The human heart is a hollow muscular organ, whose function is to pump blood, making it flow throughout a complex network of blood vessels (arteries, capillaries and veins) to provide the different body tissues and organs of the body with oxygen and nutrients, which are essential for life. This pumping function is autonomous as its muscle contractions does not depends on our cerebral cortex but on its own conducting system. The muscle of the heart is called myocardium as it is specialized striated muscle.
When filled with blood, it is about the size of a human fist and is located in the center of the chest, slightly to the left of your sternum (breastbone). As the body develops, the heart grows at the same rate as the fist. So an infant's heart and fist are about the same size at birth. The human heart is actually shaped like an upside-down pear. A double-layered membrane called the pericardium surrounds your heart like a sac. The outer layer of the pericardium surrounds the roots of your heart's major blood vessels and is attached by ligaments to your spinal column, diaphragm, and other parts of your body.
Anatomical Description
The heart has four cavities, which are called chambers; two atria and two ventricles. The two atria are not communicated but isolated from one another, with the two ventricles also being isolated from one another. Thus, the right atrium is communicated only with the right ventricle through the tricuspid valve, constituting the right side of the heart as only deoxygenated blood flows through them in its way to the lungs. Meanwhile the left atrium is connected to the left ventricle through the mitral valve as oxygen-rich blood flows through them, being pumped into every organ and tissue by the left ventricle contraction (systole).
The ventricles meet at the bottom of the heart to form a pointed base which slightly points toward the left side of your chest. The left ventricle contracts most forcefully, so you can best feel your heart pumping on the left side of your chest. A wall of muscle, called the septum, separates the right and left sides of the heart. The left ventricle is the largest and strongest chamber in your heart as its chamber walls are only about a half-inch thick, but they have enough force to push blood through the aortic valve and into your body.
Four valves controls the blood flow through your heart. 1) The mitral valve connects the left atrium with the left ventricle below it, allowing oxygen-rich blood from the lungs pass through. 2) The tricuspid valve connects the right atrium with the right ventricle. 3) The pulmonary valve controls blood flow from the right ventricle into the pulmonary arteries, which carry blood to your lungs to pick up oxygen. 4) The aortic valve opens the way for oxygen-rich blood to pass from the left ventricle into the aorta, which is the largest artery, delivering it to the rest of the body.
Electrical impulses that travels through nerves connected to the myocardium (heart muscle) cause the heart to contract. This electrical signal begins in the sinoatrial (SA) node, located at the top of the right atrium. The SA node is sometimes called the heart's natural pacemaker. An electrical impulse from this natural pacemaker travels through the muscle fibers of the atria and ventricles, causing them to contract. Although the SA node sends electrical impulses at a certain rate, the heart rate may still change depending on physical demands, stress, or hormonal factors.
From the moment of development through the moment of death, the heart pumps. The heart, therefore, has to be strong. The average heart's muscle, called cardiac muscle, contracts and relaxes about 70 to 80 times per minute without you ever having to think about it. As the cardiac muscle contracts it pushes blood through the chambers and into the vessels. The ventricles of the heart have two states: systole (contraction) and diastole (relaxation). During diastole blood fills the ventricles and during systole the blood is pushed out of the heart into the arteries.
Below, schematic image of the human heart, showing its four cavities and valves.
