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Our own beating hearts have four separate chambers, while those of frogs, toads, snakes and lizards get by with only three. Learn about the functionality of the 3-chambered heart while comparing it to your own in this lesson.

Vertebrate Class and Heart Chambers

Vertebrates, or animals with backbones, consist of several distinct classes: fish, amphibians, reptiles, mammals and birds. The vertebrate heart performs the function of pumping the organism’s life blood around its body. While similar in many ways, the hearts of different vertebrate classes possess different numbers of chambers. These chambers determine how efficiently the heart separates the flow of oxygen-rich blood and oxygen-poor blood throughout the body. Vertebrate hearts can be categorized by the number of chambers they have:

  • Two chambers: one atrium and one ventricle (fish)
  • Three chambers: two atria and one ventricle (amphibian and reptile)
  • Four chambers: two atria and two ventricles (bird and mammal)

Anatomy

Pulmonary vs. Systemic Circulation

Blood contains many things, from nutrients to wastes to antibodies.

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One vital substance, oxygen, enters the blood through gills or lungs. To achieve more efficient use of oxygen, many vertebrates have two separate stages of blood circulation: pulmonary and systemic. Let’s examine the 4-chambered human heart.In 4-chambered pulmonary circulation, the heart sends blood to the lungs to pick up oxygen. The blood starts in the right ventricle.

From there, it gets pumped through the pulmonary arteries to the lungs. It returns from the lungs through the pulmonary veins and flows into the left atrium. From there, it goes into the left ventricle, where it begins systemic circulation.Systemic circulation is when the heart distributes oxygen-rich blood around the body. The left ventricle pumps blood through the aorta, the massive artery that branches off to feed all parts of the body. Once the blood delivers oxygen, it returns through various veins that lead to either the inferior vena cava or the superior vena cava.

These two major veins then feed into the right atrium of the heart. Once there, the blood, depleted of oxygen, re-enters pulmonary circulation.Another reason for the separate stages of circulation is that the lungs cannot handle the same blood pressure that is needed to pump blood throughout the rest of the body. The right ventricle, which pumps blood to the lungs, is smaller than the left ventricle, which sends blood to the body.

Different Hearts

By keeping these two stages of circulation separate, the 4-chambered heart maximizes oxygen utilization. Only oxygen-rich blood goes to the body, and only blood in need of fresh oxygen goes to the lungs. Birds and mammals have these four chambers.

Dinosaurs probably had them, too. Crocodilians, such as alligators and crocodiles, also have four chambers, but they can shut off circulation to the lungs when they are underwater.Amphibians have a 3-chambered heart, which provides some separation of blood coming from the lungs and blood coming from the body. While there are two atria, there is one common ventricle, in which some (but not total) mixing of oxygen-rich and oxygen-poor blood occurs.

Also, amphibians tend to absorb some oxygen through their skin, which supplements regular (pulmonary) respiration. This mitigates the lesser efficiency of the single ventricle.Reptiles have a 3-chambered heart as well, but the single ventricle has a septum that provides considerable separation of blood, nearly as well as a 4-chambered heart. This separation is nearly complete when the heart contracts.

Fish have a simple, 2-chambered heart that serves their needs. There is no separation of blood from the gills and the rest of the body.

Lesson Summary

The 3-chambered heart of amphibians works because of their supplemental skin respiration, while the 3-chambered heart of reptiles works because the septum makes it almost a 4-chambered heart.

Mammals and birds have complete separation of pulmonary and systemic circulation. Crocodilians go a step further by cutting off circulation to the lungs when necessary.

Learning Outcomes

When you are finished, you should be able to:

  • Name the three classifications of vertebrate hearts
  • Describe the separate blood circulation stages of pulmonary circulation and systemic circulation
  • Explain how a 3-chambered heart is different from a 4-chambered one

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