Primary and secondary lung volumes

• Primary lung volumes are the basic volumes of air associated with breathing.
• Tidal Volume (TV) is the normal volume of air displaced during normal breathing when extra effort is not applied.
• Inspiratory Reserve Volume (IRV) is the additional amount of air that can be inhaled after a normal inhalation in a deep, forced manner.
• Expiratory Reserve Volume (ERV) is the additional amount of air that can be exhaled after a normal exhalation via forced exhalation.
• Residual Volume (RV) is the volume of air remaining in the lungs even after a forced exhalation, preventing lung collapse.
• Secondary lung volumes are calculated by combining primary lung volumes.
• Inspiratory Capacity equals TV + IRV, representing the maximum amount of air that can be inhaled after a normal exhalation.
• Functional Residual Capacity equals ERV + RV, representing the amount of air remaining in the lungs after a normal exhalation.
• Vital Capacity equals TV + IRV + ERV, representing the maximum amount of air that can be exhaled after a full inhalation.
• Total Lung Capacity equals TV + IRV + ERV + RV, representing the total amount of air the lungs can hold.

Mechanics of Breathing

• Inhalation involves the contraction of specific muscles to expand the chest cavity.
• During inhalation, the diaphragm contracts and moves downward.
• The external intercostal muscles contract, pulling the ribs up and out during inhalation.
• Internal intercostal and abdominal muscles relax during inhalation.
• Exhalation involves the relaxation of muscles to decrease chest volume.
• During exhalation, the diaphragm relaxes and moves upward.
• The external intercostal muscles relax, allowing the ribs to move downward and inward during exhalation.
• During forced exhalation the internal intercostal muscles contract, pulling the ribs down and in.
• Abdominal muscles contract during forced exhalation, pushing the diaphragm up.

Lung Volume Changes

• During inhalation (inspiration), chest volume increases.
• The diaphragm contracts and moves downward in inhalation.
• External intercostal muscles contract, lifting ribs outward and upward during inhalation.
• Increased chest cavity volume during inhalation reduces lung pressure below atmospheric pressure, causing air to rush in.
• During exhalation (expiration), chest volume decreases.
• The diaphragm relaxes and moves upward during exhalation.
• External intercostal muscles relax, and ribs move downward and inward during exhalation.
• Decreased chest cavity volume during exhalation increases lung pressure above atmospheric pressure, pushing air out.
• Forced exhalation actively involves the contraction of internal intercostal muscles, pulling ribs down.
• Abdominal muscles contract, pushing the diaphragm up further during forced exhalation.
• Chest volume decreases more forcefully, expelling more air during forced exhalation.

O2 & CO2 Percentages

• Inhaled air contains approximately 21% oxygen (O₂) and 0.03% carbon dioxide (CO₂).
• Exhaled air contains approximately 16% oxygen (O₂) and 5% carbon dioxide (CO₂).

Cilia & Mucus

• Cilia are small hairs lining the nasal cavity and airways.
• Cilia sweep debris and dirty mucus up the trachea for disposal through coughing, swallowing, or spitting.
• Mucus coats the cilia and lung tissue.
• Mucus traps dust and bacteria and allows gases to dissolve for diffusion through membranes.

Air Warming Path

• As air travels through the respiratory system, it is warmed, humidified, and filtered to prevent lung tissue damage from cold air.
• Air warming path: Nasal Cavity, pharynx, larynx, trachea, bronchi and bronchioles, alveoli.

Alveoli

• Alveoli are tiny, balloon-like air sacs in the lungs where gas exchange occurs.
• The alveolar wall is extremely thin, approximately 0.5 micrometers (µm) thick.

Types of Alveolar Cells

• Type I Alveolar Cells (Squamous Epithelial Cells):
  • Flat, thin, squamous shape.
  • Primary gas exchange cells, forming the alveolar wall for easy diffusion of O₂ and CO₂.
• Type II Alveolar Cells (Cuboidal Cells):
  • Small, cuboidal shape.
  • Produce surfactant, a substance that reduces surface tension in alveoli.

Gas exchange between capillaries:

• Oxygen (O₂) diffuses from alveoli into capillaries.
• Carbon dioxide (CO₂) diffuses from capillaries into alveoli.

Why We Need Surfactant

• Without surfactant, alveoli would collapse due to high surface tension, leading to atelectasis (alveolar collapse).

What Surfactant Does

• Reduces Surface Tension: Alveoli are lined with fluid, creating surface tension, which surfactant lowers.
• Keeps Alveoli Open: Ensures alveoli remain open for continuous gas exchange. Prevents respiratory distress, especially in premature infants lacking surfactant.
• Reduces the Work of Breathing: Makes lung expansion easier, reducing the effort needed to inhale.
• Prevents Fluid Accumulation: Helps prevent fluid from entering the alveolar space, maintaining proper lung function.