8.1 - Types of Respiratory System

Gaseous exchange involves the intake of oxygen from the environment and the expel of carbon dioxide to the environment. The process of gaseous exchange occurs in the respiratory structures, which are the respiratory surfaces that allow exchange of respiratory gases. Different organisms have different respiratory structures that will maximize their rate of gaseous exchange. Generally, smaller organisms have higher surface area to volume ratio (TSA/V) while larger organisms have lower TSA/V. This means that in larger organisms, the volume of the body that needs oxygen is greater than their surface area. Therefore, oxygen cannot diffuse across the body surface fast enough to meet the oxygen demand of the organisms. In this case, they require specialized respiratory structures for an efficient gaseous exchange.

There are four common characteristics of respiratory structures in order to ensure adequate gaseous exchange in multicellular organisms.

• High TSA/V ratio
• Cells lining the respiratory surfaces are thin (one-cell thick)
• Surface of the respiratory structure is constantly moist
• Covered by a network of blood capillaries

Figure 1 - The respiratory surfaces of unicellular organisms such as Amoeba sp. (left) and Paramecium sp. (right) are their entire plasma membranes because they have adequate TSA/V ratio for efficient gaseous exchange.

The Insect Respiratory Structure and Its Adaptations

Tracheal system is the respiratory system of insects.

• It consists of a network of tubes known as tracheae (singular, trachea).
• The tracheae branch into fine tubes called tracheoles.
• Surrounding air enters the tracheae through small openings called spiracles , which are located along the sides of the thorax and abdomen of an insect.
• Some insects such as the grasshoppers contain air sacs in their tracheal system. The air sacs help to increase the speed of air movements in and out of the tissues during body movements.

Figure 2 - The spiracles have valves which open and close to allow the movement of air in and out of the body. The tracheae are protected by rings of chitin which prevent them from collapsing.

The Fish Respiratory Structure and Its Adaptations

Gills are the specialized respiratory structures for gaseous exchange underwater. The gills are supported by gill arch and protected by the operculum. Each gill has 2 rows of thin filaments arranged in a V shape. These filaments are comprised of numerous thin-walled lamellae (singular, lamella).

• Large surface area of the filaments and lamellae facilitates effective gaseous exchange in fishes.
• The membrane of the filaments is thin which allows easy absorption of respiratory gases.
• The filaments are also covered with many blood capillaries for efficient gaseous exchange and transport.

Figure 3 - The gill filaments are constantly surrounded by water. The countercurrent exchange mechanism further enhances the efficiency of gaseous exchange. Water flows over the gills in one direction, while the blood flows in the other direction. This helps maximize oxygen transfer in the lamellae.

The Frog Respiratory Structure and Its Adaptations

Amphibians such as frogs can live on land as well as in water. Therefore, their respiratory structures are designed to adapt for gaseous exchange both on land and in water. In frogs, the exchange of respiratory gases occurs mainly through the skin and in the lungs. All the adaptations listed below help respiratory gases to dissolve and be transported easily in and out of the body.

• During period of inactivity, gaseous exchange happens through the skin.
• The skin is thin, permeable and always moist.
• Beneath the skin is a network of blood capillaries that transport respiratory gases to and from the body cells.
• In the lungs, presence of numerous inner partitions increases the surface area for gaseous exchange.
• The membranes of the lungs are also thin and moist.
• The lungs are supplied with a rich network of blood capillaries.

Figure 4 - The lungs of frogs are not as strong as human lungs. They do not have rib cage or diaphragm to support the contraction of the lungs.

The Human Respiratory Structure and Its Adaptations

In mammals like humans, gaseous exchange occurs in the alveoli (singular, alveolus) of the lungs.

• The lungs contain millions of alveoli , which increases the surface area for gaseous exchange.
• Inner surface of the alveoli is constantly moist.
• Each alveolus is covered by a dense network of blood capillaries.
• The wall of an alveolus is very thin, only one-cell thick.

Figure 5 - Alveoli are covered with a network of blood capillaries which provides a large TSA/V ratio for rapid gaseous exchange.

Similarities and Differences of Respiratory Structures in Humans and Animals

• Similarities

  • Large TSA/V in all respiratory structures for efficient gaseous exchange.
  • Thin membrane of respiratory surfaces allows easy diffusion of gases.
  • Moist respiratory surfaces allow respiratory gases to dissolve.
  • Rich supply of blood capillaries surrounding the respiratory structures (except in insects).

• Differences

  • Insects

    • Respiratory structure: Tracheoles
    • How high TSA/V is achieved: Numerous tracheoles

  • Fish

    • Respiratory structure: Filaments and lamellae
    • How high TSA/V is achieved: Numerous filaments and lamellae

  • Amphibians

    • Respiratory structure: Skin and lungs
    • How high TSA/V is achieved:
      • Lungs with numerous partitions
      • Entire surface area of the skin

  • Humans

    • Respiratory structure: Alveoli
    • How high TSA/V is achieved: Numerous alveoli