The Cardiovascular System: Blood Vessels
Te most common form of circulatory shock is
which results from large-scale blood or ﬂuid loss, as might follow
acute hemorrhage, severe vomiting or diarrhea, or extensive burns.
If blood volume drops rapidly, heart rate increases in an attempt to
correct the problem. Tus, a weak, “thready” pulse is oFen the ﬁrst
sign of hypovolemic shock. Intense vasoconstriction also occurs,
which shiFs blood from the various blood reservoirs into the major
circulatory channels and enhances venous return.
Blood pressure is stable at ﬁrst, but eventually drops if blood
loss continues. A sharp drop in blood pressure is a serious, and
late, sign of hypovolemic shock. Te key to managing hypovo-
lemic shock is to replace ﬂuid volume as quickly as possible.
Although you have not yet explored all of the body systems
that respond to hypovolemic shock, acute bleeding is such a
threat to life that it seems important to have a summary of its
signs and symptoms and the body’s attempts to restore homeo-
provides such a resource. Study it in part
now, and in more detail later once you have studied the re-
maining body systems.
, blood volume is normal, but circulation is poor
as a result of extreme vasodilation. A huge drop in peripheral re-
sistance follows, as revealed by rapidly falling blood pressure.
A common cause of vascular shock is loss of vasomotor tone
due to anaphylaxis (anaphylactic shock), a systemic allergic re-
action in which the massive release of histamine triggers body-
wide vasodilation. ±wo other common causes are failure of
autonomic nervous system regulation (
), a severe systemic bacterial infection
(bacterial toxins are notorious vasodilators).
±ransient vascular shock may occur when you sunbathe for
a prolonged time. Te heat of the sun on your skin dilates cu-
taneous blood vessels. Ten, if you stand up abruptly, blood
pools brieﬂy (because of gravity) in the dilated vessels of your
lower limbs rather than returning promptly to the heart. Con-
sequently, your blood pressure falls. Te dizziness you feel is a
signal that your brain is not receiving enough oxygen.
, or pump failure, occurs when the heart
is so ineﬃcient that it cannot sustain adequate circulation. Its
usual cause is myocardial damage, as might follow numerous
myocardial infarctions (heart attacks).
Check Your Understanding
Your neighbor, Bob, calls you because he thinks he is having
an allergic reaction to a medication. You ﬁnd Bob on the
verge of losing consciousness and having trouble breathing.
When paramedics arrive, they note his blood pressure is
63/38 and he has a rapid, thready pulse. Explain Bob’s low
blood pressure and rapid heart rate.
For answers, see Appendix H.
In theory, blood pressure—which forces ﬂuid out of the
capillaries—is opposed by the
interstitial ﬂuid hydrostatic
acting outside the capillaries and pushing ﬂuid
in. However, there is usually very little ﬂuid in the interstitial
space, because the lymphatic vessels constantly withdraw it.
may vary from slightly negative to slightly positive, but
traditionally it is assumed to be zero.
Colloid Osmotic Pressures
Colloid osmotic pressure (OP)
force opposing hydrostatic pressure, is created by large nondiﬀus-
ible molecules, such as plasma proteins, that are unable to cross the
capillary wall. Such molecules draw water toward themselves. In
other words, they encourage osmosis because the water concen-
tration in their vicinity is lower than it is on the opposite side of
the capillary wall. A quick and dirty way to remember this is “hy-
drostatic pressure pushes and osmotic pressure pulls (or sucks).”
Te abundant plasma proteins in capillary blood (primarily al-
bumin molecules) develop a
capillary colloid osmotic pressure
, also called
, of approximately 26 mm Hg.
Interstitial ﬂuid contains few proteins, so its colloid osmotic pres-
) is substantially lower—from 0.1 to 5 mm Hg. Unlike
HP, OP does not vary signiﬁcantly from one end of the capillary
bed to the other.
Hydrostatic-Osmotic Pressure Interactions
We are now ready
to calculate the
net ﬁltration pressure (NFP)
, which considers
all the forces acting at the capillary bed. As you work your way
through the right-hand page of ²igure 19.17, notice that while net
is occurring at the arteriolar end of the capillary, a negative
value for N²P at the venous end of the capillary indicates that ﬂuid
the capillary bed (a process called
a result, net ﬂuid ﬂow is
of the circulation at the arterial ends of
capillary beds and
the circulation at the venous ends.
However, more ﬂuid enters the tissue spaces than returns to
the blood, resulting in a net loss of ﬂuid from the circulation of
about 1.5 ml/min. Lymphatic vessels pick up this ﬂuid and any
leaked proteins and return it to the vascular system, which ac-
counts for the relatively low levels of both ﬂuid and proteins in
the interstitial space. Were this not so, this “insigniﬁcant” ﬂuid
loss would empty your blood vessels of plasma in about 24 hours!
Check Your Understanding
rises dramatically—say because of a severe
bacterial infection in the surrounding tissue. (a) Predict how
ﬂuid ﬂow will change in this situation. (b) Now calculate the
NFP at the venous end of the capillary in Figure 19.17 if OP
increases to 10 mm Hg. (c) In which direction does ﬂuid ﬂow
at the venous end of the capillary now—in or out?
For answers, see Appendix H.
Deﬁne circulatory shock. List several possible causes.
is any condition in which blood vessels are
inadequately ﬁlled and blood cannot circulate normally. Blood
ﬂow is inadequate to meet tissue needs. If circulatory shock per-
sists, cells die and organ damage follows.
(Text continues on p. 721.)