The size of the fist of you hand, your heart pumps 24 hours a day non-stop to feed your body. Hence it is one of the most vital organs in your body. There are lots of stories and poems that connect heart to emotions, however, it has nothing to do with your emotions at all. Simply speaking your heart is a pump, and it pumps the blood to your body and itself.
Heart is basically divided into 2 sides- the right heart and the left heart. The right heart gets the dirty blood (oxygen depleted) from the body and pumps it to the lungs for cleaning. The left side of the heart takes the clean (oxygen enriched) blood from the lungs and pumps it out to the body and itself.
There are 4 chambers in your heart two for each side of the heart. The right heart has right atrium and right ventricle while the left side has a left atrium and left ventricle. The atria and the ventricles are separated by valves (doors) that open and close in response to pressure within the chambers. The right heart door is called the tricuspid valve (valve has three leaflets) and mitral valve. When the blood comes into the atria, the valves between the atria and the ventricles are closed. The valves remain closed until the atria become so filled that they exert pressure on the valves to push them open so that the blood can now flow from the atria into the ventricles. Initially the blood flows passively into the ventricles, and then remaining blood is actively squeezed into the ventricles by the atrial kick, and after that the valvular doors shut close.
Now the blood goes from the ventricles into the pulmonary circulation via pulmonary artery. The valve (door) from the venrticle to the pulmonary artery is called pulmonic valve, and similar door from left ventricle to the aorta (blood vessel leading to the body) is called the aortic valve and they also close and open to the pressure dynamics just like the tricuspid valve and mitral valve.
The atria are like reservoirs of the heart, while ventricles are the main pumps that pump the blood out of the heart to the lungs and body, respectively. The right heart is a low pressure system because it pumps blood to the pulmonary system, which is a low pressure system. The left heart is a high pressure system, since it pumps blood to all the body and itself. Heart feeds itself only when it rests (diastole), and feeds rest of the body by contracting or pumping (systole).
Normally the heart beats at 60 to 100 times per minute, except in athletes where it is normal even at 40 beats a minute. If a heart beats too slow (< 60 bpm) it is called bradycardia and if it beats too fast (> 100bpm) itis called tachycardia. Whenever heart beats too slow or too fast you may start to feel dizzy, lightheaded or you may pass out. This happens because the amount of blood that your heart pumps out per beat (cardiac out put) is less than what you body needs for proper functioning. These arrhythmias and others can be treated with medicines, pacemakers and defibrillators or ablations.
High blood pressure, diabetes, coronary artery disease can cause problems for heart's pumping function by making it work harder to pump blood into the circulation through narrowed blood vessels. If the blockages in the hearts circulation get too bad, then you can have a heart attack. In addition when heart has to pump harder and harder, initially its walls get thick like any other muscle and later on they given in and heart becomes baggy and enlarged. This heart is now very weak, and hence cannot pump enough blood forward. This can lead to overflow of blood backwards into the lungs, legs, and gastrointestinal system casing congestive heart failure.
Other problems with circulation can occur if the valves get leaky because they cannot open or close too well. This can cause either obstruction to blood flow when valves can not open well, or it can cause regurgitation because blood starts to flow backwards through the valves that can not close too well. These problems can lead to a drop in the cardiac output and symptoms such as shortness of breath, chest pain, dizziness, and light headedness.
In summary heart is a pump that keeps our circulation going even when we are resting or sleeping. However any structural or physiologic problems can lead to strain and failure of the heart as a pump, and thus compromising our circulatory system.