CHAPTER 7 • ORGAN BLOOD FLOW
149
TABLE 7-1
BLOOD FLOW IN MAJOR ORGANS OF THE BODY
O R G A N
P E R C E N T B O D Y
W E IG H T
P E R C E N T C A R D IA C
O U T P U T A T REST
N O R M A L F L O W
(m L /m in PER 1 0 0 g )
M A X IM A L F L O W
(m L /m in PER 1 0 0 g )
Heart
0.5
5
80
4 0 0
Brain
2
14
55
150
Skeletal m uscle
4 0
18
3
60
Skin
3
4
10
150
Stom ach, intestine,
liver, spleen, pan-
creas
6
23
30
250
Kidneys
0.5
20
4 0 0
6 0 0
O ther
48
16
-
-
Normal and maximal flows are approxim ate values for the whole organ. Many organs (e.g., brain, muscle, kidney, and intes-
tine) have considerable heterogeneity of flow w ithin the organ depending on the type of tissue or region of organ being
perfused. The liver receives blood flow from the gastrointestinal venous drainage as well as from the hepatic artery (only
hepatic artery flow is included in this table). “O ther” includes reproductive organs, bone, fat, and connective tissue.
fasted, resting state and at normal environ-
mental conditions of temperature and humid-
ity). The ratio of basal flow to maximal flow
is a measure of the vascular tone, which is the
degree of vascular constriction (see Chapter 5).
The lower the basal flow relative to the maxi-
mal flow, the higher the vascular tone. The
difference between basal flow and maximal
flow represents the flow capacity or vasodila-
tor reserve for the organ. Most organs have
a relatively large vasodilator reserve, whereas
others, such as the kidneys, have a relatively
small vasodilator reserve (see Table 7-1).
The changes that occur in organ blood flow
under different conditions depend on the inter-
play between neurohumoral and local regulatory
mechanisms that govern vascular resistance.
The neurohumoral mechanisms were discussed
in Chapter 6. The following sections focus on
the local regulatory mechanisms that affect vas-
cular resistance and organ blood flow.
LOCAL REGULATION OF
BLOOD FLOW
Tissues and organs have the ability to regu-
late, to a varying degree, their own blood flow.
This intrinsic ability to regulate blood flow
is termed “local regulation” and can occur in
the complete absence of any extrinsic neuro-
humoral influences. For example, if a mus-
cle is removed from the body, perfused under
constant pressure from a reservoir containing
oxygenated blood, and then electrically stimu-
lated to induce muscle contractions, the blood
flow increases. The increase in blood flow occurs
in the absence of neurohumoral influences and
therefore is a local or intrinsic mechanism.
The mechanisms responsible for local regu-
lation originate from within the blood vessels
(e.g., endothelial factors, myogenic mecha-
nism s) and from the surrounding tissue (i.e.,
tissue factors), many of which are related to
tissue metabolism or other biochemical path-
ways (e.g., arachidonic acid metabolites and
bradykinin). Mechanical factors (e.g., com-
pressive forces during muscle contraction)
can also influence vascular resistance and
thereby alter blood flow.
Tissue Factors
Tissue factors are substances produced by the
tissue surrounding blood vessels (Fig. 7.1).
These substances act on the blood vessel to
produce either relaxation or contraction of
the smooth muscle, thereby altering resistance
and blood flow. In some cases, these sub-
stances indirectly act on the vascular smooth
muscle by affecting endothelial function or
by altering the release of norepinephrine by
sympathetic nerves. Some of these vasoactive
substances are tissue metabolites that are prod-
ucts of cellular metabolism or activity (e.g.,
adenosine, CO2, H+, K+, lactate). In addition,
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