Respiratory System: Structure, Function, and Gas Exchange

The Respiratory System

Pressures Associated with the Lungs

The movement of air into and out of the lungs is governed by various pressures within the thoracic cavity and lungs.

• Atmospheric Pressure (Patm): The pressure exerted by the air surrounding the body (typically 760 mmHg at sea level).

• Intrapulmonary (Alveolar) Pressure (Ppul): The pressure within the alveoli; fluctuates with breathing and equalizes with atmospheric pressure at the end of inspiration and expiration.

• Intrapleural Pressure (Pip): The pressure within the pleural cavity; always negative relative to Ppul to prevent lung collapse (usually about 4 mmHg less than Ppul).

• Transpulmonary Pressure: The difference between Ppul and Pip; keeps the airways open.

Transport of Oxygen and Carbon Dioxide in the Blood

Oxygen and carbon dioxide are transported in the blood by different mechanisms:

• Oxygen (O2):

  • About 98.5% is carried bound to hemoglobin in red blood cells as oxyhemoglobin (HbO2).

  • About 1.5% is dissolved directly in plasma.

• Carbon Dioxide (CO2):

  • About 70% is transported as bicarbonate ions (HCO3-) in plasma.

  • About 20-23% is bound to hemoglobin as carbaminohemoglobin (HbCO2).

  • About 7-10% is dissolved in plasma.

Stimuli for Breathing

Breathing is primarily regulated by the levels of CO2, O2, and pH in the blood.

• Most Powerful Stimulus: Increased CO2 (hypercapnia) is the most potent stimulus for breathing, detected by central chemoreceptors in the medulla oblongata.

• Decreased pH (increased H+) also stimulates breathing.

• Low O2 (hypoxemia) is a less powerful stimulus, detected by peripheral chemoreceptors in the carotid and aortic bodies.

Gas Laws and Their Application

• Boyle’s Law: The pressure of a gas is inversely proportional to its volume at constant temperature. Equation: $P_1 V_1 = P_2 V_2$ Application: Explains how changes in thoracic volume during breathing cause air to move in and out of the lungs.

• Dalton’s Law: The total pressure of a mixture of gases is the sum of the partial pressures of each individual gas. Equation: $P_{total} = P_1 + P_2 + P_3 + \ldots$ Application: Explains how gases move independently down their partial pressure gradients during gas exchange.

• Henry’s Law: The amount of gas that dissolves in a liquid is proportional to its partial pressure and its solubility in the liquid. Equation: $C = k P$ Application: Explains why CO2 is more soluble in blood plasma than O2.

Mechanisms of Inspiration and Expiration

• Quiet Inspiration: Diaphragm and external intercostal muscles contract, increasing thoracic volume and decreasing intrapulmonary pressure, causing air to flow into the lungs.

• Quiet Expiration: Passive process; diaphragm and intercostals relax, thoracic volume decreases, intrapulmonary pressure increases, and air flows out.

• Forced Inspiration: Accessory muscles (scalenes, sternocleidomastoid, pectoralis minor) further increase thoracic volume.

• Forced Expiration: Active process; abdominal and internal intercostal muscles contract to decrease thoracic volume more rapidly.

Function and Secretion of Surfactant

• Function: Surfactant reduces surface tension within the alveoli, preventing alveolar collapse during exhalation.

• Secretion: Produced and secreted by Type II alveolar cells (also called septal cells).

Anatomical Differences Between the Left and Right Lungs

• Right Lung: Has three lobes (superior, middle, inferior) and is shorter and wider due to the position of the liver.

• Left Lung: Has two lobes (superior, inferior) and a cardiac notch to accommodate the heart; it is longer and narrower.

Mechanisms Preventing Dust and Debris from Reaching the Alveoli

• Mucociliary Escalator: Ciliated epithelial cells and mucus trap and move particles upward toward the pharynx for removal.

• Macrophages: Alveolar macrophages engulf and digest particles that reach the alveoli.

• Nasal Hairs and Turbinates: Filter and trap larger particles in the nasal cavity.

Maintenance of Tracheal Patency

• C-shaped Hyaline Cartilage Rings: These rings prevent the trachea from collapsing and maintain an open airway.

The Bohr Effect and Oxygen Loading/Unloading

• Bohr Effect: Increased CO2 and H+ (lower pH) decrease hemoglobin’s affinity for O2, facilitating oxygen unloading in tissues.

• Application: Active tissues produce more CO2 and H+, promoting O2 release where it is needed most.

Mechanisms for CO2 Transport in the Blood

• As Bicarbonate Ions: CO2 reacts with water to form carbonic acid, which dissociates into bicarbonate and H+ (catalyzed by carbonic anhydrase in RBCs): $\mathrm{CO_2 + H_2O \xrightarrow{carbonic\ anhydrase} H_2CO_3 \rightarrow HCO_3^- + H^+}$

• Bound to Hemoglobin: CO2 binds to the globin portion of hemoglobin (not the heme group).

• Dissolved in Plasma: A small percentage is transported as dissolved gas.

Key Respiratory Terms

• Internal Respiration: Gas exchange between systemic blood and tissue cells.

• External Respiration: Gas exchange between alveoli and pulmonary capillaries.

• Pulmonary Ventilation: The movement of air into and out of the lungs (breathing).

Ventilation-Perfusion Coupling

This mechanism matches the amount of air reaching the alveoli (ventilation) with the blood flow in pulmonary capillaries (perfusion) to optimize gas exchange.

• Areas with low O2 receive less blood flow (vasoconstriction), while areas with high O2 receive more blood flow (vasodilation).

Types of Alveolar Cells

• Type I Alveolar Cells: Simple squamous epithelial cells that form the majority of the alveolar wall and are the primary site of gas exchange.

• Type II Alveolar Cells: Secrete surfactant and help repair the alveolar epithelium.

Anatomical Parts of the Respiratory Membrane

The respiratory membrane is the site of gas exchange between alveolar air and blood in pulmonary capillaries.

• Consists of:

  • Alveolar epithelium (Type I cells)

  • Fused basement membranes of alveolar and capillary walls

  • Capillary endothelium

Summary Table: Oxygen and Carbon Dioxide Transport

Gas	Transport Mechanism	Percentage
Oxygen (O2)	Bound to hemoglobin (HbO2)	~98.5%
Oxygen (O2)	Dissolved in plasma	~1.5%
Carbon Dioxide (CO2)	As bicarbonate ions (HCO3-)	~70%
Carbon Dioxide (CO2)	Bound to hemoglobin (HbCO2)	~20-23%
Carbon Dioxide (CO2)	Dissolved in plasma	~7-10%