How is the heart beat controlled?
The beating of the heart is controlled by a specialised group of cells called the sinoatrial node (SA node). Located in the right atrium, the SA node is commonly known as the "natural pacemaker" of the heart. It generates electrical impulses that initiate each heartbeat and then cause the atria to contract.
Here's how the heart beat is controlled:
1. Sinoatrial Node (SA Node):
- The SA node is composed of specialised cells that have the innate ability to generate electrical impulses spontaneously.
- These impulses are generated due to the flow of ions (mostly sodium and potassium) in and out of the SA node cells, leading to changes in the electrical potential across their membranes.
- The SA node's inherent rhythm of electrical discharge sets the basic rate and timing of the heartbeat.
2. Atrioventricular Node (AV Node):
- The electrical impulses generated by the SA node travel through the right atrium and reach the atrioventricular node (AV node), which is situated between the atria and the ventricles.
- The AV node delays the electrical signal slightly. This slight delay allows the atria to fill with blood before the ventricles contract.
3. Bundle of His and Purkinje Fibres:
- From the AV node, the electrical signal passes down specialised conducting pathways: the bundle of His, located at the junction between the atria and ventricles, and the Purkinje fibres, which extend throughout the ventricular muscle.
- The bundle of His and the Purkinje fibres rapidly transmit the electrical impulse, ensuring a coordinated contraction of the ventricular muscle fibres, leading to the squeezing of the ventricles and the pumping of blood out of the heart.
4. Regulation by the Nervous System:
- The heart's intrinsic electrical conduction system is influenced and regulated by the nervous system.
- The sympathetic nervous system accelerates the heart rate and increases the force of contraction, while the parasympathetic nervous system slows the heart rate and reduces its contractile force.
- These autonomic nervous inputs modulate the heart rate based on the body's needs, such as during physical activity, rest, or in response to stress.
5. Hormones:
- Certain hormones, like adrenaline (epinephrine), can also affect the heart rate. Adrenaline increases the heart rate and contractile force, preparing the body for a "fight or flight" response.
In summary, the heart's intrinsic electrical conduction system, consisting of the SA node, AV node, bundle of His, and Purkinje fibres, controls the heart's rhythm and the coordination of its pumping action. This electrical activity is influenced by the nervous system and hormones to adapt the heart's function to the body's changing demands.