Other cardiovascular reflexes: chemoreceptor, atrial (Bainbridge), muscle and pressure waves
- Regulation
- Exam priority: medium
Besides the baroreflex, sensors of blood gases, atrial stretch and brain signals also steer heart rate, vessel tone and urine output.
Also called: Low-pressure (volume) reflexes, Bainbridge reflex, Mayer waves
See it in 3D: Carotid bifurcation · Aortic arch · Right atrium · Glossopharyngeal nerve (IX) · Vagus nerve · Medulla oblongata
What it is
The vasomotor center in the medulla receives input from far more than the arterial baroreceptors. Peripheral chemoreceptors support pressure when it is very low, low-pressure stretch receptors in the atria and pulmonary artery sense blood volume, and the hypothalamus and cortex set pressure for emotion and exercise. These systems explain several classic exam distractors next to the baroreflex.
How it works, step by step
- A rise in blood volume stretches the low-pressure atria and pulmonary artery.
- Atrial receptors send signals through the vagus nerves to the medulla and hypothalamus.
- Renal sympathetic activity falls, afferent arterioles dilate and GFR rises; ADH release falls, so less water is reabsorbed.
- Atrial myocytes release ANP, which adds natriuresis; the extra urine brings blood volume back toward normal.
- In severe hypotension (< 80 mmHg) chemoreceptors, starved of flow, sense low O₂ and high CO₂ and add sympathetic drive on top of the baroreflex.
Regulation
- +Peripheral chemoreceptors (low O₂, high CO₂, low pH): Raise sympathetic vasoconstriction and arterial pressure, mostly when mean pressure is below ≈ 80 mmHg.
- −Atrial and pulmonary-artery stretch receptors: Lower renal sympathetic activity, ADH and renin, raise ANP; dilate afferent arterioles and increase urine output.
- +Bainbridge reflex (right atrial stretch): Raises heart rate by ≈ 10–15% or more with marked volume load.
- +Hypothalamus and motor cortex: Raise pressure in alarm and at the start of exercise (central command), before metabolites build up.
- −Sympathetic vasodilator fibres (studied in animals): Dilate skeletal muscle vessels in anticipation of exercise; their role in humans is uncertain.
- +Abdominal compression reflex and skeletal muscle tone: Reflex contraction of abdominal and limb muscles squeezes veins and raises venous return and output.
Clinical notes
- Obstructive sleep apnea: repeated hypoxia and hypercapnia stimulate chemoreceptors at night, raising sympathetic activity and arterial pressure.
- Hypovolemic shock: as pressure falls under the baroreceptor range, chemoreceptors and the CNS ischemic response become the main drivers of vasoconstriction.
- Vasovagal syncope is a central reflex (emotion or pain) that suddenly slows the heart and dilates vessels; it is different from the reflexes on this page.
Test yourself
Think of the answer, then tap the question to see it.
Why do peripheral chemoreceptors contribute little to pressure control at normal arterial pressure?
They are excited mainly when low flow lets PO₂ fall and PCO₂ and H⁺ rise in the bodies, which happens when mean pressure is below ≈ 80 mmHg.
A rapid saline infusion stretches the atria. Which reflex changes help return volume to normal?
Less renal sympathetic activity (afferent dilation, higher GFR), less ADH and renin, more ANP: together they increase water and sodium excretion.
What is the Bainbridge reflex?
A rise in heart rate when the right atrium is stretched by increased venous return, mediated partly by vagal afferents to the medulla and partly by direct stretch of the SA node.
Why can a volume load either slow or speed the heart?
The higher arterial pressure activates the baroreflex (slowing), while atrial stretch triggers the Bainbridge reflex (speeding); which wins depends on the initial heart rate and volume.
Connections
See it in 3D6
The structures of Durgical's 3D anatomy where it happens.
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Obstructive Sleep Apnea (StatPearls (NCBI Bookshelf), 2024); Hypovolemia and Hypovolemic Shock (StatPearls (NCBI Bookshelf), 2026); Fainting: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026).
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Representative model, for educational purposes only.