Blood pressure can be best described as a measurement of the force on your arterial walls as your heart pumps blood through the body. (Medicine Plus, 2017) A n instrument called a sphygmomanometer is used to measure blood pressure. It is a process where a cuff is placed around your arm and inflated until the blood circulation in your arm is cut off. Slowly, the cuff deflated and the instrument notes the first instance blood can be felt rushing through the arm (systolic pressure), and then the last instance (diastolic pressure). The final measurement of blood pressure is given in millimetres of mercury (mm Hg) with the systolic value noted first, followed by the diastolic value. 120/80 mm Hg would be an example of a healthy blood pressure for an individual who is resting. (llilades, 2009) Your body has the ability to override any changes in blood pressure in the long and short term. Long term pressure changes cause the body to respond through a system known as the renin-angiotensin system. However, short term changes in blood pressure activate either baroreceptors, chemoreceptors or atrial receptors. (Chapleau, 2018) I will be discussing the role of baroreceptors and how they lower blood pressure after an increase.
Baroreceptors are specialised nerve endings located in the carotid sinus and aortic arch. They allow your brain to detect and regulate blood flow and blood pressure in the larger vessels of your circulatory system. (Rodrigo, 2017) The circulatory system consists of blood vessels, including veins, and arteries. Veins are responsible for bringing blood from the rest of your body back to your lungs. Arteries in the other hand, send blood from your lungs to the rest of your body. Higher blood pressure usually indicates that more blood is travelling through that vessel to the specific organ.
Fig 1:
Effector responses to an increase in baroreceptor activity causes by an increase in blood pressure.
The figure above shows effector mechanisms and how they respond to an increase in baroreceptor activity. As shown, increases in arterial blood pressure and baroreceptor activity either increase paraSNA, decrease SNA or prevent the release of AVP, leading to cardiovascular, hormonal or renal responses.(Chapleau, 2012)
Baroreceptors are actually pressure sensing bodies, sometimes they are also referred to as stretch receptors as they detect stretch in the arterial wall. (North, 2018) When blood pressure rises, the arterial wall becomes deformed which then increases the frequency of action potentials travelling to the brain along the baroreceptor afferent fibres. Figure 2 shows how an increase in arterial pressure leads to an increase in the frequency of action potentials. The stretch of the receptors causes sodium ions to move into the nerve ending, which is what initiates the action potential. (Chapleau, 2012)
Fig.2
As the arterial pressure rises from 80 to 100, the frequency of action potentials increases, which alerts the brain that the body’s blood pressure has increased.
Baroreceptors have a standard firing pattern. This means they have an innate potential to generate action potentials at a particular frequency at all times. The frequency of these action potentials is increased when the baroreceptors receive a stretch stimulus due to an increase in blood pressure. The carotid sinuses increase the rate at which impulses are generated when the pressure in them builds up to values greater than 50 mm Hg. At any other values lower than this threshold pressure, the carotid baroreceptors do not initiate an action potential. Having said this, the arch of aorta can record drops in blood pressure up to 30 mm Hg. The upper limit for blood pressure, where frequency of action potential no longer increases, is 175 mm Hg. The normal MAP is calculated to be around 93 mm Hg. At this pressure, the baroreceptors are believed to be at their most sensitive and even slight changes in pressure will result in rapid firing of action potentials. It is important to note that at blood pressures lower than 30 mm Hg, chemoreceptors take over. These receptors work by sensing the arterial concentration of metabolites such as oxygen and carbon dioxide. So unlike baroreceptors, chemoreceptors do not actually detect changes in pressure. (Chapleau, 2012)
BARORECEPTOR REFLEX
Baroreceptor activity increases when blood pressure rises and decreases when blood pressure falls. These changes in baroreceptor activity initiate a rapid response that either mitigate or oppose changes in arterial blood pressure in a negative-feedback manner.(Step health Journal, 2018)
The Baroreceptor reflex is comprised of three distinct units; afferent nerve carrying impulses from the receptors, the central processing unit and an efferent nerve that innervates the effector. Figure 3, shows the locations of the baroreceptors and the neural pathways that cause baroreflex responses. The arterial baroreceptors innervate the carotid sinuses at the aortic arch, the right carotid artery and the right subclavial artery juncture. The cardiopulmonary baroreceptors innervate the veno-atrial juncture, the atria, ventricles and the pulmonary vasculature. The activity of cardiopulmonary baroreceptors directly correlates with the central blood volume, so these nerve endings are often referred to as ‘volume receptors’ and sometimes ‘low-pressure baroreceptors’. These reflex adjustments that are triggered by changes in cardiopulmonary baroreceptor activity regulate blood volume whilst also influencing blood pressure.(Chapleau, 2012)
Fig. 3
The image shows the location of the baroreceptors, and the various units of the baroreceptor reflex.
Sensory nerve activity from the baroreceptors travels via the vagus and glossopharyngeal nerves, to the medulla oblongata, which tells the autonomic nervous system to respond appropriately. The vasomotor control centres that are located in the medulla control the vasoconstriction and vasodilation, therefore aiding in the regulation of the total peripheral resistance. The cardiac control center in the medulla regulates the cardiac rate. (Chapleau, 2012)
The baroreceptor reflex is activated after an increase or increase of blood pressure, and is more sensitive to decreases in pressure rather than increases. It is also more sensitive to sudden changes in blood pressure than to slower, more gradual changes. (Dr Been, 2018) An example of a rapid change in blood pressure is when someone goes from a lying to standing position very quickly. Gravity means that when one goes from a supine to a orthostatic position, about 500-700ml of blood pools in the lower abdomen and legs.(Rodrigo, 2017) This is widely known as venous pooling. This pooling results in the reduction of the central venous pressure, and thus the venous return, therefore reducing the end diastolic volume (EDV). The decreased EDV reduces the stroke volume of the heart by approximately 30-40%, resulting in a fall in cardiac output and then the mean arterial blood pressure. This entire process is known as postural hypotension. (Rodrigo, 2017) However, the baroreceptor reflex quickly intervenes, as the decrease in baroreceptor sensory information traveling from the glossopharyngeal and vagus nerves to the medulla oblongata prevents parasympathetic nerve activity and stimulates sympathetic nerve activity. This produces and increase in cardiac rate and vasoconstriction, helping to maintain a normal blood pressure when one stands up. As this system may take a few seconds to kick in, you many only feel dizzy for a very short time before your blood pressure is regulated. (Step Health Journal, 2018) Information given by the baroreceptors can also give the opposite response. When blood pressure rises above the individuals regular range, the baroreflex can initiate the slowing of the cardiac rate and vasodilation. (Rodrigo, G)
To conclude, blood pressure can be regulated by various other mechanisms in the body but baroreceptors also known as stretch receptors, detect stretches in the arterial wall, indicating an increase in blood pressure. Communication via the vagus and glossopharyngeal nerves to the medulla oblungata results in the vasodialtion of the arteries, resulting in the regulation of high blood pressure.