A medical student labels a diagram of a mammalian heart as part of revision for an anatomy exam, and needs to state the type of blood found in each of the four chambers. Which row correctly describes the type of blood found in the left atrium and in the left ventricle?

IGCSE Biology 0610: Oxygenated Blood in the Left Side of the Heart

Step 1: Trace the path of blood into the left side of the heart

In the mammal’s double circulation, blood that has been oxygenated at the lungs travels along the pulmonary vein directly into the left atrium. This blood is therefore oxygenated.

Step 2: Trace the path of blood through the left atrium into the left ventricle

The left atrium contracts and pushes this same blood down through the (bicuspid) atrioventricular valve into the left ventricle. No gas exchange happens inside the heart itself, so the blood passing into the left ventricle is still oxygenated. The left ventricle then pumps this oxygenated blood out along the aorta to the rest of the body.

Step 3: Eliminate the other options

Option B describes deoxygenated blood in both chambers, which is instead the situation on the right side of the heart (right atrium and right ventricle), not the left side.
Options C and D each describe the left atrium and left ventricle as containing different types of blood, but blood does not change from deoxygenated to oxygenated (or vice versa) while simply moving from one chamber to another on the same side of the heart; that change only happens at the lungs.

Final answer

Both the left atrium and the left ventricle contain oxygenated blood, option A.