First understand the topic that we are going to discuss only the part of Arterial pressure
controlled by CNS and only which has rapid output and not long term. Arterial Pressure is
controlled by sympathetic, Parasympathetic and higher nervous system. CNS
Barometer receptor Reflex
Chemo receptor Reflex
Low pressure receptor Reflex
Kidney Volume Reflex
Brain Bridge Reflex
Brain Ischemic Reflex
Exercise Reflex
Abdominal Compression Reflex
Baro-receptors are sensitive to pressure changes and they are located in Aorta and Carotid arteries near the bifurcation of these arteries.
During normal conditions they send impulses to brain. The firing rate of nerve impulses during normal conditions is very slow (1-2 per second) but it is continuous and help to maintain pressure.
Whenever there is decrease in pressure the firing rate of impulses increases very fast. The paths of nerve impulses are as follows.
| Aortic Baroreceptor - Hering's nerve - Glossophayngeal nerve - Medulla |
| Carotid Baroreceptors - Vagus nerve - Medulla |
Then parasympathetic efferents increases the rate of impulses to heart. Keep in mind that impulses are already going to heart but to a much less rate e.g 1-2/sec however heart can beat without them but the rate will be definitely slower. Increase in parasympathetic stimulations increases the heart rate and its power of contraction as well so the output is an increase in arterial pressure. So in this way the decrease in arterial pressure is compensated.
During increase in arterial pressure the method is reversed but the efferents are sympathetic nerve fibres.
It is the most rapid pressure control mechanism in the body. But it cannot control the pressure change for long terms beacause it reset after 2-3 days. It respond very fast to changing pressures than a stationary low or high pressure for long time.
Dog with baroreceptors removed shows considerable variations in AP during 24 hours.
Mechanism of impulse efferents and afferents is same as that of baroreceptor reflex but the
only difference is the receptors which are sensitive to amount of carbon dioxide in blood instead
of pressure changes.
Two carotid bodies are located in the carotid sinus (which is a swollen part at the carotid
bifurcation) and three are located in the walls of aorta called aortic bodies.
The primary response comes from medulla.
These are present in arteries other that big arteries like aorta and carotid. These are similar with aortic and carotid baroreceptors but difference is that, these operate at low pressures so unlike aortic and carotid baroreceptors these are sensitive to small pressure changes.
If 300ml blood is injected in a dog which is de-nervated from these low pressure baroreceptors these would be an increase of 100 mmHg and if de-nervated from main barorceptors in aorta and carotid arteries then there would be an increase of 40 mmHg but in normal case these is only increase of 15 mmHg which is very small change as compared with formers.
These baroreceptors are located in arteries other than carotid and aortic, like pulmonary and
atrial receptors. They respond to volume changes with vasodilatation in vessels before any major
increase in pressure so when blood is injected in body, the room for this extra volume is made to
compensate the pressure.
The stimulus for this reflex is stretch in atria. If there is more volumes of blood in body there would be stretch in atria and it will lead to vasodilatation of kidney and also ADH (hormone responsible for the reabsorption of water and salts from the filtrate) is reduced. This in turn leads to more filtration and less reabsorption respectively. So more and more volumes of urine are excreted and volume of blood reduces which tend to make the blood pressure normal.
A decrease in blood pressure leads to less volume of urine by opposite effects.
When there is increase in pressure it increases the heart rate.
It does not tend to normal the blood pressure. Its benefit is that it does not let the blood
to stay in veins.
When there is hypoxia and oxygen is not supplied to brain, the response is svere. If brain cells begin to die, person faints and blood pressure rises to 250 mmHg very rapidly by svere vasoconstriction and increased heart rate. It is emergency situation.
Sometimes it does also happens due to an increased intra cranial pressure on brain and CSF does not circulate. In this situation it is called Cushing reflex.
Exercise increases heart rate and so is blood pressure. It is due to increased venous return due to increased muscular and fascia pressure on veins and increased cardiac output.
Abdominal muscles remain in continuous contraction when there is decreased in pressure. They increase the venous return and cardiac output.
