The heart is a muscular organ in all vertebrates responsible for pumping blood through the blood vessels by repeated, rhythmic contractions, or a similar structure in annelids, mollusks, and arthropods. The term cardiac (as in cardiology) means "related to the heart" and comes from the Greek ?a?d??, kardia , for "heart." The heart of a vertebrate is composed of cardiac muscle, an involuntary striated muscle tissue which is found only within this organ. The average human heart, beating at 72 beats per minute, will beat approximately 2.5 billion times during a lifetime (about 66 years). It weighs on average 250 g to 300 g in females and 300 g to 350 g in males. [1]

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Early development

The mammalian heart is derived from embryonic mesoderm germ-layer cells that differentiate after gastrulation into mesothelium, endothelium, and myocardium. Mesothelial pericardium forms the inner lining of the heart. The outer lining of the heart, lymphatic and blood vessels develop from endothelium. Myocardium develops into heart muscle. ^[2]

From splachnopleuric mesoderm tissue, the cardiogenic plate develops cranially and laterally to the neural plate. In the cardiogenic plate, two separate angiogenic cell clusters form on either side of the embryo. Each cell cluster coalesces to form an endocardial tube continuous with a dorsal aorta and a vitteloumbilical vein. As embryonic tissue continues to fold, the two endocardial tubes are pushed into the thoracic cavity, begin to fuse together and are completely fused at approximately 21 days. ^[3]

The human embryonic heart begins beating around 21 days after conception, or five weeks after the last normal menstrual period (LMP), which is the date normally 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. The human heart begins beating at a rate near the mother’s, about 75-80 beats per minute (BPM).

The embryonic heart rate (EHR) then accelerates approximately 100 BPM during the first month of beating, peaking at 165-185 BPM during the early 7th week, (early 9th week after the LMP). This acceleration is approximately 3.3 BPM per day, or about 10 BPM every three days, an increase of 100 BPM in the first month. ^[4] After 9.1 weeks after the LMP, it decelerates to about 152 BPM (+/-25 BPM) during the 15th week after the LMP. After the 15th week the deceleration slows reaching an average rate of about 145 (+/-25 BPM) BPM at term. The regression formula which describes this acceleration before the embryo reaches 25 mm in crown-rump length or 9.2 LMP weeks is: Age in days = EHR(0.3)+6.

There is no difference in male and female heart rates before birth. ^[5]

Structure

The structure of the heart varies among the different branches of the animal kingdom. (See Circulatory system.) Cephalopods have two "gill hearts" and one "systemic heart". Fish have a two-chambered heart that pumps the blood to the gills and from there it goes on to the rest of the body. In amphibians and most reptiles, a double circulatory system is used, but the heart is not always completely separated into two pumps. Amphibians have a three-chambered heart.

thumb of the human heart. The heart is demarcated by:
-A point 9 cm to the left of the midsternal line (apex of the heart)
-The seventh right sternocostal articulation
-The upper border of the third right costal cartilage 1 cm from the right sternal line
-The lower border of the second left costal cartilage 2.5 cm from the left lateral sternal line. ^[6]

Birds and mammals show complete separation of the heart into two pumps, for a total of four heart chambers; it is thought that the four-chambered heart of birds evolved independently from that of mammals.

In the human body, the heart is usually situated in the middle of the thorax with the largest part of the heart slightly offset to the left (although sometimes it is on the right, see dextrocardia), underneath the sternum. The heart is usually felt to be on the left side because the left heart (left ventricle) is stronger (it pumps to all body parts). The left lung is smaller than the right lung because the heart occupies more of the left hemithorax. The heart is fed by the coronary circulation and enclosed by a sac known as the pericardium and is surrounded by the lungs. The pericardium comprises two parts: the fibrous pericardium, made of dense fibrous connective tissue; and a double membrane structure (parietal and visceral pericardium) containing a serous fluid to reduce friction during heart contractions. The heart is located in the mediastinum, the central sub-division of the thoracic cavity. The mediastinum also contains other structures, such as the esophagus and trachea, and is flanked on either side by the right and left pulmonary cavities, which house the lungs. ^[7]

The apex 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. It is located posterior to the 5th intercostal space just medial of the left mid-clavicular line. In normal adults, the mass of the heart is 250-350 g (9-12 oz), or about twice the size of a clenched fist (it is about the size of a clenched fist in children), but extremely diseased hearts can be up to 1000 g (2 lb) in mass due to hypertrophy. It consists of four chambers, the two upper atria and the two lower ventricles.

Functioning

In mammals, the function of the right side of the heart (see right heart) is to collect de-oxygenated blood, in the right atrium, from the body (via superior and inferior vena cavae) and pump it, via the right ventricle, into the lungs (pulmonary circulation) so that carbon dioxide can be dropped off and oxygen picked up (gas exchange). This happens through the passive process of diffusion. The left side (see left heart) collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood moves to the left ventricle which pumps it out to the body (via the aorta). On both sides, the lower ventricles are thicker and stronger than the upper atria. The muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation.

Starting in the right atrium, the blood flows through the tricuspid valve to the right ventricle. Here it is pumped out the pulmonary semilunar valve and travels through the pulmonary artery to the lungs. From there, blood flows back through the pulmonary vein to the left atrium. It then travels through the mitral valve to the left ventricle, from where it is pumped through the aortic semilunar valve to the aorta. The aorta forks and the blood is divided between major arteries which supply the upper and lower body. The blood travels in the arteries to the smaller arterioles, then finally to the tiny capillaries which feed each cell. The (relatively) deoxygenated blood then travels to the venules, which coalesce into veins, then to the inferior and superior venae cavae and finally back to the right atrium where the process began.

The heart is effectively a syncytium, a meshwork of cardiac muscle cells interconnected by contiguous cytoplasmic bridges. This relates to electrical stimulation of one cell spreading to neighboring cells.

Some cardiac cells are self-excitable, contracting without any signal from the nervous system, even if removed from the heart and placed in culture. Each of these cells has its own intrinsic contraction rhythm. A region of the human heart called the sinoatrial node SA node, or pacemaker, sets the rate and timing at which all cardiac muscle cells contract. The SA node generates electrical impulses, much like those produced by nerve cells. Because cardiac muscle cells are electrically coupled by inter-calated disks between adjacent cells, impulses from the SA node spread rapidly through the walls of the artria, causing both artria to contract in unison. The impulses also pass to another region of specialized cardiac muscle tissue, a relay point called the atrioventricular (AV) node , located in the wall between the right artrium and the right ventricle. Here, the impulses are delayed for about 0.1s before spreading to the walls of the ventricle. The delay ensures that the artria empty completely before the ventricles contract. Specialized muscle fibers called Purkinje fibers then conduct the signals to the apex of the heart along and throughout the ventricular walls. The Purkinje fibres form conducting pathways called bundle branches. The impulses generated during the heart cycle produce electrical currents, which are conducted through body fluids to the skin, where they can be detected by electrodes and recorded as an electrocardiogram (ECG or EKG). ^[8]

Cardiac arrest is the sudden cessation of normal heart rhythm which can include a number of pathologies such as tachycardia, an extremely rapid heart beat which prevents the heart from effectively pumping blood, fibrillation which is an irregular and ineffective heart rhythm, and asystole which is the cessation of heart rhythm entirely.

Cardiac Tamponade is a condition in which the fibrous sac surrounding the heart fills with excess fluid or blood, suppressing the heart's ability to beat properly. Tamponade is treated by pericardiocentesis, the gentle insertion of the needle of a syringe into the pericardial sac (avoiding the heart itself) on an angle, usually from just below the sternum, and gently withdrawing the tamponading fluids.

History of discoveries

The valves of the heart were discovered by a physician of the Hippocratean school around the 4th century BC. However, their function was not properly understood then. Because blood pools in the veins after death, arteries look empty. Ancient anatomists assumed they were filled with air and that they were for transport of air.

Philosophers distinguished veins from arteries but thought that the pulse was a property of arteries themselves. Erasistratos observed that arteries that were cut during life bleed. He ascribed the fact to the phenomenon that air escaping from an artery is replaced with blood that entered by very small vessels between veins and arteries. Thus he apparently postulated capillaries but with reversed flow of blood.

The 2nd century AD, Greek physician Galenos (Galen) knew that blood vessels carried blood and identified venous (dark red) and arterial (brighter and thinner) blood, each with distinct and separate functions. Growth and energy were derived from venous blood created in the liver from chyle, while arterial blood gave vitality by containing pneuma (air) and originated in the heart. Blood flowed from both creating organs to all parts of the body where it was consumed and there was no return of blood to the heart or liver. The heart did not pump blood around, the heart's motion sucked blood in during diastole and the blood moved by the pulsation of the arteries themselves.

Galen believed that the arterial blood was created by venous blood passing from the left ventricle to the right by passing through 'pores' in the inter ventricular septum, air passed from the lungs via the pulmonary artery to the left side of the heart. As the arterial blood was created 'sooty' vapors were created and passed to the lungs also via the pulmonary artery to be exhaled.

The first major scientific understanding of the heart was put forth by the medieval Arab polymath Ibn Al-Nafis, regarded as the father of circulatory physiology. ^[9] He was the first physician to correctly describe pulmonary circulation, ^[10] the capillary ^[11] and coronary circulations. ^[12]

Prior to Ibn Al-Nafis, the theory that was widely accepted was that of Galen's, which was slightly improved upon by Avicenna. Ibn Al-Nafis rejected the Galen-Avicenna theory and corrected many wrong ideas that were put forth by it, and also adding his new found observations of pulse and circulation to the new theory.

Ibn Al-Nafis' major observations include (as surmised by Dr. Paul Ghalioungui):

1. "Denying the existence of any pores through the interventricular septum."

2. "The flow of blood from the right ventricle to the lungs where its lighter parts filter into the pulmonary vein to mix with air."

3. "The notion that blood, or spirit from the mixture of blood and air, passes from the lung to the left ventricle, and not in the opposite direction."

4. "The assertion that there are only two ventricles, not three as stated by Avicenna."

5. "The statement that the ventricle takes its nourishment from blood flowing in the vessels that run in its substance (i.e. the coronary vessels) and not, as Avicenna maintained, from blood deposited in the right ventricle."

6. "A premonition of the capillary circulation in his assertion that the pulmonary vein receives what comes out of the pulmonary artery, this being the reason for the existence of perceptible passages between the two."

Ibn Al-Nafis also corrected Galen-Avicenna assertion that heart has a bone structure through his own observations and wrote the following criticism on it: ^[13]

"This is not true. There are absolutely no bones beneath the heart as it is positioned right in the middle of the Thoracic cavity

—chest cavity where there are no bones at all. Bones are only found at the chest periphery not where the heart is positioned."

Healthy heart

Obesity, high blood pressure and high cholesterol can increase the risk of developing heart disease. However, fully half the amount of heart attacks occur in people with normal cholesterol levels. Heart disease is a major cause of death (and the number one cause of death in the Western World).

Of course one must also consider other factors such as lifestyle and overall health (mental and social as well as physical). ^{[14] [15] [16] [17]}

See also

• Cardiac cycle
• Heart disease
• Human heart
• Electrocardiogram
• Electrical conduction system of the heart
• Physiology

References

1. Kumar, Abbas, Fausto: ''Robbins and Cotran Pathologic Basis of Disease'', 7th Ed. p. 556
2. Animal Tissues
3. Main Frame Heart Development>
4. OBGYN.net "Embryonic Heart Rates Compared in Assisted and Non-Assisted Pregnancies"
5. Terry J. DuBose Sex, Heart Rate and Age
6. Gray's Anatomy of the Human Body - 6. Surface Markings of the Thorax
7. Human Biology and Health
8. Campbell, Reece-Biology, 7th Ed. p.873,874
9. Chairman's Reflections (2004), "Traditional Medicine Among Gulf Arabs, Part II: Blood-letting", Heart Views 5 (2): 74-85 [1]
10. S. A. Al-Dabbagh (1978). "Ibn Al-Nafis and the pulmonary circulation", The Lancet 1: 1148
11. [1] Dr. Paul Ghalioungui (1982), "The West denies Ibn Al Nafis's contribution to the discovery of the circulation", ''Symposium on Ibn al-Nafis'', Second International Conference on Islamic Medicine: Islamic Medical Organization, Kuwait (cf.) The West denies Ibn Al Nafis's contribution to the discovery of the circulation
12. Husain F. Nagamia (2003), "Ibn al-Nafis: A Biographical Sketch of the Discoverer of Pulmonary and Coronary Circulation", Journal of the International Society for the History of Islamic Medicine 1: 22–28.
13. Dr. Sulaiman Oataya (1982), "Ibn ul Nafis has dissected the human body", Symposium on Ibn al-Nafis, Second International Conference on Islamic Medicine: Islamic Medical Organization, Kuwait (cf. Ibn ul-Nafis has Dissected the Human Body, Encyclopedia of Islamic World).
14. Eating for a healthy heart
15. Deaths: Final data for 2004
16. American Heart Month, 2007
17. National Statistics Press Release 25 May 2006