Hello everybody. Once again, welcome. We are going to discuss another important
physiological concept which is cardiac output. So what is actually cardiac
output? And what is it's important? It measures the ability of the heart to
pump enough blood to the different organs. How do we measure the cardiac
output? So it is the volume of blood ejected from each ventricle per minute.
Remember, because we are going to use another parameter which is going to be
per beat and not per minute . How do we calculate the cardiac output? We multiply
what we call stroke volume -- we're going to learn about this in a minute -- by the
heart rate. We put in some numbers here and we come out with around 5 liters per
minute. So this should be the normal cardiac output, around 5 liters per
minute. So we're going to look at stroke volume alone. We're going to look at
heart rate alone and see how these will determine the cardiac output. So what is
stroke volume? It is the volume of blood ejected from each ventricle per beat.
Every time the heart beats it's going to eject some volume of blood.
So let's look here. We are looking at diastole and remember diastole is when
the ventricles are relaxed and filling with blood. So when we look here the
ventricles will start to fill and when they are absolutely filled with blood we
call this the end diastolic volume and we're going to put in a number of 120ml.
When the ventricle starts to contract in systole -- systole is contraction -- some
blood is going to start leaving the ventricle and will be
ejected outside. There is going to be a volume of blood remaining inside. This
blood remaining is called the end systolic volume. So if I subtract the end
diastolic volume, 120, from the end systolic, which is remaining here, 50, I
will know how much is ejected in that beat and this is around 70 ml. This is my
stroke volume. Now, are there circumstances that will change the
stroke volume? Is this something that is constant? Now, there are going to be
things, factors that will interfere with with stroke volume and we're going to
learn these factors together. All right. We're going to learn three important
factors. Preload, Contractility, Afterload. What do I mean by preload? I am looking
at how much volume of blood I am loading each ventricle with. How much venous
return is coming to fill in the ventricle? And this will definitely make
a difference. If I have too much or too little, it's going to change my stroke
volume. The second factor here is the
contractility, the ability of the ventricular muscle to contract and push
the blood out. Again, if there is change in contractility, it will change my
stroke volume. The last factor here is the afterload. How much resistance or
pressure is meeting the blood when it is trying to be ejected from the ventricles?
The more resistance or the less resistance, this will change my stroke
volume. Let's look at each one of these individually. This is my
preload. So I'm actually looking at the end diastolic volume. How much blood is
in the ventricles over here? If for any reason there is decrease in this volume
definitely the stroke volume will be decreased. And commonly, if somebody has
dehydration or bleeding, there is definitely less blood leaving the
ventricles and this person will have lower cardiac output. If there is
increase in the end diastolic volume, there is more venous return. This will
push on the walls of the ventricle. This will stretch out the walls of the
ventricles. And someone called Mister Starling studied this phenomenon and he
came up with the Frank-Starling law. And he said the more this stretch on the
walls of the ventricle, the stronger is going to be the contraction of this
ventricle to push this excess fluid away from the heart. So for example, if
somebody is over hydrated, they're just drinking lots of fluid and this ends up
in the heart, the heart is going to contract more and stronger to push this
excess fluid out into the system. Eventually it's going to reach the
kidneys. It's going to be filtered outside as urine and this person will excrete
this excess fluid. All right. Next we're going to talk about the Afterload. So I
am looking here. This is the blood that needs to be ejected. Is it meeting any
resistance? Is there any pressure against it that will impede the normal stroke
volume? So, is there is any increased afterload? So we're looking at the two
main valves here. We're looking at the pulmonary valve
and the aortic valves. If there is disease that decreases the diameter,
decreases the ability of the valve to open, definitely this will decrease the
ejection of blood and stroke volume will be decreased. In hypertension, blood
vessels that are farther away from the heart are constricted. And again, this
will impede the normal stroke volume and it will decrease. On the other hand, if I
if I want to decrease the afterload to help the blood to be ejected, we can give
a person vasodilators, dilate those vessels up here, and then the ejection is
going to be easier and stroke volume will increase. The third important factor here
is Contractility. I am looking at the ability of my muscle to contract and
push the blood out. So if we have diseases of the cardiac muscle, the
myocardium is weakened such as in heart failure. Or, it is too thickened because
of some hypertrophic heart diseases, it will not accommodate enough fluid to
start with or be able to eject to the outside. So those situations where I have
weakness in the cardiac muscle or stiffness, definitely the contraction,
contractility, is diminished and definitely the stroke volume will be
decreased. On the other hand, are there substances or circumstances where we can
increase the contractility? Yes, the sympathetic stimulation. So every time we
go out and exercise this sympathetic stimulation is going to increase, it's
going to increase the contractility of the ventricles. So the ventricles are
pushing blood quicker to our systems taking it to the muscles
for the exercise that we need to perform. Also, in situations where people have
heart failure we can give them what we call positive inotropic drugs. These
drugs will increase and improve the contraction of the failing ventricles.
All right, we're going to look at the other factor which is the heart rate.
What are things that will affect the heart rate? These are pretty simple. We're
talking first about the autonomic nervous system. We know that sympathetic
stimulation will increase the heart rate while the exact opposite, the
parasympathetic, will decrease the heart rate. Medication. Any medicine that
increases the heart rate we call it a positive chronotropic drug and an
example is epinephrine. Any medicine that decreases the heart
rate is called a negative chronotropic drug. An example of these are beta
blockers and we use these in difficult different clinical settings. Hormones,
epinephrine and thyroxine are known to increase the heart rate. Certain ions
will affect the heart rate, particularly potassium. Very high levels of potassium
in the blood will decrease the heart rate and accordingly decrease the
cardiac output. And this concludes our discussion of cardiac output and thank
you.