Diastole This should really be called “ventricular diastole”, but don't worry about this for GCSE. During diastole the thick muscular walls of the ventricles relax. Again, this happens to both sides of the heart. The pressure of the blood in the ventricles falls low enough for the bicuspid valve to open.

**Explanation:**

During diastole, the heart is in its relaxation phase where it is filling up with blood before the next contraction. The correct answer is option B, which states that blood enters the heart during diastole. Let's explain this in detail:

During diastole, the atria and ventricles of the heart are relaxed and their chambers expand. This expansion creates a lower pressure within the heart chambers, allowing blood to flow into the heart from various parts of the body.

**The Flow of Blood during Diastole:**
During diastole, blood flows in a specific direction within the heart:

1. **Passive Blood Flow:** As the atria relax, blood flows passively from the major veins (such as the superior and inferior vena cava) into the atria. This blood is deoxygenated and comes from various organs and tissues of the body.

2. **Atrial Contraction:** Towards the end of diastole, the atria contract, further pushing blood into the ventricles. This contraction is responsible for about 20-30% of the blood filling in the ventricles.

3. **Opening of Atrioventricular (AV) Valves:** As the atria contract and fill the ventricles, the pressure in the ventricles increases. This causes the AV valves (tricuspid valve on the right side and mitral valve on the left side) to open, allowing blood to flow from the atria into the ventricles.

4. **Ventricular Filling:** As blood enters the ventricles, they continue to relax and expand, creating a larger space for blood to fill. This process is known as ventricular filling and occurs throughout diastole.

5. **Closure of AV Valves:** Towards the end of diastole, the ventricles begin to contract slightly. This contraction causes the AV valves to close, preventing the backflow of blood into the atria.

In summary, blood enters the heart during diastole as the atria relax and passively receive blood from the major veins. The atrial contraction and opening of the AV valves allow further filling of the ventricles. This process ensures that the heart chambers are adequately filled with blood before the next contraction (systole) occurs.

During diastole blood enter in heart and during systole leave from heart.