Blood vessels in human body

The blood vessels in human body,Blood vessels come in three varieties

1 Arteries carry blood away from the heart. The largest artery is the aorta. Small
ones are called arterioles, and microscopically small ones are called metarterioles.

2 Veins carry blood toward the heart; all veins except the pulmonary veins contain
deoxygenated blood. Small ones are called venules, and large venous spaces are
called sinuses.

3 Microscopically small capillaries carry blood from arterioles to venules, but
sometimes tiny spaces in the liver and elsewhere called sinusoids replace
capillaries.

The walls of arteries and veins have three layers: the outermost tunica externa (sometimes
called tunica adventitia) composed of white fibrous connective tissue, a central
“active” layer called the tunica media composed of smooth muscle fibers and yellow
elastic fibers, and an inner layer called the tunica intima made up of endothelium that
aids in preventing blood coagulation by reducing the resistance of blood flow. Arterial
walls are very strong, thick, and very elastic to withstand the great pressure to which
the arteries are subjected. Arteries have no valves.

There are two types of arteries: elastic and muscular. In elastic arteries, found primarily
near the heart, the tunica media is composed of yellow elastic fibers that stretch
with each systole and recoil during diastole; essentially they act as shock absorbers to
smooth out blood flow. In muscular arteries, the tunica media consists primarily of
smooth muscle fibers that are active in blood flow and distribution of blood. The
larger blood vessels have smaller blood vessels, the vasa vasorum, that carry nourishment
to the vessel wall.

While larger in diameter than arteries, veins have thinner walls and are less distensible
and elastic. Veins that carry blood against the force of gravity, such as those in the legs
and feet, contain valves to prevent backsliding into the capillaries. Normally the blood
that veins are returning to the heart is unoxygenated (contains carbon dioxide); the
one exception is the pulmonary vein, which returns oxygenated blood to the heart
from the lungs.

Capillaries are breathtakingly tiny and capable of forming vast networks, or capillary
beds. Their walls are a single layer of squamous endothelial cells. Precapillary
sphincters take the place of valves to regulate blood flow. All exchange occurs at
the capillaries.

Blood from the digestive tract takes a detour through the hepatic portal vein to
the liver before continuing on to the heart. Called the hepatic portal system, this
circuitous route helps regulate the amount of glucose circulating in the bloodstream