Gas Exchange in Biology

Gas Exchange Module 5

Concept 42.5: Gas Exchange Occurs Across Specialized Respiratory Surfaces

• Definition of Gas Exchange:

  • Every organism must exchange substances with the environment.

  • Gas exchange specifically refers to the uptake of O2 from the environment and the discharge of CO2.

Partial Pressure Gradients in Gas Exchange (Slide 3)

• Driving Force:

  • Exchange relies on partial pressure gradients which act as a driving force for gas movement.

  • Different organisms consider different mediums for gas exchange:

  • Humans = air

  • Other organisms = water.

• Partial Pressure:

  • Each gas in a mixture exerts its own pressure independent of other gases.

  • Allows us to focus on necessary gases like O2.

  • Example at sea level:

  • Total pressure = 760 mmHg.

  • O2 concentration = 21% of this = $0.21 imes 760 ext{ mmHg} = 159.6 ext{ mmHg } ext{ (approx. 160 mmHg)}$.

• O2 Concentration Comparison:

  • Air O2 concentration = 210 mL/L.

  • Water O2 concentration = 7 mL/L.

  • Implication:

  • Oxygen is more available in air, making it easier to acquire compared to water, where aquatic organisms need more efficient systems for respiration.

• Density and Viscosity:

  • Water is more dense and viscous than air, which complicates gas exchange.

Respiratory Surfaces in Aquatic Organisms (Slide 4)

• Specialized Respiratory Surfaces:

  • Aquatic organisms have surfaces optimized for gas exchange.

  • Types of Gills:

  • External Gills:

    • Found in organisms like axolotls and marine worms.

    • Increase surface area to maximize O2 absorption.

  • Internal Gills:

    • Found in crayfish and fish, providing protection and improving body movement in water.

• Ventilation Methods:

  • Ventilation mechanisms vary among aquatic organisms.

  • Drawbacks of external gills include:

  • Susceptibility to damage

  • High energy expenditure to maintain ventilation.

Countercurrent Exchange (Slides 5 and 6)

• Mechanism in Bony Fish:

  • Countercurrent exchange maximizes O2 extraction.

  • Water moves from the mouth across gills and exits through the operculum (gill cover).

• Gill Structure:

  • Gill arches hold gill filaments lined with lamellae.

  • Blood flow within the lamellae is countercurrent to water flow, ensuring a continuous partial pressure gradient.

Blood and Water Dynamics:

• Oxygen Diffusion:

  • O2 always diffuses from water to blood due to lower saturation in blood compared to water.

  • **Mechanisms for Water Movement:

  1. Actively drawing water through mouth.

  2. Swimming open-mouthed.

  3. Positioning into current when at rest.

  • Blood Flow:

  • Afferent vessels carry deoxygenated blood to gills.

  • Efferent vessels deliver oxygenated blood to the body.

Tracheal System in Insects (Slide 7)

• Structure:

  • Insects have external openings (spiracles) leading to tracheae, which then branch into smaller tubules (tracheoles).

• Function:

  • Tracheoles contact almost every body cell, allowing diffusion of O2 and CO2.

  • Muscular movement adjusts ventilation according to activity level.

The Mammalian Respiratory System - Part 1 (Slide 8)

• Components:

  • Nasal cavity, mouth, branching tubes, lungs, muscles, connective tissue.

• Air Pathway:

  • Air enters through nose/mouth, gets warmed and humidified via mucus and hair filters.

  • Travels through the pharynx to the larynx, then to the trachea (supported by cartilage).

• Bronchial Structure:

  • Trachea branches into bronchi, then into bronchioles (with smooth muscle).

The Mammalian Respiratory System (Part 2)

• Two Major Zones:

  • Conducting Zone:

  • Comprises all passages until terminal bronchioles where gas exchange does not occur.

  • Respiratory Zone:

  • Comprises alveolar sacs and alveoli, which are sites of gas exchange.

  • Surrounded by pulmonary capillaries to facilitate efficient gas transfer.

Surfactants (Slide 10)

• Function:

  • Maintain moisture in alveoli to prevent surface tension collapse.

  • Surfactants are amphiphilic proteins that disrupt hydrogen bonds between water molecules, reducing surface tension.

• Importance:

  • Link to embryonic development of infants;

  • Premature infants often lack surfactants leading to Infant Respiratory Distress Syndromes (IRDS).

  • Treatment includes positive-pressure ventilation and surfactant administration as an aerosol.

Module 6 - Concept 42.6 - Breathing Ventilates the Lungs

Amphibian Breathing (Slide 14)

• Lung Structure:

  • Formed from sac-like pouches. Air moves through the oral cavity and is controlled by the glottis via positive pressure breathing (similar to mouth-to-mouth resuscitation).

Bird Breathing (Slide 15)

• Respiratory Cycle:

  • Involves two cycles: inhalation/exhalation and an efficient structure of posterior and anterior air sacs to maximize oxygen extraction.

  • Aids in respiration even at high altitudes.

Mammal Breathing (Slide 16)

• Lung Mechanics:

  • Involves negative pressure breathing.

  • Key muscles: diaphragm (downwards expansion) and intercostal muscles (rib cage movement).

Pulmonary Ventilation (Slides 17 and 18)

• Mechanical Process:

  • Driven by muscular contraction and thoracic cavity size change, creating pressure gradients for air movement.

Pulmonary Volumes and Capacities

• Types of Volumes:

  • Tidal Volume: Amount of air per breath.

  • Inspiratory Reserve Volume: Additional air beyond tidal volume.

  • Expiratory Reserve Volume: Air expelled beyond tidal volume.

  • Residual Volume: Air remaining in lungs post-exhalation.

• Types of Capacities:

  • Vital Capacity: Max air inhaled/exhaled (
    $VC = VT + IRV + ERV$).

  • Inspiratory Capacity: Max air after normal expiration ($IC = VT + IRV$).

  • Functional Residual Capacity: Air remaining after expiration ($FRC = ERV + RV$).

  • Total Lung Capacity: Total air lungs can contain ($TLC = VC + RV$).

Control of Breathing in Humans (Slide 20)

• Nervous System Regulation:

  • Involves respiratory centers in the medulla oblongata which respond to pH changes from CO2 levels.

  • Chemoreceptors:

  • Central (in brain) respond to cerebrospinal fluid pH.

  • Peripheral (in carotid/aorta) respond to blood pH.

• Breathing Response:

  • If blood pH drops due to high CO2, signals promote increased breathing rate and depth to expel CO2 and restore pH.

Module 7 - Concept 42.7 - Adaptations for Gas Exchange Include Pigments that Bind and Transport Gasses (Slides 22-28)

• Coordination of Circulation and Gas Exchange:

  • O2 moves from alveoli into the alveolar capillaries where it binds to hemoglobin for systemic circulation.

  • CO2 generated from cellular respiration diffuses into capillaries, forming bicarbonate ions which transport CO2.

Respiratory Pigments: Hemoglobin (Slide 25)

• Structure:

  • Hemoglobin: A polypeptide with a quaternary structure, contains heme groups that bind to O2. Maximum binding capacity = 4 O2 molecules.

Hemoglobin Dissociation Curves (Slide 26)

• Loading and Unloading:

  • Oxyhemoglobin (when O2 binds) and Deoxyhemoglobin (when O2 releases).

  • Cooperative binding means higher saturation with more O2 present.

Factors Influencing Oxygen Unloading (Slide 27)

• Factors include temperature (warmer tissues lead to increased unloading) and pH (lower pH leads to more O2 unloading due to the Bohr effect).

Systemic CO2 Transport (Slide 28)

• CO2 transport methods include dissolved in plasma, chemically bound to hemoglobin, or as bicarbonate ions. The enzyme carbonic anhydrase accelerates CO2 to bicarbonate conversion.

Pulmonary CO2 Transport (Slide 29)

• Conversion back to CO2 happens in the lungs, facilitated again by carbonic anhydrase.

• The Haldane effect emphasizes how reduced hemoglobin affinity for O2 enhances CO2 transport from tissues to lungs.

Respiratory Adaptations of Diving Mammals (Slide 30)

• Diving mammals store oxygen in myoglobin, allowing extended time underwater. Myoglobin has a single chain structure for high oxygen binding. They may also modify buoyancy and reroute blood to conserve oxygen, or utilize fermentation if oxygen depletes.