Which of the following types of circulation is absent in mammals?

Options

A. Hepatic portal system
B. Renal portal system
C. Hypophyseal portal system
D. Coronary system of circulation.

Answer

To determine which type of circulation is absent in mammals, we can analyze the options provided:

1. **Hepatic Portal System**: This system is present in mammals, including humans. It carries blood from the gastrointestinal tract and spleen to the liver.

2. **Renal Portal System**: This system is typically found in some non-mammalian vertebrates, such as amphibians and reptiles. In mammals, including humans, the renal portal system is absent because the kidneys receive blood directly from the heart via the renal arteries.

3. **Hypophyseal Portal System**: This system is present in mammals. It connects the hypothalamus and the anterior pituitary gland, allowing for the direct transport of hormones.

4. **Coronary System of Circulation**: This system is also present in mammals. It supplies blood to the heart muscle itself.

Based on this analysis, the type of circulation that is absent in mammals is the **Renal Portal System**.

### Step-by-Step Solution:
1. **Identify the types of circulation**: Review the options given in the question.
2. **Analyze each option**:
- Check if the hepatic portal system is present in mammals.
- Check if the renal portal system is present in mammals.
- Check if the hypophyseal portal system is present in mammals.
- Check if the coronary system is present in mammals.
3. **Determine which system is absent**: After analyzing each option, conclude that the renal portal system is the one that is absent in mammals.

### Final Answer:
The type of circulation that is absent in mammals is the **Renal Portal System**.