The Respiratory System To explore gross and minute features of the respiratory system and examine gaseous exchange.

From Last Week’s Session...  What was the name of the structure that conveys air from the pharynx towards the lungs?  How is it kept open?  What does it divide to become?

Bronchi  Primary bronchi  Secondary (lobar) bronchi  Tertiary (segmental) bronchi  Take air from the trachea into each lung  Branch from primary bronchi; take air to a respective lobe  Branch from secondary bronchi; take air into bronchopulmonary segments.

 As bronchi divide, they continue to get smaller in diameter.  In the tertiary bronchi, cartilage rings become irregular plates of cartilage. These still offer support but do not encircle the bronchi.  Tertiary bronchi divide to become bronchioles.  These have a diameter of 1mm or less. They have no cartilage in their walls.  As the cartilage disappears, the amount of smooth muscle increases, which appears as bands around the bronchioles’ lumen.  The smallest bronchioles are called terminal bronchioles.

 Cilia are present but gradually decrease in number.  Goblet cells are absent in bronchioles.  Instead, secretory cells called Clara cells are present. They secrete a fluid to prevent collapse of the bronchioles.  Terminal bronchioles mark the end of the conducting zone. All structures that lead to the terminal bronchioles are designed to allow air to flow into and out of the lungs.  The remainder of the bronchial tree is referred to as the respiratory zone and allows for gas exchange.

Respiratory bronchiole 1 2 3

TracheaAlveoli ducts Alveoli Primary bronchiRespiratory bronchioles Secondary bronchi (lobar) Terminal bronchioles Segmental (tertiary) bronchi Bronchioles

 Respiratory bronchioles are microscopic and have some single alveoli (air sacs) branching off their walls.  They eventually lead to alveolar ducts, which in turn lead to clusters of tiny air sacs called alveoli.  Alveoli are tiny air sacs and are the sites where gas exchange takes place.  There are millions of alveoli in the lungs, they increase the surface area of the lung to approximately 70m². This provides more surface for efficient gas exchange.

Alveoli  The alveoli make up the respiratory surface.  It is in the alveoli that gas exchange takes place.  Alveoli are efficient at exchanging gases because of their following adaptations:

 Thin walls The walls of alveoli are only 1 cell thick.  Large surface area The lungs contain millions of alveoli creating a huge surface (70m²) for the exchange of gases.  Moist walls Special cells in the alveoli secrete a watery liquid; O 2 dissolves in this moisture before entering the blood.  Good blood supply Each alveolus is surrounded by a capillary network (which are also only 1 cell thick).

 Good supply of air Ventilation (breathing) keeps the alveoli well supplied with air and therefore oxygen.

Composition of Air  Atmospheric air is a mixture of gases – nitrogen, oxygen, carbon dioxide, water vapour and tiny amounts of inert gases.  For the purpose of diffusion across the respiratory surface, there must be a difference in the concentration of gases for oxygen to move into the body and carbon dioxide to move out.  Gases in a mixture, are measured in partial pressures (p).

GasAtmospheric Air (sea level) Alveolar AirDeoxygenated Blood Oxygenated blood pO2pO pCO Partial pressures are measured in mmHg

Gaseous Exchange  Oxygen from inspired air within the alveoli diffuses down its concentration gradient across the thin alveolar walls to enter the blood capillaries.  Carbon dioxide moves in the opposite direction; carbon dioxide diffuses out of blood and into the air within the alveoli, to be exhaled.  The rate of diffusion is dependent on: surface area; difference in concentration gradients and thickness of the diffusion pathway.

pO 2 = 105mmHg pO 2 = 40mmHg

pCO 2 = 40mmHg pCO 2 = 45mmHg

Gas Exchange Animation  nt_view0/chapter23/animation__changes_in_the_partial_ pressures_of_oxygen_and_carbon_dioxide.html nt_view0/chapter23/animation__changes_in_the_partial_ pressures_of_oxygen_and_carbon_dioxide.html

 Overview  topics/topics/hlw/whathappens.html topics/topics/hlw/whathappens.html  lc/chap15summary.html lc/chap15summary.html