Respiratory Physiology

Processes of Respiration

The respiratory system is responsible for the exchange of gases between the body and the environment. This process involves several key steps:

• Pulmonary Ventilation: Movement of air into and out of the lungs (inspiration and expiration).

• External Respiration: Exchange of O2 and CO2 between the lungs and the blood.

• Transport of Respiratory Gases: Movement of O2 from the lungs to tissues and CO2 from tissues to the lungs via the blood.

• Internal Respiration: Exchange of gases between blood and tissue cells.

Respiratory Tract Structure

The respiratory tract consists of conducting and respiratory zones:

• Conducting Zone: Includes nose, nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles. Functions: passageway for air, moistens, warms, and filters air.

• Respiratory Zone: Site of gas exchange; includes respiratory bronchioles, alveolar ducts, and alveoli.

Respiratory Mucosa and Defense Mechanisms

• Respiratory Mucosa: Pseudostratified ciliated columnar epithelium with mucous and serous glands. Mucus traps debris; cilia move mucus toward the pharynx.

• Alveolar Macrophages: Phagocytize foreign particles in alveoli.

• Alveolar Cells: Type I (simple squamous epithelium for gas exchange) and Type II (secrete surfactant to reduce surface tension).

Mechanics of Breathing

Pressure Relationships in the Thorax

Breathing is driven by pressure differences between the atmosphere and the lungs:

• Intrapulmonary Pressure: Pressure within alveoli; fluctuates with breathing.

• Intrapleural Pressure: Pressure within pleural cavity; always negative relative to intrapulmonary pressure (approx. -4 mm Hg).

• Transpulmonary Pressure: Difference between intrapulmonary and intrapleural pressures; keeps airways open.

Inspiration and Expiration

• Inspiration: Diaphragm and external intercostal muscles contract, thoracic cavity volume increases, intrapulmonary pressure drops, air flows into lungs.

• Expiration: Muscles relax, thoracic cavity volume decreases, intrapulmonary pressure rises, air flows out of lungs.

Gas Exchange and Transport

Oxygen Transport

Oxygen is transported in the blood in two forms:

• Dissolved in Plasma: About 1.5% of O2 is dissolved directly in plasma.

• Bound to Hemoglobin (Hb): About 98.5% of O2 is carried by hemoglobin in red blood cells.

Hemoglobin binds O2 reversibly; when O2 binds, hemoglobin is called oxyhemoglobin (HbO2), and when O2 is released, it is called deoxyhemoglobin (HHb).

Oxygen-Hemoglobin Dissociation Curve

The relationship between the partial pressure of oxygen (PO2) and hemoglobin saturation is described by the oxygen-hemoglobin dissociation curve:

• At high PO2 (lungs), hemoglobin is almost fully saturated with O2.

• At low PO2 (tissues), hemoglobin releases O2 more readily.

• Factors affecting the curve: temperature, blood pH, PCO2, and BPG (2,3-bisphosphoglycerate).

Equation:

Carbon Dioxide Transport

CO2 is transported in the blood in three forms:

• Dissolved in Plasma: 7-10% of CO2 is dissolved directly in plasma.

• Bound to Hemoglobin: 20% of CO2 binds to hemoglobin as carbaminohemoglobin (HbCO2).

• Bicarbonate Ions (HCO3-): 70% of CO2 is transported as bicarbonate ions, formed by the reaction of CO2 with water catalyzed by the enzyme carbonic anhydrase in red blood cells.

Equation:

Regulation of Respiration

Neural Control

Respiratory rhythm is generated by centers in the brainstem:

• Medullary Respiratory Centers: Control the basic rhythm of breathing.

• Pontine Centers: Modify and fine-tune breathing patterns.

Chemical Control

• Chemoreceptors: Located in the brainstem and peripheral arteries; detect changes in blood levels of CO2, O2, and pH.

• Increased CO2 (hypercapnia) stimulates increased breathing rate and depth.

Pathology: Chronic Obstructive Pulmonary Disease (COPD)

Overview

COPD is a group of lung diseases characterized by chronic airflow obstruction:

• Chronic Bronchitis: Inflammation of bronchi, increased mucus production.

• Emphysema: Destruction of alveolar walls, loss of lung elasticity.

• Symptoms: Dyspnea (difficulty breathing), cough, frequent infections.

• Main cause: Smoking (90% of cases).

Defenses such as nasal hairs, mucus, and cilia are impaired, leading to accumulation of pathogens and debris.

Summary Table: Forms of Gas Transport in Blood

Additional info:

• Surfactant is secreted by type II alveolar cells and reduces surface tension, preventing alveolar collapse.

• Hyperventilation leads to decreased CO2 in blood (hypocapnia), which can cause respiratory alkalosis.

• Hypoventilation leads to increased CO2 (hypercapnia), causing respiratory acidosis.