Introduction To The Heart...^_^

The HEART is a muscular organ in all vertebrates responsible for pumping blood through the blooad vessels by repeated, rhythmic contractions. The term 'cardiac' (as in cardiology) means "related to heart" and comes from the Greek kardia, for "heart". The heart of a vertebrate is composed of cardiac muscle, an involuntary muscle tissue which is found only within this organ. Average human heart beating at 72 beats per minute, and will beat approximately 2.5 billion times during a lifetime spanning 66 years.

EARLY DEVELOPMENT...^_^
Heart is derived from embryonic mesoderm germ-layer cells that differentiate after gastrulation into mesothelium, endothelium, and myocardium. Mesothelial pericardium forms outer lining of the heart. The inner lining of the heart, lymphatic and blood vessels develop from endothelium. Myocardium develops into heart muscle.

From splachnopleuric mesoderm tissue, the cardiogenic plate develops cranially and laterally to the neural plate. In the cardiogenic plate, 2 separate angiogenic cell clusters form on either side of the embryo. Each cell cluster coelasces to form an endocardial tube continuous with a dorsal aorta and a vitteloumbilical vein. As embryonic tissue continues to fold, the 2 endocardial tube are pushed into the thoracic cavity and begin to fuse together and are completely fused at approximately 21 days of embryonic life.

The human embryonic heart begins beating around 21 days after conception, or 5 weeks after the last normal menstrual period, which is the normal date used to date pregnancy. It is unknown how blood in the human embryo circulates for the first 21 days in the absence of a functioning heart (hopefully still dlm kajian...^_^). The human heart begins beating at a rate near the mother's, about 75-80 beats per minute. The embryonic heart then accelerates linearly for the first month of beating, peaking at 165-185 beats per minute during the early 7th week (usually early 9th week after normal menstrual period). This acceleration is approximately 3.3 beats per minute per day, or about 10 beats per minute every 3 days, an increase of 100 beats per minute in the first month.

After peaking at about 9.2 weeks after normal menstrual cycle, heart decelerates to about 152 beats per minute during the 15th week efter normal menstrual cycle. After the 15th week the deceleration slows reaching an average rate about 145 beats per minute at term. The regression formula which describes this acceleration before the embryo reaches 25 mm in crown-rump length or 9.2 normal menstrual cycle weeks is Age in days. There is no difference in male and female heart rates before birth.

HEART STRUCTURE...^_^
The picture shows the basic structure of the heart. Mammals show complete separation of heart into 2 pumps, for a total of four heart chambers.

In the human body, the heart usually situated in the middle of the thorax with the largest part of the heart slightly offset to the left, underneath the breastbone. The heart is usually felt to be on the left side because the left ventricle is stronger than the right ventricle (it pumps blood to all body part). The left lung is smaller than the right lung because the heart occupies more of the left hemithorax. The heart is enclosed by a sac known as the pericardium and is surrounded by the lungs. The pericardium comprises 2 parts : the fibrous pericardium, made of dense fibrous connective tissue; and a double membrane structure containing a serous fluid to reduce friction during heart contractions (the serous pericardium). The mediastinum, a subdivision of the thoracic cavity, is the name of the heart cavity.

The apex of the heart is the blunt point situated in an inferior (pointing down and left) direction. A stethoscope can be placed directly over the apex so that the beats can be counted. Is is located posterior to the 5th intercostal space in the mid-clavicular line. In normal adults, the mass of the heart is 250-350 g, or about 3/4 the size of a clenched fist, but extremely diseased heart can be up to 1000 g in mass due to hypertrophy. It consists of 4 chambers : the 2 upper atria (singular : atrium) and the 2 lower ventricles.

LOVE OUR HEART...PEACE NO WAR!!! ^_^