heart and blood vessel disorders
Biology of the Heart
Regulation of the Heart
The contraction of the muscle fibers in the heart is very organized and highly controlled. Every heart muscle fiber does not contract at exactly the same time. Instead, the fibers contract in a sequence that best pumps blood out of each heart chamber. The contraction sequence is controlled by rhythmic electrical impulses (discharges) that flow through the heart in a precise manner along distinct pathways and at a controlled speed. The impulses originate in the heart's natural pacemaker (the sinus or sinoatrial node—a small mass of tissue in the wall of the right atrium), which generates a tiny electrical current. The sinoatrial (sinus) node (1) initiates an electrical impulse that flows through the right and left atria (2), making them contract. When the electrical impulse reaches the atrioventricular node (3), it is delayed slightly. The impulse then travels down the bundle of His (4), which divides into the right bundle branch for the right ventricle (5) and the left bundle branch for the left ventricle (5). The impulse then spreads through the ventricles, making them contract. The heart rate, or pulse, is the number of times the heart beats within a minute. The heart rate goes up when the body needs more oxygen (such as during exercise). The heart rate goes down when the body needs less oxygen (such as during rest). The rate at which the sinus node sends out its impulses governs the heart rate. The sinus node has its own rate of sending out impulses. This rate can be modified by two opposing parts of the Autonomic nervous system —one part speeds the heart rate up (the sympathetic division of the nervous system) and one slows it down (the parasympathetic division). Thesympathetic divisionworks through a network of nerves called the sympathetic plexus and through the hormonesepinephrine(adrenaline) andnorepinephrine(noradrenaline), which are released by the adrenal glands and the nerve endings. Theparasympathetic divisionworks through a single nerve—the vagus nerve—which releases the neurotransmitter acetylcholine. 