The heart beats at a resting rate close to 52 beats per minute.
The heart is a muscular organ in humans and other animals , which pumps blood through the blood vessels of the circulatory system .. muscular. muscle. organ. Organ ( anatomy ). blood. blood. circulatory system. circulatory system. Blood provides the body with oxygen and nutrients , as well as assists in the removal of metabolic wastes .. oxygen. oxygen. In humans , the heart is located between the lungs , in the middle compartment of the chest .. middle compartment. mediastinum. chest. thorax. lungs. lungs. In humans , other mammals , and birds , the heart is divided into four chambers  : upper left and right atria ; and lower left and right ventricles .. atria. Atrium ( heart ). ventricles. Ventricle ( heart ). Commonly the right atrium and ventricle are referred together as the right heart and their left counterparts as the left heart .. Fish , in contrast , have two chambers , an atrium and a ventricle , while reptiles have three chambers .. In a healthy heart blood flows one way through the heart due to heart valves , which prevent backflow .. blood. blood. backflow. cardiac regurgitation. The heart is enclosed in a protective sac , the pericardium , which also contains a small amount of fluid .. pericardium. pericardium. fluid. pericardial fluid. The wall of the heart is made up of three layers  : epicardium , myocardium , and endocardium .. epicardium. epicardium. myocardium. myocardium. endocardium. endocardium. The heart pumps blood with a rhythm determined by a group of pacemaking cells in the sinoatrial node .. blood. blood. sinoatrial node. sinoatrial node. These generate a current that causes contraction of the heart , traveling through the atrioventricular node and along the conduction system of the heart .. atrioventricular node. atrioventricular node. conduction system of the heart. conduction system of the heart. The heart receives blood low in oxygen from the systemic circulation , which enters the right atrium from the superior and inferior venae cavae and passes to the right ventricle .. blood. blood. oxygen. oxygen. systemic circulation. systemic circulation. superior. superior vena cava. inferior. inferior vena cava. venae cavae. venae cavae. From here it is pumped into the pulmonary circulation , through the lungs where it receives oxygen and gives off carbon dioxide .. oxygen. oxygen. pulmonary circulation. pulmonary circulation. lungs. lungs. carbon dioxide. carbon dioxide. Oxygenated blood then returns to the left atrium , passes through the left ventricle and is pumped out through the aorta to the systemic circulation − where the oxygen is used and metabolized to carbon dioxide .. blood. blood. oxygen. oxygen. systemic circulation. systemic circulation. aorta. aorta. metabolized. metabolized. carbon dioxide. carbon dioxide. The heart beats at a resting rate close to 72 beats per minute .. rate. heart rate. Exercise temporarily increases the rate , but lowers resting heart rate in the long term , and is good for heart health .. rate. heart rate. Exercise. Physical exercise. resting heart rate. resting heart rate. Cardiovascular diseases -LRB- CVD -RRB- are the most common cause of death globally as of 2008 , accounting for 30 % of deaths .. Of these more than three quarters are a result of coronary artery disease and stroke .. coronary artery disease. coronary artery disease. stroke. stroke. Risk factors include  : smoking , being overweight , little exercise , high cholesterol , high blood pressure , and poorly controlled diabetes , among others .. blood. blood. smoking. smoking. overweight. overweight. high cholesterol. high cholesterol. high blood pressure. hypertension. diabetes. diabetes mellitus. Cardiovascular diseases frequently have no symptoms or may cause chest pain or shortness of breath .. chest. thorax. chest pain. chest pain. shortness of breath. shortness of breath. Diagnosis of heart disease is often done by the taking of a medical history , listening to the heart-sounds with a stethoscope , ECG , and ultrasound .. stethoscope. stethoscope. medical history. medical history. listening. auscultation. ECG. electrocardiogram. ultrasound. ultrasound. Specialists who focus on diseases of the heart are called cardiologists , although many specialties of medicine may be involved in treatment .. cardiologists. cardiologists