Blood Vessels

Different types of blood vessels in the body with their structural composition related to their function. Features:

  • Elastic fibres → composed of elastin and can stretch and recoil, providing vessel walls with flexibility

  • Smooth muscles → contracts or relaxes, which changes the size of the lumen ( tube that carries the blood )

  • Collagen → provides structural support to maintain the shape and volume of the vessel. A strong, inelastic protein. In arteries, its job is to stop the artery from stretching too far and prevent it from bursting when blood pressure is high

Arteries and arterioles

arteries carry blood away from the heart to the tissues of the body. They carry oxygenated blood EXCEPT IN THE PULMONARY ARTERY, which carries deoxygenated blood from the heart to the lungs. ALSO (during pregnancy) THE UMBILICAL ARTERY, carrying deoxygenated blood from the fetus to the placenta

  • ARTERY WALLS = elastin fibres which enable them to withstand high force and pressure of blood pumped out of the heart and stretch to take the larger blood volume, smooth muscle and collagen.

  • In between contractions of the heart, elastic fibres return back to original length = surges of blood pumped from the heart to given a continuous flow. (pulse = a surge of blood).

  • Lining of an artery (endothelium) = smooth so blood flows easily over it.

  • ARTERIOLES = link arteries and capillaries together. More smooth muscle and less elastin in their walls than arteries as have weaker pulse surge, but can constrict or dilate to control flow of blood into individual organs.

  • When the smooth muscle in the arterioles contracts, it prevents blood flowing into capillary bed = VASOCONSTRICTION.

  • When smooth muscle relaxes, blood flows through into capillary bed = VASODILATION

Capillaries

  • Microscopic blood vessels that link arterioles with venules.

  • Form an extensive network through all the tissues of the body. The lumen of a capillary = really small (red blood cells have to travel through in single file (7.5-8 um)

  • Substances are exchanged through capillary walls between tissue cells and blood - the gaps between the endothelial cells that make up the capillary walls. EXCEPTION → capillaries in the CNS, which have very tight connections between the endothelial cells..

  • In most organs, blood entering the capillaries from arterioles is oxygenated. When it leaves the capillaries for the venules, it has less O and more CO2 (deoxygenated). EXCEPTION → lungs and placenta with deoxygenated blood entering the capillaries and oxygenated blood leaving into the venules.

Ways which capillaries are adapted to their role

  • large surface area for diffusion of substances

  • Single endothelial cell thick - very thin layer for diffusion

  • Thin lumen = slower movement of blood through capillaries therefore more time for diffusion to take place.

Veins and Venules

  • Veins carry blood away from the cells of the body towards the heart , and with exception, carry deoxygenated blood.

  • The Pulmonary Vein carries oxygenated blood from lungs to the heart, (during pregnancy) the umbilical vein carries oxygenated blood from the placenta to the fetus

  • Deoxygenated blood flows from the capillaries into very small veins called Venules and then into larger veins. Then reaches 2 main vessels carrying deoxygenated blood back to the heart - THE INFERIOR VENA CAVA →from lower parts of the body. THE SUPERIOR VENA CAVA → From the head and upper body

  • Veins do not have a pulse - surges from the heart pumping are lost as blood passes through narrow capillaries. They do hold large reservoir of blood - 60% of blood volume in the veins at any one time.

  • Veins = low blood pressure compared to arteries. Medium sized veins have valves to prevent backflow of blood.

  • Vein walls have a lot of collagen and little elastic fibre, vessels have a wide lumen and a smooth thin lining - so blood flows easily.

  • Venules link capillaries with the veins - very thin walls with a little smooth muscle - several venules join to form a vein

Deoxygenated blood must be returned to the heart to be pumped to the lungs and oxygenated again - but blood is low pressure and needs to move against gravity - adaptations to counter this problem

  • majority of veins have ONE WAY VALVES at intervals (flaps or infoldings of inner lining of the vein).

  • Many bigger veins run between big, active muscles in the body (arms, legs). When muscles contract, they squeeze the veins - forcing blood towards the heart. Valves prevent backflow when muscles relax

  • Breathing movements of the chest act as a pump. The pressure changes and the squeezing actions move blood in the veins of the chest and abdomen towards the heart

THESE ALL WORK IN COMBINATION