Unit 2: Nutrition                                                                  1   2   3 

2.2. Breathing

Inside every cell, cellular respiration takes place. Thanks to this chemical reaction

all living beings obtain energy from the organic matter. This energy is used to stay alive and to grow.

To carry out cellular respiration, living beings need oxygen that they have to take

from the exterior through breathing. By the same mechanism, they expel the carbon dioxide that is produced during the process as waste substance.

In animals, breathing is performed by the respiratory system, because not all cells are able to take the oxygen from the external medium directly.

a) Characteristics of the respiratory surfaces:

   Breathing requires surfaces where gas exchange can take place effectively.

   These surfaces are very thin, moist and covered in blood vessels.

   This makes it easier to take oxygen into the blood stream and to expel

   carbon dioxide out.

b) Types of respiratory systems:

   The simplest animals (Cnidarians and Poriferans) do not have respiratory

   system. Gas exchange is performed by the whole body surface.

   The rest of animals have a respiratory system adapted to the medium

   they live in, and associate to the circulatory system that transport gases

   to the cells and from them to the outside of the organism.

- Cutaneous respiration.

    It belongs to Annelids (earthworms) and adult Amphibians, although

    they have pulmonary respiration too.

    The skin of these animals is very thin and they keep it always moist.

    In addition, there are numerous blood vessels close to it, which makes easier

    the gas exchange.

- Tracheal respiration.

    It is exclusive of terrestrial Arthropods (Arachnids, Insects and Myriapods).

    It is made through tracheae. This respiratory system consists on a system

    of tiny tubules that branch into the body. Tracheae open to the exterior

    by an orifice called spiracle through which the air enters. As they enter into

    the body they turn more and more thin.

    Tracheae carry oxygen directly to cells. So that, it is not necessary

    that the circulatory system transport neither the oxygen nor the carbon

    dioxide.

- Branchial respiration.

    It belongs to aquatic animals, such as some Annelids (Bristle worms),

    Molluscs (clams, squids, sea snails, etc), Crustaceans (prawns, lobsters,

    crabs, etc), fish and young of Amphibians (tadpoles).

    Gas exchange is made through the gills, thin extensions of the body surface

    with a lot of blood vessels. They can be internal or external gills.

- Pulmonary respiration.

    It is typical of most Vertebrates (Mammals, Birds, Reptiles,
    Amphibians), but also of some Invertebrates such as snails. 

    It is made through the lungs. These organs are internal cavities

    which have very thin and damp walls, full of blood vessels.

    Lungs are connected with the exterior by airways.

    Respiration takes place in two different movements. Inhalation,

    when air is taken into the lungs and exhalation when air is expelled

    from them.

           - Amphibians’ lungs are simple hollow sacs. Their pulmonary
             respiration is very inefficient so that they complement it

   with cutaneous respiration.

- Reptiles’ lungs have some internal folds. The exchange surface 

  is larger and their respiration is more efficient.

- Mammals’ lungs have a large exchange surface due to they have
   numerous tiny sacs (alveoli).

- Birds have very specialised lungs with air capillaries and expansions

  (air sacs) that make the exchange much more efficient than in other

   groups of Vertebrates.

2.6. Answer these questions:

a. Why do living beings need oxygen?

b. Why do sponges and jellyfish not have a respiratory system?

c. Why do Amphibians need to have cutaneous respiration

    if they have lungs?

2.7. List the characteristic that exchange gaseous surfaces have to have.

        Why do they have them?

2.8. When there is a lack of oxygen, some aquatic Annelids stretch their bodies

          until they have almost twenty times their original size.

a. What type of respiratory system do theses worms have?

b. What advantage do they obtain to stretch their bodies?