Lecture 15 - Breathing and Gas Exchange

Lecture 15 - Breathing and Gas Exchange I. Introduction to Breathing • Animals breathe to obtain oxygen (O2 ) for cellular respiration and to expel carbon dioxide (CO2 ). • Key Concept: – Oxygen is required for ATP production in mitochondria. – Carbon dioxide (CO2 ) is a byproduct of the Krebs cycle and must be removed to prevent acidification of tissues. • Breathing involves both external respiration (exchange between environment and lungs/gills) and internal respiration (exchange between blood and tissues). II. Differences Between Air and Water Breathers • Comparison of Oxygen Availability: – Air has approximately 21% oxygen content, while water typically has only 0.7% dissolved oxygen. – Oxygen solubility in water decreases with increasing temperature and salinity. • Energy Expenditure: – Water breathers (e.g., fish) must move large volumes of water across gills, leading to high energy costs. – Air breathers (e.g., mammals, birds) use less energy for ventilation due to lower density and viscosity of air. • Evolutionary Adaptations: – Fish developed highly efficient gills with a counter-current exchange mechanism. – Mammals evolved alveoli for increased surface area and efficient gas exchange. 1 III. General Respiratory Structures • Cutaneous Gas Exchange: – Occurs in amphibians and some reptiles. – Limited by surface area and skin permeability. • Lungs, Gills, and Tracheal Systems: – Lungs (e.g., mammals, birds): Composed of alveoli for high surface area. – Gills (e.g., fish): Utilize counter-current flow to maximize oxygen extraction. – Tracheal System (insects): Gases are transported directly to tissues through spiracles. IV. Animal Ventilation Patterns • Passive Ventilation: – Utilized by smaller aquatic animals that rely on water currents. • Active Ventilation: – Requires muscular effort to move air or water over respiratory surfaces. • Types of Active Ventilation: – Unidirectional (e.g., birds, fish): Continuous flow in one direction. – Tidal (e.g., mammals): Air moves in and out through the same pathway. V. Gas Exchange Mechanisms • Tidal Exchange (Mammals): – Airflow is bidirectional, reducing overall efficiency. • Cross-Current Exchange (Birds): – Air flows perpendicularly to blood flow, optimizing oxygen uptake. • Counter-Current Exchange (Fish): – Water and blood flow in opposite directions, creating a steep oxygen gradient. 2 VI. Factors Influencing Gas Exchange • Surface Area: Larger surface areas facilitate more efficient gas exchange. • Diffusion Distance: Shorter distances between air/water and blood vessels increase efficiency. • Partial Pressure of Oxygen (PO2 ): High PO2 levels favor oxygen binding to hemoglobin. • Environmental Conditions: Temperature, pH, and humidity affect gas solubility and diffusion. VII. Respiratory Pigments • Hemoglobin (Hb): – Has a sigmoidal oxygen dissociation curve due to cooperative binding. – Bohr Effect: Oxygen affinity decreases with lower pH and higher carbon dioxide (CO2 ) levels. • Myoglobin (Mb): Stores oxygen in muscle tissues. • Hemocyanins (invertebrates): Use copper instead of iron for oxygen binding. VIII. Circulatory Systems • Open vs. Closed Systems: – Open systems: Hemolymph bathes tissues directly. – Closed systems: Blood is confined to vessels, allowing higher pressure. • Heart Structures: – Fish: 2 chambers (1 atrium, 1 ventricle). – Amphibians: 3 chambers (2 atria, 1 ventricle). – Mammals/Birds: 4 chambers (2 atria, 2 ventricles). 3