Number of chambers in human heart.

## The reasoning The human heart has **4 chambers** — two on top and two at the bottom. Think of it like a duplex building with two flats on each floor: - **Upper chambers (atria):** Right atrium and Left atrium — these receive blood coming INTO the heart - **Lower chambers (ventricles):** Right ventricle and Left ventricle — these pump blood OUT of the heart This 4-chamber design is crucial because it separates oxygen-poor blood (right side) from oxygen-rich blood (left side), making circulation efficient. The right side handles blood going to the lungs; the left side pumps blood to your entire body. ## Why the wrong options tempt you **Option A (2):** You might confuse this with simpler animals like fish, which have 2-chambered hearts. **Option B (3):** Amphibians (like frogs) have 3 chambers — don't mix up animal groups! **Option D (5):** No vertebrate has 5 chambers. This is just a distractor. ## Quick takeaway **Remember: Humans have a "double duplex" heart — 2 atria upstairs, 2 ventricles downstairs = 4 total chambers for efficient double circulation.**