The heart atria are the upper chambers of the heart that receive blood from the veins and deliver it to the lower chambers, or ventricles. There are two atria in the heart: the right atrium receives oxygen-poor blood from the body and pumps it into the right ventricle, which then sends it to the lungs to be oxygenated; and the left atrium receives oxygen-rich blood from the lungs and pumps it into the left ventricle, which then sends it out to the rest of the body. The atria contract before the ventricles during each heartbeat, helping to fill the ventricles with blood and prepare them for contraction.

In medical terms, the heart is a muscular organ located in the thoracic cavity that functions as a pump to circulate blood throughout the body. It's responsible for delivering oxygen and nutrients to the tissues and removing carbon dioxide and other wastes. The human heart is divided into four chambers: two atria on the top and two ventricles on the bottom. The right side of the heart receives deoxygenated blood from the body and pumps it to the lungs, while the left side receives oxygenated blood from the lungs and pumps it out to the rest of the body. The heart's rhythmic contractions and relaxations are regulated by a complex electrical conduction system.

Heart rate is the number of heartbeats per unit of time, often expressed as beats per minute (bpm). It can vary significantly depending on factors such as age, physical fitness, emotions, and overall health status. A resting heart rate between 60-100 bpm is generally considered normal for adults, but athletes and individuals with high levels of physical fitness may have a resting heart rate below 60 bpm due to their enhanced cardiovascular efficiency. Monitoring heart rate can provide valuable insights into an individual's health status, exercise intensity, and response to various treatments or interventions.