Amphibians blood and removing wastes became more efficient.

Amphibians use a double circulation system, but what does that mean? As an organism suited to live both on land and in water, amphibians have unique ecological characteristics.

Learn more in this lesson!

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Purpose of the Circulatory System

What does a circulatory system do anyway, and why is it important enough to dedicate an entire lesson to? Well, the circulatory system is responsible for moving blood and nutrients around the body – that’s a pretty important task! In fact, the circulatory system has two main jobs. First, it transports blood with oxygen and nutrients to cells all over the body. Second, it picks up waste products from the cells to be transported out of the body, and it takes the de-oxygenated blood back to the lungs so it can reload.

Types of Circulatory Systems

The animal kingdom is pretty diverse, but if we go back to the basics, we find two types of circulatory systems: open systems and closed systems.

With an open circulatory system, blood and nutrients are not restricted to moving within blood vessels. Body fluids can be pumped right from the heart into the body cavity, where they cover the internal structures and the oxygen and nutrients needed are directly absorbed. This is common in invertebrates like crustaceans and insects.Then, there is the closed circulatory system – this is the type vertebrates like fish, mammals, and amphibians have. With this type, the blood and other fluids stay inside a set of blood vessels and are never freely released into the body cavity.

Blood vessels vary in size, and oxygen and nutrients are exchanged between blood vessels and tissues through the tiniest ones.Looking closely at closed circulatory systems, they can actually categorize them further based on the path the blood takes through the body.In some animals, like fish, blood takes a single path around the body and moves in one direction in a circuit called the systemic circuit. Because of this unidirectional path, or circuit, (also called single circulation) the heart of a fish only needs two chambers: one to receive blood and one to pump it back out again.As animals evolved, the process of delivering blood and removing wastes became more efficient. Mammals, reptiles, and amphibians use a system called double circulation.

This means that there are now two pathways, or circuits, for blood to move around the body.

  • The first circuit circulates blood between the heart and the oxygen source (typically the lungs) (this is called pulmonary circulation),
  • The second circuit circulates blood between the heart and the rest of the body (this is called systemic circulation).

Amphibians and Circulation

Amphibians are a pretty unique type of animal, and their circulatory system is adapted to fit their ecological traits. As juveniles, amphibians live entirely in water, breathing through gills and swimming with the help of a tail. After metamorphosis, they develop lungs and grow legs; adults split their time between land and water. What makes amphibians especially cool is that they breathe in two ways.

First, they breathe through lungs, just like mammals and reptiles, but they also absorb oxygen through their skin!Like we learned above, amphibians have a double circulatory system, but we also just learned that they can get oxygen from their skin, so how does this oxygen get into the circulatory system? Don’t worry – evolution has an answer. Instead of a pulmonary circuit, amphibians have a pulmocutaneous circuit(pulmo- refers to the lungs, and -cutaneous refers to skin). The pulmocutaneous circuit carries deooxygenated blood to both the lungs and the skin’s surface so that additional gas exchange can occur.

Amphibian Circulatory Pathway

Here’s a closer look at the path of blood through an amphibian; an amphibian’s heart has three chambers. Two of the chambers are atria; atria receive blood returning from the lungs, skin, or body.

The third chamber is a ventricle; the ventricle pumps blood from the heart to either the lungs and skin or the body. The atria take in blood, the ventricle pumps it out.Deoxygenated blood travels from the body back to the heart, entering the right atrium. Simultaneously, oxygenated blood from the lungs and skin is delivered to the left atrium. Both atria empty into the ventricle, which causes the blood to mix (this mixing decreases the oxygen efficiency a little bit, but the ventricle has a partial separation that limits the amount of mixing that can take place).

From the ventricle, deoxygenated blood enters the pulmocutaneous circuit to go to the lungs and the skin to pick up more oxygen and drop off waste materials. Oxygenated blood enters the systemic circuit to be delivered to the rest of the body. A single ventricle sends blood to both the respiratory organs and the body.

In the diagram, the red loop is the pulmocutaneous circuit, and the blue loop is the systemic circuit.

Though oxygen efficiency is decreased by the mixing that takes place in the single ventricle, this type of circulation allows blood to move quickly throughout the circulatory system.

Lesson Summary

Amphibians are unique animals in that they can breathe both through the use of lungs and by exchanging oxygen through their skin. To make use of this oxygen, the amphibian circulatory system has to route blood to the skin’s surface to pick up oxygen. As such, amphibians have a double circulatory system composed of two circuits. The systemic circuit circulates blood between the heart and the rest of the body, and the pulmocutaneous circuit circulates blood between the heart and the lungs and skin.

In general, a circulatory system transports blood with oxygen and nutrients to cells, it picks up waste products from the cells to be moved out of the body, and it takes de-oxygenated blood back to the lungs.


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