Heart
Heart
A heart is a means to circulate blood through the body of an animal. Among the lower species such as insects, arachnids, and others, the heart may simply be an expanded area in a blood vessel and may occur a number of times. The earthworm, for example, has 10 such “hearts.” These areas contract rhythmically to force the blood through the aorta, or blood vessel.
Not until the evolution of the higher vertebrates does the heart achieve its ultimate form, that of a chambered organ with differentiated purposes. Even the lower chordates, such as amphioxus, possess hearts not more advanced than those in the earthworm. It is simply a pulsating blood vessel that moves blood through the body.
Blood
In a complex organism such as a vertebrate, with multiple cell layers and complex organ systems, blood distributes nourishment and oxygen to the cells and removes waste products. Cells within the blood, such as the red blood cells (erythrocytes) and white blood cells (leukocytes) serve specialized functions. The red cells hold and distribute oxygen to release in the cells and return carbon dioxide to the lungs to be eliminated. The white cells carry out immune functions to destroy invading bacteria and other foreign material. Still other components of the blood are involved in forming clots when a blood vessel is opened. The liquid medium, the plasma, carries vitamins and other nutrients throughout the body.
Animals require a means for the blood to obtain oxygen, whether through gills or lungs, and a means to propel the blood through those structures. The heart is central to that purpose.
The multiform heart
The heart is a pulsating organ that pushes a liquid medium throughout the body. It may be as simple as one chamber or as complex as four, as in the higher mammals. In all animals, however, it is an organ that must function day after day without pause to keep the blood moving.
In general, blood from the body or from the oxygen-exchanging structures returns to an atrium, which is simply a holding chamber. The atrium (plural is atria) empties into another chamber called the ventricle, a muscular chamber that contracts rhythmically to propel the blood through the body. Movement of the blood between chambers and in and out of the
heart is controlled by valves that allow movement only in one direction.
Lower vertebrates such as hagfish and other fish have two-chambered hearts. The ventricle pumps blood forward through the gills to obtain oxygen and dispose of carbon dioxide. From there blood enters the dorsal aorta and is carried through the body. The blood returns to the heart by means of the sinus venosus, which empties into the auricle or atrium. From there it is passed into the ventricle and the cycle begins again.
Terrestrial vertebrates such as amphibians have three-chambered hearts and a more complex circulatory system. The third chamber is another auricle or atrium. Unoxygenated blood from the body of the animal returns to the right auricle and the oxygenated blood from the lungs goes into the left auricle. Both contract simultaneously and empty their contents into the single ventricle. The oxygenated and unoxygenated blood does not mix to any degree because of specialized muscle strands in the ventricle. Also, the ventricle contracts immediately after the auricles so the bloods do not have time to mix. When the ventricle contracts, the unoxygenated blood is forced from the heart first and enters the pulmocutaneous vessels leading to the lungs and skin for oxygen exchange. The oxygenated blood enters the truncus arteriosus and flows to the head, arms, body, and hind legs.
Reptiles demonstrate a further step in heart development—a divided ventricle. The wall in the ventricle dividing left from right is incomplete except in crocodiles
and alligators. Blood from the body enters the right auricle from which it passes into the right side of the ventricle and is pumped through the lungs. Oxygenated blood from the lungs enters the left auricle and passes into the left side of the ventricle. From there it moves out through the two aortic arches to the body.
Among the crocodilia, however, a peculiar structure of the aortae allows mixing of arterial and venous blood. The left aortic arch rises from the right side of the heart, as does the pulmonary artery. Thus, unoxy-genated blood pumped from the right ventricle goes to the lungs through the pulmonary artery and also into the aortic arch and out into the body where it will mix with oxygenated blood. The right aortic arch rises from the left ventricle and carries only oxygenated blood.
Birds also possess four-chambered hearts, in this case with complete separation of the two ventricles. They have only one aortic arch, however, and that is the right arch.
Mammals, including humans, also have a four-chambered heart, but the aortic arch curves up and to the left as it leaves the heart. Here, as with birds, there is no mixing of venous and arterial blood (under normal circumstances). Though the heart appears to be a simple organ, it requires a complex series of nerve stimulations, valve openings, and muscle contractions to achieve its purpose.
The human heart
Located in the thoracic cavity, the heart is a four-chambered muscular organ that serves as the primary pump or driving force within the circulatory system. The heart contains a special form of muscle, appropriately named cardiac muscle, that has intrinsic contractility (i.e., is able to beat on its own, without nervous system control).
The Chinese were aware more than 2,000 years ago that the heart is a pump that forces blood through a maze of arteries. The Greeks, however, believed the blood did not circulate at all, and their ideas dominated medical science until the seventeenth century. The Greek physician Galen (c. 130–c. 200) published a great deal on the human body and its functions, much of which was incorrect, but his doctrines held sway for hundreds of years. Not until the 1600s did William Harvey (1578–1657), through human and animal experiments, discover that the heart circulates the blood. He published his findings in 1628, thus bringing Western medical science in line with that of the ancient Chinese.
The human heart on the average weighs about 10.5 oz (300 g). It is a four-chambered, cone-shaped organ about the size of a closed fist that lies in the mid-thorax, under the breastbone (sternum). Nestled between the lungs, the heart is covered by a fibrous sac called the pericardium. This important organ is protected within a bony cage formed by the ribs, sternum, and spine.
In its ceaseless work, the heart contracts some 100,000 times a day to drive blood through about 60,000 mi (96,000 km) of vessels to nourish each of the trillions of cells in the body. Each contraction of the ventricles forces about 2.5 oz (0.075 l) of blood into the circulation, which adds up to about 10 pt (4.7 l) of blood every minute. On average, the heart will pump about 2,500 gal (9,475 l) of blood in a day, and that may go up to as much as 5,000 gal (18,950 l) with exertion. In a lifetime the heart will pump about 100 million gal of blood.
The chambers of the human heart are divided into two upper (superior) atrial chambers and two thicker-walled, heavily muscular inferior ventricular chambers. The right and left sides of the heart are divided by a thick septum. The right side of the heart is on the same side of the heart as is the right arm of the patient. The atrial and ventricular chambers on each side of the septum constitute separate collection and pumping systems for the pulmonary (right side) and systemic circulation (left side). The coronary sulcus or grove separates the atria from the ventricles. The left and right side atrial and ventricular chambers each are separated by a series of one way valves that, when properly functioning, allow blood to move in one direction, but prohibit it from regurgitating (flowing back through the valve).
Deoxygenated blood—returned to the heart from the systemic circulatory venous system—enters the right atrium of the heart through the superior and inferior vena cava. Auricles lie on each atrium and are most visible when the atria are drained and deflated. The auricles (so named because they resembled ear flaps) allow for greater atrial expansion. Pectinate muscles on the auricles assist with atrial contraction. Small contractions within the right atrium, and pressure differences caused by evacuation of blood in the lower (inferior) right ventricle, cause this deoxygenated blood to move through the tricuspid valve during diastole (the portion of the heart’s contractile cycle between contractions, and a period of lower pressure as compared to systole) into the right ventricle. When the heart contracts, a sweeping wave of pressure forces open the pulmonic semilunar valve that allows blood to rush from the right ventricle into the pulmonary artery where it is travels to the lungs for oxygenation and other gaseous exchanges.
Freshly oxygenated blood returns to the heart from the pulmonary circulation through the pulmonary vein the empties into the left atrium. During diastole, the oxygenated blood moves from the left atrium into the left ventricle through the mitral valve. During systolic contraction, the oxygenated blood is pumped under high pressure through the semilunar aortic valve into the aorta and thus, enters the systemic circulatory system.
As the volume and pressure rise during the filling of the right and left ventricles, the increased pressure snaps shut the flaps of the atrioventricular valves (tri-cuspid and mitral valves) anchored by fibrous connection to the left and right ventricles. The pressure in the ventricles seals the valves and as the pressure increases during systole, the valves seal becomes further compressed. A prolapse in one of the valves (a pushing through of one of the cusps) leads to blood flow back through the valve. The cusps are held against prolapse by the chordae tendineae, thin cords that attach the cusps to papillary muscles.
The heart and great vessels attached to it are encased within a multi-layered pericardium. The outer layer is fibrous and covers a double mem-braned inner sac-like structure termed the pericar-dial cavity that is filled with pericardial fluid. The pericardial fluid acts to reduce friction between the heart, the pericardial membranes, and the thoracic wall as the heart contracts and expands during the cardiac cycle.
The heart muscle is composed of three distinct layers. The outermost layer, the outer epicardium, is separated from the inner endocardium by the middle pericardium. The outer epicardium is continuous and in some places the same as the visceral pericardium. Epicardium protects the heart and is invested with capillaries, nerves, and lymph vessels. The middle myocardium is a thick layer of cardiac muscle. The innermost endocardium contains connective tissue and Purkinje fibers. The endocardium is continuous with the lining of the great vessels attached to the heart and it lines all valve and cardiac inner surfaces.
Heart muscle does not directly take up oxygen from the blood it pumps. A specialized set of vessels (e.g., the left and right coronary arteries and their branches) supply oxygenated blood to the heart muscle and constitute the coronary circulation. A heart attack occurs whenever blood flow is occluded (blocked).
The fossa ovalis is a remnant or the embryonic foramen ovale that allows blood to flow between the left and right atria in the developing fetus.
Regulation of the heart
Various intrinsic, neural, and hormonal factors act to influence the rhythm control and impulse conduction within the heart. The rhythmic control of the cardiac cycle and its accompanying heartbeat relies on the regulation of impulses generated and conducted within the heart. Regulation of the cardiac cycle is also achieved via the autonomic nervous system. The sympathetic and parasympathetic divisions of the autonomic system regulate heart rhythm by affecting the same intrinsic impulse conducting mechanisms that lie within the heart in opposing ways.
Cardiac muscle is self-contractile because it is capable of generating a spontaneous electrochemical signal as it contracts. This signal induces surrounding cardiac muscle tissue to contract and a wave-like contraction of the heart can result from the initial contraction of a few localized cardiac cells.
The cardiac cycle describes the normal rhythmic series of cardiac muscular contractions. The cardiac cycle can be subdivided into the systolic and diastolic phases. Systole occurs when the ventricles of the heart contract and diastole occurs between ventricular contractions when the right and left ventricles relax and fill. The sino-atrial node (S-A node) and atrioventricular node (AV node) of the heart act as pacemakers of the cardiac cycle.
The contractile systolic phase begins with a localized contraction of specialized cardiac muscle fibers within the sinoatrial node. The S-A node is composed of nodal tissue that contains a mixture of muscle and neural cell properties. The contraction of these fibers generates an electrical signal that then propagates throughout the surrounding cardiac muscle tissue. In a contractile wave originating at the S-A node, the right atrium muscle contracts (forcing blood into the right ventricle) and then the left atrium contracts (forcing blood into the left ventricle).
Intrinsic regulation is achieved by delaying the contractile signal at the atrioventricular node. This delay also allows the complete contraction of the atria so that the ventricles receive the minimum amount of blood to make their own contractions efficient. A specialized type of neuromuscular cells, named Purkinje cells, form a system of fibers that covers the heart and which conveys the contractile signal from S-A node (which is also a part of the Purkinje system or suben-docardial plexus). Because the Purkinje fibers are slower in passing electrical signals (action potentials) than are neural fibers, the delay allows the atria to finish their contractions prior to ventricular contractions. The signal delay by the AV node lasts about a tenth (0.1) of a second.
The contractile signal then continues to spread across the ventricles via the Purkinje system. The signal travels away from the AV node via the bundle of His before it divides into left and right bundle branches that travel down their respective ventricles.
Extrinsic control of the heart rate and rhythm is achieved via autonomic nervous system (ANS) impulses (regulated by the medulla oblongata) and specific hormones that alter the contractile and or conductive properties of heart muscle. ANS sympathetic stimulation via the cervical sympathetic chain ganglia acts to increase heart rate and increase the force of atrial and ventricular contractions. In contrast, parasympathetic stimulation via the vagal nerve slows the heart rate and decreases the vigor of atrial and ventricular contractions. Sympathetic stimulation also increases the conduction velocity of cardiac muscle fibers. Parasympathetic stimulation decreases conduction velocity.
The regulation in impulse conduction results from the fact that parasympathetic fibers utilize acetylcho-line, a neurotransmitter hormone that alters the transmission of an action potential by altering membrane permeability to specific ions (e.g., potassium ions [K+ ]). In contrast, sympathetic postganglionic neurons secrete the neurotransmitter norepinephrine that alters membrane permeability to sodium (Na+) and calcium ions (Ca2+).
The ion permeability changes result in parasympathetic induced hypopolarization and sympathetic induced hyperpolarization.
Additional hormonal control is achieved principally by the adrenal glands (specifically the adrenal medulla) that release both epinephrine and norepinephrine into the blood when stimulated by the sympathetic nervous system. As part of the fight or flight reflex, these hormones increase heart rate and the volume of blood ejected during the cardiac cycle.
The electrical events associated with the cardiac cycle are measured with an electrocardiogram (EKG). Disruptions in the impulse conduction system of the heart result in arrhythmias.
Variations in the electrical system can lead to serious, even dangerous, consequences. When that occurs an artificial electrical stimulator, called a pacemaker, must be implanted to take over regulation of the heartbeat. The small pacemaker can be implanted under the skin near the shoulder and long wires from it are fed into the heart and implanted in the heart muscle. The pacemaker can be regulated for the number of heartbeats it will stimulate per minute. Newer pacemakers can detect the need for increased heart rate when the individual is under exertion or stress and will respond.
See also Angiography; Artificial heart and heart valve; Heart diseases; Heart-lung machine; Thoracic surgery; Transplant, surgical.
Resources
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Brenda Wilmoth Lerner
Larry Blaser