Download Cells of the Vessels

Cells of the Vessels
Within a blood vessel there are three main layers, which we will consider
in vascular tissue, described later. Endothelial cells are on the inside
(lumen) of a blood vessel. In some vessels, smooth muscle cells wrap
them. Fibroblasts secrete extracellular matrix in the large vessels,
creating a connective tissue outer layer.
Figure 12
Endothelial Cells
Any surface that comes in contact with blood, including the inside of the
heart and every blood vessel, is lined with a special cell called
an endothelial (endo-inside) cell. Vascular endothelium line the
blood vessels, and there are special endothelial cells of the lymph
system. Endothelium of the interior surfaces of the heart chambers is
called endocardium.
The layer of endothelium is only one layer thick, and these cells have
several important functions.
Mechanical-chemical Stimulation
Endothelial cells sense changes in local chemical and mechanical
environment, and signal to the underlying muscle cells. For example,
under certain conditions, increased force on the vessels cause
endothelial cells to release vasodilator chemicals (such as nitric oxide),
which cause the smooth muscle cells to relax, leading to a dilated
(expanded) vessel.
Immune Response
When tissues have been compromised by damage or invasion of
pathogens, the immune system releases chemical signals (cytokines)
into the region. These cytokines activate the endothelial cells to promote
association with circulating immune cells. Also, pores open up between
endothelial cells to allow immune cells from the circulation to leave the
lumen of the vessel and invade the tissue.
Forming New Blood Vessels
As we grow or repair damaged tissue, we need new blood vessels to carry
nutrients and oxygen into new tissue. The process of blood vessel
formation is called angiogenesis, and involves stimulation of endothelial
cells. Vascular endothelial growth factor (VEGF) stimulates the
normally quiescent (non-dividing) endothelial cells to begin to divide.
The cells then break down the extracellular matrix and form new blood
vessels. Without angiogenesis, we could not repair any damaged tissue
or adapt to changes. However, cancer cells will often secrete VEGF or
other angiogenic factors to cause the ingrowth of vessels into the tumor,
supporting its rapid growth.
Preventing Clot Formation
One important function of endothelial cells is only really observable as a
dysfunction: thrombus prevention. A thrombus, or a blood clot inside
the vessel, can occur because of the easily activated coagulation proteins
that flow through the blood stream. Endothelial cell dysfunction, such as
loss or inflammation, can cause clots to form on the inner surface of the
blood vessel. Large thrombi can occlude (or completely block) blood
vessels, so material cannot pass through. Just as dangerous, these
thrombi can dislodge from the vessel wall and may travel to the vessels
of the heart or brain and cause heart attacks or strokes, respectively.
Smooth Muscle Cells
Smooth muscle tissue is found throughout the body, including around
organs in the digestive, respiratory, and reproductive tracts and around
blood vessels. In the cardiovascular system, smooth muscle cells form a
layer outside of the endothelial cells. Here they provide strength to
blood vessels and provide a mechanism for vessel adaptation.
Smooth muscle cells contract via the actin-myosin contraction
mechanism described in the muscular unit. Smooth muscle cells adjust
their contraction based on sensed stretch or the concentration of various
chemical factors. For example, hormones related to exercise or anxiety
can cause vessel constriction and nitric oxide can cause vasodilation.
Extra-Cellular Matrix of the Vessels
Besides the endothelial lining found in all blood vessels, and the smooth
muscle cell layer found in most vessels, all vessels except capillaries also
have one or more layers of connective tissue associated with them.
These vessels have an outer connective tissue layer (tunica externa, or
adventitia), while some also have a thin layer of connective tissue, called
the internal elastic lamina that is found between the endothelial layer
and smooth muscle layer. The types and concentrations of molecules in
these connective tissue layers provide structure, support, and determine
how easily the vessels will passively (without relaxation of muscle cells)
expand.
Elastin
Even without active changes in smooth muscle tension, blood vessels
need an ability to deform and return to form when internal or external
pressures are applied. The elastin molecule in the connective tissue
layers of blood vessels gives them this property. Elastin, when
combined with other extracellular components, helps blood vessels and
other tissues, including skin, to return to their original shapes after
expansion or deformation. Elastin molecules are disorganized,
entangled and crosslinked. When stretched, these fibers straighten out,
but will recoil back to normal when force is released on the tissues.
Collagen
Collagen is a stiff, filamentous extracellular protein found in most
connective tissues. Collagen filaments are composed of twisted
protofilaments, and protofilaments themselves are also twisted. Thus,
collagen has ropelike qualities that provide stiffness. In blood vessels,
collagen is found in the tunicia externa and provides stiffness to the
blood vessel to prevent rupture. Arteries contain blood at higher
pressure than veins so they have a thicker layer of collagen than veins.
Blood as a Connective Tissue
Blood is a liquid connective tissue. Connective tissues are defined as
tissues with multiple cell types and a large extracellular matrix
component. The blood cells, described before, serve different functions
and the plasma acts as the extracellular matrix. Blood is a unique,
complex fluid with various physical characteristics and functions needed
to maintain a stable internal environment in the body. Blood
volume is the combination of hematocrit (45 percent) and plasma
volume (55 percent). Hematocrit (hct) is the fraction of blood that is
composed of red blood cells (RBCs). Besides RBC, leukocytes (white
cells) and platelets make up the rest of the formed elements. The process
of producing blood cells is calledhematopoiesis. Stem cells are
nascent cells in the bone marrow that differentiate into various types of
formed elements in blood.