Maintaining Blood Pressure
Is Critical for Survival
LEARNING OBJECTIVES
1.
Identify
the changes in blood pressure as blood
flows through the cardiovascular system.
2.
Describe
the factors that influence blood pressure.
3.
Explain
how blood pressure is regulated.
n many ways, the cardiovascular system is
much like a water balloon:
Water exerts outward pressure on the bal-
loon’s thin elastic walls just as the blood exerts
pressure on the walls of the blood vessels.
If you increase the volume of water in the balloon, you
increase the pressure. Likewise, increasing blood volume
increases blood pressure.
If you squeeze the balloon, you increase the pressure
inside. When the heart contracts, the fluid pressure
inside increases.
If you squeeze one end of the balloon (increase pressure),
the water flows to the other side (low pressure). The
heart increases the pressure at the beginning of the
circulation, and the blood flows through the systemic
and pulmonary circulations.
If you constrict the balloon in the middle
(increase
resistance) and then squeeze one end, you have to squeeze
harder (more pressure) to get the water to move than
without the constriction. Similarly, narrowing the arteries
increases the resistance to flow and yields higher pressure.
In addition, the amount of fluid that you put into the wa-
ter balloon affects pressure, just as varying cardiac output
alters the pressure within the vessels. Dilation of arteri-
oles and precapillary sphincters allows fluid to move more
quickly toward the venous circulation, also changing the
pressure of the blood remaining in the arterial circulation.
So blood flow in the cardiovascular system is subject to the
same physical principles as the flow of any other fluid, or
the flow of water in a balloon. Let’s look at the dynamics of
blood flow.
Pressure Drives the Flow of Blood
Like any other fluid, blood flows from areas of higher pres-
sure to areas of lower pressure. Whether it’s water in plumb-
ing or blood in vessels, there is always some resistance to
flow (such as friction of the fluid against the walls or width
of the lumen). The general relationship describing blood
flow is as follows:
Change in blood pressure
Blood flow =
Vascular resistance
So the heart does not “push” the blood through the car-
diovascular system but increases the pressure of the blood.
The blood then flows according to the principle described
above (Figure 11.18). The heart can also be compared to
the chain that pulls a roller coaster car (stroke volume of
blood) to the top of the first hill (aorta); once it reaches
the top, the car proceeds through the rest of the ride on
its own until it returns to the starting point (right atrium).
In the arteries, blood pressure fluctuates periodically
with the systole and diastole phases of the cardiac cycle.
The normal systolic pressure (SP) is about 120 mm Hg, and
normal diastolic pressure (DP) is about 80 mm Hg. (Blood
pressure is usually written as SP/DP and expressed as
“systolic over diastolic,” such as 120/80, or “120 over
80.”) Often, this fluctuating pressure is expressed as
mean arterial pressure (MAP), which is calculated as
follows: MAP = !/3 (SP - DP) + DP. This calculation pro-
vides an “average” pressure at any given time and takes
into consideration the fact that the heart remains at rest
for a longer period of time than it is contracted.
Because the blood encounters narrower and nar-
rower vessels as it proceeds through the circulation, the
resistance increases, thereby causing blood pressure to
drop along the way. In addition, the elasticity of the arter-
ies causes the blood pressure to change to a more steady,
non-fluctuating pressure, no longer showing systolic and
diastolic pressure variations.
For the most part, the blood flow through the car-
diovascular system (cardiac output) remains consistent.
Therefore, the blood pressure remains steady. However,
many factors can influence blood pressure. Let’s take a
closer look at a few of these.
Many Factors Influence
Blood Pressure
Blood pressure is focused on the pressures within the
arteries. Many factors, including the following, increase
blood pressure:
Increasedforce o f contraction (contractility).
Norepinephrine
from sympathetic nerves to the heart and epinephrine
332 CHAPTER 11
The Cardiovascular System: Heart, Blood Vessels, and Circulation
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