How Organisms Developed Different Specializations

Instructor: Amanda Robb
In this lesson, we'll be learning about the evolution of different specializations to maintain homeostasis. We'll be looking at examples of respiratory and excretory functions in mammals, fish, and amphibians.

What Functions Do Organisms Need to Perform?

Look around you. What animals can you see? You might have a pet fish or maybe a pet frog. Humans are technically animals as well. Although we all look and behave differently, there are essential functions that animals must do to stay alive.

Animals need oxygen to perform cellular respiration and make energy. However, different animals have evolved different ways to accomplish this task depending on their environment. For example, the way fish get their oxygen is quite different from humans. Animals also need to excrete wastes and similar to respiration, different organisms do this quite differently. These specializations allow organisms to survive and thrive in their respective environments.

Respiratory Systems

Animals need to take in oxygen and release carbon dioxide which is a toxic waste product made during cellular respiration. However, different animals have evolved different strategies to accomplish this same function.


Mammals, such as humans, can only exchange oxygen by air through lungs - a spongy network of air-filled tubes and sacs. Air enters through our nose and mouth and is taken to the right and left lungs through a series of tubes. Inside the lungs are alveoli which are the sites of gas exchange. Each alveoli is wrapped with a system of capillaries, or thin blood vessels which allow the transfer of fluids, gases, nutrients and waste products. These capillaries are responsible for moving carbon dioxide into the lungs to be exhaled and oxygen to move from the lungs into the blood.

In mammals, gas exchange occurs in the alveoli


Fish are specialized to exchange gases through water. Most fish have respiratory organs called gills located behind their jaws. Gills are made of flaps of tissue which are filled with capillaries and they are typically covered by flaps of skin or gill covers. Water passes through the fish's mouth and over the gills. Dissolved oxygen diffuses into the capillaries and carbon dioxide in the blood diffuses out into the water.

In fish, gas exchange is performed by the gills

Gills need to work a lot harder than lungs. About 20% of the air we breathe is oxygen, whereas water might only contain 1% dissolved oxygen. Fish need to have a tremendous amount of capillaires in their gills to extract as much oxygen as possible. Fish use a technique called counter current exchange. In this process, blood flows through the capillaries in the opposite direction of water flow, maximizing the diffusion of oxygen into the blood.


Amphibians have a unique lifestyle that is partially terrestrial and partially aquatic. At different stages of their life, they use gills, skin, and lungs to exchange gases. Amphibians generally start their life in an aquatic environment. During this stage they use gills to exchange gases, much like fish. Amphibians may also perform gas exchanges through their skin. Beneath the skin is an extensive network of capillaries which allow oxygen to diffuse into the blood and carbon dioxide to diffuse into the water. Frog tadpoles use gills and also their large tail fin for gas exchange. As tadpoles become mature, they will lose their gills and develop lungs. When adult frogs are on land, they use small lungs to gulp in air where gas exchange can happen, similar to mammals. When adult frogs are completely submerged in water, gas exchange occurs almost entirely through their skin.

When submerged, frogs can breathe through their skin

Excretory Systems

Animals must maintain a water balance and get rid of nitrogenous waste which is produced during metabolic reactions. This is the job of the excretory system.


When protein is metabolized it produces ammonia, a highly toxic compound. Most mammals have limited tolerance for ammonia and it is converted to urea or ureic acid and excreted by the kidneys. Blood is filtered through the kidney's functional unit, called the nephron. Each nephron filters toxins like urea and any excess water out of the blood. The waste liquid is taken to the bladder and then expelled from the urethra.

The kidneys also help to regulate water balance in mammals in a process called osmoregulation. If we have excess water in our bodies, our kidney secretes more water as urine to maintain the balance. If we are low on water, the kidney secretes less water.

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