Circulation and Gas Exchange

Introduction

  • Source: Campbell Biology in Focus, Fourth Edition, Chapter 34 "Circulation and Gas Exchange" Lecture Presentations.

  • Presenters: Kathleen Fitzpatrick (Simon Fraser University) & Nicole Tunbridge (Kwantlen Polytechnic University).

  • Copyright: Pearson Education, Inc. (Copyright 2025, 2020, 2016).

Concept 34.5: Gas Exchange Occurs Across Specialized Respiratory Surfaces

  • Definition: Gas exchange involves the uptake of oxygen (O₂) from the environment and the discharge of carbon dioxide (CO₂) to the environment.

Partial Pressure Gradients in Gas Exchange

General Concepts

  • Partial Pressure: The pressure exerted by a specific gas in a mixture of gases.

  • Example of Atmospheric Composition:

    • The atmosphere consists of approximately 21% oxygen by volume.

    • The partial pressure of oxygen can be calculated based on atmospheric pressure.

Application to Liquids

  • Dissolved Gases: Partial pressures also apply to gases that are dissolved in liquids, such as water.

  • Equilibrium in Water and Air: When water is exposed to air, an equilibrium is achieved where the partial pressures of each gas are equal in both water and air.

Respiratory Surfaces

  • Mechanism of Exchange: Gas exchange occurs via diffusion across respiratory surfaces.

  • Characteristics of Respiratory Surfaces:

    • Large surface area.

    • Thin structure.

    • Must remain moist to facilitate gas exchange.

  • Variability Across Species: Different animals have adapted varying types of respiratory surfaces, including:

    • Gills

    • Tracheaem

    • Lungs

Lungs

  • Structure: Lungs are infoldings of the body surface, divided into multiple pockets.

  • Functionality: The circulatory system can be either open or closed to transport gases between the lungs and the rest of the body.

  • Adaptations in Vertebrates: The use of lungs for gas exchange varies in vertebrates that do not possess gills.

Mammalian Respiratory Systems: A Closer Look

Overview of the Air Pathway (1 of 4)

  • Air Conduction System: A branching system of ducts directs air into the lungs.

  • Nasal Functions: Air inhaled through nostrils is filtered by hairs, warmed, humidified, and sampled for odors.

  • Pharynx Role: The pharynx serves as an intersection between the air and food paths.

  • Laryngeal Function: Swallowing causes the larynx to move upward, which tips a flap of cartilage over the trachea.

Air Pathway Continuation (2 of 4)

  • Path through Airways: After the larynx, air passes into the trachea, bronchi, and bronchioles.

  • Sound Production: Exhaled air passes over vocal cords in the larynx, which allows the creation of sound.

  • Mucus and Cilia Function: The epithelium of air ducts is lined with cilia and mucus that transport particles upward to the pharynx (known as the "mucus escalator"). This mechanism cleans the respiratory system and facilitates swallowing into the esophagus.

Gas Exchange Mechanisms (3 of 4)

  • Site of Gas Exchange: Gas exchange occurs in the alveoli, tiny air sacs located at the tips of the smallest bronchioles.

  • Oxygen Diffusion: Oxygen diffuses through the moist epithelium of the alveoli and enters capillaries.

  • Carbon Dioxide Diffusion: Conversely, carbon dioxide diffuses from capillaries across the epithelium back into the air space of the alveoli.

Alveoli Characteristics (4 of 4)

  • Cilia Absence: Alveoli do not have cilia, making them susceptible to contamination.

  • Defense Mechanism: White blood cells patrol alveoli, engulfing foreign particles to protect the lungs.

  • Surfactants: Secretions known as surfactants coat alveolar surfaces. Preterm infants may lack surfactant, leading to respiratory distress syndrome, which is treated with artificial surfactants.

How a Mammal Breathes

Mechanism of Breathing (1 of 2)

  • Negative Pressure Breathing: Mammals ventilate their lungs by creating negative pressure in the thoracic cavity, which draws air into the lungs.

  • Volume Changes: Lung volume increases as rib muscles and the diaphragm contract.

  • Tidal Volume: The tidal volume represents the volume of air inhaled with each individual breath.

Breathing Dynamics (2 of 2)

  • Vital Capacity: The maximum tidal volume is referred to as vital capacity.

  • Residual Volume: After exhalation, a residual volume remains in the lungs.

  • Air Mixing: Each inhalation mixes fresh air with oxygen-depleted residual air, resulting in a significantly lower maximum oxygen concentration in the alveoli compared to atmospheric levels.

Control of Breathing in Humans

Breathing Regulation (1 of 2)

  • Neural Control: The neurons in the medulla oblongata primarily regulate breathing, situated near the base of the brain.

  • pH Response: The medulla adjusts both the rate and depth of breathing in response to changes in pH levels of cerebrospinal fluid.

  • Metabolic Demand Matching: Breathing rates and depths are adapted to meet metabolic requirements.

Blood and Breathing (2 of 2)

  • Sensor Locations: Sensors monitoring oxygen and carbon dioxide concentrations are located in the aorta and carotid arteries.

  • Matching Ventilation to Blood Flow: The ventilation rate is adjusted to align with blood flow through alveolar capillaries.

  • Negative Feedback Mechanisms: A negative feedback mechanism is in place to prevent overexpansion of the lungs during ventilation.

Coordination of Circulation and Gas Exchange

  • Air Mixing Dynamics: During inhalation, fresh air mixes with remaining air in the lungs.

  • Partial Pressure Variance: The resultant mixture in alveoli has a higher oxygen concentration than that in the blood flowing through capillaries:

    • O₂ diffuses into the blood.

    • CO₂ diffuses into the alveolar space.

  • Systemic Capillary Exchange: In systemic capillaries, partial pressure gradients facilitate the diffusion of CO₂ into interstitial fluids and O₂ into the blood.

Respiratory Pigments

Function and Importance (1 of 4)

  • Gas Transport Enhancement: Animals typically transport the majority of their O₂ bound to proteins known as respiratory pigments.

  • Increased Transport Capacity: These pigments circulate in blood or hemolymph, greatly increasing the amount of oxygen that can be transported efficiently.

  • Evolutionary Diversity: Various respiratory pigments have evolved across animal species.

  • Metal-Protein Structure: Most respiratory pigments consist of a metal ion bound to a protein framework.

Hemoglobin (2 of 4)

  • General Information: Hemoglobin is the respiratory pigment found in nearly all vertebrates and many invertebrates.

  • Composition: Comprising four polypeptide chains, each associated with an iron-containing heme group.

  • Carrying Capacity: A single hemoglobin molecule can transport four molecules of O₂, one per heme group.

  • Reversible Binding: Hemoglobin binds oxygen reversibly, loading at gills/lungs and releasing it at other body areas.

Cooperative Binding (3 of 4)

  • Cooperativity Feature: Hemoglobin exhibits cooperative binding properties.

  • Increased Affinity Upon Binding: When O₂ binds to one subunit, it causes conformational changes in other subunits, thereby increasing affinity for additional O₂ binding.

  • Ease of Release: Release of O₂ by one subunit facilitates release by others, a phenomenon illustrated by the dissociation curve for hemoglobin.

pH Influence (4 of 4)

  • Bohr Shift Concept: CO₂ generated during cellular respiration lowers blood pH, decreasing hemoglobin's O₂ affinity, termed the Bohr effect.

  • Additional Roles: Hemoglobin also plays a minor role in CO₂ transport and is involved in stabilizing blood pH against drastic changes.

Carbon Dioxide Transport

  • Transport Mechanism: Most CO₂ from respiring cells diffuses from plasma into erythrocytes and subsequently reacts with water to form carbonic acid (H₂CO₃).

  • Dissociation: Carbonic acid readily dissociates into bicarbonate ions (HCO₃⁻) and protons (H⁺).

  • Plasma Transport: The majority of this CO₂ diffuses back into the plasma for transport to the lungs.

  • Alveolar Exchange: In the lungs, CO₂ diffuses out of the blood and into alveoli for exhalation.

Respiratory Adaptations of Diving Mammals

  • Oxygen Storage Abilities: Deep-diving mammals can store significant amounts of O₂ in their muscles, mainly via myoglobin proteins.

  • Oxygen Conservation Strategies: These mammals exhibit several strategies to conserve oxygen:

    • Swimming efficiently with minimal muscular effort.

    • Gliding passively to reduce energy expenditure.

    • Prioritizing blood flow to vital organs, limiting respiratory function to non-essential muscles.

    • Utilizing ATP production through fermentation when oxygen levels are low.

Conclusion

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