Respiration is defined as the movement of oxygen from the outside environment to the cells within tissues, and the transport of carbon dioxide in the opposite direction.

The physiological definition of respiration is differs from the biochemical definition, which refers to cellular respiration, a metabolic process by which an organism obtains energy (in the form of ATP) by oxidizing nutrients and releasing waste products. Although physiologic respiration is necessary to sustain cellular respiration and thus life in animals, the processes are distinct: cellular respiration takes place in individual cells of the organism, while physiologic respiration concerns the diffusion and transport of metabolites between the organism and the external environment.

In animals with lungs, physiological respiration involves respiratory cycles of inhaled and exhaled breaths. Inhalation (breathing in) is usually an active movement. The contraction of the diaphragm muscle cause a pressure variation, which is equal to the pressures caused by elastic, resistive and inertial components of the respiratory system. In contrast, exhalation (breathing out) is usually a passive process. Breathing in, brings air into the lungs where the process of gas exchange takes place between the air in the alveoli and the blood in the pulmonary capillaries.

The process of breathing does not fill the alveoli with atmospheric air during each inhalation (about 350 ml per breath), but the inhaled air is carefully diluted and thoroughly mixed with a large volume of gas (about 2.5 liters in adult humans) known as the functional residual capacity which remains in the lungs after each exhalation, and whose gaseous composition differs markedly from that of the ambient air. Physiological respiration involves the mechanisms that ensure that the composition of the functional residual capacity is kept constant, and equilibrates with the gases dissolved in the pulmonary capillary blood, and thus throughout the body. Thus, in precise usage, the words breathing and ventilation are hyponyms, not synonyms, of respiration; but this prescription is not consistently followed, even by most health care providers, because the term respiratory rate (RR) is a well-established term in health care, even though it would need to be consistently replaced with ventilation rate if the precise usage were to be followed.

Respiratory organs of the animal breathing


The respiratory system in insects consists of a network of tubes, called tracheae, which directly ventilate the tissues. Actively moving air to the site of gas exchange is called ventilation. The tubes divide and branch out into smaller and smaller tubes extending into all parts of the insect, similar to the way arteries branch out into tiny capillaries in a closed circulatory system.

Insects have openings scattered throughout its body called spiracles. Spiracles are openings to the tracheae. In small insects, gas exchange occurs by diffusion only. Larger insects will actively breathe to pump air into the tubes.

Aquatic insects must seal their spiracles when they are under water to prevent flooding their tubes. Amazingly, some aquatic insects even have specialized spiracles that can puncture underwater plants and access those plants’ oxygen storage centers. Think of it like an underwater vampire bug that sucks oxygen.


The chief organ in mammalian respiration is the lungs. The lungs are actively ventilated via a suction-pump mechanism of inhalation and exhalation. Breathing is dependent upon the rib muscles and the diaphragm, a structure shaped like a dome-shaped floor just beneath the lungs.

Inhalation happens when the rib cage opens up and the diaphragm flattens and moves downward. The lungs expand into the larger space, causing the air pressure inside to decrease. The drop in air pressure inside the lung makes the outside air rush in.

Exhalation is the opposite process. The diaphragm and the rib muscles relax to their neutral state, causing the lungs to contract. The squashing of the lungs increases their air pressure and forces the air to flow out.

Most mammals are nose breathers. Inhaling through the nose warms and moistens the air. The air is filtered by cilia and mucus membranes, which trap dust and pathogens. Air then reaches the epiglottis, the tiny leaf-shaped flap at the back of the throat. The epiglottis regulates air going into the windpipe and closes upon swallowing to prevent food from being inhaled. It’s the gatekeeper to the lungs.

Reptiles and Amphibians

Reptiles and amphibians have lungs and exchange gases in the capillaries like mammals, but there are some differences in how they ventilate their respiratory systems. Reptiles don’t typically breathe the same way as mammals, since many reptiles lack a diaphragm. Reptiles use their axial muscles, the ones attached to their ribs, to expand their ribcage for breathing. During periods of intense activity, reptiles might be forced to hold their breath, as they use those muscles for running away.

Some reptiles get around this by buccal pumping while they run. Buccal pumping is when an animal uses the muscles of the mouth and throat to pull air into the lungs. Muscles pull air through the mouth or nose into a buccal cavity. Throat muscles then pump and move the floor of the mouth up in a way that’s visible from the outside. This forces air out of the mouth and into the lungs. This is what amphibians do, by puffing up their chinny-chin-chins to get the air in. Look at this frog’s constantly moving throat .

Apart from their capillaries, amphibians perform gas exchange directly through their skin. This works for them because their skin has lots of blood vessels very close to the permeable skin surface. Diffusion can take place right through the skin. In fact, some salamanders have no lungs at all, and they get all of their oxygen through their skin.


The respiratory system of birds is similar to that of mammals. Air is pulled in using a suction-type pull. Gases are exchanged in the capillaries. The major difference is the route of airflow through the body. Birds have air sacs that collect air. They then force the air through their lungs like bellows stoking a fire.

When a bird inhales, air is brought into the posterior air sacs, which expand. Then the bird exhales and the air is forced from the posterior air sacs into the lungs, where gas exchange occurs. The bird inhales a second time, moving the air from the lungs to the anterior air sac. A second exhalation pushes the air out of the body.


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