Components of Blood (Cambridge O Level Biology)

• Blood consists of red blood cells, white blood cells, platelets and plasma

Blood composition diagrams

Human blood contains red and white blood cells, platelets, and plasma

Components of the blood table

• You need to be able to identify red and white blood cells in photomicrographs and diagrams
  • Red blood cells have a concave disc shape with no nucleus
  • White blood cells are usually round in shape with a nucleus
    • There are different types of white blood cell, including lymphocytes and phagocytes

Blood micrograph diagram

Blood cells can be seen using a microscope. The white blood cells labelled here can be identified as a lymphocyte (top) and a phagocyte (bottom)

Lymphocytes

• Lymphocytes produce antibodies
  • Antibodies are Y-shaped proteins with a shape that is complementary to the antigens on the surface of the pathogen
  • This is a specific immune response as the antibodies produced will only fit one type of antigen
• Lymphocytes can easily be recognised under the microscope by their large, round nucleus which takes up nearly the whole cell and their clear, non-granular cytoplasm

Lymphocyte diagram

Lymphocytes can be recognised under a microscope by their large, round nucleus

Phagocytes

• Phagocytes carry out phagocytosis by engulfing and digesting pathogens
  • Phagocytes have a sensitive cell surface membrane that can detect chemicals produced by pathogenic cells
  • Once they encounter the pathogenic cell, they will engulf it and release digestive enzymes to digest it
    • This is a non-specific immune response
• Phagocytes can be easily recognised under the microscope by their multi-lobed nucleus and their granular cytoplasm

Phagocyte diagram

Phagocytes have a multilobed nucleus; this gives the cell flexibility for engulfing pathogens in phagocytosis

• Plasma is important for the transport of blood cells, carbon dioxide, glucose, amino acids, vitamins, mineral ions, hormones, plasma proteins and urea
• Red blood cells transport oxygen around the body from the lungs to cells which require it for aerobic respiration
  • They carry oxygen in the form of oxyhaemoglobin
• White blood cells defend the body against infection by pathogens 
  • Lymphocytes produce antibodies
  • Phagocytes engulf and digest pathogens
• Platelets are involved in helping the blood clot

Platelet function

• Platelets are fragments of cells that are involved in blood clotting where the skin has been cut or punctured
• When the skin is broken, platelets arrive to stop the bleeding
• A series of reactions occur within the blood plasma
• Platelets release chemicals that cause soluble fibrinogen proteins to convert into insoluble fibrin and form an insoluble mesh across the wound, trapping red blood cells and therefore forming a clot
• The clot eventually dries and develops into a scab; this prevents further blood loss and acts as a barrier to pathogens

Platelet function diagram

Soluble fibrinogen is converted into insoluble fibrin to form a blood clot