Capillaries are the smallest blood vessels where gas exchange takes place between blood and tissues. They have only a single layer of endothelium and are arranged in capillary beds. Venules are slightly larger blood vessels formed by the joining of capillaries. They have thin walls and carry blood at low pressure away from tissues towards veins. Veins are the largest blood vessels and carry blood back to the heart. They have thin, expandable walls to hold large blood volumes and contain valves to prevent backflow.

1. Structure & Functions of
Capillaries, Venules and
Veins
By; Kashif Nadeem Khokhar

2. Overview
• VEINS - carry blood toward the heart
• VENULES - small veins
• CAPILLARIES - site of gas exchange

3. 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
the heart;
• Veins - return blood to the heart.
• Veins are composed of three layers (tunics):

4. Three Layers;
• 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.

5. • 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
of blood.
• Valves are flaps that are comprised of folds of tunica
intima.
• Valves are most abundant in the veins of the limbs.

6. Continued……..
• Muscular Pump - contracting skeletal muscles press
against veins and forces blood through the one-way
venous valves.
• 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 cavity.
• Abdominal veins are squeezed during inhalation,
• The unequal pressures create an upward sucking
effect that pulls blood toward the heart.

7. • 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.

8. Vein & Muscular Pump

9. Capillaries;
• 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
Tissues.
• Most capillaries are arranged in capillary
beds.

10. Continued………….
• 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
capillary.
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

11. Capillaries..

12. Types of Capillaries;
• Continuous Capillaries
• Most tissues of the body are supplied by continuous
capillaries.
• 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
interstitial fluid
• Prevent the loss of RBCs and plasma proteins

13. Types continued……….
• 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.

14. Venules;
• Venules –
• Form When Capillaries Unite
• Larger Venules Have Small Tunica Medias And
Tunica Externas
• The Smallest Venules Consist Of Only A Tunica
Intima
• Blood Is Under Low Pressure In The Venules
• Venules Join To Form Veins
• Venules have same functions as of veins.

15. Venule..

16. THANK YOU SO MUCH…..
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