8.1 - Types of Respiratory System

Respiratory Structures

• A respiratory structure is a respiratory surface that enables gaseous exchange between respiring organism cells and the outer environment.
• Unicellular organisms such as Amoeba sp. do not require a special respiratory structure because their large total surface area to volume ratio permits gaseous exchange through diffusion.
• As organism size increases, its total surface area to volume ratio decreases.
• Large, complex organisms require special respiratory structures because diffusion through the body surface alone cannot meet their oxygen requirement.

Shared Characteristics of Respiratory Structures

• Large total surface area to volume ratio for efficient respiratory gaseous exchange.
• Thin structure, only one cell thick, for rapid diffusion of respiratory gases.
• Always-moist surface so respiratory gases can dissolve.
• Network of blood capillaries, except in insects, for efficient transport of respiratory gases.

Insect Respiratory Structure and Adaptations

• The insect breathing system is the tracheal system.
• Spiracles are small pores in the thorax and abdomen that allow air into the tracheal system.
• The trachea branches into finer tubes called tracheoles.
• The tracheole is the respiratory surface:
  • Numerous tracheoles provide a large total surface area for gaseous exchange.
  • Thin, moist tracheole walls allow oxygen to diffuse into cells and carbon dioxide to diffuse from cells into the tracheoles rapidly.
• Some insects have air sacs in the tracheal system. Air in these sacs accelerates the delivery of respiratory gases during active body movements.

Fish Respiratory Structure and Adaptations

• The fish respiratory structure is the gill.
• Each gill consists of rows of filaments supported by a gill arch.
• Each filament has numerous thin, flat projections called lamellae.
• Numerous filaments and lamellae provide a large total surface area for efficient gaseous exchange.
• Thin gill-lamella membranes with many blood capillaries facilitate diffusion and transport of oxygen and carbon dioxide.

Frog Respiratory Structures and Adaptations

• An inactive frog uses its skin for gaseous exchange:
  • Thin and highly permeable to respiratory gases.
  • Moist so respiratory gases can dissolve.
  • Many blood capillaries beneath the skin transport respiratory gases.
• The frog also uses its lungs:
  • Folded lung surface increases the total surface area for gaseous exchange.
  • Thin lung membrane facilitates diffusion of respiratory gases.
  • Always-moist lung wall allows respiratory gases to dissolve.
  • Rich blood-capillary network transports respiratory gases rapidly.

Human Respiratory Structure and Adaptations

• The human respiratory structure is the alveolus.
• Numerous alveoli provide a large total surface area for diffusion of respiratory gases.
• Always-moist alveolus walls allow oxygen and carbon dioxide to dissolve and diffuse through the walls.
• Each alveolus is surrounded by many blood capillaries, accelerating diffusion of respiratory gases.
• The alveolus wall is thin, only one cell thick, facilitating gaseous diffusion.

Comparison of Human and Animal Respiratory Structures

• Similarities:
  • Large total surface area to volume ratio.
  • Thin respiratory structure.
  • Always-moist respiratory surface.
  • Blood-capillary network, except in insects.
• Respiratory structures:
  • Insect: tracheoles.
  • Fish: gill filaments and lamellae.
  • Frog: skin and lungs.
  • Human: alveoli.
• How a large total surface area to volume ratio is obtained:
  • Insect: numerous tracheoles.
  • Fish: numerous gill filaments and lamellae.
  • Frog: folded inner lung surface and the whole skin surface.
  • Human: numerous alveoli.