Gas Exchange

Gas Exchange in Vertebrates

Outline of Circulation and Gas Exchange

  • Introduction to gas exchange

  • Vertebrate circulatory system

  • Mammalian circulation

  • Gas exchange

  • Overview of the vertebrate respiratory system

Introduction to Gas Exchange

  • Purpose: The primary role of gas exchange is to supply O2 for cellular respiration and dispose of CO2.

  • Mechanism: Gas exchange occurs across specialized respiratory surfaces, involving the following:

    • Presentation of a large, thin surface area to the environment

    • Diffusion Process: Gas exchange across these surfaces occurs via diffusion, wherein gases move from areas of higher partial pressure to areas of lower partial pressure, facilitating the entry of O2 and the elimination of CO2, maintaining a steep partial-pressure gradient.

Partial Pressure Gradients in Gas Exchange

  • Definition of Partial Pressure: The pressure exerted by a particular gas in a mixture of gases, applicable to gases dissolved in liquids (e.g., water).

  • Diffusion Dynamics: Gases will undergo net diffusion from regions of higher partial pressure to regions of lower partial pressure, influencing gas exchange efficiency.

Respiratory Media

  • Source Options: Animals may utilize either air or water as sources for O2 (respiratory medium).

  • Comparison of O2 Levels: In any given volume, water holds significantly less O2 than air, making the extraction of O2 from water more energy-intensive than from air.

  • Variability of Respiratory Surfaces: The respiratory systems correspond to the habitat and physiology of different animals, including:

    • Skin

    • Gills

    • Tracheae

    • Lungs

Gills in Aquatic Animals

  • Structure of Gills: Gills are outfoldings of the body that enhance the surface area for gas exchange.

  • Ventilation Mechanism: Ventilation moves the respiratory medium (water) over the respiratory surface, and aquatic animals either:

    • Move through water, or

    • Pump water over their gills (e.g., crayfish gills).

Fish Gills

  • Countercurrent Exchange System:

    • Blood flows in the opposite direction to the water passing over the gills.

    • This arrangement ensures that the blood remains less saturated with O2 than the water it encounters, optimizing the exchange of gases.

Tracheal Systems in Insects

  • Network Structure: Comprising a network of branching tubes throughout the body, supplying O2 directly to tissues.

  • Separation of Systems: The respiratory and circulatory systems operate independently in insects.

Lungs in Terrestrial Vertebrates

  • Basic Structure: Lungs are infoldings of the body surface that serve as the primary organs for gas exchange.

  • Role of Circulatory System: The circulatory system is responsible for transporting gases between the lungs and other body regions.

  • Correlation with Metabolic Rate: The size and complexity of lungs in different species correlate with the animal's metabolic rate, allowing for adequate gas exchange.

How Amphibians Breathe

  • Breathing Process: Ventilation of their lungs involves alternative inhalation and exhalation of air (called positive pressure breathing), which forces air down the trachea, inflating the lungs effectively.

Mammalian Respiratory Systems

  • Air Passage: Air inhaled through the nostrils travels through:

    • Pharynx

    • Larynx

    • Trachea

    • Bronchi

    • Bronchioles

    • Alveoli (site of gas exchange)

  • Gas Exchange in Alveoli:

    • O2 diffuses through the moist film of the epithelium into blood capillaries.

    • CO2 moves from capillaries across the epithelium into the alveolar air space.

  • Surfactants: Secretions known as surfactants coat the alveolar surface, reducing surface tension and preventing collapse.

Mammalian Breathing Mechanism

  • Type: Mammals ventilate their lungs via negative pressure breathing.

  • Lung Mechanics:

    • Lung volume increases when rib muscles and diaphragm contract.

    • Tidal Volume: Refers to the volume of air inhaled and exhaled with each breath.

    • Vital Capacity: The maximum amount of air that can be expelled after maximum inhalation.

    • Residual Volume: The air that remains in the lungs after exhalation, cannot be expelled.

How Birds Breathe

  • Structure of Respiratory System: Birds possess eight or nine air sacs.

  • Airflow Dynamics:

    • Air moves through the lungs in a unidirectional flow, ensuring complete renewal of air during exhalation.

    • Gas Exchange: Takes place in parabronchi (parallel tubes), with inhalation inflating both sets of air sacs, and exhalation deflating them, thus completely renewing lung air.

Control of Breathing in Humans

  • Regulatory Centers: Located in the medulla oblongata of the brain, controlling the rate and depth of breathing.

  • Response Mechanisms:

    • The centers respond to pH changes in cerebrospinal fluid, adjusting ventilation to meet metabolic requirements.

Coordination of Circulation and Gas Exchange

  • In the Lungs: Oxygen diffuses into the blood, and carbon dioxide diffuses from the blood into the alveolar space.

  • In Tissue Capillaries: Partial pressure gradients favor the diffusion of O2 into interstitial fluids and CO2 into the blood.

  • Blood Characteristics: Blood entering the lungs has low partial pressure of O2 and high partial pressure of CO2 relative to alveolar air.

Respiratory Pigments

  • Function: Proteins that transport oxygen and significantly enhance the capacity for oxygen transport in blood.

  • Types:

    • Hemocyanin: Found in arthropods and mollusks, utilizes copper as the oxygen-binding component.

    • Hemoglobin: Present in most vertebrates and some invertebrates, using iron as the oxygen-binding component within erythrocytes.

Hemoglobin Dynamics

  • Oxygen Capacity: A single hemoglobin molecule can transport four O2 molecules, each linked to an iron-containing heme group.

  • Hemoglobin Dissociation Curve: Illustrates that small changes in the partial pressure of oxygen can lead to significant changes in the amount of O2 delivered by hemoglobin.

  • Bohr Shift: CO2 produced during cellular respiration lowers blood pH, decreasing hemoglobin's affinity for O2, facilitating better oxygen delivery where needed.

Carbon Dioxide Transport in Body Tissues

  • Transport Methods: CO2 from active cells diffuses into blood and is transported by three main pathways:

    • As dissolved gas in blood plasma

    • Bound to hemoglobin

    • As bicarbonate ions (HCO3–)

Carbon Dioxide Transport in Lungs

  • Diffusion and Conversion: Most HCO3– diffuses into plasma, traveling toward the lungs!

  • Conversion Process: In the lungs, HCO3– enters red blood cells, combines with H+, forming carbonic acid, which then converts to CO2 and water. CO2 diffuses into plasma, interstitial fluid, and alveolar spaces for exhalation.

Key Concepts in Gas Exchange

  1. Bird Lung Function: In bird lungs, air flows unidirectionally through parabronchi.

  2. Tidal Ventilation: Non-bird terrestrial vertebrates employ tidal ventilation, where inhaled air and exhaled gases flow in the same path, making it less efficient than unidirectional flow.

  3. Oxygen Binding to Hemoglobin: Oxygen binds reversibly to hemoglobin in red blood cells, with each hemoglobin molecule capable of carrying a maximum of four oxygen molecules, influenced by the partial pressure through positive cooperativity.

  4. Carbon Dioxide Transport: The dominant method of carbon dioxide transport in the blood occurs as bicarbonate ions (HCO3–).