Smooth muscle cells lack a well-developed transverse tubule (T-tubule) system. Instead, their plasma membrane possesses flask-shaped invaginations called caveolae, which functionally serve as the smooth muscle counterpart of T-tubules for calcium signaling.

At a resting heart rate of approximately 75 beats per minute, the total duration of a single complete cardiac cycle (atrial systole 0.1 s, ventricular systole 0.3 s, complete cardiac diastole 0.4 s) is $60 / 75 = 0.8$ seconds.

The left atrium receives oxygenated blood returning from the lungs via the pulmonary veins.