Physiology: The Circulatory System Part 3 (Revisited)
The heart nodes - These special bundles of unique tissue are simply astounding. The first is embedded in the wall of the right atrium, and is called sinoatrial node (S.A. node) and is the "pacemaker" of the heart. (Artificial pacemakers derive their name from it.) The other bundle is in the lower part of the septum, and is the atrioventricular node (A.V. node). The bundle of His is also in this area, and is called the coordinator.
Heartbeat - The heartbeat originates in the S.A node, and immediately the entire auricle contracts. Then the A.V node picks up the message and relays the signal to the muscle fibers of the ventricle, which contracts. Heart block is a condition which occurs when disease of the bundle of His interrupts communication between the auricle and the ventricles. A proper balance of calcium, sodium, and potassium is needed for the heart to function properly. If you eat only good food, you should have the proper nutritional balance.
Heart rate - The heart rate is controlled by the medulla in the brain and sensory nerve impulses to the heart. It is speeded up by emotional reaction, fever, or physical exertion. It is decreased by increased blood pressure, a lack of oxygen, or excess carbon dioxide.
Heart valves - The openings into the aorta and the pulmonary artery are fitted with flaps, called valves. There are similar valves at the openings between the auricles and ventricles. The valve between the right auricle and right ventricle is called the tricuspid valve. The valve between the left auricle and left ventricle is called the mitral, or bicuspid valve. After the blood is squeezed into the ventricles, these valves close, and those over the artery outlets open. This keeps the blood from flowing back when the heart relaxes.
Blood vessels - These are the tubes which carry blood throughout the body. Those that carry blood away from the heart are the arteries; those that take it to the heart are the veins. (One oddity in following this rule is the pulmonary circulation, which sends blood to the lungs and back to the heart. Arteries carry deoxygenated blood away from the heart and veins carry fresh, oxygenated blood to the heart. Elsewhere in the body, only arteries carry the fresh blood.)
Coronary arteries - These are especially important arteries because they are the ones supplying food and oxygen to the heart muscle itself. If these arteries become narrowed, or if a blood clot blocks part of them, the heart is not supplied with blood and serious trouble may occur. We call this coronay heart disease; it is the cause of most of the deaths from heart trouble after middle age.
Brain arteries - If a blockage occurs in a brain artery, a stroke can occur.
Capillaries - Blood travels out from the heart through the aorta, and then through smaller arteries, and finally through very small arteries (arterioles) into the capillaries (the smallest blood vessels). From there, the blood travels into the smallest veins (venules), then into veins, and finally to superior and inferior venae cava and back into the heart.
Special systems - Pulmonary circulation: takes blood from the heart to the lungs and back again - so the blood can pick up oxygen. Another important one is the portal system. All the veins from the stomach, intestines, spleen, and pancreas empty into the portal vein, which leads to the liver - so the blood can pick up food. Blood leaves the liver through the hepatic vein and goes to the heart.
Lymphatic system - Your body is filled with lymphatic vessels. They do not carry blood; they carry excess fluids and waste and empty into the thoracic duct and right lymphatic duct, from when it goes into veins at the back of the neck. Body muscles keep the lymph following; valves in the vessels keep it from flowing backward.
Lymph nodes - There are lymph nodes at several places in your body; these filter out harmful substances such as bacteria and cancer cells. They also manufacture lymphocytes (one type of white blood cell). There are six places where these nodes are found: under the arms, on the right and left side of the groin, and the right and left side of the neck. If they become infected, the disease is called adenitis.
Spleen - The spleen lies directly below the diaphragm, above the left kidney, and behind the stomach. It destroys old red blood cells, makes one type of white blood cell, and filters toxins from the blood. It also produces antibodies, which give us immunity to certain diseases. It stores iron, bile pigments, and antibodies. It becomes enlarged in anemia, malaria, and leukemia.
Tonsils - The three tonsils in the pharyngeal wall at the back of your throat strain out toxins and make lymphocytes. It is significant that the tonsils guard the entrance to the gastro-intestinal tract and the appendix guards the outlet of the small intestine.