14.structure & functions of capillaries, venules and veins
Structure & Functions of
Capillaries, Venules and
By; Kashif Nadeem Khokhar
• VEINS - carry blood toward the heart
• VENULES - small veins
• CAPILLARIES - site of gas exchange
Basic Structure of Veins;
• Veins –
• Thinner walled and larger lumen than
arteries which allows them to hold large
volumes of blood; these vessels experience
the least pressure as they are farthest from
• Veins - return blood to the heart.
• Veins are composed of three layers (tunics):
• Tunica Intima – composed of simple squamous
epithelium (endothelium); it is the innermost
layer to the lumen (blood-filled space)
• Tunica Media – middle layer; contains sheets
of smooth muscle, but less than arteries
• Tunica Externa (=tunica adventitia) –
composed of connective tissue; outermost layer;
this is the predominant layer of the vein wall.
• Blood is under low pressure in veins; thus, there are
3 mechanisms that help blood to move forward
through these low pressure vessels:
• VALVES - veins have valves which prevent backflow
• Valves are flaps that are comprised of folds of tunica
• Valves are most abundant in the veins of the limbs.
• Muscular Pump - contracting skeletal muscles press
against veins and forces blood through the one-way
• Respiratory Pump - there are pressure changes
associated with breathing.
• At the beginning of inhalation, pressure decreases
in the thoracic cavity and increases in the
• Abdominal veins are squeezed during inhalation,
• The unequal pressures create an upward sucking
effect that pulls blood toward the heart.
• The systemic veins and venules contain about
60%, of the blood volume:
• Veins are 8 times more expandable than arteries.
• Veins are capacitance vessels, meaning they expand
easily with very little pressure.
• Venoconstriction - this response to blood loss causes
systemic veins to constrict; it reduces the amount of
blood in the venous system and increases blood in
the arterial and capillary system.
• Venous Reserve - veins in the liver, skin, and lungs
can constrict and redistribute about 20% of the total
blood volume to the general circulation.
• Capillaries - smallest of the blood
vessels; very thin walled; composed of
only a lumen and tunica intima
(endothelium) only; no smooth muscle;
• Capillaries are where the exchange of
materials takes place between blood and
• Most capillaries are arranged in capillary
• The capillary wall consists of only the tunica intima
(endothelium); this thin wall allows for the
exchange of materials between blood and tissues.
• Pre-capillary sphincters - smooth muscle fibers
surround the beginning of each capillary; these
sphincters control the flow of blood through the
Blood pressure is relatively low in the capillaries;
higher blood pressure would end up forcing
excessive amounts of solute-containing fluids out of
blood and into the interstitial fluid
Types of Capillaries;
• Continuous Capillaries
• Most tissues of the body are supplied by continuous
• The capillary endothelium is a complete lining
• This is important in the brain and comprises part of
the blood-brain barrier
• Permit the diffusion of water, small molecules, and
lipid soluble materials into the surrounding
• Prevent the loss of RBCs and plasma proteins
• Fenestrated Capillaries
• Contain pores that span the endothelial lining.
• These capillaries allow for a quick exchange of water and
relatively large solutes between plasma and interstitial fluid.
• They are located in the Choroid Plexus Of The
Brain, Hypothalamus, Pituitary Gland, And Thyroid Gland.
• SINUSOIDS are similar to fenestrated capillaries but they
have gaps between adjacent endothelial cells, and allow a
free exchange of water and even larger solutes between
blood and interstitial fluid; sinusoids are found in the
Liver, Bone Marrow, And Spleen.
• Venules –
• Form When Capillaries Unite
• Larger Venules Have Small Tunica Medias And
• The Smallest Venules Consist Of Only A Tunica
• Blood Is Under Low Pressure In The Venules
• Venules Join To Form Veins
• Venules have same functions as of veins.