Blood and Blood Vessels

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  • Blood and blood vessels
    • Blood
      • An average adult has 4-6 litres of blood. Blood it made up of';
        • Plasma (55%)
          • Consists of 90% water. The other 10% is made up of dissolved solutes e.g. CO2, hormones, fibrinogen and antibodies
            • Plasma also has a role in the distribution of heat throughout the body
        • Cells (45%)
      • Large diameter of red blood cells going through a small diameter capillary means there is a large surface area of contact and more diffustion
    • White blood cells
      • Granulocyte
        • These are phagocytic and engulf bacteria to fight infection. They have lobed nuclei
      • Agranulocyte
        • These produce antibodies and antitoxins to fight infection and provide immunity to diseases. They have spherical nuclei
    • Platelet cells are involved with blood clotting
    • Types of blood vessels
      • Arteries
        • Small, round lumen
          • Increases resistance to blood flow, which in turn helps to maintain a high pressure as the blood gets furthur away from the influence of the heart
        • Thick layer of smooth muscle in walls
          • Needed to withstand the high pressure of blood coming from the heart
            • Can contract to narrow the lumen furthur, helping to maintain pressure
        • Thick layer of elastic tissue fibres in walls
          • These allow the artery walls to expand with each pulse of blood
            • They undergo elastic recoil which pushes the blood onwards. This produces a unidirectinal flow, so the arteries don't need valves
        • All arteries carry deoxygenated blood except the pulmonary artery
        • All arteries have blood with high pressure
        • No arteries have valves (except aorta and pulmonary artery)
      • Arterioles
        • Aterioles branch off from arteries and are similar in structure but have less elastic tissue and more smooth muscle
          • This enables them to constrict and reduce blood flow through the tissue so it can be directed to where it is needed most
            • During exercise more blood is required by the working muscle, blood is therefore redistributed around the body
              • When the smooth muscle contracts this causes vasoconstriction (the lumen of the  arteriole become narrower and restricts the blood flow through to the capillaries) - this happens in the gut
                • When the smooth muscle relaxes, this is called vasodilation (the lumen become wider) and increases the blood flow through to the capillaries - happens in muscle and skin surface
      • Capillaries
        • Arterioles furthur subdivide into thin walled capillaries. The capillaries form a vast network which will penetrate all the tissues and organs of the body
          • It is at capillaries that the exchange of materials between the blood and tissues actually takes place
            • Blood from the capillaries collects in venules, which in turn empty blood into veins which return it to the heart
        • There are millions of capillaries, they form a dense network of tissues
          • Capillary walls have just one single layer of endothelial cells forming their structure so they are permeable to water and other dissolved substances like glucose
            • The capillaries have a narrow lumen. This causes friction between the blood and the capillary walls which slows down the rate of blood flow
              • Although capillaries are small in diameter, there are lots of them, so overall there is a large cross sectional area. This furthur reduces the rate of blood flow
      • Venules
      • Veins
        • Large, irregular shaped lumen
          • Wide lumen offers less resistance to blood flow  and since the blood the veins is under low pressure, it is not prevented from returning to the heart
        • Thin walls with a thin layer of smooth muscle
          • Less muscle to contract, so the lumen will not be narrowed and there will be no increase in resistance to blood flow
        • Thin layer of elastic tissue fibre in wall
          • No need to expand as there are no surges of blood and no need to contract
    • Venous return
      • As skeletal muscles contract, blood beyond the site of contraction is forced towards the heart, opening the valves
        • Valves behind the site of contraction prevent blood from being forces away from the heart
          • As skeletal muscles relax, blood pushes back against the valves, causing them to close and prevent the blood from being forced away from the heart again
      • Blood is returned to the heart due to;
        • Residual pressure of blood during ventricular systole
        • Massaging effect of skeletal muscles on veins
        • Negative pressure in the pressure during inspiration
        • Negative pressure due to atrial diastole
    • Exchange of materials from capillary to tissue fluid is possible because;
      • Blood entering the capillary network through arterioles is under high pressure
      • The endothelial cells 'fit' together loosely, leaving small spaces through which materials in the blood can be forces out by the high hydrostatic pressure of the blood
        • The leakage furthur decreases the rate of blood flow


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