398
U N I T 5
Circulatory Function
Therefore, it has been proposed that when the arterial
pressure stretches the vessel, this in turn causes reactive
vascular constriction that reduces the blood flow nearly
back to normal. Conversely, at low pressures, the degree
of stretch of the vessel is less, so that the smooth mus-
cle relaxes, reducing vascular resistance and helping to
return blood flow toward normal.
A phenomenon called
reactive hyperemia
is a mani-
festation of local metabolic regulation of blood flow.
When the blood supply to an area has been occluded
and then restored, local blood flow through the tissues
increases within seconds to restore the metabolic equi-
librium of the tissues. This increased flow is called reac-
tive hyperemia. The transient redness seen on an arm
after leaning on a hard surface is an example of reactive
hyperemia. Local control mechanisms rely on a continu-
ous flow from the main arteries; therefore, hyperemia
cannot occur when the arteries that supply the capillary
beds are narrowed. For example, if a major coronary
artery becomes occluded, the opening of channels sup-
plied by that vessel cannot restore blood flow.
Endothelial Control of Blood Flow
One of the important functions of the endothelial cells
lining the arterioles and small arteries is the synthe-
sis and release of factors that can affect the degree of
relaxation or contraction of the arterial wall. The most
important of the
endothelial relaxing factors
is
nitric
oxide
. The normal endothelium maintains a continu-
ous release of nitric oxide, which is synthesized from
the amino acid arginine and oxygen and reduction of
inorganic nitrate. The production of nitric oxide can be
stimulated by a variety of endothelial agonists, includ-
ing acetylcholine, bradykinin, histamine, and throm-
bin. Shear stress on the endothelium resulting from an
increase in blood flow or blood pressure also stimulates
Right lymphatic
duct
Right lymphatic duct
Right
brachiocephalic
vein
Superior
vena
cava
Thoracic
duct
Inferior
vena cava
Aorta
Intercostal
trunks
Subclavian
vein
Left lymphatic
duct
Internal jugular vein
Right broncho-
mediastinal trunk
Thoracic (left
lymphatic) duct
Left subclavian
trunk
Left jugular trunk
Right subclavian trunk
Right jugular trunk
FIGURE 17-22.
Lymphatic system
showing the thoracic duct and position of
the left and right lymphatic ducts (inset).
Smooth muscles
Arteriole
Precapillary
sphincters
Venule
Venous
capillary
Thorough-
fare channel
Arterial
capillary
FIGURE 17-23.
Capillary bed. Precapillary sphincters control
the flow of blood through the capillary network. Thoroughfare
channels (i.e., arteriovenous shunts) allow blood to move
directly from the arteriole into the venule without moving
through nutrient channels of the capillary.
