The<unk>  is a muscular organ in most animals that pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to the lungs. In humans, the<unk>  is approximately the size of a closed fist and is located between the lungs, in the middle compartment of the chest. In humans, other mammals, and birds, the<unk>  is divided into four chambers: upper left and right atria and lower left and right ventricles. Commonly the right atrium and ventricle are referred together as the right<unk>  and their left counterparts as the left<unk> . Fish, in contrast, have two chambers, an atrium and a ventricle, while reptiles have three chambers. In a<unk> hy<unk>  blood flows one way through the<unk>  due to<unk>  valves, which prevent backflow. The<unk>  is enclosed in a protective sac, the pericardium, which also contains a small amount of fluid. The wall of the<unk>  is made up of three layers: epicardium, myocardium, and endocardium. The<unk>  pumps blood with a rhythm determined by a group of pacemaker cells in the sinoatrial node. These generate a current that causes the<unk>  to contract, traveling through the atrioventricular node and along the conduction system of the<unk> . In humans, deoxygenated blood enters the<unk>  through the right atrium from the superior and inferior venae cavae and passes it to the right ventricle. From here it is pumped into pulmonary circulation to the lungs, where it receives oxygen and gives off carbon dioxide. Oxygenated blood then returns to the left atrium, passes through the left ventricle and is pumped out through the aorta into systemic circulation, traveling through arteries, arterioles, and capillaries—where nutrients and other substances are exchanged between blood vessels and cells, losing oxygen and gaining carbon dioxide—before being returned to the<unk>  through venules and veins. The<unk>  beats at a resting rate close to 72 beats per minute. Exercise temporarily increases the rate, but lowers resting<unk>  rate in the long term, and is good for<unk> <unk> h. Cardiovascular diseases (CVD) 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<unk> onary artery disease and stroke. Risk factors include: smoking, being overweight, little exercise, high cholesterol, high blood pressure, and poorly controlled diabetes, among others. Cardiovascular diseases frequently do not have symptoms or may cause chest pain or shortness of breath. Diagnosis of<unk>  disease is often done by the taking of a medical history, listening to the<unk> -sounds with a stethoscope, ECG, echocardiogram, and ultrasound. Specialists who focus on diseases of the<unk>  are called cardiologists, although many specialties of medicine may be involved in treatment.
Heart
Kidney
Lung
Brain
Liver
Central nervous system
Peripheral nervous system
Skin