Respiratory System Physiology Review Sheet

Respiratory System Physiology: A Comprehensive Review

This review sheet provides a comprehensive overview of respiratory system physiology, covering key concepts from basic anatomy to complex regulatory mechanisms. Understanding the respiratory system is crucial, as it's responsible for gas exchange, vital for sustaining life. This guide will delve into the mechanics of breathing, gas transport, and the regulatory controls that maintain homeostasis. Whether you're a student preparing for an exam or a healthcare professional brushing up on your knowledge, this detailed review will equip you with a strong foundation in respiratory physiology.

I. Introduction: The Marvel of Breathing

The respiratory system is far more than just lungs; it's a complex network of organs and tissues working in concert to facilitate gas exchange – the process of acquiring oxygen (O2) and expelling carbon dioxide (CO2). This intricate process involves several key components:

• The Upper Respiratory Tract: Includes the nose, nasal cavity, pharynx (throat), and larynx (voice box). These structures filter, warm, and humidify incoming air, protecting the delicate lower respiratory tract.
• The Lower Respiratory Tract: Consists of the trachea (windpipe), bronchi, bronchioles, and alveoli (tiny air sacs where gas exchange occurs). The branching structure of the bronchi and bronchioles ensures efficient distribution of air to the vast surface area of the alveoli.
• The Lungs: A pair of spongy organs housed within the thoracic cavity, the lungs are the primary sites of gas exchange. Their elastic nature allows for expansion and contraction during breathing.
• The Pleura: A double-layered membrane surrounding the lungs, creating a lubricating space (pleural cavity) that reduces friction during breathing.

Understanding the anatomy lays the foundation for comprehending the physiological processes involved in respiration.

II. Mechanics of Breathing: Inspiration and Expiration

Breathing, or pulmonary ventilation, is the mechanical process of moving air into and out of the lungs. This involves two phases:

A. Inspiration (Inhalation):

Inspiration is an active process, requiring muscular effort. The diaphragm, the primary muscle of inspiration, contracts and flattens, increasing the volume of the thoracic cavity. Simultaneously, external intercostal muscles (located between the ribs) contract, lifting the rib cage upward and outward. This expansion decreases the intrathoracic pressure, creating a pressure gradient that draws air into the lungs.

• Pressure Changes: Intrapulmonary pressure (pressure within the alveoli) decreases below atmospheric pressure, causing air to flow from an area of higher pressure (atmosphere) to an area of lower pressure (lungs).
• Lung Compliance: The ease with which the lungs can expand is called compliance. Factors like lung elasticity and surface tension affect compliance. Reduced compliance (e.g., in pulmonary fibrosis) makes breathing more difficult.
• Airway Resistance: The friction encountered by air as it flows through the airways influences the ease of breathing. Factors such as bronchoconstriction and mucus accumulation increase resistance.

B. Expiration (Exhalation):

Expiration is typically a passive process at rest. As the diaphragm and external intercostal muscles relax, the elastic recoil of the lungs and thoracic cage causes the chest cavity to decrease in volume. This increase in intrathoracic pressure forces air out of the lungs.

• Active Expiration: During forceful exhalation (e.g., during exercise or coughing), internal intercostal muscles and abdominal muscles contract, further reducing the thoracic volume and increasing the pressure gradient.
• Surface Tension: Surfactant, a lipoprotein produced by alveolar cells, reduces surface tension within the alveoli, preventing their collapse during expiration. Lack of surfactant (e.g., in respiratory distress syndrome) severely impairs lung function.

III. Gas Exchange: The Alveolar-Capillary Membrane

The primary function of the respiratory system is gas exchange – the movement of O2 from the alveoli into the blood and CO2 from the blood into the alveoli. This occurs across the alveolar-capillary membrane, a thin barrier composed of alveolar epithelium, interstitial space, and capillary endothelium.

A. Partial Pressures:

Gas exchange is driven by differences in partial pressures. Partial pressure is the pressure exerted by a specific gas in a mixture of gases. Alveolar PO2 (partial pressure of oxygen) is typically around 100 mmHg, while alveolar PCO2 (partial pressure of carbon dioxide) is about 40 mmHg. Pulmonary capillary blood entering the lungs has a lower PO2 and higher PCO2, creating a gradient for diffusion.

B. Diffusion:

Oxygen diffuses from the alveoli (high PO2) into the pulmonary capillaries (low PO2), binding to hemoglobin in red blood cells. Simultaneously, CO2 diffuses from the pulmonary capillaries (high PCO2) into the alveoli (low PCO2) for exhalation. The efficiency of diffusion is influenced by factors like the surface area of the alveolar-capillary membrane, the thickness of the membrane, and the partial pressure gradients.

C. Perfusion and Ventilation:

• Perfusion: refers to the blood flow through the pulmonary capillaries.
• Ventilation: refers to the airflow into and out of the alveoli.

Optimal gas exchange requires a good match between perfusion and ventilation (ventilation-perfusion matching). Imbalances can lead to reduced efficiency of gas exchange.

IV. Gas Transport in the Blood

Once oxygen diffuses into the blood, it's transported primarily bound to hemoglobin (Hb) within red blood cells. Hemoglobin's affinity for oxygen is influenced by several factors:

• PO2: Higher PO2 leads to increased hemoglobin saturation.
• pH: Lower pH (acidity) reduces hemoglobin's affinity for oxygen (Bohr effect).
• Temperature: Higher temperature reduces hemoglobin's affinity for oxygen.
• 2,3-Bisphosphoglycerate (2,3-BPG): This molecule reduces hemoglobin's affinity for oxygen.

Carbon dioxide is transported in the blood in three main ways:

• Dissolved in plasma: A small fraction of CO2 is dissolved directly in the plasma.
• Bound to hemoglobin: CO2 binds to hemoglobin, forming carbaminohemoglobin.
• As bicarbonate ions: Most CO2 is converted to bicarbonate ions (HCO3-) within red blood cells, catalyzed by the enzyme carbonic anhydrase. This reaction also produces hydrogen ions (H+), contributing to the Bohr effect.

V. Respiratory Control: Maintaining Homeostasis

Respiratory rate and depth are precisely regulated to maintain blood gas homeostasis (optimal levels of O2 and CO2). This regulation involves several components:

A. Central Chemoreceptors:

Located in the medulla oblongata of the brainstem, central chemoreceptors are highly sensitive to changes in cerebrospinal fluid (CSF) pH. Increased CO2 in the blood leads to increased H+ in the CSF, stimulating chemoreceptors to increase respiratory rate and depth.

B. Peripheral Chemoreceptors:

Located in the carotid bodies and aortic bodies, peripheral chemoreceptors monitor arterial blood PO2, PCO2, and pH. Decreased PO2, increased PCO2, or decreased pH stimulate these receptors to increase respiratory activity.

C. Higher Brain Centers:

The cerebral cortex and limbic system can exert voluntary control over respiration (e.g., holding your breath), but this is ultimately overridden by the involuntary control mechanisms.

D. Lung Receptors:

Stretch receptors in the lungs prevent overinflation, while irritant receptors trigger protective reflexes such as coughing and bronchoconstriction.

VI. Common Respiratory Disorders

Many diseases affect the respiratory system, impacting its structure and function. A few examples include:

• Asthma: A chronic inflammatory disorder characterized by airway narrowing and bronchospasm.
• Chronic Obstructive Pulmonary Disease (COPD): A group of diseases, including emphysema and chronic bronchitis, characterized by airflow limitation.
• Pneumonia: An infection of the lungs that causes inflammation and fluid accumulation in the alveoli.
• Pulmonary Embolism: A blockage of a pulmonary artery by a blood clot, often originating from deep vein thrombosis.
• Cystic Fibrosis: A genetic disorder that causes thick mucus buildup in the lungs and other organs.
• Pulmonary Fibrosis: A condition characterized by progressive scarring of lung tissue, leading to reduced lung compliance.

VII. Clinical Assessment of Respiratory Function

Assessing respiratory function often involves:

• Spirometry: Measures lung volumes and flows to assess lung function.
• Arterial Blood Gas (ABG) analysis: Measures the partial pressures of O2 and CO2, pH, and bicarbonate levels in arterial blood.
• Pulse oximetry: Measures the oxygen saturation of hemoglobin in arterial blood.
• Chest X-ray: Provides an image of the lungs to identify abnormalities.
• Computed Tomography (CT) scan: Provides a more detailed image of the lungs than a chest X-ray.

VIII. Frequently Asked Questions (FAQ)

Q1: What is the difference between external and internal respiration?

A: External respiration refers to gas exchange between the alveoli and the pulmonary capillaries. Internal respiration refers to gas exchange between systemic capillaries and tissues.

Q2: What is the role of surfactant?

A: Surfactant reduces surface tension in the alveoli, preventing their collapse during expiration and maintaining lung compliance.

Q3: How does the body compensate for acidosis and alkalosis?

A: The respiratory system plays a key role in acid-base balance. In acidosis (low blood pH), the respiratory system increases ventilation to remove CO2, thus reducing H+ concentration. In alkalosis (high blood pH), ventilation is decreased to retain CO2, increasing H+ concentration.

Q4: What is the difference between restrictive and obstructive lung diseases?

A: Restrictive lung diseases are characterized by reduced lung compliance (difficulty expanding the lungs), while obstructive lung diseases are characterized by increased airway resistance (difficulty expelling air).

IX. Conclusion: The Breath of Life

This comprehensive review of respiratory system physiology has explored the intricate mechanisms that enable gas exchange, a fundamental process supporting life. From the mechanics of breathing to the complex regulatory pathways, a thorough understanding of these processes is vital for healthcare professionals and students alike. This knowledge is crucial for diagnosing and managing a wide range of respiratory disorders and promoting respiratory health. Further exploration into specific areas and clinical applications will build upon this foundation, allowing for a deeper understanding of the breath of life.