So now that we've talked about the regulation of stroke
volume, let's look at the regulation of heart rate.
So if for example, the stroke volume decreases as a
result of a decreased blood volume or weakened heart,
the body responds and maintains our cardiac output by
increasing the heart rate and the heart's contractility.
Positive chronotropic agents or
factors can increase the heart rate
and negative chronotropic
factors decrease the heart rate.
So heart rate can be regulated by our autonomic nervous
system, certain chemicals and also some other factors.
So first, looking at the autonomic
nervous systems regulation of heart rate,
we find that the sympathetic nervous system can be
activated by our emotional or physical stressors.
This causes the release of norepinephrine which then
binds to beta-adrenergic receptors on the heart.
The binding of these receptors causes the pacemaker cells
to fire more rapidly and this increases our heart rate.
As the heart rate is increased, the
end diastolic volume is decreased
because there is a decrease in the
amount of time that the heart has to refill.
As well, norepinephrine can also lead to an
increase in the contractility of our muscle cells.
This increase in contractility is going to lead to a
decrease in the end systolic volume and this is because
the increased volumes of ejected blood, so we are ejecting
more blood and so there's less left in the ventricle.
Because both the end diastolic volume
and the end systolic volume decrease,
the stroke volume will remain unchanged
because they are balancing each other out.
When it comes to the parasympathetic nervous system
which is going to oppose sympathetic effects,
acetylcholine will lead to the hyperpolarization of our
pacemaker cells by causing the opening of potassium channels.
This in turn will slow the heart rate
but has a very little effect on contractility.
At this point, the heart at rest
is going to exhibit vagal tones.
So during rest, the parasympathetic
system or parasympathetic nervous system
is going to be the dominant influence on our heart rate.
The vagal tone will lead to a decrease in the
heart rate to about 25 beats per minute at rest.
If you were to cut the vagal nerve
or if you had damage to this nerve,
this could lead to heart rates going up to about 100 beats per
minute since you are lacking this parasympathetic activity.
So when the sympathetic nervous system is activated,
the parasympathetic nervous system is inhibited
and vice versa so they kinda work off of each other.
The atrial reflex also referred to as the
Bainbridge reflex is a sympathetic reflex
that is going to be initiated by increased
venous return thus it increases atrial filling.
It is caused by the atrial walls being
stretched because of this increased volume
which is going to simulate that SA node,
which is then going to increase the heart rate.
It's also gonna stimulate atrial stretch receptors
that are going to activate sympathetic reflexes.
So now let's look at how the heart rate
can be regulated by way of chemicals.
So there are two main chemicals
that are gonna regulate our heart rate.
We have our hormones such as
epinephrine from the adrenal medulla
which is going to increase
heart rate and contractility,
and then we have thyroxine or thyroid
hormone which is going to increase heart rate
and also enhance the effects of norepinephrine
and epinephrine from the adrenal medulla.
Other chemicals include ions.
You have your intracellular and extracellular ion
concentrations of things like calcium and potassium
that must be maintained for a normal heart
If there is any imbalance in your ions,
this can be very dangerous to the heart
because it's going to affect the ability of polarization and
depolarization to occur and for that contraction to occur.
Other factors are gonna be things like age.
So as you're growing or as you're developing
in utero, the fetus has the fastest heart rate
and as you get older starting from birth and beyond,
your heart rate is going to decline with age.
Also, females generally have a
faster heart rate than that of males,
and also with exercise you're gonna
get an increase in your heart rate
and you will find however, that if you are
trained athlete even though you're exercising,
your heart rate does not go
up the way an untrained athlete
or just a normal person who doesn't work out all
the time, the way their heart rate would increase.
And finally, heart rate can also
be influenced by body temperature.
There is a direct relationship between
heart rate and body temperature
so as your body heats up, the heart rate also goes up.