>>Dr. Ketchum: In this lecture video we will discuss the concept of central venous pressure
and the factors that affect it. To begin, let’s define a few terms: venous capacity,
venous return, and central venous pressure. First let’s discuss venous capacity. Think
about going to a restaurant, and they always have the sign that says, “Maximum Capacity
is ___” and some number. This sign tells you the maximum number of people you should
have in that establishment. That is similar to venous capacity. Venous capacity indicates
the volume of blood that the veins can accommodate. Now let’s focus on venous return. Venous
return is the volume of blood that returns to the heart. Finally, you will need to be
familiar with the term central venous pressure, or what we call CVP. Central venous pressure
is the pressure in the large veins of the thoracic cavity. This would include the inferior
vena cava and the superior vena cava.
All right, so let’s start thinking about how increasing or decreasing venous capacity
or venous return affects central venous pressure, and vice versa. For example, if central venous
pressure increases, what will happen to venous return? Will a patient with an increased central
venous pressure have more blood flowing into the heart or less? More blood, you’re correct.
So increasing central venous pressure increases venous return. Okay, so if the patient has
an increased venous return, how is that going to effect the volume in the ventricles before
contraction? It increases, right? You have just defined what’s called the end-diastolic
volume—EDV. The end-diastolic volume is the maximum amount of blood in the ventricles
before contraction. So let’s continue with the idea of increasing the venous return.
Again, if you increase venous return, that will increase the end-diastolic volume. So
what will that do to the patient’s cardiac muscle?
Is the cardiac muscle going to stretch or not? A greater volume of blood causes more
stretch toward the optimal length, which will increase the number of crossbridges that
can form, and will cause the force of contraction to increase. As a result, this will cause
the stroke volume to increase. Okay, let’s return for a moment to the increased force
of contraction. How did this affect the patient’s end-systolic volume? Remember, the end-systolic
volume or the ESV is the maximum amount of blood in the ventricles after contraction.
The patient just had a huge contraction, so is there going to be more or less blood in
the ventricles after that contraction? Less, therefore the end-systolic volume will be lower. Okay,
let’s return to the stroke volume.
Earlier, we said that the patient’s stroke volume would increase due to the increased
central venous pressure. How does stroke volume affect cardiac output? Remember that cardiac
output is equal to heart rate times the stroke volume. So if stroke volume increases, so
does your cardiac output. And remember that this will also increase the mean arterial
pressure, which is the pressure in the aorta during one cardiac cycle. All right, so to recap.
If we alter central venous pressure, that will indirectly influence your mean arterial
pressure. And remember, that mean arterial pressure influences how much blood is delivered
to your organs.