West Chapter 1: Structure and Function - How the Architecture of the Lung Subserves Its Function

How do oxygen and carbon dioxide move between air and blood?

By simple diffusion, from an area of high to low partial pressure

Fick’s Law of Diffusion

The amount of gas that moves across a sheet of tissue is proportional to the area of the sheet but inversely proportional to its thickness

How thick is the blood-gas barrier and what is its area?

Thin

Area between 50 and 100 m²

What is the sequence of the conducting zone from largest to smallest?

Trachea → right and left main bronchi → lobar bronchi → segmental bronchi → terminal bronchioles

What is the smallest airway without alveoli?

Terminal bronchioles

Conducting Airways

Function is the lead inspired air to the gas-exchanging regions of the lung

Constitute anatomical dead space because they contain no alveoli and therefore take no part in gas exchange

Dead Space

Areas of lung that receive ventilation but no blood flow

Structures of the Conducting Airways

Larger proximal airways are lined by a ciliated columnar epithelium and have a lot of cartilage in their walls

• As airways progress distally, the proportion of cartilage decreases and smooth muscle increases so that the very small distal airways are composed mostly of smooth muscle

What is the sequence of the transitional and respiratory zones zone from largest to smallest?

Terminal bronchioles → respiratory bronchioles (occasional budding from walls) → alveolar ducts (completely lined with alveoli)

Respiratory Zone

Alveolated region of the lung where gas exchange occurs

• Makes up most of the lung

Acinus

Portion of lung distal to a terminal bronchiole

How does the cross sectional area of the thorax increase during inspiration?

The volume of the thoracic cavity increases, in part from contraction of the diaphragm causing it to descend and in part from the action of the intercostal muscles which raise the ribs

Where does inhaled dust frequently settle out?

In the terminal bronchioles because the velocity of gas falls rapidly there

What is the dominant mechanism of gas movement in the respiratory zone?

Diffusion of gas within the airways

• Rate of diffusion of gas molecules within the airways is so rapid and distances to be covered so short that the differences in concentration within the acinus are virtually abolished within a second

What is the dominant mechanism of gas movement in the conducting zone?

Inspired air flows down to about the terminal bronchioles by bulk flow

• Beyond that point, the combined cross sectional area of the airways is so large due to the number of branches that the forward velocity of the gas becomes small

Structure of Blood Vessels in the Lungs

• Arteries, veins, and bronchi run close together initially but toward the periphery of the lung, the veins move away to pass between lobules and arteries and bronchi travel together down the centers of the lobules in what is referred to as the bronchovascular bundle

• Capillaries form a dense network in the walls of the alveoli

  • Lengths of segments are short so the dense network forms an almost continuous sheet of blood in the alveolar wall, making it efficient for gas exchange

What can damage the pulmonary capillaries?

• Because the blood-gas barrier is so thin, the capillaries are easily damaged

  • Increasing the pressure in the capillaries to high levels or inflating the lung to high volumes can raise the wall stresses of the capillaries to a point where ultrastructural changes can occur and the capillaries leak plasma and even RBCs into the alveolar spaces

Bronchial Circulation

Additional blood supply for the lung which provides blood to the conducting airways down to about the terminal bronchioles

• Most of this blood is carried away from the lung via the pulmonary veins, while a small amount reaches the left side of the heart and enters the systemic circulation

How much time does blood spend in the pulmonary capillaries?

About 0.75 seconds

Role of Surfactant

• Because of the surface tension of the liquid lining the alveoli, relatively large forces develop that tend to collapse alveoli

• Surfactant dramatically lowers the surface tension of the alveolar lining layer and markedly increases the stability of the alveoli

Removal of Inhaled Particles

• Large particles are filtered out in the nose

• Smaller particles that deposit in the conducting airways are removed by a moving staircase of mucus that continually sweeps debris up to the epiglottis where it is swallowed or expectorated

  • Mucus is secreted by mucous gland and also goblet cells in the bronchial walls and is propelled by cilia

• Alveoli had no cilia and particles that deposit there are engulfed by macrophages

  • Foreign material is removed from the lung via the lymphatics or blood flow

Removal of Material from the Pulmonary Blood

The branching network of very small blood vessels traps small pieces of infected material or blood clots which prevents such material from getting to the left side of the circulation where they can travel to various organs and cause problems