At the end of joint diastole ventricle is almost 70% filled with blood because the atrio ventricular valve is open.
the rest 30% blood fills when atria contracts.

At the end of joint diastole, the ventricle is two-thirds filled with blood. This is due to several factors that contribute to the filling of the ventricle during diastole.

1. Ventricular relaxation: During diastole, the ventricles relax, allowing the chambers to expand and create a low-pressure environment. This relaxation phase is known as joint diastole.

2. Atrial contraction: Prior to ventricular filling, the atria contract, pushing blood into the ventricles. This atrial contraction, also known as atrial systole, contributes to the filling of the ventricles.

3. Passive blood flow: As the ventricles relax, blood flows passively from the atria into the ventricles due to the pressure difference. This passive filling accounts for a significant portion of ventricular filling.

4. Venous return: The venous return, the volume of blood returning to the heart from the systemic circulation, also plays a role in ventricular filling. The venous return is influenced by factors such as skeletal muscle contraction, respiratory movements, and venous tone.

5. End-diastolic volume: The volume of blood present in the ventricle at the end of joint diastole is known as the end-diastolic volume (EDV). The EDV is determined by the balance between ventricular filling and ejection during the cardiac cycle.

6. Frank-Starling mechanism: The Frank-Starling mechanism describes the relationship between the EDV and stroke volume (SV). According to this mechanism, an increase in EDV leads to an increase in SV. This relationship ensures that the heart pumps an adequate amount of blood based on the initial filling volume.

In summary, at the end of joint diastole, the ventricle is two-thirds filled with blood due to a combination of factors including ventricular relaxation, atrial contraction, passive blood flow, venous return, and the Frank-Starling mechanism. These mechanisms ensure that the ventricles receive an adequate amount of blood to be pumped out during systole.