Posted by DAM on February 3, 2012 in IONISING RADIATIONS | Short Link
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Types of ionizing radiation:

  1. Electromagnetic radiation: X-rays and Gamma rays.
  1. Particulate radiation:

    alpha particles – beta particles – protons – electrons and neutrons.

    – All of these radiations can be administered to animals and human beings: Externally or Internally.

    1. External sources include X-ray machines, electron accelerators.
    2. Internal irradiation arises from the ingestion of any of the hundreds of known radionucleotides (radioactive isotopes) many of which are used for medical diagnostic or therapeutic purposes.

    Mode of action of ionizing radiation

    A) Direct action:

    Radiation causes its effects by transferring energy to the substance through which it passes.

    This energy can produce two changes:

  2. Ionization: Loss or gain of an electron.
    1. Excitation: Movement of an electron into an outer orbit. Occurs in some DNA molecules (genes) and this results in a change in their chemical structure (mutation).

    (B) In direct action:

    Diffuse ionization of water in the cell leads to the formation of oxidizing agents such as H202 which inactivate cellular enzymes.

    Forms of Radiation injury of cells:

  3. Cloudy swelling, hydropic degeneration or necrosis.
  4. Temporary or permanent arrest of mitosis results from: a) Interference with synthesis of DNA.

    b) Injury of mitotic spindle.

    1. Fragmentation of chromosomes with failure of correct reconstruction results in necrosis of daughter cells.
    2. Mutation: Mutation may be lead to neoplastic transformation as follows:


  • If mutations are in the germ cells, the children of irradiated parents can be affected.
  • If the mutations are in somatic cells malignant tumors can be induced by radiation. The commonest radiation-induced tumors are leukemia and tumors of thyroid, breast, bone and lung.

Radiosensitivty of cells or tissues:

Two general rules govern the variable susceptibility of different tissues to the effects of ionizing radiations:

1- Embryonic and undifferentiated cells are more susceptible to radiation injury than differentiated cells.

2- Rapidly proliferating cells are more easily damaged by ionizing radiation during interphase or early phase of mitosis.

Human tissues are classified into three groups:

1-    Radio-sensitive Tissues:

a- Lymphoid tissue b- Hemopoietic tissue
c- Germ cells    d- Intestinal epithelium.

2-    Radio-responsive tissues (intermediate):

a- Skin    b- Vascular endothelium

c- Conjunctiva, cornea and lens.

3-    Radio-resistant tissues

a- Bones    b- lungs    c- Nerve cells

d- Skeletal muscles e- kidneys, f- Endocrine glands.

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