When cardiac muscle is stimulated to shorten, the heart chamber (lumen) gets smaller and forces the blood out through a valve due to the contraction of the cardiac muscle fibers.
The contraction of the cardiac muscle fibers is caused by the release of calcium ions into the muscle cells. This causes the muscle fibers to slide past each other, which shortens the muscle and reduces the size of the heart chamber.
As the heart chamber gets smaller, the pressure inside the chamber increases, which forces the blood out through a valve and into the blood vessels. The valve prevents the blood from flowing back into the heart chamber, ensuring that the blood flows in the correct direction through the circulatory system.
This process is known as systole, which is the phase of the cardiac cycle during which the heart muscle contracts and pumps blood out of the heart.