Structure & Functions of Capillaries, Venules and Veins

By; Kashif Nadeem Khokhar

Overview

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

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.

• 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.

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.

• 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.

Vein & Muscular Pump

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.

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

Capillaries..

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

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.

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.

Venule..

THANK YOU SO MUCH…..