5 Steps to a 5 AP Biology, 2014-2015 Edition

178 ❯ STEP 4. Review the Knowledge You Need to Score High

Circulatory System

Heart

Welcome to the heart. The human heart is a four-chambered organ whose function is to

circulate blood by rhythmic contraction. The heart pumps oxygenated blood from the left

ventricle out to the aorta (Figure 15.1). From there it travels through arteriesto feed the

organs, muscles, and other tissues of the body. The blood returns to the heart via the veins.

The superior and inferior vena cavae return deoxygenated blood from the body to the heart.

The blood reenters the heart through the right atrium, passes through to the right ventri-

cle, and from there to the lungs to exchange carbon dioxide for more oxygen. At this point,

the blood has made a complete cycle through the body. The blood is at its most oxygenated

stage just after leaving the lungs as it enters the left side of the heart and travels into the

aorta. The blood is in its least oxygenated stage as it reenters the right atrium of the heart.

KEY IDEA

Deoxygenated blood arrives via vena cavae into right

atrium Oxygenated blood returns to heart via pulmonary veins

Right atrium (blood is

least oxygenated here) Left atrium (blood is most oxygenated here)

Left ventricle

Right ventricle

Aorta (sends blood to rest of body)

To lungs via

pulmonary artery

Figure 15.1 Oversimplified diagram of the heart and blood flow.

Structure–function relationships come up often on the AP Biology exam, and the circula-

tory system provides a good example you could add to an essay on the topic. The left ven-

tricle of the heart is the thickest and most muscular part of the heart, and the most pressure

is exerted on it. Why does this make sense functionally? Because the left ventricle is the por-

tion of the heart that needs to pump the blood into the aorta and to the rest of the body.

The left ventricle is structurally designed to fit its function. The right ventricle is smaller

and less muscular because it only pumps blood a short distance to the lungs for gas

exchange (the picking up of oxygen and the release of CO 2 ).

Blood

As we continue on our journey, if you look off to your right, you will see some of the blood

and its components passing us now. If you look closely, you will see little red blood cells,

which carry oxygen, traveling in the bloodstream. Thanks to a molecule termed hemoglo-

bin,the red blood cells are able to carry and deliver oxygen throughout the body to hard-

working organs and tissues. Iron is a major component of hemoglobin. If you do not have

enough iron in your diet, your ability to deliver oxygen via the blood can be compromised,

and you may develop anemia.