The heart has 4 chambers: Right Atrium, Right Ventricle, Left Atrium, Left Ventricle (in the picture, the right ventricle is bigger than the left, but it’s really the opposite). The two Atria pump together (sending blood to the Ventricles), and the two Ventricles pump together (sending blood to either the lungs, or out to the body).
Following the path of blood as it’s pumped through the heart:
1. Blood returns to heart from all over the body, through the veins. This is “venous blood,” [dark blue in the diagram], with carbon dioxide (CO2) in it, but without oxygen (O2).
2. Venous blood enters the Right Atrium, & gets sent to the Right Ventricle. From there it’s pumped into the Lungs through the Pulmonary Artery.
3. In the Lungs, blood drops off CO2 to be breathed out, and picks up O2 that we’ve breathed in, becoming “oxygenated” [bright red in the diagram].
4. Oxygenated blood returns to the heart through the Pulmonary Vein, enters the Left Atrium, and gets sent to the Left Ventricle.
5. The Left Ventricle pumps it into the Aorta, the largest artery in the body. From there the oxygenated blood travels away from the heart, through arteries, delivering O2 to every part of the body.
6. All cells of the body use O2 to make their energy to live. They drop off CO2, which returns to the heart through the veins, to get breathed out.
Note that there are 4 valves in the heart, between the Atria and the Ventricles, between the Right Ventricle and the Pulmonary Artery, and between the Left Ventricle and the Aorta. The valves keep the blood flowing in its single direction. If they get damaged (Valvular Heart Disease), there’s more stress on the heart muscle as a whole, which can cause Heart Failure.
Not seen in the picture above is the Coronary Artery. This is the first artery to branch off from the Aorta, so it can deliver oxygenated blood to the heart muscle itself (see below). When the Coronary Artery gets diseased, and its blood flow gets obstructed, the heart can’t get the oxygen it needs, and you have a Heart Attack.
The heart consists of 3 layers, from inside to out. The thin layer that lines the inside of the chambers, in contact with blood flow, is the Endocardium (if bacteria get in the blood and latch on, a serious infection Endocarditis occurs). The bulk of the heart, all muscle, is called the Myocardium, so if a part of it dies, it’s a “Myocardial Infarction (M.I.),” i.e. a “heart attack”. The outermost thin layer is the Epicardium, which is then covered by a membrane called the Pericardium.
The heart is unique in that it has its own electrical system, that begins when a fetus is around 2 months old. A cluster of cells in the muscle of the Right Atrium, called the Sinoatrial Node, generates electricity a little quicker than every second (usually). The current follows a natural pathway through both Atria, then to the Atrioventricular Node, from where it’s shunted down to both ventricles. As the electricity passes through it, heart muscle contracts in a coordinated way, and thus the heart pumps blood.
The electrocardiogram (EKG) traces the electricity as it runs. It can identify interferences, like from a heart attack (dead myocardium), or damaged muscle that’s still trying to pump, or chronic muscle stress from long-standing hypertension. The normal electrical current is shown here:
Certain diseases interrupt the circuit, causing Arrhythmias (now called dysrhythmias). These include types of tachycardia (heart beats too fast), bradycardia (too slow), extra beats (usually harmless), skipped beats (harmful or not, depending), or a completely chaotic irregular rhythm called Atrial Fibrillation (“A. Fib”). We can live with A Fib, because the ventricles still pump blood. But Ventricular Fibrillation (“V. Fib”), random fluttering without coordinated pumping, equals cardiac arrest (a shock, “defibrillation,” can often interrupt V. Fib & let the SA Node take over again). “Flat Line,” i.e. no electricity at all, is true death.