During systole of heart

Options

A. only atria contract
B. only ventricles contract
C. atria and ventricles contract seperately
D. Atria and ventricles contract simultaneously

Answer

### Step-by-Step Solution:

1. **Understanding the Heart's Structure**:
- The heart consists of four chambers: two atria (auricles) and two ventricles.
- The right atrium receives deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs.

2. **Defining Systole**:
- Systole is the phase of the heartbeat when the heart muscle contracts. This phase is responsible for pumping blood out of the heart.

3. **Contraction of Atria**:
- During systole, the atria (auricles) contract first. This contraction pushes blood from the atria into the ventricles.

4. **Contraction of Ventricles**:
- Following the contraction of the atria, the ventricles contract. The right ventricle pumps deoxygenated blood into the pulmonary artery, which carries it to the lungs for oxygenation.

5. **Oxygenation of Blood**:
- In the lungs, the blood receives oxygen and releases carbon dioxide. This oxygenated blood then returns to the heart.

6. **Return to the Heart**:
- The oxygenated blood enters the left atrium from the lungs, preparing for the next cycle of contraction.

7. **Simultaneous Contraction**:
- It's important to note that while the atria and ventricles contract, they do so in a coordinated manner. The atria contract first, followed by the ventricles, ensuring efficient blood flow.