The heart is a pump made of four chambers: the right atrium and right ventricle, and the left atrium and left ventricle. The two sides are separated by a muscular septum. In a normal heart, blood cannot pass from one side to the other without first going through either the general, systemic, or pulmonary circulation. Four valves keep the blood flowing in one direction. When the valves are diseased, blood leaks backward, causing the heart to pump less effectively. If there is a hole in the septum, blood can also leak backward. Blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium. It then flows through the open tricuspid valve into the right ventricle. When the right ventricle is full, the tricuspid valve shuts, preventing blood from flowing backward into the atrium while the right ventricle contracts. Blood leaves the right ventricle through the pulmonic valve and flows into the pulmonary artery. The pulmonary artery branches into smaller vessels and finally into capillaries around the air sacs. Oxygen passes through the walls of the capillaries and into the blood. At the same time, carbon dioxide, a waste product of metabolism, passes from the blood into the air sacs of the lungs and leaves the body when the cat exhales. The oxygenated blood flows through the pulmonary veins to the left atrium. It then flows through the open mitral valve into the left ventricle. When the left ventricle is full, the mitral valve shuts, preventing blood from flowing backward into the atrium while the left ventricle contracts. Blood then leaves the heart through the aortic valve, flowing into the aorta. It passes through progressively smaller arteries until it reaches the capillary beds of the skin, muscle, brain, and internal organs. At these end locations, oxygen is released and carbon dioxide is collected. Blood is carried back to the heart through progressively larger veins, thus completing the cycle. The arteries and veins are under the control of the nervous system and of the hormones. They can expand or contract to maintain a correct blood pressure. The heartbeat is controlled by an internal nerve system that releases electrical impulses. This system is responsive to outside influences, so the heart speeds up when the cat exercises, becomes frightened, overheats, goes into shock, or requires greater blood flow to tissues. Heart rhythms follow a fixed pattern that can be seen on an electrocardiogram (EKG orECG). Whether the beat is fast or slow, the sequence in which the various muscle fibers contract remains the same. This sequence causes a synchronized beat, allowing both ventricles to empty at the same time. If the heart rate is very slow, this is called bradycardia. If the heart rate is too fast, this is called tachycardia. When the rate is so fast that the normal sequence of contraction is disturbed, the condition is called fibrillation.Arrhythmia, an absence of regular rhythm, upsets the normal pattern of the heart muscle contraction, causing inefficient pump action. Pacemakers have been used in cats with arrhythmias, but not very successfully. This may be due, in part, to the small size of cats. Some cats with arrhythmias develop secondary cephalothorax (accumulation of lymph fluid in the chest)
