Baroreceptors are like pressure sensors in the body. It's important for the body to keep things in balance, and the body has to put a lot of work into keeping things like pressure at a steady level. It's a bit like inflating car tyres; if the pressure gets to high, it can cause damage, but if the pressure is too low, things won't work properly. The same is true in the body.
Baroreceptors are particularly involved in sensing blood pressure in blood vessels. The main ones are in the aorta (the first artery coming out of the heart) and in the carotid sinus (part of the carotid artery, the artery which supplies the brain). These are particularly good places to have baroreceptors because they sense the blood pressure as soon as blood has come out of the heart; this helps to give a really good idea of how high the pressure can get.
There are also some blood pressure sensors in other parts of the circulatory system. Sensors in the veins (and also in the right atrium of the heart) give a good idea of how much blood there is in the system; understanding blood volume can be useful for making sure the body can react to increase it or decrease it if there is the wrong amount.
Like many other receptors, baroreceptors don't just sense changes - they also cause something to happen about it. So as well as noticing a change in blood pressure, they also send messages to the brain to get it to bring the pressure back to normal. This is part of the baroreceptor reflex.
The baroreceptor reflex or baroreflex is the body's way of responding to a change in blood pressure. Baroreceptors in the arteries notice that something has changed. When they sense a change in pressure, they send out signals through nerves to a part of the brain called the brainstem.
The increase in pressure causes more signals to be sent to the brainstem. These act to inhibit a part of the brain which increases the blood pressure. So if you send more signals, there will be more inhibition, and the blood pressure will be lowered. In some ways, you could think of it like eating. If someone is getting hungrier and hungrier, you can give them some food and they'll feel less hungry. The more food you give them, the less hungry they feel. The same is true with blood pressure - if the blood pressure is getting higher and higher, the baroreceptors send signals which basically lower the blood pressure. The higher the blood pressure gets, the more signals are sent, and the lower the blood pressure gets. This is called negative feedback. Instead of being encouraging, this feedback discourages something from happening - and the more feedback it gets, the less it does it.
So, as the blood pressure gets high, the baroreceptors stretch more, they send more signals to the brain, which means the brain is inhibited more, which means the blood pressure drops. It's an important part of maintaining a steady blood pressure, although there are other ways that the blood pressure is controlled.
One of the most fascinating things about this area of science is understanding how something is turned into a signal. The way that this signal is transported is covered elsewhere; here the question is how that signal is created in the first place. Baroreceptors need to turn the stretching effect of pressure into an electrical signal or action potential.
Action potentials are always achieved by changing the membrane potential from its resting state. The membrane is happily sat there with a normal concentration of sodium and potassium on either side. The way you change that is by opening a sodium or potassium channel (or both!). This allows some of these metal ions to move in or out of the cell, changing the concentration and therefore changing the potential across the membrane.
When pressure changes the shape of the membrane, channels open up. In the case of baroreceptors in arteries, it's thought that on the baroreceptor nerve endings you can find channels for sodium and calcium. When there is a change in the pressure stretching the wall of the blood vessel, these channels open, allowing sodium and calcium to flood into the cell. This big change in concentration causes a big change in the membrane potential - a depolarisation, which spreads down the nerve in the form of an action potential.
Hey presto! You've got a signal that the body can interpret as the effect of pressure. The more pressure there is, the more the membrane is going to be stretched or deformed, and the more these channels get opened up. Therefore the more these signals are being produced, the higher the pressure must be.