The Human Heart
The Human Heart
The heart is one of the most important organs in the   entire human body. It is really nothing more than a pump, composed of muscle   which pumps blood throughout the body, beating approximately 72 times per   minute of our lives. The heart pumps the blood, which carries all the vital   materials which help our bodies function and removes the waste products that we   do not need. For example, the brain requires oxygen and glucose, which, if not   received continuously, will cause it to loose consciousness. Muscles need   oxygen, glucose and amino acids, as well as the proper ratio of sodium, calcium   and potassium salts in order to contract normally. The glands need sufficient   supplies of raw materials from which to manufacture the specific secretions. If   the heart ever ceases to pump blood the body begins to shut down and after a   very short period of time will die.
The heart is essentially a muscle(a little larger than the fist). Like   any other muscle in the human body, it contracts and expands. Unlike skeletal   muscles, however, the heart works on the "All -or-Nothing Law". That is, each   time the heart contracts it does so with all its force. In skeletal muscles,   the principle of "gradation" is present. The pumping of the heart is called the Cardiac Cycle, which occurs about 72 times per minute. This means that   each cycle lasts about eight-tenths of a second. During this cycle the entire   heart actually rests for about four-tenths of a second.
Make-up of the Heart.
The walls of the heart are made up of three layers,   while the cavity is divided into four parts. There are two upper chambers,   called the right and left atria, and two lower chambers, called the   right and left ventricles. The Right Atrium, as it is called, receives   blood from the upper and lower body through the superior vena cava and   the inferior vena cava, respectively, and from the heart muscle itself   through the coronary sinus. The right atrium is the larger of the two   atria, having very thin walls. The right atrium opens into the right ventricle   through the right atrioventicular valve(tricuspid), which only allows   the blood to flow from the atria into the ventricle, but not in the reverse   direction. The right ventricle pumps the blood to the lungs to be reoxygenated.   The left atrium receives blood from the lungs via the four pulmonary   veins. It is smaller than the right atrium, but has thicker walls. The   valve between the left atrium and the left ventricle, the left   atrioventicular valve(bicuspid), is smaller than the tricuspid. It opens   into the left ventricle and again is a one way valve. The left ventricle pumps   the blood throughout the body. It is the Aorta, the largest artery in   the body, which originates from the left ventricle.
The Heart works as a pump moving blood around in our bodies to nourish   every cell. Used blood, that is blood that has already been to the cells and   has given up its nutrients to them, is drawn from the body by the right half of   the heart, and then sent to the lungs to be reoxygenated. Blood that has been   reoxygenated by the lungs is drawn into the left side of the heart and then   pumped into the blood stream. It is the atria that draw the blood from the   lungs and body, and the ventricles that pump it to the lungs and body. The   output of each ventricle per beat is about 70 ml, or about 2 tablespoons. In a   trained athlete this amount is about double. With the average heart rate of 72   beats per minute the heart will pump about 5 litres per ventricle, or about 10   litres total per minute. This is called the cardiac output. In a trained   athlete the total cardiac output is about 20 litres. If we multiply the normal,   non-athlete output by the average age of 70 years, we see that the cardiac   output of the average human heart over a life time would be about 1 million   litres, or about 250,000 gallons(US)!