Blood pressure: sensing and reflex mechanisms to maintain normal blood pressure

2021-05-27 02:58 PM

This is a reflex initiated by receptor tension, distributed in specific areas in the walls of several large arteries. An increase in blood pressure stretches the baroreceptors and induces signal transduction into the central nervous system.

In addition to the function of increasing blood pressure during physical activity and stress, many neuroregulatory mechanisms are specifically at the subconscious level to maintain blood pressure at or near-normal levels. Almost all are negative feedback mechanisms, described in the next section.

Known as the neural mechanism for reflex blood pressure regulation of baroreceptors. Basically, this is a reflex initiated by stretching receptors, called receptors, which are distributed in specific areas in the walls of several large arteries. An increase in blood pressure stretches the baroreceptors and induces signal transduction into the central nervous system. The “feedback” signal is sent back through the autonomic nervous system to the circulation to lower blood pressure to normal.

Physiological anatomy and distribution of pressure receptors 

Baroreceptors are spray-type nerve endings located in the arterial wall and are stimulated under tension, distributed in the great artery walls of the thoracic and neck, and the receptors are abundantly distributed in (1(1) ) wall of each internal carotid artery above the bifurcation of the carotid artery (carotid sinus), (2) wall of the aortic arch.

Sensory receptor system for arterial pressure control

Figure. Sensory receptor system for arterial pressure control.

The figure shows signals from the carotid sinus pressure receptor being transmitted via the Hering's nerve to the vasopharyngeal nerve in the superior neck, and then to the solitary nucleus pulposus in the medulla.

Response of the receptor to blood pressure

The figure shows the effect of different blood pressure levels on the rate of signal conduction in the Hering carotid nerve. Note that the carotid sinus receptors are not stimulated when blood pressure is between 0 and 50-60 mmHg, but above this, they respond rapidly increasing rapidly and peaking at about 180 mmHg. The response of the aortic receptor is similar to that of the carotid artery except that in normal functioning, blood pressure is at a threshold higher than 30 mmHg.

Activation of receptors for different levels of arterial pressure

Figure. Activation of receptors at different levels of arterial pressure. ΔI, carotid sinus nerve impulse change per second; ΔP, change in arterial blood pressure in mmHg.

Of particular note is that normal blood pressure is regulated in the range of 100 mmHg, even a small change in blood pressure dramatically alters the signal from the receptor to readjust blood pressure to normal levels. 

The receptor responds very quickly to changes in blood pressure, the pulse rate increases during each systole and decreases again during diastole. Furthermore, the receptor responds to changes in blood pressure much more rapidly than unchanged blood pressure, i.e. the same level of blood pressure 150 but if it is fluctuating the signal conduction velocity may be greater. 2 times the blood pressure at the fixed threshold.

The circulatory reflex begins at the receptor

After the signal from the receptor is sent to the solitary nucleus pulposus, the second signal inhibits the vasoconstrictor centre of the medulla and stimulates the X cord parasympathetic centre. varicose veins and arterioles in the peripheral circulatory system, (2) decrease in heart rate and myocardial contractility. Thus, stimulation of the receptor by high blood pressure causes a decrease in blood pressure because it reduces peripheral resistance and decreases cardiac output. Conversely, low blood pressure causes the opposite effect to raise blood pressure to the normal range.

The figure shows specific reflexes in blood pressure changes caused by occlusion of the two common carotid arteries. A decrease in pressure in the carotid sinus reduces the signal from the receptor and causes a decrease in inhibition of the vasomotor centre. As a result, the vasomotor centre becomes more active than normal, causing a rise in blood pressure that remains elevated for 10 minutes when the carotid artery is clamped. Removal of the occlusion causes pressure in the carotid sinus to rise, and the carotid reflex causes an immediate drop in blood pressure, which briefly falls below normal and then returns to normal.

Typical carotid sinus reflex

Figure. The typical carotid sinus reflex acts on aortic pressure by clamping both common nerves (after bifurcation of the vagus nerves).

Sensory receptors reduce orthostatic changes in blood pressure

The role of receptors in maintaining blood pressure in the upper body is important when standing up from lying down. Immediately upon standing up blood pressure in the head and the upper body tends to drop, a sharp drop can cause loss of consciousness. However, the decrease in blood pressure at the receptor immediately causes a reflex. As a result, the sympathetic system exerts a powerful effect on the whole body and causes the blood pressure in the head and upper body to decrease to a minimum. .

Pressure “buffers” the system control function of the receptor

Because the receptor system has the ability to raise and lower blood pressure, it is called the blood pressure buffer system, and the nerve from the receptor is called the glial.

Two-hour recorded arterial pressure

Figure. Arterial pressures were recorded two hours (top) and (bottom) several weeks after the receptor was desensitized.

The figure shows the important buffering function of the receptor, the top is a 2-hour graph of the arterial blood pressure of a normal dog, and the bottom is a record of the arterial blood pressure of a dog with cerebral palsy. cut all the nerves to the receptor. We found that in dogs with neurectomy there is a variation in blood pressure caused by normal activities of the day such as lying down, standing up, eating, defecating...

Frequency distribution curve of arterial pressure

Figure. The frequency distribution curve of arterial pressure

Over a 24-hour period in a normal dog and in the same dog several weeks after the receptor was reduced.

The figure shows the frequency distribution of blood pressure recorded over 24 h in both normal and denervated dogs. When the receptor is functioning normally, blood pressure values ​​remain within a narrow range of 85-115 mmHg throughout the day, and mostly around 100 mmHg. After denervation of the receptor, the blood pressure distribution frequency curve is wider and lower, representing a 2.5-fold variation from normal, often down to 50 mmHg, and higher than 160 mmHg. Thereby showing the large variation of blood pressure in the absence of the arterial pressure sensing system.

Important receptor in long-term regulation of blood pressure?

Although arterioreceptors have an important role in the immediate regulation of blood pressure, their role in long-term regulation is controversial. Some physiologists argue that it is not important in the long-term regulation of a blood pressure level, it can change the blood pressure threshold when that blood pressure persists for 1-2 days. Specifically, if blood pressure rises from the normal threshold of 100 to 160 mmHg, at first the signal conduction from the receptor is very high. Over the next few minutes, the pulse drops significantly, and then drops off a lot for a day or two, eventually returning to near normal, even though the blood pressure value remains at 160 mmHg. In contrast, when blood pressure drops to a very low threshold, the receptor does not initially transmit the signal, but gradually, over 1-2 days, the receptor's pulse rate returns to the control level.

The "reset" of the receptor may impair their ability to regulate in the presence of long-term fluctuations in blood pressure. Research has shown that the receptor is not completely reset and thus contributes to long-term blood pressure regulation, especially in the presence of active renal sympathetic support. For example, when blood pressure is persistently elevated, the receptor reflex immediately activates the renal sympathetic nervous system to increase the renal excretion of sodium and water. This reduces blood volume, helping blood pressure return to normal. Therefore, the long-term regulation of arterial blood pressure by receptor requires the coordination of activities of many systems, playing the main role of the renal-humoral-blood pressure regulation system (along with the combination of blood pressure and blood pressure). neural and humoral mechanisms).

Regulation of arterial blood pressure by chemoreceptors - the effect of decreased oxygen levels on blood pressure

Closely associated with the receptor in blood pressure regulation.

Chemoreceptors are chemosensitive cells that are sensitive to hypoxia, excess CO2 and H+. Forms small clumps about 2mm (two carotid bodies, located at the bifurcation of each common carotid artery, and 1 to 3 trunks at the aortic junction). Chemoreceptors stimulate nerve fibres, along with those of the receptor, via the hering and X nerves to the vasomotor centre in the brain stem.

Each carotid and client is supplied with blood by a small feeding artery. When arterial blood pressure drops to dangerous levels, chemoreceptors are stimulated by decreased conduction blood flow causing hypoxia, and excessive formation of co2 and h+ since they are not removed by the bloodstream. slow flow.

Signals from chemoreceptors stimulate the vasomotor centre to raise blood pressure to normal. However, this reflex is not strong until the blood pressure falls below 80mmHg. Therefore, when blood pressure is low this reflex is very important to prevent blood pressure from falling further.

Chemoreceptors are discussed in relation to respiratory regulation, which plays a more important role than blood pressure regulation.