The
blood vessels carry
blood to and from the
heart and make up the bulk of the circulatory
system.
Major tissue layers
All
blood vessels have an inner layer of
epithelial tissue called the
tunica intima. It consists of
simple squamous epithelium resting on basement
membrane.
The middle layer of
blood vessel tissue is the
tunica media. It consists mostly of
smooth muscle and
loose connective tissue. The
tunica media is only found in
arteries and
veins.
The outermost layer of
blood vessel tissue is the
tunica externa. It consists mostly of
connective tissue rich in
collagen fibers.
Arteries
Arteries carry
blood leaving the
ventricles under
high pressure. The largest artery in the body is the
aortic arch, which carries
blood from the
left ventricle towards smaller
arteries leading to the head and body.
Arteries are designed to accommodate
high pressure blood with thick, elastic walls and strong
connective tissue layers.
Artery structure:
Artery function:
Arteries carry
blood under
high pressure away from the
heart towards the
capillaries.
Arteries stretch during systole to accommodate the
high pressure blood, then return to their original shape during
diastole, pushing the
blood forward.
Veins
Veins carry
blood returning to the
atria under
low pressure. The largest
veins in the body are the
superior and
inferior vena cava, which drain
blood from the
systemic circuit to the right atrium.
Veins are designed to accommodate large volumes of
blood under
low pressure with strong but supple walls and
valves to prevent backflow.
Vein structure:
Vein function:
Veins carry
blood under
low pressure. By the time the
blood has squeezed through the
capillaries, its
pressure is near zero. The
muscular and
respiratory systems "pump" the
blood back towards the
heart by squeezing the
veins. The
valves of the
veins prevent
blood from flowing back towards the
capillaries.
Capillaries
Capillaries are the smallest of all
blood vessels. They are found in
capillary beds at every vascularized
tissue in the body. At the
capillaries materials are exchanged with the surrounding
tissues through the thin single endothelium layer.
Capillaries are so small that
red blood cells must pass through them single file!
Capillary structure:
Tunica
intima - endothelium
Microcirculation flows through interconnected networks of
capillaries called
capillary beds. At the arterial end of each capillary bed
precapillary sphincters comtrol the flow of
blood into the bed. The opening and closing of
precapillary sphincters is under control of the ANS (
autonomic nervous system).
Capillary function:
Capillaries exchange
solutes between the
blood and surrounding
interstitial fluid in four different ways:
Net fluid movement into and out of
capillaries is regulated by two factors:
The
net fluid movement is the amount of fluid lost into the interstitial fluid by
blood pressure minus the amount of fluid returned to the
blood by
osmotic pressure. Typically there is a net fluid loss from the
blood to the interstitial fluid. This excess interstitial fluid is returned to the
blood by the
lymphatic system.
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1 - Anatomy and Physiology Basics