eliminate the tumor completely) in mice. Angiogenesis inhibi-
tors are currently under study in people with cancer.
Anatomy of the Capillary Network
Capillary structure varies considerably from organ to organ,
but the typical capillary (
) is a thin-walled tube
of endothelial cells one layer thick resting on a basement mem-
brane, without any surrounding smooth muscle or elastic tis-
sue. Capillaries in several organs (e.g., the brain) have a second
set of cells that surround the basement membrane and affect
the ability of substances to penetrate the capillary wall.
The ﬂ at cells that constitute the endothelial tube are not
attached tightly to each other but are separated by narrow,
water-ﬁ lled spaces termed
lial cells generally contain large numbers of endocytotic and
exocytotic vesicles, and sometimes these fuse to form continu-
across the cell (
Blood ﬂ ow through capillaries depends very much on
the state of the other vessels that constitute the microcircula-
). For example, vasodilation of the arte-
rioles supplying the capillaries causes increased capillary ﬂ ow,
whereas arteriolar vasoconstriction reduces capillary ﬂ ow.
In addition, in some tissues and organs, blood does
not enter capillaries directly from arterioles but from vessels
which connect arterioles to venules.
Metarterioles, like arterioles, contain scattered smooth muscle
cells. The site at which a capillary exits from a metarteriole
is surrounded by a ring of smooth muscle, the
which relaxes or contracts in response to local
metabolic factors. When contracted, the precapillary sphincter
closes the entry to the capillary completely. The more active
the tissue, the more precapillary sphincters are open at any
moment and the more capillaries in the network are receiving
blood. Precapillary sphincters may also exist where the capil-
laries exit from arterioles.
Velocity of Capillary Blood Flow
illustrates a simple mechanical model of a
series of 1-cm-diameter balls being pushed down a single tube
that branches into narrower tubes. Although each tributary
tube has a smaller cross section than the wide tube, the sum of
the tributary cross sections is three times greater than that of
the wide tube. Let us assume that in the wide tube, each ball
moves 3 cm/min. If the balls are 1 cm in diameter and they
move two abreast, six balls leave the wide tube and enter the
narrow tubes per minute, and six balls leave the narrow tubes
per minute. At what speed does each ball move in the small
tubes? The answer is 1 cm/min.
This example illustrates the following important prin-
ciple: When a continuous stream moves through consecutive
sets of tubes, the velocity of ﬂ ow decreases as the sum of the
cross-sectional areas of the tubes increases. This is precisely
the case in the cardiovascular system (
blood velocity is very great in the aorta, slows progressively
in the arteries and arterioles, and then slows markedly as the
blood passes through the huge cross-sectional area of the
Endothelial cell 1
Endothelial cell 2
(a) Diagram of a capillary cross section. There are two endothelial cells in the ﬁ gure, but the nucleus of only one is seen because the other is
out of the plane of section. The fused-vesicle channel is part of endothelial cell 2. (b) Electron micrograph of a capillary containing a single
erythrocyte; no nuclei are shown in this section. The long dimension of the blood cell is approximately 7 μm.
Figure adapted from Lentz. EM courtesy of Dr. Michael Hart.