vascular system

• three major types of vessels — artery, capillary, and vein — carry blood throughout the body
• artery consists of:

  1. tunica externa
  2. tunica media
  3. elastic layer
  4. tunica intima
  5. endothelium
  6. lumen
• vein consists of:

  1. tunica externa
  2. tunica media
  3. tunica intima
  4. valve
  5. endothelium
  6. lumen
• capillary consists of endothelial cell
• arteriole consists of smooth muscle and precapillary sphincter
• arteries carry blood from the heart to trillions of body cells that make up an organism
• the walls of the arteries must be able to withstand the surge of blood that results from each contraction of the heart
• the walls of the large arteries have three layers to provide the necessary strength and flexibility
• three layers of arteries:

  • tunica externa
  • tunica media
  • tunica intima
• tunica externa is a tough, outer coat of connective tissue that provides strength
• tunica media is a layer of smooth muscular tissue
• depending on the needs of the body, tunica media can cause vasoconstriction or vasodilation
• vasoconstriction is the reduction of the lumen diameter caused by smooth muscle contraction
• vasodilation is the widening of the lumen caused by smooth muscle relaxation
• tunica intima is a thin, inner lining composed of endothelial cells that provides a smooth surface so blood can flow easily through its lumen
• pulse is the surge of blood felt in the arteries when blood is pumped from the heart
• because of the pressure against arterial walls associated with the pumping action of the heart, a cut or severed artery may lead to profuse bleeding
• arterial blood (except in pulmonary artery) contains a high concentration of oxygen
• arterial blood appears bright red and is oxygenated
• oxygenated blood travels to smaller arteries called arterioles and finally to the smallest vessels, the capillaries
• capillaries are microscopic vessels that join the arterial system with the venous system
• capillaries may be the smallest vessel, but they are actually the most important because of their function
• at the capillary level, nutrients and oxygen in the blood are exchanged for waste products formed by the surrounding cells
• because capillary walls are composed of a single layer of endothelial cells, they are very thin
• the thinness of capillary walls enables the exchange of water, macromolecules, metabolites, and wastes
• because vast numbers of capillaries branch from arterioles, blood flows slowly through them, providing sufficient time for exchange of necessary substances
• blood flowing through the capillary system is regulated by the contraction of smooth muscle precapillary sphincters that lead into the capillary bed
• precapillary sphincters are smooth muscles that contract to regulate blood flow, leading it into the capillary bed
• when tissues require more blood, precapillary sphincters open; when less blood is required, they close
• once the exchange of products is complete, blood enters the venous system for its return cycle to the heart
• veins return blood to the heart
• veins are formed from smaller vessels called venules
• venules develop from the union of capillaries
• the extensive network of capillaries absorbs the propelling pressure exerted by the heart
• veins use other methods to propel blood to the heart:

  • skeletal muscle contraction
  • gravity
  • respiratory activity
  • valves
• valves are small structures within veins that prevent the backflow of blood
• valves are found in the extremities and are important for returning blood from the legs to the heart, because blood must travel a long distance against the force of gravity to reach the heart
• large veins (e.g. in abdomen) contain smooth muscle that contract peristaltically to propel blood toward the heart against gravity
• Blood carried in the veins (except in pulmonary veins) contains a low concentration of oxygen with a high concentration of carbon dioxide
• veins contain deoxygenated blood