Review Sheet Respiratory System Physiology

Respiratory System Physiology: A Comprehensive Review

Understanding respiratory system physiology is crucial for comprehending how our bodies obtain the oxygen necessary for survival and eliminate the waste product, carbon dioxide. This review sheet delves into the intricate mechanisms involved, covering key aspects from pulmonary ventilation to gas exchange and the regulatory control of breathing. We'll explore the anatomy involved, the mechanics of breathing, and the underlying physiological principles that govern this vital process. This comprehensive guide aims to provide a solid foundation for students and anyone interested in learning more about the respiratory system.

I. Introduction: The Respiratory System's Vital Role

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. This exchange, the uptake of oxygen (O₂) and the release of carbon dioxide (CO₂), is essential for cellular respiration, the process that provides energy for our bodies. The system encompasses the upper respiratory tract (nose, pharynx, larynx) and the lower respiratory tract (trachea, bronchi, bronchioles, alveoli, and lungs). Each component plays a specific role in ensuring efficient gas exchange and maintaining homeostasis.

II. Pulmonary Ventilation: The Mechanics of Breathing

Pulmonary ventilation, or breathing, involves two main phases: inspiration (inhalation) and expiration (exhalation). These processes are driven by pressure differences between the lungs and the atmosphere.

A. Inspiration:

• The diaphragm, a major respiratory muscle, contracts and flattens, increasing the vertical dimension of the thoracic cavity.
• The external intercostal muscles contract, raising the ribs and expanding the chest cavity laterally.
• These actions increase the volume of the thoracic cavity, decreasing the intrapulmonary pressure (pressure within the lungs).
• Air rushes into the lungs from the atmosphere, down its pressure gradient, until intrapulmonary pressure equals atmospheric pressure.

B. Expiration:

• During normal, quiet expiration, the diaphragm and external intercostal muscles relax.
• The elastic recoil of the lungs and thoracic wall causes the chest cavity to decrease in volume.
• This decrease in volume increases intrapulmonary pressure above atmospheric pressure.
• Air is passively expelled from the lungs until intrapulmonary pressure equals atmospheric pressure.

C. Forced Breathing:

• During strenuous activity or when breathing is compromised, accessory muscles are recruited to assist in both inspiration and expiration.
• Accessory muscles of inspiration include sternocleidomastoid, scalenes, and pectoralis minor.
• Accessory muscles of expiration include abdominal muscles (rectus abdominis, obliques, transversus abdominis) and internal intercostal muscles.

III. Gas Exchange: Alveolar and Capillary Interaction

Gas exchange occurs primarily in the alveoli, tiny air sacs within the lungs. The alveoli are surrounded by a dense network of pulmonary capillaries. This close proximity allows for efficient diffusion of gases across the respiratory membrane, a thin barrier consisting of the alveolar epithelium, interstitial space, and capillary endothelium.

A. Partial Pressures and Diffusion:

• Gases move down their partial pressure gradients. The partial pressure of a gas is the pressure exerted by that gas in a mixture of gases.
• In the alveoli, the partial pressure of oxygen (PO₂) is higher than in the pulmonary capillaries, driving oxygen diffusion into the blood.
• Simultaneously, the partial pressure of carbon dioxide (PCO₂) is higher in the pulmonary capillaries than in the alveoli, driving carbon dioxide diffusion into the alveoli for exhalation.

B. Oxygen Transport:

• Oxygen is transported in the blood primarily bound to hemoglobin (Hb) in red blood cells. Hemoglobin's affinity for oxygen is influenced by factors such as PO₂, pH, temperature, and 2,3-bisphosphoglycerate (2,3-BPG).
• A small amount of oxygen is dissolved in the plasma.

C. Carbon Dioxide Transport:

• Carbon dioxide is transported in the blood in three ways:
  • Dissolved in plasma (small amount)
  • Bound to hemoglobin (carbaminohemoglobin)
  • As bicarbonate ions (HCO₃⁻), the primary mechanism. This process involves carbonic anhydrase, an enzyme that catalyzes the reversible reaction between CO₂ and water to form carbonic acid (H₂CO₃), which then dissociates into H⁺ and HCO₃⁻.

IV. Respiratory Control: Maintaining Homeostasis

Breathing is a complex, involuntary process regulated by the respiratory centers located in the brainstem (medulla oblongata and pons). These centers receive input from various chemoreceptors and mechanoreceptors, allowing for precise control of ventilation to meet metabolic demands.

A. Chemoreceptors:

• Central chemoreceptors in the medulla oblongata are sensitive to changes in PCO₂ and pH of the cerebrospinal fluid (CSF). Increased PCO₂ (hypercapnia) or decreased pH (acidosis) stimulates ventilation.
• Peripheral chemoreceptors in the carotid and aortic bodies are sensitive to changes in PO₂, PCO₂, and pH of arterial blood. Decreased PO₂ (hypoxia), increased PCO₂, or decreased pH stimulates ventilation.

B. Mechanoreceptors:

• Stretch receptors in the lungs and airways monitor lung inflation. The Hering-Breuer reflex prevents overinflation of the lungs by inhibiting inspiration when the lungs are overly stretched.
• Irritant receptors in the airways respond to irritants (dust, smoke, etc.), triggering bronchoconstriction and coughing.

C. Higher Brain Centers:

• The cerebral cortex exerts voluntary control over breathing, allowing us to consciously alter our breathing patterns (e.g., holding our breath, taking deep breaths).
• Emotional factors can also influence breathing patterns.

V. Lung Volumes and Capacities: Measuring Respiratory Function

Pulmonary function tests (PFTs) measure various lung volumes and capacities to assess respiratory function. These measurements are crucial in diagnosing and monitoring respiratory diseases.

• Tidal Volume (TV): The volume of air inhaled or exhaled in a single breath during quiet breathing.
• Inspiratory Reserve Volume (IRV): The additional volume of air that can be forcefully inhaled after a normal inhalation.
• Expiratory Reserve Volume (ERV): The additional volume of air that can be forcefully exhaled after a normal exhalation.
• Residual Volume (RV): The volume of air remaining in the lungs after a maximal exhalation.
• Inspiratory Capacity (IC): The total volume of air that can be inhaled (TV + IRV).
• Functional Residual Capacity (FRC): The volume of air remaining in the lungs after a normal exhalation (ERV + RV).
• Vital Capacity (VC): The total volume of air that can be forcefully exhaled after a maximal inhalation (TV + IRV + ERV).
• Total Lung Capacity (TLC): The total volume of air that the lungs can hold (TV + IRV + ERV + RV).

VI. Respiratory Diseases: Common Disorders Affecting Function

Numerous diseases can impair respiratory function, affecting various aspects of the respiratory system. Some common examples include:

• Asthma: A chronic inflammatory disorder of the airways characterized by bronchoconstriction, inflammation, and mucus hypersecretion.
• Chronic Obstructive Pulmonary Disease (COPD): A group of progressive lung diseases that obstruct airflow from the lungs, including chronic bronchitis and emphysema.
• Pneumonia: An infection of the lungs that causes inflammation and fluid accumulation in the alveoli.
• Pulmonary Edema: Fluid accumulation in the lungs, often caused by heart failure or other conditions.
• Cystic Fibrosis: A genetic disorder affecting the mucus-producing glands throughout the body, leading to thick mucus buildup in the airways.
• Lung Cancer: Uncontrolled growth of abnormal cells in the lungs.

VII. Alveolar Gas Exchange in Detail: Understanding the Partial Pressures

Let's delve deeper into the dynamics of alveolar gas exchange. Remember that the driving force for gas exchange is the difference in partial pressures between the alveoli and the pulmonary capillaries. Oxygen, with a higher partial pressure in the alveoli (approximately 100 mmHg) moves into the capillaries where the partial pressure is lower (approximately 40 mmHg). Conversely, carbon dioxide, with a higher partial pressure in the capillaries (approximately 45 mmHg) diffuses into the alveoli where the partial pressure is lower (approximately 40 mmHg). This constant movement of gases ensures the continuous supply of oxygen to the body's tissues and the efficient removal of carbon dioxide. The efficiency of this exchange is highly dependent on the surface area of the alveoli, the thickness of the respiratory membrane, and the diffusion capacity of the gases.

VIII. The Role of Hemoglobin in Oxygen Transport: Beyond Simple Binding

Hemoglobin's role extends beyond simple oxygen binding. It's a complex protein with allosteric properties, meaning its affinity for oxygen changes based on various factors. The oxygen-hemoglobin dissociation curve illustrates this relationship. A shift to the right indicates decreased hemoglobin affinity for oxygen (e.g., increased temperature, decreased pH, increased 2,3-BPG), facilitating oxygen unloading to tissues. A shift to the left indicates increased affinity, promoting oxygen loading in the lungs. This intricate control mechanism ensures that oxygen is delivered efficiently to tissues with high metabolic demands.

IX. The Bohr Effect and Haldane Effect: Interplay of Gases and pH

The Bohr effect describes how changes in blood pH affect hemoglobin's affinity for oxygen. A decrease in pH (increased acidity) reduces hemoglobin's affinity, promoting oxygen release to tissues that are metabolically active and producing more acid. The Haldane effect is the converse, describing how the binding of oxygen to hemoglobin affects carbon dioxide transport. Oxygen binding to hemoglobin decreases its ability to bind carbon dioxide, facilitating carbon dioxide release in the lungs. These two effects work in concert to optimize gas exchange and maintain acid-base balance.

X. Respiratory System Disorders and their Impact on Gas Exchange

Many respiratory diseases directly impair gas exchange. For example, in pneumonia, fluid accumulation in the alveoli increases the thickness of the respiratory membrane, hindering diffusion. Emphysema, characterized by the destruction of alveolar walls, reduces the surface area available for gas exchange. Asthma causes bronchoconstriction, limiting airflow and reducing the amount of oxygen reaching the alveoli. Understanding the pathophysiology of these diseases is key to developing effective treatment strategies aimed at restoring or maintaining efficient gas exchange.

XI. Clinical Assessment of Respiratory Function: Beyond Lung Volumes

Clinical assessment of respiratory function involves more than just measuring lung volumes and capacities. It includes evaluating arterial blood gases (ABGs) to determine PO₂, PCO₂, and pH; chest auscultation to listen for abnormal breath sounds; and imaging techniques like chest X-rays and CT scans to visualize lung structures and identify abnormalities. Spirometry is a commonly used PFT that measures airflow and lung volumes, providing valuable information about airway resistance and lung capacity.

XII. Conclusion: A Complex System, Vital for Life

The respiratory system is a marvel of biological engineering, a complex interplay of anatomy, mechanics, and regulation all working together to provide the life-sustaining process of gas exchange. From the mechanics of breathing to the intricate control mechanisms governing ventilation, a thorough understanding of respiratory physiology is paramount for understanding health, disease, and the body's remarkable ability to maintain homeostasis. This review sheet provides a foundation for further exploration into this vital system. Continued study and clinical experience are necessary to fully grasp the complexities and clinical relevance of respiratory physiology.

XIII. Frequently Asked Questions (FAQ)

Q: What is the difference between ventilation and respiration?

A: Ventilation refers to the mechanical movement of air into and out of the lungs. Respiration refers to the gas exchange process, including the uptake of oxygen and release of carbon dioxide at both the lungs (external respiration) and the tissues (internal respiration).

Q: What is the role of surfactant?

A: Surfactant is a lipoprotein complex produced by alveolar type II cells. It reduces surface tension in the alveoli, preventing them from collapsing during expiration and ensuring efficient gas exchange.

Q: How does altitude affect respiratory function?

A: At high altitudes, the partial pressure of oxygen is lower, leading to hypoxia. The body compensates by increasing ventilation, increasing red blood cell production (erythropoiesis), and increasing 2,3-BPG levels.

Q: What are some common signs and symptoms of respiratory distress?

A: Common signs and symptoms include shortness of breath (dyspnea), rapid breathing (tachypnea), wheezing, coughing, chest pain, and cyanosis (bluish discoloration of the skin due to low blood oxygen).

Q: How is respiratory failure diagnosed?

A: Respiratory failure is typically diagnosed through arterial blood gas analysis, chest X-ray, and evaluation of clinical symptoms.

This in-depth review sheet provides a comprehensive overview of respiratory system physiology. Remember to consult relevant textbooks and other learning resources for further study and clarification.