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Regulation of the Heart

 

A. Introduction:

1.

The heart needs a lot of regulations to allow it to perform its task of pumping blood adequately.

2.

There control mechanisms can be classified into two groups:

  1. Intrinsic regulation mechanisms
  2. Extrinsic regulation mechanisms

 

B. Intrinsic regulation:

1.

This mechanism is located in the heart muscle itself (intrinsic = belonging to the organ itself).

2.

There is essentially one very important intrinsic mechanism:
The Frank-Starling system

B1. The Frank-Starling mechanism.

1.

Remember, in skeletal muscles, that when you stretched the muscle before the contraction, that that would increase the force of contraction?

2.

The same mechanism applies to the heart. The more the cardiac wall is stretched, the stronger the next contraction will be.

3.

But, in the heart, the stretching is done by the blood that is accumulating in the chambers. The more blood in a chamber (atria or ventricle), the more stretch.

4.

So, during diastole, blood flows into the ventricles, stretching the walls. The volume that is reached just before the contraction starts is the end-diastolic volume.

5.

If there is a lot of blood accumulating in the ventricles, then the end-diastolic volume will be higher, there will be more stretch and the resulting contraction will be stronger.

6.

A stronger contraction means that more blood will be pumped into the arteries. Therefore, the cardiac output (CO = SV*HF) has increased with this mechanism.

7.

The reverse is also true. If less blood flows into the heart, then stretch and contraction force will decrease and the cardiac output will be less.

8.

This is therefore a very nice system of automatic regulation; if the heart receives a lot of blood, it will pump more. If it gets less blood, it will pump less.

C. Extrinsic Regulations:

1.

Because a correct function of the heart is so critical, there are many extrinsic regulatory systems.

2.

The major systems are:

  1. the autonomic nervous system
  2. the hormones (local and at a distance)

C1. The Autonomic Nervous System:

1.

As indicated in the diagram, there are two nervous systems innervating the heart; the parasympathetic and the sympathetic nervous system.

 

cardiac innervation

 

 

2.
The parasympathic system (the vagus nerve) only innervates the sinus node and the AV-node. It inhibits the firing rate of the sinus node (longer P-P) and delays the propagation though the AV-node (longer P-Q).

3.

The sympathetic system also innervates both nodes, but also the myocardium itself, both in the atria and

 

4.
An increase in sympathetic activity will therefore:

        1. increase heart rate
        2. increase propagation velocity in the AV-node
        3. increase contraction force in the cardiac muscle.
 

 

C2. The Bainbridge Reflex:

As an example of how the autonomic nervous system work on the heart, consider this very nice reflex: the Bainbridge reflex, also called the Respiratory Sinus Arrhythmia

1.

The reflex is very simple: when the pressure in the right atrium increases, the heart beats faster. If the pressure decreases, the heart rate goes down.

2.
As the pressure increases in the right atrium, possibly due to an increase venous return, the atrial stretch receptors are activated which send their signals to the medullary centre in the brain.  This in turn activates the sympathetic system.

3.

An increase in heart rate is then useful, as it makes the heart pump more blood, which will decrease the pressure in the

4.

You see this reflex very well during forcefu (=strong) in- and expiration.
5.
As you inhale, the pressure in the thorax decreases, which induces an increase in venous return and an increase in blood flow to the right atrium. This increases the atrial pressure -> increase heart rate.

6.

After inhalation follows expiration, the thoracic pressure increases, the venous return decreases, less blood flows into the atria, the pressure decreases and the heart rate decreases.

 

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