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The Heart: The centre of the whole body.

The heart is a muscular organ in humans and other animals, which pumps blood through the blood vessels of the circulatory system. Blood provides the body with oxygen and nutrients, and also assists in the removal of metabolic wastes. The heart is located in the middle compartment of the mediastinum in the chest. In humans, other mammals and birds the heart is divided into four chambers: upper left and right atria; and lower left and right ventricles. Commonly the right atrium and ventricle are referred together as the right heart and their left counterparts as the left heart.

The heart pumps blood through both circulatory systems. Blood low in oxygen from the systemic circulation enters the right atrium from the superior and inferior vena cavae and passes to the right ventricle. From here it is pumped into the pulmonary circulation, through the lungs where it receives oxygen and gives off carbon dioxide. Oxygenated blood then returns to the left atrium, passes through the left ventricle and is pumped out through the aorta to the systemic circulation−where the oxygen is used and metabolized to carbon dioxide. In addition the blood carries nutrients from the liver and gastrointestinal tract to various organs of the body, while transporting waste to the liver and kidneys. Normally with each heartbeat, the right ventricle pumps the same amount of blood into the lungs as the left ventricle pumps out into the body. Veins transport blood to the heart, while arteries transport blood away from the heart. Arteries always carries oxygenated blood except the pulmonary artery while Veins always carries deoxygenated blood except the pulmonary vein. Veins normally have lower pressures than arteries. Exercise temporarily increases this rate, but lowers resting heart rate in the long term, and is good for heart health.

Cardiovascular diseases were the most common cause of death globally in 2008, accounting for 30% of cases. Of these deaths more than three quarters were due to coronary artery disease and stroke. Risk factors include: smoking, being overweight, not enough exercise, high cholesterol, high blood pressure, and poorly controlled diabetes among others. Diagnosis of CVD is often done by listening to the heart-sounds with a stethoscope, ECG or by ultrasound. The back surface of the heart lies near to the vertebral column, and the front surface sits deep to the sternum and costal cartilages. The heart is typically the size of a fist: 12 cm in length, 8 cm wide, and 6 cm in thickness. These enclose the pericardial cavity. The pericardial cavity contains fluid which lubricates the surface of the heart.

Chambers The heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers. The atria are connected to the ventricles by the atrioventricular valves and separated from the ventricles by the coronary sulcus. There is an ear-shaped structure in the upper right atrium called the right atrial appendage, or auricle, and another in the upper left atrium, the left atrial appendage. The right atrium and the right ventricle together are sometimes referred to as the right heart and this sometimes includes the pulmonary artery. Similarly, the left atrium and the left ventricle together are sometimes referred to as the left heart. The ventricles are separated by the anterior longitudinal sulcus and the posterior interventricular sulcus. The cardiac skeleton is made of dense connective tissue and this gives structure to the heart. It forms the atrioventricular septum which separates the atria from the ventricles, and the fibrous rings which serve as bases for the four heart valves. The cardiac skeleton also provides an important boundary in the heart’s electrical conduction system since collagen cannot conduct electricity. The interatrial septum separates the atria and the interventricular septum separates the ventricles.

Coronary circulation Cardiomyocytes like all other cells need to be supplied with oxygen, nutrients and a way of removing metabolic wastes. This is achieved by the coronary circulation. The coronary circulation cycles in peaks and troughs relating to the heart muscle relaxing or contracting. Between the third and fourth week, the heart tube lengthens, and begins to fold to form an S-shape within the pericardium. This places the chambers and major vessels into the correct alignment for the developed heart. Further development will include the septa and valves formation and remodelling of the heart chambers. By the end of the fifth week the septa are complete and the heart valves are completed by the ninth week. After 9 weeks it starts to decelerate, slowing to around 145 bpm at birth. There is no difference in female and male heart rates before birth.

Physiology Blood flow The heart functions as a pump in the circulatory system to provide a continuous circulation of blood throughout the body. This circulation includes the systemic circulation and the pulmonary circulation of the lungs. Blood in the pulmonary circulation collects oxygen from the lungs and delivers carbon dioxide for exhalation. Blood in the systemic circuit transports oxygen to the body and returns relatively deoxygenated blood and carbon dioxide to the lungs. The signal then travels to the atrioventricular node. This is found at the bottom of the right atrium in the atrioventricular septum–the boundary between the right atrium and the left ventricle. The septum is part of the cardiac skeleton, tissue within the heart that the electrical signal cannot pass through, which forces the signal to pass through the atrioventricular node only.

Heart rate The resting heart rate of a newborn can be 120 beats per minute and this gradually decreases until maturity and then gradually increases again with age. The adult resting heart rate ranges from 60 to 100 bpm. Exercise and fitness levels, age and basal metabolic rate can all affect the heart rate. An athlete’s heart rate can be lower than 60bpm. During exercise the rate can be 150bpm with maximum rates reaching from 200 and 220 bpm.

Influences The normal sinus rhythm of the heart is influenced by several factors. The resting heart rate is influenced by the central nervous system through sympathetic and parasympathetic nerves from two paired cardiovascular centres in the medulla. The parasympathetic nervous system acts by the vagus nerve to decrease heart rate, and the sympathetic nerve system by the sympathetic trunk to increase heart rate. These intermingle in the cardiac plexus near the base of the heart. Normally the parasympathetic stimulation predominates to decrease the heart rate. Without parasympathetic input, the sinoatrial node would beat at approximately 100 bpm. S2 may split into two distinct sounds, either as a result of inspiration or different valvular or cardiac problems. One example of a murmur is Still's murmur, which presents a musical sound in children, has no symptoms and disappears in adolescence. A different type of sound, a pericardial friction rub can be heard in cases of pericarditis where the inflamed membranes can rub together.

Clinical significance Being such a complex organ the heart is prone to several cardiovascular diseases some becoming more prevalent with ageing. Heart disease is a major cause of death, accounting for an average of 30% of all deaths in 2008, globally. Doctors that specialise in the heart are called cardiologists. Many other medical professionals are involved in treating diseases of the heart, including doctors such as general practitioners, cardiothoracic surgeons and intensivists, and allied health practitioners including physiotherapists and dieticians. Obesity, high blood pressure, and high cholesterol can all increase the risk of developing heart disease. However, half the number of heart attacks occur in people with normal cholesterol levels. It is generally accepted that factors such as exercise or the lack of it, good or poor diet, and overall well-being, affect heart health. Exercise results in the addition of protein myofilaments and this can result in hypertrophy where the size of individual cells are increased but not their number. It is also the main cause of angina. Cardiomyopathy and most commonly dilated cardiomyopathy, is a noticeable deterioration of the heart muscle's ability to contract, which can lead to heart failure. Other common causes of heart failure, are heart attacks, valve disorders and high blood pressure. This happens happens when the heart is pumping insufficiently and cannot meet the body's blood flow demands. Because the heart is a double pump, each side can fail independently of the other, resulting in heart failure of the right heart or the left heart, either of which through causing strain in the other side can result in the failure of the whole heart. Congestive heart failure results in blood backing up in the systemic circulation. Edema of the feet, ankles and fingers is the most noticeable symptom. Pulmonary congestion results from left heart failure. The right side of the heart continues to propel blood to the lungs, but the left side is unable to pump the returning blood into the systemic circulation. As blood vessels within the lungs become swollen with blood, the pressure within them increases, and fluid leaks from the circulation into the lung tissue, causing pulmonary edema. If untreated, the person will suffocate because they are drowning in their own blood. Heart murmurs are abnormal heart sounds which can be either pathological or benign and there are several kinds. Murmurs are graded by volume, from 1) the quietest, to 6) the loudest, and evaluated by their relationship to the heart sounds and position in the cardiac cycle. Defibrillation is used to treat serious arrhythmias. Artificial pacemakers used to regulate the heartbeat can also incorporate a defibrillator.

Surgery Coronary artery bypass surgery to improve the blood supply to the heart is often the only treatment option for coronary heart disease. Heart valve repair or valve replacement are options for valvular heart disease.

History Ancient The valves of the heart were discovered by a physician of the Hippocratean school around the 4th century BC, although their function was not fully understood. On dissection, arteries are typically empty of blood because blood pools in the veins after death. It was subsequently assumed they were filled with air and served to transport air around the body. Philosophers distinguished veins from arteries, but thought the pulse was a property of arteries. Erasistratos observed that arteries cut during life bleed. He ascribed the fact to the phenomenon that air escaping from an artery is replaced with blood which entered by very small vessels between veins and arteries. Thus he apparently postulated capillaries, but with reversed flow of blood. The Greek physician Galen knew blood vessels carried blood and identified venous and arterial 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 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 the arterial blood was created by venous blood passing from the left ventricle to the right through 'pores' in the interventricular septum, while 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.

Pre-modern The earliest descriptions of the coronary and pulmonary circulation systems can be found in the Commentary on Anatomy in Avicenna's Canon, published in 1242 by Ibn al-Nafis. In his manuscript, al-Nafis wrote that blood passes through the pulmonary circulation instead of moving from the right to the left ventricle as previously believed by Galen. His work was later translated into Latin by Andrea Alpago. In Europe, the teachings of Galen continued to dominate the academic community and his doctrines were adopted as the official canon of the Church. Andreas Vesalius questioned some of Galen's beliefs of the heart in De humani corporis fabrica, but his magnum opus was interpreted as a challenge to the authorities and he was subjected to a number of attacks. Michael Servetus wrote in Christianismi Restitutio that blood flows from one side of the heart to the other via the lungs. Otto Frank was a German physiologist; among his many published works are detailed studies of this important heart relationship. Ernest Starling was an important English physiologist who also studied the heart. Although they worked largely independently, their combined efforts and similar conclusions have been recognized in the name "Frank–Starling mechanism." By the middle of the 20th century, heart disease had surpassed infectious disease as the leading cause of death in the United States, and it is currently the leading cause of deaths worldwide. Since 1948, the ongoing Framingham Heart Study has shed light on the effects of various influences on the heart, including diet, exercise, and common medications such as aspirin. Although the introduction of ACE inhibitors and beta blockers has improved the management of chronic heart failure, the disease continues to be an enormous medical and societal burden, with 30 to 40% of patients dying within a year of receiving the diagnosis.

Society and culture Symbolism As one of the vital organs, the heart was long identified as the center of the entire body, the seat of life, or emotion, or reason, will, intellect, purpose or the mind. Martin Hunt

London Personal Trainer

Oracle Fitness

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