Organs: any part or structure of an organism adapted for a special function or functions. (Lawrence) A unique macroscopic anatomic structure that performs specific functions. It is composed of various tissues. A part of an anatomic system or a body region. (NCIt)

Composed of two or more types of "tissue." (Brooker, 4) A part of an animal or plant adapted for a particular function, such as "digestion," "respiration," excretion, "reproduction," locomotion, "perception," etc. (Oxford)

Bladder: general term for a membranous "sac" filled with air or fluid. In humans, the urinary bladder, the structure in which urine collects before release from the body. (Lawrence) The organ that stores urine. (NCI3) Functions as a reservoir of urine, collecting from the kidneys and eliminating via the urethra. (NCIt)

Esophagus: the muscular tube through which food passes from the “throat” to the “stomach.” (NCIt) A muscular tube about 23 cm long, that extends from the “pharynx” to the stomach. It is lined with “mucus membrane,” whose secretions lubricate food as it passes from the mouth to the stomach. Waves of (coordinated rhythmic muscle contractions) assist the passage of food. (OxfordMed)

Gall Bladder: spherical sac that stores "bile." It drains via the 'bile duct' into the "duodenum." (Lawrence) Pear-shaped organ located under the “liver” that... concentrates “bile” secreted by the liver. From the gallbladder the bile is delivered through the bile ducts into the “intestine” thereby aiding the digestion of fat-containing foods. (NCI)

Heart: a hollow muscular cone-shaped organ, lying between the lungs, with the pointed end directed downwards, forwards and to the left. Its wall consists largely of ‘cardiac’ “muscle” (myocardium), lined and surrounded by “membranes.” It is divided into separate right and left halves, each of which is divided into an upper “atrium” and a lower “ventricle.” (OxfordMed) Consists of four chambers, an upper thin-walled atrium and a lower thick-walled ventricle on either side. By rhythmic contractions, it pumps blood around the body. The contraction is self-sustaining, and is synchronized by 'pacemaker cells' located in the "sinoatrial node." (Lawrence)

Aorta: the great trunk artery that carries blood from the heart to the body. (Lawrence) The main artery of the body, from which all others derive. It arises from the "left ventricle," arches over the top of the heart and descends in front of the backbone, giving off large and small branches and finally dividing to form the right and left ‘iliac’ arteries. (OxfordMed)

Atri: prefix denoting an atrium, especially the atrium of the heart. (OxfordMed)

Atrium: either of the two upper chambers of the heart. Their muscular walls are thinner than those of the ventricles. (OxfordMed) The paired upper chambers of the heart. (NCIt) Plural - 'atria.'

Left Atrium: receives "oxygenated" blood from the “pulmonary vein” and pumps blood into the left ventricle. (NCIt)  Oxygenated blood re-enters the heart through the left atrium and leaves the heart from the left ventricle. (Lawrence)

Right Atrium: receives... 'deoxygenated' blood from the entire body via the “superior” and “inferior vena cavae" and pumps blood into the “right ventricle.” (NCIt) "Venous" blood from the body enters the right atrium and leaves for the "lungs" from the right ventricle. (Lawrence)

Heart Electrical System: the atria and ventricles work together, alternately contracting and relaxing to pump blood through the heart. The electrical system of the heart is the power source that makes this possible. (Cleveland Clinic, Diseases & Conditions)

Accessory Pathway: extra impulse-conducting tissue in the heart that creates abnormal impulse-conducting connections between atria and ventricles. (MeSH) An extra electrical conduction pathway between the atria and ventricles of the heart that predisposes to “re-entry tachycardia.” The pathway may be contained within the “atrioventicular node” or may be anatomically separate, giving rise to a ‘delta wave’ characteristic of the “Wolff-Parkinson-White syndrome.” (OxfordMed)

Atrioventricular Bundle: a bundle of modified heart muscle fibers (called ‘Purkinje fibers’) passing from the atrioventricular node forward to the “septum” between the ventricles, where it divides into right and left bundles, one for each ventricle. The fibers transmit "contraction" ("signals") from the atria, via the “AV node” to the ventricles. (OxfordMed) Also referred to as 'bundle of HIS.'

Atrioventricular Node: a mass of modified heart muscle situated in the lower middle part of the right atrium. It receives a (“signal”) to contract from the “sinoatrial node”, via the atria, and transmits it... to the ventricles (OxfordMed) Acts like a gate that slows the electrical signal before it enters the ventricles. This delay gives the atria time to contract before the ventricles do. (Cleveland Clinic, Diseases & Conditions) Also referred to as ‘AV Node.’

Cardiac: of, or relating to, the heart (Oxford) Near, or supplying the heart. (Lawrence)

Cardiac Cycle: the sequence of events between one heartbeat and the next, normally occupying less than a second. The atria contract simultaneously and force blood into the relaxed ventricles. The ventricles then contract very strongly and pump blood out through the aorta and "pulmonary artery." (OxfordMed)

Cardiac Reflex: reflex control of the heart rate. Sensory fibers in the walls of the heart are stimulated when the heart rate increases above normal. Impulses are sent to the cardiac centre in the brain, stimulating the ‘vagus nerve’ and leading to slowing of the heart rate. (OxfordMed)

Heart Beat: a normal heartbeat begins when a tiny cluster of cells called the (“sinoatrial node”) sends an electrical signal. The signal then travels through the atria and passes through another group of cells called the atrioventricular node. The signal (then) travels through the ventricles, causing them to contract and pump out blood. After this, the process starts over. (Mayo Clinic, Normal Heartbeat)

Heart Rate: the number of times the ventricles contract per unit of time, usually per minute. (MeSH) The number of times your heart beats in one minute. Varies from person to person. Slower when at rest and increases during exercise, since more oxygen-rich blood is needed by the body during exercise. (Cleveland Clinic, Diseases & Conditions) Also referred to as ‘pulse.’

Heart Rhythm: regular beating, or contraction, of the heart (enabling movement of) the blood throughout the body. Each heartbeat is controlled by electrical impulses traveling through the heart. In the normal heart these electrical impulses occur in regular intervals. When something goes wrong with the heart’s electrical system, the heart does not beat regularly. (EMedHealth, Heart Rhythm Disorders)

HIS-Purkinje Network: pathway of fibers (that) sends the impulse into the muscular walls of the ventricles and causes them to contract. This contraction forces blood out of the heart to the lungs and body. (Cleveland Clinic, Diseases & Conditions) Modified cardiac muscle fibers composing the (end) of the heart conduction system. (MeSH)

Sinoatrial Node: small mass of modified cardiac muscle fibers located at the junction of the superior vena cava and right atrium. Contraction impulses... start in this node, spread over the atrium and are then transmitted by the atrioventricular bundle to the ventricle. (MeSH) The natural ‘pacemaker’ of the heart. A microscopic area of specialized cardiac muscle located in the upper wall of the right atrium near the entry of the “vena cava.” It controls the (heart rate and hearth rhythm). (It is controlled by) the “autonomic nervous system.” Contains (nerve) fibers (which are) 'self-excitatory,' contracting rhythmically at around 70 times per minute. Following each contraction, the impulse spreads throughout the atrial muscle and into fibers connecting it with the atrioventricular node. (OxfordMed) With each heartbeat, the electrical impulse begins at the sinoatrial node. The electrical activity spreads through the walls of the atria and causes them to contract. (Cleveland Clinic, Diseases & Conditions) Also referred to as the ‘SA node’ and 'sinus node.'

Heart Valves: (provide) the heart’s ability to push blood from chamber to chamber. (HRS, Heart Rhythm Foundation Glossary) Flaps of tissue that prevent (the backflow) of blood from the ventricles to the atria or from the pulmonary arteries or aorta to the ventricles. (MeSH)

Aortic Valve: a valve that is located between and controls the flow of blood from the left ventricle of the heart and the aorta. (NCIt) Prevents backflow into the left ventricle. (MeSH)

Aortic Regurgitation: the backward flow of blood from the aorta into the left ventricle, owing to insufficiency of the aortic valve; it may be "chronic" or "acute." (NCIt) Most commonly due to degenerative ‘wear and tear’ of the aortic valve. Mild cases are symptom-free, but patients more severely affected develop breathlessness, ‘angina pectoris,’ and enlargement of the heart. (OxfordMed)

Mitral Valve: a dual-flap valve of the heart that regulates the flow of blood between the left atrium and the left ventricle of the heart. (NCIt) Allows blood to pass from the atrium to the ventricle, but prevents any backward flow. (OxfordMed) Also referred to as ‘bicuspid valve.’

Mitral Regurgitation: the backward flow of blood from the left ventricle into the left atrium, owing to insufficiency of the mitral valve; it may be acute or chronic, usually due to 'mitral valve prolapse,' "heart disease" or a complication of cardiac 'dilatation.' (NCIt) Mild cases are symptomless and require no treatment, but in severe cases the affected valve should be repaired or replaced with an artificial one. (OxfordMed)

Pulmonary Valve: a valve situated at the entrance to the pulmonary (artery) from the right ventricle. (MeSH) Controls the flow of blood from the right ventricle of the heart and the pulmonary artery. (NCIt) Prevents blood returning to the ventricle. (OxfordMed)

Pulmonary Regurgitation: the backflow of blood from the pulmonary artery into the right ventricle, owing to insufficiency of the pulmonic valve. (NCIt) Leakage of the pulmonary valve in the heart. Mild regurgitation is a common normal finding, but severe "congenital" pulmonary regurgitation may require surgical correction. (OxfordMed)

Tricuspid Valve: valve consisting of three cusps (leaflets that seal the valve when closed), situated between the right atrium and right ventricle of the heart. (MeSH) A dual-flap valve of the heart that regulates the flow of blood between the right atrium and the right ventricle of the heart. (NCIt) Channels the flow of blood...  and prevents any backflow. (OxfordMed) Also referred to as ‘atrioventricular valve.’

Myocardium: the middle of the three layers forming the wall of the heart. It is composed of cardiac muscle and forms the greater part of the heart wall, being thicker in the ventricles than in the atria. (OxfordMed) The muscle tissue of the heart. Composed of 'striated...' muscle cells connected to form the... pump to generate blood flow. (MeSH) Enveloped by the 'epicardium' and the 'endocardium.' (NCIt)

Palpitations: the sensations of feeling the heart beat. (HRS, Glossary) An unpleasant sensation of irregular and/or forceful beating of the heart. (NCIt)

Pulmonary Artery: an artery arising from the right ventricle of the heart that carries deoxygenated blood to the lungs. (NCI) Conveys blood from the heart to the lungs for oxygenation: the only artery in the body containing deoxygenated blood. It leaves the right ventricle and passes upwards before diving into two, one branch going to each lung. Within the lungs each pulmonary artery divides into many fine branches, which end in “capillaries” in the lung walls. (OxfordMed)

Pulmonary Vein: one of four veins that carries oxygen-rich blood from the lungs to the cardiac left atrium. (NCIt) Returns oxygenated blood from the lungs to ... the heart. (MeSH)

Vena Cavae: two main veins conveying deoxygenated blood from the other veins to the right atrium of the heart. (OxfordMed) Return blood from the head, neck and extremities to the heart. (NCIt) The 'inferior vena cava' receives blood from parts of the body below the ‘diaphragm.’ The 'superior vena cava' drains blood from the head, neck, thorax, and arms. (OxfordMed)

Ventricle(s): either of the two lower chambers of the heart, which have thick muscular walls. (OxfordMed) The lower right and left chambers of the heart. The right ventricle pumps venous blood into the lungs and the left ventricle pumps oxygenated blood into the systemic arterial circulation. (MeSH)

Left Ventricle: the left lower chamber of the heart that receives blood... and pumps it through the aorta to the body. (NCIt) Receives blood from the pulmonary vein via the left atrium. (OxfordMed)

Right Ventricle: the lower chamber of the heart located in the right side. It receives blood from the right atrium that is no longer oxygenated and it pumps it to the pulmonary artery. (NCIt) Pumps blood received from the venae cavae. (OxfordMed)

Intestine(s): the long, tube-shaped organ in the abdomen that completes the process of digestion. (NCI3) The portion of the gastrointestinal tract between the "stomach" and the 'anus.' It includes the "small intestine" and "large intestine." (NCIt) The lining of the intestines (acts as a) barrier. It is the major interface between the environment—what we ingest—and ourselves. Its ability to keep out harmful subabstances is key. (Dorsey, 40) Also referred to as ‘bowel.’

Large Intestine: a segment of the lower “gastrointestinal” tract. (MeSH) Long, tube-like organ that is connected to the small intestine at one end and the anus at the other. Partly digested food moves through the 'cecum' into the 'colon,' where water and some nutrients and 'electrolytes' are removed. The remaining material moves through the colon, is stored in the rectum, and leaves the body through the anal canal and anus. (NCIt)

Small Intestine: the part of the digestive tract that is located between the stomach and the large intestine. Approximately 20 feet long. Its main function is to absorb nutrients from food as the food is transported to the large intestine. (NCIt)

Duodenum: part of the small intestine that connects to the stomach. (NCIt) First portion of the small intestine immediately beyond the stomach. (Oxford)

Kidney(s): body organ that filters blood for the secretion of urine and that regulates "ion" concentrations. (MeSH) Responsible for the formation of urine in a mammal. (Indge, 153) One of a pair of organs in the abdomen. The kidneys remove waste and extra water from the blood and help keep chemicals (such as "sodium," "potassium," and "calcium") balanced in the body. The kidneys also make “hormones” that help control blood pressure and stimulate “bone marrow” to make “red blood cells.” (NCI13)

Liver: a very large reddish-brown organ found in the body of a mammal. (Indge, 161) A triangular-shaped organ located under the “diaphragm.” It is the largest internal organ of the body, weighting up to 2 kg. “Metabolism” and "bile" "secretion" are its main functions. It is composed of cells which have the ability to regenerate. (NCIt) Key organ in the metabolism of foodstuffs after digestion and storage of "carbohydrate" as "glycogen." (Lawrence) The liver cleanses the blood and aids in digestion by secreting bile. (NCI13)

Bile: a solution produced in the liver which empties into the small intestine. It is a slightly “alkaline” liquid containing, apart from water and various “inorganic” ions, "bile salts" which play an important part in the digestion of “fat,” through the ‘bile duct.‘ Bile does not contain any digestive "enzymes." Between meals, bile is stored in the gall bladder. When partly digested food from the stomach enters the first part of the small intestine, a hormone is released that results in the emptying of the gall bladder. (Indge, 32)

Bile Salts: “steroids” that “emulsify” fats. Generated in the liver and stored in the gall bladder for later use. (Norman, 6/17/09)

Pancreas: a large globular gland near the stomach which secretes digestive fluid into the duodenum, and “insulin” and other hormones into the blood. (Oxford) 'Pancreatic juice' contains many digestive enzymes. (Indge, 199) Located in the abdomen. It makes pancreatic juices, which contain enzymes that aid in digestion, and it produces several hormones, including insulin. The pancreas is surrounded by the stomach, intestines, and other organs. (NCI3)

Spleen: vascular organ in vertebrates in which "immune reactions" are initiated and red blood cells destroyed. (Lawrence) An abdominal organ which serves as a reservoir for blood (Oxford) An organ that is part of the “lymph system.” The spleen makes “lymphocytes,” filters the blood, stores blood cells, and destroys old blood cells. It is located on the left side of the abdomen near the stomach. (NCI13)

Stomach: an organ of digestion situated in the left upper quadrant of the abdomen. (MeSH) Part of the digestive system. (NCI3) The stomach helps in the digestion of food by mixing it with digestive juices and churning it into a thin liquid. (NCIt) Sac-like organ. Its function is to continue the process of digestion that begins in the mouth. ‘Gastric juice’ secreted by ‘gastric glands’ contributes to chemical digestion. This, together with the churning action of the muscular layers of the stomach, reduces the food to a semi-liquid, partly digested mass that passes on to the “duodenum.” (OxfordMed)

Thymus: a glandular organ near the base of the neck. (Oxford) Part of the lymph system, in which T-lymphocytes grow and multiply. The thymus is in the chest behind the breastbone. (NCI13) In humans, the thymus begins to "atrophy" after "puberty." (Lawrence) Also referred to as ‘thymus gland.’