Bainbridge reflex is due to
**Core Concept**
The Bainbridge reflex, also known as the atrial stretch reflex, is a physiological reflex that helps regulate heart rate in response to changes in blood volume or pressure. It is mediated by stretch receptors in the right atrium, which sense changes in atrial pressure and send signals to the brain to adjust heart rate accordingly.
**Why the Correct Answer is Right**
The Bainbridge reflex is activated when the right atrium is stretched due to increased blood volume or pressure. This stretching of the atrial wall activates mechanoreceptors, specifically the stretch receptors or pacemaker cells, which release neurotransmitters that stimulate the sinoatrial (SA) node to increase heart rate. This reflex helps to maintain cardiac output and blood pressure by increasing heart rate in response to increased blood volume or pressure.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the Bainbridge reflex. While the autonomic nervous system does play a role in regulating heart rate, the Bainbridge reflex is a specific response to atrial stretch, not a general response to sympathetic or parasympathetic stimulation.
* **Option B:** This option is incorrect because it refers to a different reflex, the baroreceptor reflex, which is activated by changes in blood pressure, not atrial stretch.
* **Option C:** This option is incorrect because it refers to a different mechanism, the Frank-Starling mechanism, which describes the relationship between ventricular preload and contractile function, not the Bainbridge reflex.
**Clinical Pearl / High-Yield Fact**
The Bainbridge reflex is an important mechanism for maintaining cardiac output and blood pressure, especially in response to changes in blood volume or pressure. It is an example of a negative feedback mechanism that helps to regulate heart rate in response to changes in the body.
**Correct Answer:** C. The Bainbridge reflex is activated by stretch receptors in the right atrium.