Systemic heart refers to

Options

A. heart in lower vertebrates
B. the two ventricles together in humans
C. the heart that contracts under stimulation from nervous system
D. left auricle and left ventricle in higher vertebrates.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Heart Structure**: The human heart is a four-chambered organ, consisting of two atria (upper chambers) and two ventricles (lower chambers).

2. **Identifying the Chambers**: The right side of the heart includes the right atrium (or auricle) and the right ventricle. The left side consists of the left atrium (or auricle) and the left ventricle.

3. **Blood Flow in the Heart**:
- Deoxygenated blood from the body enters the right atrium via the superior and inferior vena cava.
- This blood moves into the right ventricle and is then pumped to the lungs through the pulmonary artery for oxygenation.

4. **Oxygenated Blood Return**: After the blood is oxygenated in the lungs, it returns to the heart through the pulmonary veins into the left atrium.

5. **Systemic Circulation**: The oxygenated blood from the left atrium moves into the left ventricle, which then pumps it into the aorta. This distribution of oxygenated blood to the body is known as systemic circulation.

6. **Defining Systemic Heart**: The term "systemic heart" specifically refers to the left side of the heart (left atrium and left ventricle) responsible for pumping oxygen-rich blood to the body. In higher vertebrates, including humans, both the left atrium and left ventricle can be considered part of the systemic heart.

7. **Conclusion**: Therefore, the systemic heart refers to the left side of the heart, particularly the left atrium and left ventricle, and can also encompass both ventricles in the context of humans.

**Final Answer**: The systemic heart refers to the left auricle and left ventricle in higher vertebrates, including humans.

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