The Respiratory System

 Karthik Gurumurthy

Most animals cannot survive without a constant supply of oxygen. If you cells are without it for even a few minutes, they- and you-start to die. That is why you never stop breathing, even when you sleep.

Oxygen is a gas found in both air and water. Animals living in lakes, rivers, and oceans get oxygen by breathing water, and animals living on land get it by breathing air. Breathing is also the way animals get rid of carbon dioxide, a gas which cells produce a waste. Animals regulate their breathing to maintain homeostasis. For example, they breathe faster and deeper when exercising because the cells need more oxygen.They breathe slower and shallower when sleeping because less oxygen is needed. 

In tiny animals like flatworms, most cells are in direct contact with the outside environment. That means oxygen and carbon dioxide can pass freely between the cells and the air or water. In larger or more complex animals, most cells are inside their bodies. Only the surface cells can get oxygen directly from the air or water. These animals have respiratory systems to transport oxygen and carbon dioxide between the outside environment and their inner cells.

Insects have body openings called spiracles to let in oxygen. Air enters the spiracles and flows through a network of branching tubes called tracheae. Oxygen passes through the tubes walls and into the inner cells through their membrances. Carbon dioxide passes out of the cells and exits the body through the same tubes. Insects continually pump fresh air into their spiracles by expanding and contracting their bodies. Similarly, sponges take in water through small holes called pores, which lead to a system of tubes that reach all the cells.

Many animals, including humans, have special organs to pump oxygen into the body. Aquatic animals, such has fish, have gills, and land animals, such as humans, have lungs.