Cells make up tissues, organs and systems

Cells make up tissues, organs and systems

  • Tissue: a tissue (e.g. muscle tissue) is a group of similar cells that work together to carry out a particular function.
  • Organ: An organ is a group of different tissues that work together to perform a particular function.
  • Organ system: An organ system is a group of organs working together to perform a particular function.


  • Blood may be a liquid but it is an organ containing many different types of specialised cells that carry out particular functions in the body.
  • Blood is made up of four components: plasma (55%), white blood cells and platelets (<1%) and red blood cells (45%).


  • This is a yellow liquid which keeps the blood fluid and transports many things:
    • Red and white blood cells and platelets.
    • Nutrients like glucose and amino acids. They are absorbed from the gut and taken to body cells.
    • Carbon dioxide – transported by the blood from the cells to the lungs where it is removed.
    • Urea – blood transports it to the kidneys where it’s removed.
    • Hormones – transported from glands to target organs.
    • Antibodies and antitoxins produced by the white blood cells.

White blood cells

  • These help fight disease.
  • Some types can engulf and digest pathogens (phagocytes). Others (lymphocytes) produce antibodies (proteins that bind to pathogens to destroy then) and antitoxins to neutralise any toxins produced byt he microorganisms.
  • A low WBC count could increase risk of infection while a high count could mean you have an infection or even leukaemia.


  • Fragments of cells that help with clotting of the blood.
  • Blood clots at wounds stop blood pouring out and stop microorganisms getting in.
  • Lack of platelets can caused excessive bleeding and bruising.

Red blood cells

  • The job of red bloods cells is to carry oxygen around the body. It contains the red pigment haemoglobin which reacts with oxygen to make oxyhaemoglobin.
  • In cells the reverse happens so the cells can get oxygen.
  • Red blood cells are biconcave discs which means they have dimples on both sides to give a large surface area for absorbing oxygen.
  • They also don’t have a nucleus to make more room for haemoglobin.
  • Anaemia is a lack of iron which means the blood doesn’t carry enough oxygen as haemoglobin contains a lot of iron.