The human heart
In the human body the heart is situated slightly to the left of the middle of the thorax, behind the sternum (breastbone). It is enclosed by a sac known as the pericardium and is surrounded by the lungs. In adults, it weighs about 300~350 g. It consists of four chambers, the two upper atria (singular: atrium) and the two lower ventricles.
A thick, muscular wall, the septum, divides the right atrium and ventricle from the left atrium and ventricle, keeping blood from passing between them. Valves between the atria and ventricles maintain coordinated unidirectional flow of blood from the upper atria to the lower ventricles.
The ventricles are the parts of the heart that pump blood around the body or to the lungs. They are thicker walled than the atria, and the contraction of the ventricle wall is much more important to move blood around.
Oxygen-depleted blood from the body enters the right atrium through two veins, the superior vena cava and the inferior vena cava. The blood then passes to the right ventricle. The right ventricle pumps the deoxygenated blood to the lungs, through the pulmonary artery. After the blood loses carbon dioxide and picks up oxygen in the lungs, it flows through pulmonary veins to the left atrium. From the left atrium the newly oxygenated blood enters the left ventricle. The left ventricle is the main pumping chamber, sending blood through the aorta to all of the body except the lungs.
The left ventricle is much thicker than the right because it must pump blood around the entire body, which involves exerting a considerable force to overcome the pressure caused by the body. As the right ventricle must just pump blood to the lungs it requires less muscle.
Even though the ventricles lie below the atria, the two vessels through which the blood exits the heart (the pulmonary artery and the aorta) leave the heart at its top side.
The wall of the heart is very muscular and does not tire. It consists of three distinct layers. The first is the outer epicardium which is composed of a layer of flattened epithelial cells and connective tissue. Beneath this is a much thicker myocardium made up of cardiac muscle. The endocardium is a further layer of flattened epithelial cells and connective tissue.
A large blood supply is necessary to power the heart itself. It is supplied by the left and right coronary arteries, which branch off from the aorta.
The function of the heart is to pump blood around the body, in cycles. The cycle is explained below.
The cardiac cycle
Every beat of the heart involves a sequence of events called the cardiac cycle. This consists of three major stages: the atrial systole, the ventricular systole, and the complete cardiac diastole. The atrial systole consists of the contraction of the atria and the corresponding influx of blood in to the ventricles. Once the blood has fully left the atria, the atrioventricular valves, which are situated between the atria and ventricular chambers, close. This prevents any backflow into the atria. It is the sound of the valves closing which produces the familiar beating sounds of the heart.
The ventricular systole consists of the contraction of the ventricles and flow of blood into the circulatory system. Again, once all the blood has left, the pulmonary and aortic semilunar valves close. Finally complete cardiac diastole involves the relaxation of the atria and ventricles in preparation for new blood to enter the heart.
Regulation of the cardiac cycle
Cardiac muscle is myogenic, which means that it is self-exciting. This is in contrast with skeletal muscle, which requires either conscious or reflex nervous stimuli. The heart's rhythmic contractions occur spontaneously, although the frequency or heart rate can be changed by nervous or hormonal influences such as exercise or the perception of danger.
The rhythmic sequence of contractions is coordinated by the sinoatrial node and atrioventricular nodes. The sinoatrial node, often known as the cardiac pacemaker, is located in the upper wall of the right atrium and is responsible for the wave of electrical stimulation (See action potential) that initiates the atria to contract. Once the wave reaches the atrioventricular node, situated in the wall between ventricular chambers, it is conducted through the bundles of His and causes contraction of the ventricles. The time taken for the wave to reach this node from the sinoatrial nerve creates a delay between contraction of the two chambers and ensures that each contraction is coordinated simultaneously throughout all of the heart. In the event of severe pathology, the Purkinje fibers can also act as a pacemaker; this is usually not the case because their rate of spontaneous firing is considerably lower than that of the other pacemakers and hence is overridden.
Diseases and treatments
The study of diseases of the heart is known as cardiology. Important diseases of the heart include:
Beta blockers are drugs that lower the heart rate and blood pressure and reduce the heart's oxygen requirements. Nitroglycerin and other compounds that give off nitric oxide are used to treat heart disease as they cause the dilation of coronary vessels.
At Groote Schuur Hospital in Cape Town, South Africa, 53-year-old Lewis Washkansky on December 3, 1967 became the first human to receive a heart transplant (however he died 18 days later from double pneumonia). The transplant team was headed by Christiaan Barnard.
See cardiac arrest for emergencies involving the heart
The hearts of other animals
The structure of the heart of other mammals is quite similar to that of humans, with four chambers. Birds also have a four-chambered heart, however it is thought that this evolved independently of that of mammals. Amphibians have a three-chambered heart. Reptilia also have a three-chambered heart, except from crocodylians, which have four. Fish have a single circulation system and a heart with two chambers. The hearts of arthropods and mollusks have a single chamber.
Smaller animals have faster heartbeat. This is evident within a species as well, as the young beat their hearts faster than the adults. Gray Whale beats 9 times per minute, Harbour Seal 10 when diving, 140 when on land, elephant 25, human 70, sparrow 500, shrew 600, and hummingbird 1,200 when hovering.
The earthworm has a series of multiple primitive hearts.
The heart in literature and metaphor
In the Bible, and in much later literature, the heart is used to refer to the moral core of a human being. This is true from the earliest passages; Genesis 6:5 situates the thoughts of evil men in their hearts, and Exodus 5 through 12 speak repeatedly of the LORD "hardening Pharaoh's heart;" by this it is meant that God made Pharaoh resolve not to let the Israelite slaves leave Egypt, in order to bring judgment against him. In Egyptian mythology, the heart was weighed in a balance against the feather of Maat, symbolising truth, in the judgment of the dead in the Egyptian Book of the Dead. Similarly, in Jeremiah 17:9, we are told that the "heart is deceitful above all things, and desperately wicked"; and that the LORD is the judge who "tries" the human heart.
The Roman physician Galen considered the heart to be the seat of the emotions; the Stoics taught that the heart was the seat of the human soul. (Galen also located the seat of the passions in the liver, and the seat of reason in the brain.) While Galen's identification with the heart and emotion were proposed as a part of his theory of the circulatory system, the Biblical text, this traditional Western medicine, and similar literary usages have caused the heart to be identified as the source of human emotions; and especially, the emotion of love.
In European traditional art and folklore, the heart is drawn in this shape:
This shape also appears on playing cards as the pip of the suit of hearts. What the traditional "heart shape" actually depicts is a matter of some controversy. It only vaguely resembles the human heart. Some claim that it actually depicts the hearts of cattle; while beef hearts resemble the heart shape somewhat more closely, the resemblance is still small. The shape does resemble that of the three-chambered heart of the turtle, and that of the human male prostate gland, but is surely not patterned after either of these organs. Others claim that the "heart" shape actually depicts the human female pubic mound. A Sumerian cuneiform symbol for "woman" closely resembles the heart shape, and is believed to directly depict the pubic mound.