RESPIRATORY ZONES

Conducting zone

Conducts the air into and out of the lungs; this portion is lined with ciliated mucous membranes that remove foreign materials from the respiratory tract.

Conducting Zone Function:

• Regulate airflow; this is most easily achieved by altering the diameter of the bronchi and bronchioles, which have layers of smooth muscle in their walls.
  *Bronchial tubes respond to local oxygen concentration by altering their diameter to increase or decrease oxygen levels.
  – A decrease in diameter will increase the resistance to airflow, and, therefore, reduce the rate of airflow;
  – An increase in bronchial diameter will decrease the resistance to airflow and increase the rate of airflow.
• Clinical correlation:
  Patients with asthma suffer from narrowed airways, which prevents sufficient airflow and oxygen intake to meet tissue demands. Asthma is often treated with drugs that induce bronchial dilation to increase airflow.

Conducting Zone Anatomy:

• Nasal cavity
• Pharynx
• Larynx
• Trachea
• Bronchi (Primary, secondary, and tertiary bronchi)
• Bronchioles

Respiratory zone

Comprises the terminal end of the tract; this is where gas exchange with the pulmonary capillaries occurs.

Respiratory Zone Function:

• Provides the site of gas exchange between the pulmonary blood and the external environment.

Respiratory Zone Anatomy:

• Respiratory bronchioles
• Alveolar ducts and sacs (on average, there are 300 million alveoli in each lung).
• Alveoli facilitate gas exchange; sac-like pouches made of type I and II alveolar cells (aka, pneumoncytes).
  – The type I cells are flat epithelial cells with a large surface area to facilitate gas exchange; they are the predominant cell type.
  – Type II cells are cuboidal; they synthesize pulmonary surfactant, which reduces the surface tension of alveoli (so they can “open” more easily and fill with air).
  Alveolar macrophages capture debris and move to the conducting zone, which expels it from the body.
• The alveolar sacs are surrounded by pulmonary capillaries, which allows for gas exchange between the circulatory and respiratory systems.

PULMONARY BLOOD CIRCULATION.

Pulmonary blood flow = cardiac output of the right ventricle.

Key pathways:

• Right atrium sends deoxygenated blood to the right ventricle, which ejects it through the pulmonary trunk and arteries.
• Within the lungs, the pulmonary arteries form branching patterns that parallel the tracheobronchial tree.
• Blood flow travels through the pulmonary capillary networks that surround the alveoli.
• Carbon dioxide is released from the bloodstream and fresh oxygen enters it.
• Oxygenated blood returns to the heart via the pulmonary veins, then drains through the left atrium to the ventricle.
• Upon contraction, the left ventricle sends the oxygenated blood through the aorta to the systemic tissues.

Bronchial circulation:

• Blood supply to the conducting zone of the respiratory tract; thus, it is the very small portion of total blood flow that does not take part in gas exchange.

Gravitational effects on pulmonary blood flow

• In upright position, pulmonary blood flow is not distributed evenly throughout the lungs.
• Gravitational pull results in a lower blood flow at the apex of the lung than at the base.