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Short-Term Regulation of the CVS

 

A. Introduction:

1.

Short-term regulation of the blood pressure includes those regulators that work very fast, from minutes to seconds. There are also long-term regulators but these take days to weeks to work (see next page).

2.

There are several short-term regulators:

  • Baro-reflex
  • Chemo-reflex
  • Vascular shift

 

B. The Baro-Reflex:

1.

It is a real nervous reflex. This means it has a reflex arc. This reflex arc consist (in general) of:

  1. a sensor
  2. afferent  nerves
  3. a centre in the Central Nervous System (=CNS)
  4. efferent nerves
  5. an effector

In the case of the baro-reflex, these components are as follows:

2.

Sensors: the baro-receptors, located in the internal carotid arteries (located in the carotid sinus) and along the aortic arch. These are stretch sensors. When they are stretched (because of higher blood pressure), they will send more action potentials to the CNS. If the blood pressure is decreased, then the firing frequency is decreased.

 

3.

The afferent nerves; the nerves from the aortic receptors run through the vagus nerve. The nerves from the carotid sinus go through Hering’s nerve to the glossopharyngeal nerve.

 

structure of the baro reflex

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4.

Both nerves end in the cardiovascular centre (= vasomotor centre) in the medulla of the brain.

5. The efferent nerves are

  • the vagus (the para-sympathetic system) and
  • the sympathetic system.

6. The effectors are

  • the heart
  • the arteries
  • the veins

C. How does the Baro-Reflex work?

Step 1:

Suppose the blood pressure decreases suddenly.

 

 

diagram of the workings of the baroreflex

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Step 2:

The baro-receptors will sense this and respond by producing less action potentials

Step 3:

The Vasomotor centre will respond by influencing two systems:

Step 4A:

The parasympathetic system is inhibited: this will increase the heart rate by increasing the frequency of the cardiac sinus node.

Step 4B:

It will stimulate the sympathetic system. This has multiple effects:

  • increase in heart rate (= chronotropie)
  • increase in contraction force of the heart (= inotropie)
  • vasoconstriction of the arterioles (this leads to an increase in the peripheral resistance).
  • vasoconstriction of the veins: this has two effects:

step 4a)

The capacity of the veins is decreased (remember that the veins are also being used as a buffer, a capacity or a reserve). If these large veins constrict more, more blood will be pushed into the circulation, effectively increasing the circulating blood volume.

step 4b)

Increase in the venous return. This will make the heart beat stronger (Frank-Starling effect) and faster (sinus node stretch).

5.

Note: Our heart beats, inside the body, at about 60-70 beats/min. If you take the heart out of the body and keep it beating (immersed in a special fluid), the heart rate will be higher; approx.. 110 b/min. Why?
This heart rate is called the intrinsic rate.

 

D. Chemo-reflex:.

1.

The chemo-reflex is very similar to the baro reflex. The only difference is that the sensors are not sensitive to stretch but sensitive to blood concentrations of oxygen, carbon dioxide and pH.

2.

These chemo receptors are located in the aortic bodies and the carotid bodies, close to the stretch receptors.

3.

Because the receptors are a bit slower than the stretch receptors, this reflex works a bit slower.

4.

Because they measure blood gasses, their function is much more important in the regulation of the respiration.

 

E. The stupid Physiologist:.

1.

There was once a physiologist who thought he had a clever idea.

2.

This idea, he thought, was so good he would become famous, win the Nobel prize and earn him a lot of money

3

As you know, hypertension is a big problem and difficult to treat.

4.

Our soon-to-become world-famous physiologist, with the help of the Baro-reflex, had invented a clever way to treat hypertension.

5.

His idea was to stimulate the Hering’s nerve with an artificial (implanted) pacemaker!

6.

In this manner, the vasomotor centre would think that the blood pressure was too high and would react by exciting the parasympathetic system and inhibiting the orthosympathetic system to decrease the blood pressure

7.

In his first group of hypertensive patients, his plan worked!! Indeed, the blood pressure decreased as soon as the pacemaker was switched on!

8.

But after a few weeks, the blood pressure started to creep back to hypertensive levels, even when the pacemaker was switched on. In fact, when the pacemaker was switched off, the blood pressure becomes even higher than before the implantation of the pacemaker!

9.

The poor physiologist, instead of treating his patients, was actually making things worse. Gone were his dreams of the Nobel prize, richness and fame.

10.

Why had he failed? Why did it not work?

Because stimulating through Hering’s nerve is a short term solution and NOT a long term solution. Hypertension is a problem with the long-term regulation of the blood pressure (see next page).

 

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