Introduction to Respiratory System

Introduction to Respiratory System

• The respiratory system is crucial for metabolic reactions, releasing energy from nutrients and producing ATP, and also releases carbon dioxide.
• This system regulates blood pH and contributes to homeostasis by exchanging gases (oxygen and carbon dioxide) between the air, blood, and tissue cells.

Learning Objectives

• Students will learn the functional anatomy of the respiratory system, including the airways, lungs, and thoracic cavity.
• Students will understand the mechanisms of inspiration and expiration, focusing on the roles of respiratory muscles and pressure changes related to airflow.

Structure of Respiratory System

• The respiratory system is categorized structurally into:
  • Upper respiratory system (nose, nasal cavity, pharynx, and associated structures)
  • Lower respiratory system (larynx, trachea, bronchi, and lungs)
• Functionally, the system is comprised of:
  • Conducting zone (nose, nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles)
  • Respiratory zone (respiratory bronchioles, alveolar ducts, alveolar sacs, alveoli)

Breathing (Pulmonary Ventilation)

• Breathing, or pulmonary ventilation, involves two cycles:
  • Inhalation (inspiration) - brings gases into the lungs.
  • Exhalation (expiration) - expels gases from the lungs.

Respiratory Events

• Pulmonary ventilation: exchange of gases between lungs and atmosphere.
• External respiration: exchange of gases between alveoli and pulmonary capillaries.
• Internal respiration: exchange of gases between systemic capillaries and tissue cells.

Phases of Pulmonary Ventilation

• Inspiration (Inhalation): an active process requiring energy; the diaphragm contracts and moves downward, and the rib cage rises, increasing lung volume and reducing intrapulmonary pressure below atmospheric pressure.
• Expiration (Exhalation): a passive process; the diaphragm relaxes and the rib cage lowers, decreasing lung volume and increasing intrapulmonary pressure above atmospheric pressure, forcing air out.

Physiology of Respiration

• Respiration encompasses three fundamental processes:
  • External pulmonary ventilation: the exchange of air between the atmosphere and the alveoli of the lungs.
  • Internal (pulmonary) ventilation: gas exchange between the alveoli and the blood in pulmonary capillaries across the respiratory membrane. During this process, pulmonary capillaries absorb O2 and release CO2.
  • Internal (tissue) respiration: the exchange of gases between systemic capillaries and tissue cells. During this process, blood releases O2 and absorbs CO2. In cells, these metabolic reactions that consume O2 and release CO2 during ATP production are known as cellular respiration.

Composition of Air

• Nitrogen (N2) - 78%
• Oxygen (O2) - 20.9%
• Other Gases (e.g., Argon) - >0.17%
• Carbon Dioxide (CO2) - 0.03%

Inhalation (Inspiration)

• During inhalation, intrapulmonary pressure decreases to become lower than atmospheric pressure, allowing air to flow into the lungs.
• This is achieved by increasing the size of the lungs.

Events of Inspiration

• Inspiratory muscles contract (diaphragm descends, ribcage rises).
• Thoracic cavity volume increases.
• Lungs stretch; intrapulmonary volume increases.
• Intrapulmonary pressure drops (to –1 mm Hg).
• Air flows into lungs down its pressure gradient until intrapulmonary pressure equals 0 (equal to atmospheric pressure).

Exhalation (Expiration)

• Expiration occurs when the inspiratory muscles relax. As the diaphragm relaxes, the dome moves superiorly due to its elasticity, and the rib cage lowers.
• Pressure in the lungs becomes greater than the external atmospheric pressure, causing air flow out of the lungs.

Steps of Exhalation

• Inspiratory muscles relax (diaphragm rises, rib cage descends).
• Thoracic cavity volume decreases
• Elastic lungs recoil; intrapulmonary volume decreases.
• Intrapulmonary pressure rises (+1 mm Hg).
• Air flows out of lungs down its pressure gradient until intrapulmonary pressure is 0.

Minute Respiratory Volume

• Minute respiratory volume represents the total amount of new air entering the respiratory passages each minute.
• A typical value for this volume is around 6 L/min.

Alveolar Ventilation

• Gas exchange primarily occurs at the alveoli.
• Air that doesn't reach the gas exchange areas is called "dead space air".
• The amount of dead space air in a young adult male is typically ~150 ml.

External Respiration (Pulmonary Gas Exchange)

• External respiration in the lungs transforms deoxygenated blood (low in O2) into oxygenated blood (high in O2) to send back to the left side of the heart.
• This process involves the pickup of O2 from the alveoli and the release of CO2 into the alveoli by the blood as it flows through the pulmonary capillaries. This process is governed by diffusion.

Internal Respiration (Systemic Gas Exchange)

• The left ventricle pumps oxygenated blood into the aorta and systemic arteries to systemic capillaries.
• The exchange between systemic capillaries and tissue cells is called internal respiration or systemic gas exchange.

Surface Tension

• Surface tension is a force from the attraction between water molecules at the air-liquid interface within the alveoli.
• In the lungs, alveoli are lined with fluid, leading to a tendency for water molecules to stick together. This creates tension, which could cause alveolar collapse.

Surfactant

• Surfactant is a lipoprotein, secreted by type II pneumocytes in the lungs.
• Its primary functions are to reduce surface tension within alveoli, improving lung compliance (expansion and contraction), and preventing alveolar collapse.

Effects of Surface Tension Without Surfactant

• Smaller alveoli experience greater surface tension, increasing pressure and more prone to collapse, leading to instability in the lung.
• This makes breathing more difficult and impacts oxygenation.

Increased Work of Breathing

• Without surfactant, breathing becomes significantly harder due to increased resistance from surface tension, needing more effort for lung expansion during inhalation.
• Uneven alveolar inflation occurs with small alveoli collapsing into larger ones, affecting the overall gas exchange and oxygenation.

Functions of Surfactant

• Reduction of Surface Tension: Surfactant lowers the cohesive forces between water molecules lining alveoli, preventing collapse during exhalation, and maintaining open alveoli for efficient gas exchange..
• Improved Lung Compliance: Reduced surface tension makes lungs more expandable and contractile, making breathing easier.
• Prevention of Alveolar Collapse: Maintaining stability of alveoli during exhalation, keeping them open during breathing.
• Facilitation of Uniform Alveolar Expansion: Ensuring that small alveoli do not collapse into larger ones due to pressure differences.
• Role in Host Defense: Surfactant contains proteins that improve immune defenses by clearing pathogens and debris in alveoli.

Boyle's Law

• Intrapulmonary pressure changes in relation to lung volume, inversely proportional to volume.
• Increase in lung volume leads to a decrease in intrapulmonary pressure, causing air to enter
• Decrease in lung volume leads to an increase in intrapulmonary pressure, causing air to exit

Quiet Inspiration

• Active process: Diaphragm, parasternal and internal intercostals contraction increases thoracic volume vertically and laterally. Lung volume expansion decreases pressure within alveoli causing air to flow in.
• Pressure changes are around 0- to –3 mm Hg (alveolar),
• 4 to -6 mm Hg (intrapleural), and +3 mm Hg (Transpulomary).

Quiet Expiration

• Passive process: After lung stretching, recoil of lungs and chest wall decreases lung volume increasing intrapulmonary pressure. This pushes air out.
• Pressure changes are around +3 to 0 mm Hg (alveolar)
• –3 to -6 mm Hg (intrapleural), and +6 mm Hg (Transpulmonary).