Respiratory system in animals

RESPIRATORY SYSTEM IN ANIMALS

The need to take in oxygen and expel carbon dioxide is almost universal among organisms.
The movement of these gases between an organism and its environment, called gas exchange, is accomplished in a variety of ways by different organisms.
In unicellular aquatic organisms, such as protozoans, and in seaweeds, sponges, jellyfish, and other aquatic organisms that are only a few cell layers thick, oxygen and carbon dioxide diffuse directly between the water and cells.
In higher animals, there are special organs that help in breathing and respiration.

Animals breathe in many ways:

(a) Through skin:

The earliest development of these gas exchange structures is seen in roundworms, microscopic invertebrates abundant in water and moist soil.
In animals such as earthworms and leeches, exchange of gases takes place through the moist and slimy surface of the skin.
In amphibians such as frogs, newts, and salamanders, the exchange of gases takes place through the skin (when in water), in addition to the lungs (when on land).

(b) Through air holes:

Insects have a unique respiratory system made up of small tubes called tracheae.
The tracheae connect all parts of the body to small openings called spiracles on the surface of the insect.

Spiracles in cockroach

Oxygen and carbon dioxide are transported through the tracheae and from the tracheae to the blood of the insect by diffusion.
The blood of most insects is contained in an open circulatory system and is moved around the internal organs by a heart.

(c) Through gills:

Gills provide a more extensive surface area for gas exchange than the body surface alone, enabling clams and larger organisms to obtain the amount of oxygen they need.
Fish have gills, a heart, and a closed circulatory system, one in which blood is transported in vessels by the pumping action of the heart.

Gills in fish

Most aquatic animals such as fishes use special organs called gills for breathing. These gills are like slits and are made up of a large number of thread-like structures called filaments.
These filaments are richly supplied with tiny blood vessels called capillaries.
When water enters the body of the fish through the mouth, it flows over the gills.
The blood in the capillaries absorbs oxygen from the water, and gives out carbon dioxide.
This oxygen is transported by the blood.

(d) Through lungs:

Most mammals such as cats, dogs, monkeys, horses, human beings, etc., breathe with the help of special sac-like, spongy organ called lungs.
Baby frogs (Tadpoles) also breathe through gills, though they breathe through skin and lungs when they grow up to become frogs.