For discussing the functions of different structures of all organisms.
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Well it's simple, your heart doesn't need more pressure as its rate increases. For example, when someone is losing a lot of blood and is going into shock, breathing increases and the heart beats faster. As the heart beats faster, the blood pressure goes down. And while rigorous activity like running, blood vessels dilate so I don't believe that more pressure is needed, but just a more constant supply of oxygenated is, as provided by the increased heart rate. But that's just my hypothesis.
I think it beats faster because the pressure is low, it's an effect not a cause.
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The amount of O2 carried with low pressure is not enough for your tissues, so heart beats faster to pump more blood to tissues.
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This isn't exactly related to the original question, but just want to make a note on this here...
In early stages of shock, blood pressure is maintained at a relatively constant level (compensated shock), even though heart rate and respirations increase (to increase perfusion of oxygen and blood flow), because blood is shunted to the vital/core organs (in other words, distal/peripheral blood vessels constrict, which prevents blood pressure from falling further).
In decompensated (late) shock, blood pressure begins to fall due to a loss of blood and overall decreased volume.
- The influence of arterial blood pressure on heart rate is dependent upon the baroreceptors, which are found in the aortic arch and the carotid sinus
- Changes in HR from baroreceptor activation is due to reciprocal changes in both the sympathetic and parasympathetic nervous systems
- These changes occur with little or great changes in pressure
- With small changes in pressure, there are minor autonomic changes
- Great changes in pressure can totally shut down the sympathetic nervous system, and the opposite is true in cases of severe hypotension
This law can be simply explained by the following mechanism
firstly we know that baroreceptors (which found in aortic arch and carotid sinus) are sensetive to changes in Blood pressure
when the blood pressure is elevated...baroreceptors are activated and sending afferent vagal impluses to the CAC (Cardio-acceleratory center) and CIC (cardio-inhibitory center) in the brain stem.....as a result CAC is inhibited and subsequently CIC is activated by reciprocal inhibition
so efferent vagal impluses are sent to the SAN(Sino-atrial node) in the heart inhibiting its excitatory impluses and by this way the heart rate is decreased
The significance of this reflex is that to allow more time for the heart to be filled with blood so increasing the initial lenght of the cardiac muscle and according to starling law...stronger contraction of the cardiac muscle to overcome the resistance existed in the aorta by the elevated Blood pressure
the mechanism is just as Dr.Mostafa explained . i'll comment on the significance and to do so i'll add that the afferent impulses from the baroreceptors are carried through the 9th(glossopharangeal) and 10th ( vagus ) cranial nerves which are called the buffer nerves .. the significance is to maintain normal value of arterial blood pressure (ABP) rapidly as hypotention may cause fainting and coma if severe and hypertention may cause cerebral haemorrhage if sudden and severe .. Actually marey's law is only one mechanism done by the buffer nerves to maintain ABP at normal value.. First , the general equation to calculate the ABP is C.O.P (CARDIAC OUT PUT ) × PR ( peripheral resistance of the blood veseles)..
other mechanisms done through these buffer nerves ( if ABP is elevated for example ) are :
1- inhibition of the vasomotor centers in the brain and spinal cord leading to peripheral vasodilatation ( decreasing the peripheral resistance )
2- inhibition of adrenaline secretory center in the medulla oblongata of the brain stem ( adrenaline is a potent vasoconstrictor ) so, decreasing the peripheral resistance .
3- inhibition of respiratory center to decrease venous blood which returns to the heart mainly by sucction blood through the negativity of the intra-thoracic cavity during inspiration . so decreasing the C.O.P
4- C.O.P is also decreased by by decreasing heart rate ( marey's law )
so hte net result is decreasing both C.O.P and PR . thus decreasing the elevated ABP ..
all these reflexes are done at the same time ..
this is only for the significance and how the body will deal with such circumistance as long as the cause of hypertention doesn't affect these mechanisms .. marey's law explained before is applied when all other factors affecting blood pressure remain constant
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