In frog each cardiac cycle begin with the contraction of

Options

A. right auricle
B. left auricle
C. interauricular septum
D. sinus venosus

Answer

To answer the question "In frog each cardiac cycle begins with the contraction of," we will analyze the options provided and understand the role of each structure in the frog's heart.

### Step-by-Step Solution:

1. **Understanding the Cardiac Cycle**: The cardiac cycle refers to the sequence of events in the heart during one complete heartbeat, which includes contraction (systole) and relaxation (diastole).

2. **Identifying the Structures**: The options given are:
- Right Auricle
- Left Auricle
- Interauricular Septum
- Sinus Venosus

3. **Function of Each Structure**:
- **Right Auricle**: This chamber receives deoxygenated blood from the body via veins. Its contraction helps to push blood into the right ventricle.
- **Left Auricle**: This chamber receives oxygenated blood from the lungs. Its contraction pushes blood into the left ventricle.
- **Interauricular Septum**: This is a wall that separates the right and left auricles. Its primary function is to prevent the backflow of blood between the two auricles.
- **Sinus Venosus**: This is a structure that collects deoxygenated blood from the body and is responsible for initiating the contraction of the heart. It acts as a pacemaker in frogs, setting the rhythm for the heart's contractions.

4. **Determining the Starting Point of the Cardiac Cycle**: In frogs, the cardiac cycle begins with the contraction of the sinus venosus. This contraction sends a wave of depolarization that spreads through the heart, leading to the contraction of the auricles and subsequently the ventricles.

5. **Conclusion**: Based on the functions of the structures, the correct answer to the question is that each cardiac cycle in frogs begins with the contraction of the **Sinus Venosus**.

### Final Answer:
In frogs, each cardiac cycle begins with the contraction of the **Sinus Venosus**.

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