KD

Gas Exchange and Circulation Notes

Gas Exchange and Circulation

Introduction

  • Animals must limit contact with their environment to maintain water and electrolyte homeostasis.
  • However, limited contact poses a challenge for oxygen acquisition and carbon dioxide excretion due to aerobic metabolism.
  • Aerobic metabolism involves oxygen as the final electron acceptor in the electron transport chain, and carbon dioxide is produced as a waste product during pyruvate processing and the citric acid cycle.
  • Gas exchange is the process of moving gases across cell membranes.

Respiratory Surface and System

  • To minimize water and electrolyte loss, gas exchange occurs at a designated respiratory surface.
  • The path to the respiratory surface is often protected by body structures.
  • At the respiratory surface, oxygen enters the body, and carbon dioxide exits through diffusion.
  • The respiratory system includes organs and structures that contain and protect the respiratory surfaces.

Circulation

  • Animals require a circulatory system to transport exchanged gases to and from body cells.
  • Gases are transported in a fluid: blood in vertebrates and hemolymph in invertebrates.
  • Circulation refers to the movement of gases dissolved in these fluids.
  • The circulatory system comprises organs and structures involved in moving blood/hemolymph throughout the body.

Gas Exchange and Diffusion

  • Gas exchange at a membrane is governed by diffusion.
  • Diffusion allows the body to move ions and molecules without expending energy.
    • However, diffusion rate is limited by surface area and the volume of the cell or structure.
  • Diffusion of gases is driven by the pressure gradient, moving from high to low pressure areas.
  • Air and blood contain a mixture of gases, each described by its partial pressure.
  • Partial pressure is the percentage of total pressure exerted by each gas.
  • For instance, at sea level (760 mm Hg), with air being 21% oxygen, the partial pressure of oxygen is 160 mm Hg.

Fick's Law of Diffusion

  • Fick's Law governs the rate of diffusion:


    Rate \propto k \cdot A \cdot \frac{(P2 - P1)}{D}

    • Where:

      • k is a constant specific to each gas.
      • A is the surface area available for gas exchange.
      • D is the diffusion distance.
      • (P2 – P1) is the partial pressure gradient.

    • The rate of diffusion is directly proportional to surface area and the pressure gradient, but inversely proportional to distance.

    • Increased surface area and a larger pressure gradient increase the rate of diffusion.

    • Increased distance decreases the rate of diffusion.

Gas Exchange Mechanisms

  • The respiratory system must facilitate ventilation and gas exchange with the environment.
  • Ventilation is the movement of air or water across a specialized respiratory surface.
  • Gas exchange is driven by diffusion, with oxygen moving from the environment (high partial pressure) into the blood (low partial pressure) and carbon dioxide moving from the blood (high partial pressure) into the environment (low partial pressure).

Gills in Aquatic Animals

  • Gills are the respiratory surface in aquatic animals, and can be either external or internal.
  • External gills are ventilated by water currents, while internal gills require ventilation by a respiratory system structure.

Ventilation in Fish

  • Two methods for ventilating internal gills:

    1. Buccal pumping:

      • During inhalation, the operculum (gill covers) are closed, and water is drawn into the mouth.
      • During exhalation, the mouth squeezes the water over the gills and out through the open operculum.

    2. Ram ventilation:

      • The fish swims with its mouth open, allowing water to flow directly over the gills and out of the operculum.

  • Some fish use a combination of both methods; some sharks (e.g., great white, mako) rely exclusively on ram ventilation and must swim to breathe.

Gill Structure

  • Gills are divided into arches with thousands of tiny gill filaments which increase the surface area for diffusion.
  • The gill filaments contain numerous blood vessels that carry blood close to the gill surface, reducing diffusion distance.
  • These structural adaptations enhance diffusion rates as described by Fick's Law.

Respiratory Systems in Terrestrial Animals

  • In terrestrial animals, the respiratory medium changes from water to air.
  • Air exposure can lead to significant water loss through evaporation, making osmoregulation and water balance more challenging.
  • Terrestrial animals have evolved fully internal and enclosed respiratory systems to minimize water loss and protect the gas exchange surface.

Tracheal Systems in Terrestrial Invertebrates

  • Terrestrial invertebrates utilize an internal tracheal system for gas exchange.
  • This system is distributed throughout the body, with openings called spiracles that can be opened and closed to regulate air flow.

Ventilation of Tracheae

  • Ventilation occurs through physical movements.
  • Muscle contractions squeeze the tracheae, pushing air out.
  • Muscle relaxation expands the tracheae, pulling air in, essentially breathing via body muscles.

Lungs in Terrestrial Vertebrates

  • Terrestrial vertebrates use lungs as their respiratory surface.
  • Lung complexity varies among organisms.
  • Frog lungs are simple sacs with little branching.
  • Human lungs branch extensively (23 times), ending in alveoli (small respiratory membranes).

Alveoli

  • Human lungs contain approximately 150 million alveoli per lung.
  • This extensive branching dramatically increases surface area for diffusion.
  • Alveoli are composed of very thin cells, with only two thin layers separating the blood from the air, minimizing diffusion distance.
  • In the alveoli, oxygen diffuses into the blood, and carbon dioxide diffuses out into the air.

Lung Ventilation

  • Lung ventilation occurs through the movement of the diaphragm.

    • Inhalation:
      • The diaphragm contracts and pulls downward, creating a