Reflexes on HR

Baroreceptor and Bainbridge reflex

Baroreceptor reflex

Baroreceptors in aortic arch and carotid sinus affect HR

Increase in BP
--> Increase vagal tone/decrease sympathetic tone
--> Decrease HR and contractility

Bainbridge reflex

... an increase in HR caused by an infusion of saline

Increased volume
--> Increased venous return to RA
--> Stimulation of atrial receptor
--> Increased HR

By similar mechanisms,

During inspiration
--> Increase venous return to RA
--> Increased HR

Bainbridge vs Baroreceptor

When blood volume is high (low HR),
--> Bainbridge reflex > Baroreceptor reflex

When blood volume is low (high HR),
--> Baroreceptor reflex > Bainbridge reflex

Other reflexes

Stretch receptor in lung

Moderate lung inflation may increase HR

Peripheral chemoreceptor

[BL8:p95]

Stimulation of peripheral chemoreceptor

--> Stimulation of medullary vagal centre (primary effect)

--> Decreased HR

But,

Stimulation of peripheral chemoreceptor

--> Increased respiratory activity

--> Hypocapnia and increased lung stretch

Both leads to increased HR (secondary effect)

When pulmonary hyperventilation is not prevented
--> Primary and secondary effect tend to cancel out
--> Minimal effect on HR

But if patient is ventilated, cessation of ventilation causes immediate bradycardia because of primary effect and lack of secondary effect

Effects of respiration

Rhythmic changes in heart rate --> Almost entirely due to oscillations in vagal tone.