In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The atrium (an adjacent/upper heart chamber that is smaller than a ventricle) primes the Pump.
The right ventricle pumps blood into the pulmonary circulation to/for the lungs, and the left ventricle pumps blood into the systemic circulation through the aorta (systemic circulation).
Ventricles have thicker walls than atria and generate higher blood pressures. The physiologic load on the ventricles requiring pumping of blood throughout the body and lungs is much greater than the pressure generated by the atria to fill the ventricles. Further, the left ventricle has thicker walls than the right because it needs to pump blood to most of the body while the right ventricle fills only the lungs.