Final answer:
The heart is a hollow, muscular organ that pumps blood throughout the body using cardiac muscle tissue and rhythmic contractions. It has four chambers and valves to ensure blood flows in one direction.
Step-by-step explanation:
The heart is a hollow, muscular organ that pumps blood throughout the body. It consists mainly of cardiac muscle tissue and has four chambers: two atria and two ventricles. The heart uses rhythmic contractions to keep the blood flowing in just one direction, aided by valves between chambers.