48. Reflex control mechanisms of circulation
Cardiovascular reflexes
There are many cardiovascular reflexes involved in the normalization of rapidly changing blood pressure. These reflexes are not involved in the long-term regulation of blood pressure.
Like all reflexes these reflexes follow the following sequence of events:
- A stimulus occurs, which stimulates a receptor
- An afferent nerve is activated
- The afferent nerve stimulates a reflex centre, often in the CNS
- The reflex centre stimulates an efferent nerve
- The efferent nerve causes a response
This is called the reflex arc.
The most important reflexes are:
- Carotid sinus reflex
- Depressor reflex
- Chemoreceptor reflex
- Bezold-Jarisch reflex
- Bainbridge reflex
- Loven reflex
- Cushing reflex
- Oculocardiac reflex
- Atrial-renal reflex
| Name of reflex | Stimulus | Receptor | Afferent nerve | Efferent nerve | Effect |
| Carotid sinus reflex | Increased stretch of arterial wall | Baroreceptors in the carotid sinus | Glossopharyngeal nerve (IX) (Hering’s nerve) | Vagal nerve (X) | Parasympathetic activation (vasodilation, decreased heart rate) |
| Depressor reflex | Increased stretch of arterial wall | Baroreceptors in the aortic arch | Vagal nerve (X) | Vagal nerve (X) | Parasympathetic activation (vasodilation, decreased heart rate) |
| Chemoreceptor reflex | Hypoxia | Chemoreceptors in the carotid body | Vagal nerve (X), glossopharyngeal nerve (IX) | Sympathetic trunk | Sympathetic activation (vasoconstriction, increased heart rate and contractility) |
| Bezold-Jarisch reflex | Increased stretch of ventricular wall | Baroreceptors, pain receptors in ventricular wall | Vagal nerve (X) | Vagal nerve (X) | Parasympathetic activation (vasodilation, decreased heart rate) |
| Bainbridge reflex | Increased stretch of right atrial wall | Baroreceptors in atrial wall | Vagal nerve | Sympathetic trunk | Sympathetic activation (vasoconstriction, increased heart rate and contractility) |
| Lovén reflex | Pain | Pain receptors | Pain fibres | Sympathetic trunk | Sympathetic activation (vasoconstriction, increased heart rate and contractility) |
| Cushing reflex | Intracranial hypertension | Baroreceptors in cerebral arteries | Vegetative fibres | Vagal nerve (X), sympathetic trunk | Parasympathetic and sympathetic activation, resulting in decreased heart rate but increased blood pressure |
| Oculocardiac reflex | Compression of the eyeball | Mechanoreceptors in the eyeball | Ophthalmic nerve (I) | Vagal nerve (X) | Parasympathetic activation (vasodilation, decreased heart rate) |
| Atrial-renal reflex | Increased stretch of atrial wall | Baroreceptors in right atrial wall | Vagal nerve (X) | Decreased ADH production | Increased diuresis, decreased blood pressure |
| Hering-Breuer reflex | Increased stretch of the lungs (inspiration) | Stretch receptors in the lungs | Vagal nerve (X) | ? | Sympathetic activation (vasoconstriction, increased heart rate) |
These reflexes are not just involved in decreasing abnormally high blood pressure but also in increasing abnormally low blood pressure. If the blood pressure decreases the stretch of the vessels will be decreased, which decreases the stimulation of the baroreceptors. This will decrease the activation of the reflex.
Changes in body position
Cardiovascular reflexes are essential in regulating the blood pressure when the body position changes from lying to standing or vice versa. When standing up the arterial pressure in the head and upper part of the body falls as gravity pulls blood towards the legs. Baroreceptors in the carotid sinus and aortic arch immediately detect the decreased blood pressure, resulting in a strong sympathetic activation which returns the blood pressure in the upper body back to normal.