particles.

Anatomical dead space is defined as air that is inhaled but does not participate in gas exchange.   Not all the air in one breath is able to be used for exchange of oxygen and carbon dioxide.   Normal dead space volume is approximately 150mls. 

Alveolar ventilation (VA) is the volume of air breathed in per minute that reaches the alveoli and takes part in gas exchange. It is not just the volume of air that reaches the alveoli.   Speaking physiologically: VA is the volume of alveolar air (per minute) that takes part in gas exchange (transfer of oxygen and carbon dioxide ) within the pulmonary capillaries.   Air that does not take part in exchange (eg: within unperfused alveoli) does not count.


(tidal volume - dead space) * respiratory rate

What is pulmonary compliance and vascular resistance?

Compliance is the ability of the lungs to accommodate and expand during a change in volume related to a change in pressure.  Where as vascular resistance is defined as the flow that must be overcome to push blood through the circulatory system.

Boyles Law states that pressure and volume are inversely related.  During normal inhalation the diaphragm contracts, which causes the thoracic cavity to increase in size.  The decrease in air pressure causes air to be sucked into the lungs. In normal exhalation the diaphragm relaxes.  The lungs recoil from the lung elasticity decreasing the size of the thoracic cavity therefore decreasing the air pressure.  Air is then forced out the lungs.

Ventilation/perfusion ratio (V/Q ratio) is a measurement used to assess the efficiency and adequacy of the matching of two variables.  Defined as the ratio of air reaching the alveoli to the amount of blood also reaching the alveoli.

V - Air
Q - Blood.


I hope this helps!  Replies and ASKs welcome


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