The Cardiovascular System: Blood Vessels
Types of Capillaries
Structurally, there are three types of capillaries—
, abundant in the skin and muscles, are
most common (Figure 19.3a). Tey are continuous in the sense
that their endothelial cells are joined together by
providing an uninterrupted lining. However, these junctions are
usually incomplete and leave gaps of unjoined membrane called
, which are just large enough to allow limited
passage of ﬂuids and small solutes. ±ypically, the endothelial
cell cytoplasm contains numerous pinocytotic vesicles that ferry
ﬂuids across the capillary wall.
Brain capillaries, however, are unique. Tere the tight junc-
tions of the continuous capillaries are complete and extend
around the entire perimeter of the endothelial cells, constituting
the structural basis of the
blood brain barrier
that we described
in Chapter 12.
es-tra-tid) are similar to the con-
tinuous variety except that the endothelial cells in fenestrated
capillaries are riddled with oval
window) (Figure 19.3b). A delicate membrane, or diaphragm
(probably condensed basal lamina material), usually covers the
fenestrations. Even so, fenestrated capillaries are much more
permeable to ﬂuids and small solutes than continuous capillar-
Fenestrated capillaries are found wherever active capillary
absorption or ﬁltrate formation occurs. For example, fenes-
trated capillaries in the small intestine receive nutrients from
digested food, and those in endocrine organs allow hormones
rapid entry into the blood. Fenestrated capillaries with perpetu-
ally open pores occur in the kidneys, where rapid ﬁltration of
blood plasma is essential.
Sinusoid Capillaries (Sinusoids)
nŭ-soyds), are highly
modiﬁed, leaky capillaries found only in the liver, bone marrow,
spleen, and adrenal medulla. Sinusoids have large, irregularly
shaped lumens and are usually fenestrated. Teir endothelial
lining has fewer tight junctions and larger intercellular cle²s
than ordinary capillaries (Figure 19.3c).
Tese structural adaptations allow large molecules and even
blood cells to pass between the blood and surrounding tissues.
In the liver, the endothelium of the sinusoids is
and its lining includes large
macrophages), which remove and destroy any bacteria. In other
organs, such as the spleen, phagocytes located just outside the
sinusoids stick cytoplasmic extensions through the intercellular
cle²s into the sinusoid lumen to get at their “prey.” Blood ﬂows
sluggishly through the tortuous sinusoid channels, allowing
time for it to be modiﬁed in various ways.
cell in lumen
cell in lumen
cell in lumen
(a) Continuous capillary.
Least permeable, and most
common (e.g., skin, muscle).
(b) Fenestrated capillary.
Large fenestrations (pores)
increase permeability. Occurs in areas of active
absorption or filtration (e.g., kidney, small intestine).
(c) Sinusoid capillary.
Most permeable. Occurs in special
locations (e.g., liver, bone marrow, spleen).
Note that the basement
membrane is incomplete only in (c) and that pericytes most often
occur on continuous capillaries.