How the heart rate is kept within normal limits?

As blood is pumped out of the heart it first goes up into the biggest artery in the body called the aorta. It has nerves in it that measure blood pressure called baroreceptors. Then the blood goes up through the carotid arteries in the neck which have more baroreceptors and another kind that measures the carbon dioxide content in blood. These are called chemoreceptors. Both of these nerve receptors send their information to a part of the brain known as the medulla oblongata which has two regulatory regions called the cardio-acceleratory and the cardio-inhibitory centers. Both of these control mechanisms are linked to the sinoatrial node at the top of the right side of the heart. This is where the electrical impulse that starts every heart beat cycle is generated. If the heart needs to pump out more blood, or increase cardiac output, a message is sent to the senatorial node to make it accelerate and cause the heart beat faster. If it needs to slow down, or decrease cardiac output, a message is sent to inhibit the sinoatrial node which then makes the heart beat slower. As blood is pumped out of the heart it first goes up into the biggest artery in the body called the aorta. It has nerves in it that measure blood pressure called baroreceptors. Then the blood goes up through the carotid arteries in the neck which have more baroreceptors and another kind that measures the carbon dioxide content in blood. These are called chemoreceptors. Both of these nerve receptors send their information to a part of the brain known as the medulla oblongata which has two regulatory regions called the cardio-acceleratory and the cardio-inhibitory centers. Both of these control mechanisms are linked to the sinoatrial node at the top of the right side of the heart. This is where the electrical impulse that starts every heart beat cycle is generated. If the heart needs to pump out more blood, or increase cardiac output, a message is sent to the senatorial node to make it accelerate and cause the heart beat faster. If it needs to slow down, or decrease cardiac output, a message is sent to inhibit the sinoatrial node which then makes the heart beat slower.