What is the youngest unit or feature in this figure? 
Definitions:
Right Atrium
The right upper chamber of the heart that receives deoxygenated blood from the body and pumps it to the right ventricle.
Left Ventricle
The chamber within the heart that receives oxygen-rich blood from the left atrium and pumps it to the body through the aorta, playing a crucial role in systemic circulation.
Right Ventricle
The lower right chamber of the heart that pumps deoxygenated blood to the lungs for oxygenation.
Decreased Immunity
A reduction in the body's ability to fight infections and diseases, which can be caused by factors such as malnutrition, illness, or certain medications.
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