Baroreceptors are a type of nerve receptor that sense pressure changes within your circulatory system. They send feedback to your medulla and pituitary gland, which react by changing your blood pressure.
Baroreceptors are a type of mechanoreceptor -- specifically a type of stretch receptor -- which fire in proportion to the distention of the artery walls. This distention, while constantly changing with the pulse of the heart beat, is still an accurate indicator of blood pressure. Baroreceptors are divided into two or three types, which work together to maintain healthy blood pressure levels.
- Aortic baroreceptors are the least sensitive (sensing 100-200 mmHg), and are found, as you might expect, in the aorta of the heart. These are sensitive to changes in pressure in the blood supplying the heart, and send signals up the vagus nerve; these signals inhibit the vasomotor center of the medulla oblongata from sending impulses to stimulate the smooth muscle in the artery walls; the lack of stimulation causes vasodilation, a lower heart rate, and a resulting drop in blood pressure. Likewise, low pressure causes these fibers to fire less and less, causing the vasomotor center to become more active, and the heartbeat to strengthen.
- Carotid sinus baroreceptors are more sensitive than the aortic ones (sensing 50-200 mmHg), but do essentially the same thing. These receptors monitor the blood going to the brain, sending impulses up the glossopharyngeal nerve. These signals work exactly the same way as the ones from the aortic baroreceptors.
- Buffer nerves or arterial baroreceptors: Together the aortic and carotid receptors are known as the buffer nerves, because they buffer any sudden changes in arterial pressure throughout the day. This process is known as the baroreflex, and is only one of the feedback loops your body uses to maintain optimal blood pressure. It is, however, the fastest-acting of your blood pressure monitoring systems, and can adjust your internal pressure in the time span of just a couple of heartbeats. If high or low blood pressure continues for a number of days, the buffer nerves will start to adjust to the new norm, and will maintain blood pressure at the new levels, even if these levels are not healthy.
- Low-pressure baroreceptors or cardiopulmonary receptors can sense even lower arterial pressures, down to about 70 mmHg (average arterial blood pressure is 80-90 mmHg). These receptors are found in the large veins close to your heart and in the atria.
While the buffer nerves are designed to respond quickly to fluctuation in blood pressure, the low-pressure baroreceptors moderate the blood pressure over the course of hours and days, rather than seconds. They do this by changing your blood volume, and give feedback both to the medulla and the pituitary gland. When high blood volume triggers these nerves the pituitary gland releases hormonal cues that cause the kidneys to start removing salt and water from the bloodstream, slowly reducing the overall blood volume.
Fun With Baroreceptors
Your low-pressure baroreceptors are the reason why many people need to pee after spending an a hour in the pool; the external pressure exerted on your body by the water causes your baroreceptors to sense that you have too high blood pressure, and your kidney starts moving water into your bladder. This is also why you start to feel so thirsty after coming out of the pool, even if you haven't been exercising hard. The bodies of serious swimmers start to adjust to these effects, and these effects are less extreme in a well-toned cardiovascular system, but even so, triathletes will still report a pressing need to pee after a workout.
This effect also takes place in reverse -- during extended bed rest, or in zero gravity, blood is distributed comparatively evenly throughout the body, rather than collecting in the lower extremities. This even distribution of blood raises the arterial blood pressure, which triggers the kidneys to excrete the 'excess' fluid, and over the course of days this can lead to a 10% or greater decrease in blood volume. When you return to a vertical position, or return to Earth, gravity causes your blood to pool in your legs again. Orthostatic hypotension occurs, the same effect as when you stand up too quickly and feel dizzy. Your buffer nerves send off emergency signals to your brain, increasing your heart rate. This would normally bring your blood pressure back to normal, but as you are lacking in adequate blood volume, you will continue to feel dizzy or even pass out.
And this is why no spaceships should contain swimming pools.
Less Fun With Baroreceptors
Your barorecepters also play a part in the body's response to shock, particularly in cases where there is lots of blood loss; in this case the body will work to make sure that the heart and brain continue to receive enough blood by both increasing heart rate and slowing blood flow to unnecessary veins in non-critical body parts. This response is called the Cushing reflex.
Baroreceptors can be damaged during heart surgery, and a good portion of the nerves are generally sliced completely during a heart transplant. Chemoreceptors that detect the level of oxygen in the blood take up some of the slack, causing the heart beat and respiratory rate to increase when blood oxygen is low. Some baroreceptor nerve fibers may regenerate, although this has not been observed in the aortic baroreceptors. The lack of functioning baroreceptors may result in sudden spikes and drops in blood pressure, although these are usually not a major problem.
Baroreceptors in Myth and Medicine
Some doctors believe, and some science publications have reported, that baroreceptors somehow mediate between changes in barometric pressure and joint pain such as arthritis. This would explain why so many people report increased joint pain before a thunderstorm; the low atmospheric pressure would lower, ever so slightly, the pressure inside your body, and this would hurt. Unfortunately, that is the only explanation ever given. At least one medical doctor blames baroreceptors in the joints, which as far as I can determine do not actually exist.
There are other comments on popular medicine sites that suggest that baroreceptors exist in your bladder, intestines, and joints. As far as I have been able to find, these writers are simply reading things out-of-context or with limited understanding. For example, eHow.com claims that there are baroreceptors in the lungs; this is true in amphibians, but not in mammals. (Humans do have pulmonary stretch receptors, but they are not referred to as baroreceptors).
I would be interested in any information that anyone can give me about baroreceptors outside of the circulatory system and/or the role of baroreceptors in weather prediction. But for now, I am chalking these up to muddy thinking.