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ConnectingCONCEPTSCircul<strong>at</strong>ory System The heart isa muscle for pumping bloodthrough the body. It is made oftwo different types of chambers.The right and left <strong>at</strong>ria collectblood from the body and lungs,and the right and left ventriclespump blood to the lungs andbody. You will learn more aboutthe heart in Chapter 30.Like amphibians, reptiles have a three-chambered heart. A reptile’s hearthas two <strong>at</strong>ria and one ventricle, as shown in FIGURE <strong>26</strong>.3. One <strong>at</strong>rium collectsoxygen-poor blood from the body. The other collects oxygen-rich blood fromthe lungs. Both <strong>at</strong>ria send blood into the ventricle, which pumps blood intothe pulmonary and systemic circuits. This unique an<strong>at</strong>omy lets these animalstemporarily “turn off” their lungs. Like amphibians, amniotes such as lizardsand turtles do not bre<strong>at</strong>he continuously. Remember th<strong>at</strong> sprawling amniotesstop bre<strong>at</strong>hing when they run. Others spend a lot of time under w<strong>at</strong>er. Ineither case, a single ventricle can divert blood away from the lungs when theanimal is not using them. This str<strong>at</strong>egy lets these animals adjust blood flowin response to their oxygen needs.Mammals and birds have a four-chambered heart. As you can see inFIGURE <strong>26</strong>.3, four-chambered hearts have two <strong>at</strong>ria and two ventricles. Thisan<strong>at</strong>omy keeps oxygen-poor and oxygen-rich blood separ<strong>at</strong>e, but it cannotshift blood away from the lungs when the animal is not bre<strong>at</strong>hing. Keepingoxygen-rich and oxygen-poor blood separ<strong>at</strong>e effectively increases the flow ofoxygen-rich blood to tissues. This adapt<strong>at</strong>ion gives these active animals a largeand constant supply of oxygen. The development of the four-chambered heartallowed for an increase in energy use and eventually gave organisms increasedcontrol over their body temper<strong>at</strong>ure.Infer Many reptiles are ambush pred<strong>at</strong>ors, hiding and waiting for prey to come tothem as opposed to actively hunting. Explain how this behavior may be rel<strong>at</strong>ed totheir circul<strong>at</strong>ory system.FIGURE <strong>26</strong>.3 Amniote HeartsThe heart, the pump th<strong>at</strong> moves blood around an organism’sbody, has developed differently in reptiles and mammals.Oxygen-poor bloodOxygen-rich bloodTHREE-CHAMBERED HEARTReptile hearts have three chambers. A septum th<strong>at</strong> only partiallydivides the heart helps direct oxygen-rich and oxygenpoorblood.to bodyto bodyright<strong>at</strong>riumfrom lungsFOUR-CHAMBERED HEARTBirds and mammals have a heart divided into four chambers,which keeps oxygen-rich and oxygen-poor blood separ<strong>at</strong>e.from bodyto bodyfrom lungsto lungsto lungsleft <strong>at</strong>riumright<strong>at</strong>riumleft <strong>at</strong>riumfrom bodyventricleseptumrightventricleseptumleft ventricleContrast How do differences in the septum affect blood flow in these hearts?790 Unit 8: Animals
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