14.structure & functions of capillaries, venules and veins
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14.structure & functions of capillaries, venules and veins






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14.structure & functions of capillaries, venules and veins 14.structure & functions of capillaries, venules and veins Presentation Transcript

  • 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….. 