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Learning Objectives

• Major Gases in Air: Examine the relative amounts of major gases in the atmosphere, noting that the composition comprises approximately 21% oxygen (O2), 78% nitrogen (N2), and 1% other gases like carbon dioxide (CO2) and argon (Ar).

• Partial Pressure: Define and calculate partial pressure, demonstrating how it varies with altitude and gas composition using examples such as PO2 = 0.21 x 760 mmHg = 160 mmHg at sea level.

• Oxygen Acquisition: Explain various methods by which animals obtain oxygen, including inhalation in mammals, diffusion across respiratory surfaces in invertebrates, and specialized systems like gills and tracheae.

• Countercurrent Exchange: Diagram and explain the countercurrent exchange mechanism in gill ventilation, highlighting its efficiency in increasing oxygen uptake in aquatic species.

• Mammalian Respiratory System: Identify components (such as bronchi, bronchioles, and alveoli) and explain lung structure, including the alveolar design which facilitates gas exchange through thin, one-cell-thick walls.

• Ventilation Process: Outline the ventilation mechanism in mammals, distinguishing between negative pressure systems (like in humans) and positive pressure systems (like in amphibians).

• Respiratory Pigments: Describe the characteristics and differences among respiratory pigments, such as hemoglobin (contains iron) and hemocyanin (contains copper), including their roles in enhancing oxygen transport in the blood.

Gas Exchange

• Definition: Gas exchange refers to the movement of CO2 and O2 in opposite directions between the environment, bodily fluids, and cells, which is vital for cellular respiration.

• Respiratory System: All structures involved in the gas exchange process such as lungs, gills, and tracheae across different animal groups.

Physical Properties of Gases

• Composition of Air: Air is primarily composed of 21% oxygen and 78% nitrogen, with trace amounts of other gases, including carbon dioxide, which is critical for photosynthesis and respiration.

• Diffusion Dynamics: Gas exchange is driven by partial pressure gradients, with gases moving from high-pressure regions to low-pressure regions, a principle that illustrates why oxygen diffusion is less efficient at high altitudes due to lower atmospheric pressure.

Types of Respiratory Systems

• Mechanisms for Gas Exchange: Gas exchange occurs across various structures including body surfaces for some invertebrates, gills in aquatic organisms, tracheae in insects, and lungs in terrestrial animals.

• Ventilation Role: This is the process of bringing O2-rich water or air into direct contact with gas-exchange organs, essential for efficient respiration.

Adaptations for Gas Exchange

• Common Features of Respiratory Organs: Effective respiratory systems share traits such as:

  • Moist surfaces that facilitate gas diffusion.

  • High surface area relative to volume for maximum gas exchange efficiency.

  • Extensive blood circulation systems that ensure rapid transport of gases.

  • Thin, delicate structures to minimize diffusion distance.

• Water-Breathing vs. Air-Breathing:

  • Aquatic animals contend with challenges like lower oxygen availability and temperature fluctuations leading to varying oxygen levels, necessitating energy expenditure to move water across respiratory surfaces.

  • Terrestrial animals must manage desiccation risks that can affect their respiratory membranes, leading to adaptations like moist internal surfaces or specialized structures like lungs.

Gas Exchange Across Body Surface

• Invertebrate Respiration: Some invertebrates use diffusion across their thin body surfaces, which enables gas exchange without specialized respiratory organs, demonstrating adaptability in simpler organisms.

Gills

• Types of Gills:

  • External Gills: Covered extensions from the body surface, prominent in invertebrates and larval amphibians. They possess a large surface area for effective gas absorption.

  • Internal Gills: Found in fish, these are protected by an operculum and consist of gill arches and filaments with lamellae to increase surface area.

• Countercurrent Exchange System: This mechanism maximizes oxygen diffusion efficiency by allowing water and blood to flow in opposite directions across gill surfaces.

• Limitations of External Gills: Although they provide increased surface area, external gills are vulnerable to damage and require significant energy to maintain water movement, potentially attracting predators.

Tracheal Systems

• Structure: Tracheal systems consist of a network of tubes (spiracles leading to tracheae) that branch into tracheoles close to body cells, allowing direct oxygen delivery to tissues.

• Functionality and Efficiency: These systems enable high metabolic rates, particularly in insects, with air moving in and out through muscle contractions.

Lungs

• Internal Structure: Lungs are paired organs designed to receive deoxygenated blood and return it as oxygenated blood through extensive networks of alveoli.

• Ventilation Mechanisms:

  • Negative pressure mechanisms in reptiles, birds, and mammals utilize intercostal muscles and the diaphragm to change lung volume and draw air in.

  • Positive pressure ventilation in amphibians involves creating pressure gradients to push air into their lungs.

• Tidal Ventilation: This refers to the inefficient mechanism where air enters and exits through the same path, leading to residual air in the lungs. However, birds possess a unique lung structure that utilizes both tidal and flow-through ventilation systems, ensuring efficient gas exchange as their lungs never fully deflate.

Structural Components of Mammalian Respiratory System

• Pathway: Air enters through the nose/mouth, travels through the pharynx, larynx, and trachea, then branches into bronchi and bronchioles terminating in alveoli.

• Alveolar Structure: Alveoli are composed of thin walls (one-cell thick) that facilitate gas exchange; type I cells enable gas diffusion while type II cells secrete surfactant to reduce surface tension and maintain alveolar integrity.

Surfactant Function

• Role: Produced by type II cells, surfactant forms a stable barrier that reduces surface tension and prevents alveolar collapse, which is critical during exhalation and can be a lifesaver in infants who experience respiratory distress syndrome (RDS) due to surfactant deficiency.

Mechanisms of Oxygen Transport in Blood

• Limitations without Pigments: Minimal oxygen dissolves in body fluids for metabolism, necessitating the presence of respiratory pigments.

• Respiratory Pigments: Hemoglobin and hemocyanin enhance oxygen availability, employing reversible binding to effectively transport oxygen through the bloodstream.

Control of Ventilation

• Regulation: Ventilation is regulated by respiratory centers in the brainstem that modulate diaphragm and intercostal muscle activity based on blood pH and gas levels.

• Factors Affecting Breath Rate: Breath rate increases with physical exercise, lung stretching, and a decrease in O2 levels, while it decreases during sleep or under the influence of certain drugs.

Chemoreceptors and Their Role

• Location: Chemoreceptors are located in the aorta, carotid arteries, and brainstem, playing a key role in monitoring blood pH and gas concentrations.

• Function: They relay critical information to the brain to finely regulate breathing rates in response to fluctuations in CO2 and O2 levels.

Adaptations to Extreme Conditions

• High Altitudes: Physiological adaptations include increased hemoglobin affinity for oxygen, larger lung volume, and heightened red blood cell count to enhance oxygen uptake.

• Extended Diving: Diver species may exhibit enhanced oxygen transport capabilities through elevated myoglobin levels in muscles and larger blood volumes.

• Erythropoiesis: The process of erythrocyte formation occurs in red bone marrow, driven by erythropoietin in response to tissue hypoxia, typically completing in about 7 days.

Impact on Public Health

• Asthma: A chronic condition characterized by hyperexcitable bronchial muscles; can be triggered by exercise, exposure to cold air, or allergens, managed with bronchodilator aerosols to ease breathing difficulties.

• Smoking and Emphysema: Smoking is a leading cause of lung cancer and emphysema, contributing to lung damage and diminished oxygenation capability, with treatment options including oxygen therapy for affected patients.

• COVID-19 Effects on the Body: The SARS-CoV-2 virus adversely impacts pulmonary, vascular, and cardiac systems by disrupting membrane protein functions, which can lead to conditions such as pneumonia, vascular complications, and heart failure.