Revision Notes

9.6 Blood

Components of Blood

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


Composition of human blood, IGCSE & GCSE Biology revision notesComposition of human blood


Components of the blood:

Blood table, IGCSE & GCSE Biology revision notes


Blood micrograph, IGCSE & GCSE Biology revision notesBlood micrograph

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Types of White Blood Cell

  • White blood cells are part of the body’s immune system, defending against infection by pathogenic microorganisms
  • There are two main types, phagocytes and lymphocytes


  • Carry out phagocytosis by engulfing and digesting pathogens

Phagocytosis, IGCSE & GCSE Biology revision notesPhagocytosis

  • 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
  • They can be easily recognised under the microscope by their multi-lobed nucleus and their granular cytoplasm


  • Produce antibodies to destroy pathogenic cells and antitoxins to neutralise toxins released by pathogens
  • They 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

Functions of the Parts of the Blood

  • Plasma is important for the transport of carbon dioxide, digested food (nutrients), urea, mineral ions, hormones and heat energy
  • Red blood cells transport oxygen around the body from the lungs to cells which require it for aerobic respiration
  • They carry the oxygen in the form of oxyhaemoglobin
  • White blood cells defend the body against infection by pathogens by carrying out phagocytosis and antibody production
  • Platelets are involved in helping the blood to clot
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Blood Clotting

  • Platelets are fragments of cells which are involved in blood clotting and forming scabs where skin has been cut or punctured
  • Blood clotting prevents continued / significant blood loss from wounds
  • Scab formation seals the wound with an insoluble patch that prevents entry of microorganisms that could cause infection
  • It remains in place until new skin has grown underneath it, sealing the skin again


How the blood clots, IGCSE & GCSE Biology revision notesHow the blood clots


  • When the skin is broken (i.e. there is a wound) 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 to protect the wound from bacteria entering

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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