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Various Modes of Gaseous Exchange  in animals:

• In simple unicellular animals like Amoeba, respiration takes place by the simple diffusion of gases through the cellmembrane. Most of the animals have, however, specific organs for respiration.
• The animals like earthworms which live in the soil use their skin to absorb oxygen from air and remove carbon dioxide. So, the respiratory organ in the earthworm is the skin.
• The aquatic animals like fish, prawns and mussels have gillsas the respiratory organs which extract oxygen dissolved in water and take away carbon dioxide from the body.
• In the insects like grasshopper, cockroach, housefly and a mosquito, the tiny holes called spiracleson their body and the air tubes called tracheae are the respiratory organs.
• The respiratory organs of the land animals such as man (humans), birds, lizard, dog and frog etc. are the lungs. However, Frogs breathe both by lungs and skin.

Therefore, we can say that all the respiratory organs whether skin, gills, trachea or lungs have three common features:

• All the respiratory organs have a large surface area to get enough oxygen.
• All the respiratory organs have thin walls for easy diffusion and exchange of respiratory gases.
• All the respiratory organs like skin, lungs and gills have a rich blood supply for transporting gases. But only in tracheal system of respiration, air reaches the cells directly.

Systems of Gas Exchange

Respiratory processes that help organisms exchange O₂ and CO₂ range from simple direct diffusion to complex respiratory systems.

Breathing is an involuntary event. How often a breath is taken and how much air is inhaled or exhaled are tightly regulated by the respiratory center in the brain. Under normal breathing conditions, humans will breathe approximately 15 times per minute on average. A respiratory cycle consists of an inhalation and an exhalation: with every normal inhalation, oxygenated air fills the lungs, while with every exhalation, deoxygenated air rushes back out. The oxygenated air crosses the lung tissue, enters the bloodstream, and travels to organs and tissues. Oxygen (O₂) enters the cells where it is used for metabolic reactions that produce ATP, a high-energy compound. At the same time, these reactions release carbon dioxide (CO₂) as a by-product. CO₂ is toxic and must be eliminated; thus, CO₂ exits the cells, enters the bloodstream, travels back to the lungs, and is expired out of the body during exhalation.

The primary function of the respiratory system is to deliver oxygen to the cells of the body’s tissues and remove carbon dioxide. The main structures of the human respiratory system are the nasal cavity, the trachea, and the lungs. All aerobic organisms require oxygen to carry out their metabolic functions.

Along the evolutionary tree, different organisms have devised different means of obtaining oxygen from the surrounding atmosphere. The environment in which the animal lives greatly determines how an animal respires. The complexity of the respiratory system correlates with the size of the organism. As animal size increases, diffusion distances increase and the ratio of surface area to volume drops. In unicellular (single-celled) organisms, diffusion across the cell membrane is sufficient for supplying oxygen to the cell. Diffusion is a slow, passive transport process. In order to be a feasible means of providing oxygen to the cell, the rate of oxygen uptake must match the rate of diffusion across the membrane. In other words, if the cell were very large or thick, diffusion would not be able to provide oxygen quickly enough to the inside of the cell. Therefore, dependence on diffusion as a means of obtaining oxygen and removing carbon dioxide remains feasible only for small organisms or those with highly-flattened bodies, such as flatworms (platyhelminthes). Larger organisms have had to evolve specialized respiratory tissues, such as gills, lungs, and respiratory passages, accompanied by a complex circulatory system to transport oxygen throughout their entire body.

Direct Diffusion

For small multicellular organisms, diffusion across the outer membrane is sufficient to meet their oxygen needs. Gas exchange by direct diffusion across surface membranes is efficient for organisms less than 1 mm in diameter. In simple organisms, such as cnidarians and flatworms, every cell in the body is close to the external environment. Their cells are kept moist so that gases diffuse quickly via direct diffusion. Flatworms are small, literally flat worms, which ‘breathe’ through diffusion across the outer membrane. The flat shape of these organisms increases the surface area for diffusion, ensuring that each cell within the body is close to the outer membrane surface and has access to oxygen. If the flatworm had a cylindrical body, then the cells in the center would not be able to get oxygen.

Skin, Gills, and Tracheal Systems

Respiration can occur using a variety of respiratory organs in different animals, including skin, gills, and tracheal systems.

Skin and Gills

There are various methods of gas exchange used by animals. As seen in mammals, air is taken in from the external environment to the lungs. Other animals, such as earthworms and amphibians, use their skin (integument) as a respiratory organ. A dense network of capillaries lies just below the skin, facilitating gas exchange between the external environment and the circulatory system. The respiratory surface must be kept moist in order for the gases to dissolve and diffuse across cell membranes.

Organisms that live in water also need a way to obtain oxygen. Oxygen dissolves in water, but at a lower concentration in comparison to the atmosphere, which has roughly 21 percent oxygen. Fish and many other aquatic organisms have evolved gills to take up the dissolved oxygen from water. Gills are thin tissue filaments that are highly branched and folded. When water passes over the gills, the dissolved oxygen in the water rapidly diffuses across the gills into the bloodstream. The circulatory system can then carry the oxygenated blood to the other parts of the body. In animals that contain coelomic fluid instead of blood, oxygen diffuses across the gill surfaces into the coelomic fluid. Gills are found in mollusks, annelids, and crustaceans.

The folded surfaces of the gills provide a large surface area to ensure that fish obtain sufficient oxygen. Diffusion is a process in which material travels from regions of high concentration to low concentration until equilibrium is reached. In this case, blood with a low concentration of oxygen molecules circulates through the gills. The concentration of oxygen molecules in water is higher than the concentration of oxygen molecules in gills. As a result, oxygen molecules diffuse from water (high concentration) to blood (low concentration). Similarly, carbon dioxide molecules diffuse from the blood (high concentration) to water (low concentration).

Oxygen transport and gills: As water flows over the gills, oxygen is transferred to blood via the veins.

Tracheal Systems

Insect respiration is independent of its circulatory system; therefore, the blood does not play a direct role in oxygen transport. Insects have a highly-specialized type of respiratory system called the tracheal system, which consists of a network of small tubes that carries oxygen to the entire body. The tracheal system, the most direct and efficient respiratory system in active animals, has tubes made of a polymeric material called chitin.

Insect bodies have openings, called spiracles, along the thorax and abdomen. These openings connect to the tubular network, allowing oxygen to pass into the body, regulating the diffusion of CO₂ and water vapor. Air enters and leaves the tracheal system through the spiracles. Some insects can ventilate the tracheal system with body movements.

Insect respiration: Insects perform respiration via a tracheal system, in which openings called spiracles allow oxygen to pass into the body

Amphibian and Bird Respiratory Systems

Birds and amphibians have different oxygen requirements than mammals, and as a result, different respiratory systems.

Amphibian Respiration

Amphibians have evolved multiple ways of breathing. Young amphibians, like tadpoles, use gills to breathe, and they do not leave the water. As the tadpole grows, the gills disappear and lungs grow (though some amphibians retain gills for life). These lungs are primitive and are not as evolved as mammalian lungs. Adult amphibians are lacking or have a reduced diaphragm, so breathing through the lungs is forced. The other means of breathing for amphibians is diffusion across the skin. To aid this diffusion, amphibian skin must remain moist. It has vascular tissues to make this gaseous exchange possible. This moist skin interface can be a detriment on land, but works well under water.

Avian Respiration

Birds are different from other vertebrates, with birds having relatively small lungs and nine air sacs that play an important role in respiration. The lungs of birds also do not have the capacity to inflate as birds lack a diaphragm and a pleural cavity. Gas exchange in birds occurs between air capillaries and blood capillaries, rather than in alveoli.

Flight poses a unique challenge with respect to breathing. Flying consumes a great amount of energy; therefore, birds require a lot of oxygen to aid their metabolic processes. Birds have evolved a respiratory system that supplies them with the oxygen needed to sustain flight. Similar to mammals, birds have lungs, which are organs specialized for gas exchange. Oxygenated air, taken in during inhalation, diffuses across the surface of the lungs into the bloodstream, and carbon dioxide diffuses from the blood into the lungs, and is then expelled during exhalation. The details of breathing between birds and mammals differ substantially.

In addition to lungs, birds have air sacs inside their body. Air flows in one direction from the posterior air sacs to the lungs and out of the anterior air sacs. The flow of air is in the opposite direction from blood flow, and gas exchange takes place much more efficiently. This type of breathing enables birds to obtain the requisite oxygen, even at higher altitudes where the oxygen concentration is low. This directionality of airflow requires two cycles of air intake and exhalation to completely get the air out of the lungs.