2.5

• Blood vessels

  The heart together with blood vessels and the blood they contain form the cardiovascular system
• Blood circulation from the heart through the arteries, capillaries and veins back to the heart

  1. Blood circulates from the heart through the arteries to the capillaries to the veins and back to the heart

  2. Capillaries connect arteries to veins

  3. There is a decrease in blood pressure as blood moves away from the heart
• Structure and function of arteries, capillaries and veins

  • The endothelium lining and the central lumen of blood vessels are surrounded by layers of tissue
• Arteries

  • Arteries have an outer layer of connective-tissue containing elastic fibres and a middle layer containing smooth-muscle with more elastic fibres
  • Contraction of the heart forces blood at high-pressure into the arteries
  • The elastic fibres in the outer walls of the arteries stretch and recoil to accommodate the search of but after each contraction
• Artery layers

  • Endothelium
  • Central lumen
  • Outer layer of connective-tissue containing a elastic fibres
  • A thick middle layer containing smooth-muscle with more elastic fibres
• The role of vasoconstriction and vasodilation in controlling blood flow in arteries

  1. To control blood flow, the smooth-muscle in the middle layer of the artery wall can contract causing visible construction, a decrease in the diameter of the central lumen

  2. Vasoconstriction decreases the diameter of the central lumen and decreases the blood flow into the capillaries

  3. Relaxation of the smooth-muscle and the middle layer of the artery wall causes the opposite affect called vasodilation

  4. Vasodilation Increases the diameter of the central lumen and increases blood flow to the capillaries
• Structure and function of capillaries

  • Capillaries allow Exchange of substances with tissues through their thin walls
  • Their walls are only one cell thick to allow easy movement of materials from blood cells and vice versa
  • They have a large surface area for maximum exchange of material with body cells
  • They connect arteries to veins, blood comes into the capillaries from arteries at high-pressure but leaves the capillaries in veins at low pressure
• Structure and function of veins

  • Veins have an outer layer of connective tissue containing elastic fibres but, a much thinner muscular wall than arteries
  • Veins have a thinner muscular wall than arteries because the blood in veins is under less pressure
  • Veins have a much wider lumen than arteries
  • A structural feature of a vein is that they have valves
  • The function of valves as to prevent the backflow of blood
• Vein structure

  • Outer layer of connective tissue and elastic fibres
  • Thin middle layer of smooth muscle and more elastic fibres
  • Endothelium
  • Wide lumen
• Exchange of material between tissue fluid and sales through pressure filtration and the role of the lymphatic system

  1. When the blood enters the capillaries at high-pressure plasma (the liquid part of blood) is forced out of the capillary walls – this process is called pressure filtration

  2. Pressure filtration causes the plasma to pass through the capillary walls into the tissue fluid surrounding the cells
• Tissue fluid

  • Cells are surrounded by a liquid called tissue fluid
  • Tissue fluid supplies cells with glucose, oxygen, and other substances by the process of diffusion
  • Carbon dioxide and other metabolic wastes diffuses out of the cells and into the tissue fluid to be excreted
  • Much of the tissue fluid returns to the blood
• Lymphatic system

  • Lymphatic vessels absorb excess tissue fluid and return it as lymph to the circulatory system
  • Lymph is a clear fluid containing white blood cells and other substances
  • The lymphatic system transports lymph
• Composition of tissue fluid and blood plasma

  Tissue fluid and blood plasma are similar in composition, with the exception of plasma proteins, which are too large to be filtered through the capillary wall