The normal resting heart rate is called the cardiac rhythm and is made in the sinoatrial node, a group of pacemaker cells found in the wall of the right atrium. The cells in the sinoatrial node do this by making an action potential, and this is done through a special movement of certain solute ions into and out of these cells. The action potential then spreads to neighboring cells. [40]
When the sinoatrial node cells are resting, they have a negative charge on their membranes, but a rapid influx of sodium ions will charge the membrane with a positive charge. This phenomenon is called depolarization and occurs spontaneously.[9] When the cells become highly charged, the sodium channels close, and calcium ions begin to enter the cell, and shortly thereafter, potassium ions begin to leave the cell. All these ions travel through the ion channels in the membrane of the sinoatrial node cells. The movement of potassium and calcium ions begins when the cell becomes highly charged, and calcium channels are called voltage gates. Shortly after the movement of calcium begins, the calcium channels close and the potassium channels open to allow potassium to leave the cell. This causes the cell to have a negative charge while at rest, a process called repolarization. When the membrane potential reaches approximately -60 mV, the potassium channels close and the process begins again.[9]
Electrolytes move from areas of high concentration to areas of low or non-existent concentration. For this reason, sodium ions move from outside the cell to the inside, while potassium moves from inside the cell to the outside. Calcium also plays a crucial role, as its slow flow through its channels means that the sinus node cells will go into a “plateau” phase when they are positively charged, part of this is called the runaway period. Calcium ions also combine with the regulatory protein tropin C in the tropin complex to enable muscle cells to contract and separate from the protein to allow them to relax. [41]
An adult’s heart rate ranges from 60 to 100 beats per minute. The resting heart rate of a newborn can reach 129 beats per minute and this number gradually decreases until maturity.[42] The heart rate of the athlete can reach 60 beats per minute, but during exercise it may reach 150 beats per minute with a maximum of 200 or 220 beats per minute.[9]
