Human heart chambers.

**The reasoning** The human heart is a double pump divided into **four chambers**: two upper chambers called **atria** (singular: atrium) and two lower chambers called **ventricles**. Think of it like a duplex with an upstairs and downstairs on both sides. - **Right atrium** receives deoxygenated blood from the body - **Right ventricle** pumps it to the lungs for oxygen - **Left atrium** receives oxygenated blood from the lungs - **Left ventricle** pumps it out to the entire body This four-chamber system separates oxygen-poor and oxygen-rich blood completely, making our circulation highly efficient. **Why the wrong options tempt you** **2 chambers** — You might think "left and right" and stop there, forgetting each side has two parts. **3 chambers** — Some animals like amphibians (frogs) actually have three chambers, so this confuses students who mix up different species. **5 chambers** — Just a distractor; no vertebrate has five heart chambers. **Quick takeaway** Remember: **"Two up, two down" — humans have 4 heart chambers** (2 atria receiving, 2 ventricles pumping), which keeps our blood circulation separate and efficient.