RESPIRATORY SYSTEM:

 

 

 

I. RESPIRATORY SYSTEM FUNCTION:

 

A. Why a respiratory system?

• Respiratory function: Gas transport for metabolism
• Non-Respiratory function: Filtering and metabolism

 

B. Gas transport: Oxygen in and Carbon Dioxide out

• Ventilation: (atmosphere <-> respiratory zone)
• Lung diffusion: (respiratory zone <-> erythrocyte/plasma)
• Circulation: (blood flow carries erthyrocyte/plasma <-> tissue)
• Tissue diffusion: (erythrocyte/plasma <-> tissue cells)
• Internal respiration: (cellular metabolism using O2 & producing CO2)

 

II. VENTILATION: Structure and Function

 

A. Gas conduction:

Upper respiratory tract: Gas humidification, filtration, and warming

Conducting airways: Gas distribution to respiratory zone

• Trachea: Cartilage rings
• Bronchi: Cartilage plates
• Bronchioles: No cartilage -> depend upon lung structure
• Total cross-sectional area of conducting airways INCREASES as we move from the trachea to the respiratory zone.
• VELOCITY (cm/sec) = FLOW (cm3/sec) / CROSS-SECTION AREA(cm2). Therefore in the periphery of the lung near the respiratory zone gas velocity drops to zero and there is no CONVECTIVE movement of gas. Movement of gas in the respiratory zone is by DIFFUSION alone.
• Terminal bronchioles of conducting airways have ciliated and goblet cells to help clear particulate matter.
• Respiratory bronchioles mark the beginning of the respiratory zone and have alveoli in their walls.

 

B. Negative Pressure Breathing:

 

• Pump Structure:
• Rib cage and spine
• Diaphragm
• Intercostal muscles
• Respiratory control is a complex, integrated phenomenon with voluntary and involuntary controls.

 

III. DIFFUSION: Structure and Function

 

A. Diffusion: Basic concepts

 

• Gas diffusion is Passive from high partial pressure to low partial pressure.
• Alveolar O₂ > Pulmonary capillary O₂: O₂ diffuses to capillary.
• Pulmonary capillary CO₂ > Alveolar CO₂: CO₂ diffuses to alveolar gas.

 

 

B. The respiratory zone:

 

Respiratory bronchioles, alveolar ducts and alveoli.

The zone is designed to maximize efficient gas exchange by diffusion:

• Large surface area: 70 m² (300 million alveoli)
• Large volume of gas to maintain diffusion pressure gradient (150ml in conduction zone vs 4,000 ml in respiratory zone)
• Very thin membrane (0.5 micron)

 

Alveolar blood-gas barrier:

 

• Respiratory epithelium
• Interstitial space
• Capillary endothelium
• Plasma
• Erythrocyte

 

C. Pulmonary blood flow:

 

All of cardiac output (+/-5%) passes through the respiratory zone.

. Pulmonary arteries carry blood from the right heart (de-oxygenated).

Pulmonary arteries distribute blood to the pulmonary capillary system to maximize erythrocyte exposure to alveolar gas tensions.

The churning/pumping action of the heart aids in gas diffusion in the respiratory zone.

The bronchial vessels arise from the aorta and carry nutrient blood to the airways.

Thus the lung is efficiently designed to conduct and distribute gas to a very large, thin diffusion membrane with good blood supply.

 

IV. NON-RESPIRATORY FUNCTION OF THE LUNG:

 

A. Basic concepts:

• The lung performs other functions than gas exchange:
• Maintenance and defense
• Filtering
• Chemical processing

Maintenance and defense:

• Protein and connective tissue synthesis
• Surfactant synthesis
• Mucociliary clearance
• Immune function

Filtering:

The lung is ideally designed to filter the bloodstream of particulate matter (clots, bacteria, etc.) before passing that blood on to the body.