KEY POINTS

1. The cardiac cycle is the sequence of electrical and mechanical events during the course of a single heartbeat.
2. Cardiac output is determined by the heart rate, myocardial contractility, and preload and afterload.
3. The majority of cardiomyocytes consist of myofibrils, which are rodlike bundles that form the contractile elements within the cardiomyocyte.
4. The basic working unit of contraction is the sarcomere.
5. Gap junctions are responsible for electrical coupling of small molecules between cells.
6. Action potentials have four phases in the heart.
7. The key player in cardiac excitation-contraction coupling is the ubiquitous second messenger calcium.
8. β-Adrenoreceptors stimulate chronotropy, inotropy, lusitropy, and dromotropy.
9. Hormones with cardiac action can be synthesized and secreted by cardiomyocytes or produced by other tissues and delivered to the heart.
10. Cardiac reflexes are fast-acting reflex loops between the heart and the CNS that contribute to regulation of cardiac function and maintenance of physiologic homeostasis.