All animals have some mechanism for removing oxygen from the air and transmitting it into the bloodstream, and this same mechanism typically is used to expel carbon dioxide from the bloodstream into the surrounding environment. Types of animal respiration, in order of complexity, include direct diffusion, diffusion into blood, tracheal respiration, respiration with gills, and finally, respiration through lungs. Microbes, fungi, and plants all obtain the oxygen they use for cellular respiration directly from the environment, meaning that there are no intermediate organs or bodily chemicals, such as lungs or blood. More complex organisms, such as sponges, jellyfish, and terrestrial (land) flatworms, all of which have blood, also breathe through direct diffusion. The latter term describes an exchange of oxygen and carbon dioxide directly between an organism, or its bloodstream, and the surrounding environment. through capillary walls (capillaries are small blood vessels that form a network throughout the body) and into the bloodstream.
Once oxygen is in the blood, it moves throughout the body to different tissues and cells. Among the organisms that rely on diffusion into blood are annelids, a group that includes earthworms, various marine worms, and leeches. In tracheal respiration air moves through openings in the body surface called spiracles. It then passes into special breathing tubes called tracheae that extend into the body. The tracheae divide into many small branches that are in contact with muscles and organs. In small insects, air simply moves into the tracheae, while in large insects, body movements assist tracheal air movement.
Insects and terrestrial arthropods (land-based organisms with external skeletons) use this method of respiration. Much more complicated than tracheae, gills are specialized tissues with many infoldings. Each gill is covered by a thin layer of cells and filled with blood capillaries. These capillaries take up oxygen dissolved in water and expel carbon dioxide dissolved in blood. Fish and other aquatic animals use gills, as did the early ancestors of humans and other higher animals. A remnant of this chapter from humans evolutionary history can be seen in the way that an embryo breathes in its mother s womb, not by drawing in oxygen through its lungs but through gill-like mechanisms that disappear as the embryo develops.
In order to maintain proper metabolism, the body has to maintain a balance in temperature, proper fluid volume, proper fluid concentration of solutes, proteins (most important is albumin), and proper Ph balance (between 7. 35 and 7. 45) among other things.
The Ph balance is maintained by two mechanisms, namely respiratory and metabolic. Proper breathing refers to not breathing with shallow breaths (for example when one is in pain), because that would lead to retaining too much CO2, which would lead to a respiratory acidosis, nor breathing too deeply, because that will bring too much O2 on board and will lead to respiratory alkalosis. Also, the oxygen saturation of the blood, ideally, needs to be above 95%, so correct breathing is important.