What does the sinoatrial node do when body temperature is increased?
The sinoatrial node (SA node) is the natural pacemaker of the heart, responsible for initiating and maintaining a regular heart rhythm. When body temperature increases, the SA node responds by increasing the heart rate. This physiological response is known as tachycardia.
The relationship between body temperature and heart rate is mediated by several mechanisms:
1. Direct Effect on the SA Node:
- Increased body temperature directly affects the SA node cells, causing them to depolarize more rapidly. This results in an increased rate of electrical impulse generation, leading to a faster heart rate.
2. Autonomic Nervous System Activation:
- Elevated body temperature stimulates the sympathetic nervous system, which increases the release of catecholamines (such as adrenaline and noradrenaline) into the bloodstream. These hormones act on the SA node and other heart tissues, enhancing impulse formation and conduction, thereby increasing heart rate.
3. Changes in Blood Viscosity:
- Higher body temperature leads to a decrease in blood viscosity. This reduction in blood thickness improves blood flow and oxygen delivery to the heart. Better oxygenation of the SA node cells enhances their electrical activity, contributing to a faster heart rate.
4. Metabolic Rate:
- Increased body temperature elevates the body's metabolic rate. This increased energy demand requires more oxygen and nutrients to be delivered to the tissues. The heart rate increases to meet the enhanced metabolic demands and ensure adequate circulation.
The acceleration of heart rate in response to increased body temperature serves several purposes:
- It enhances blood flow to deliver more oxygen and nutrients to tissues that have an increased metabolic demand due to the higher temperature.
- It promotes heat dissipation by increasing blood flow to the skin, where the body can release excess heat.
- It helps maintain blood pressure and prevent hypotension that can occur with excessive vasodilation associated with high temperatures.
However, an excessively elevated heart rate due to prolonged or extreme temperature increases can strain the heart and lead to complications, particularly in individuals with underlying cardiovascular conditions.