1.2 : The Circulatory System

Biology Form 5 

Chapter 1 : Transport

Subtopic 1.2 : The Circulatory System (Blood)

What is a circulatory system ?

  • A mass flow of system conveying materials to all cells of the body
  • Consists of a network of tubes (vessels) that deliver useful materials to all cells of the body and then take away their waste products
  • In mammals the circulatory system consists of :
    1. Cardiovascular system : consists of the heart, blood vessels and blood
    2. Lymphatic system : consists of lymphatic vessels and lymphoid tissues within the spleen, thymus, tonsils, lymph nodes and lymph
Diagram 1 : Cardiovascular system

Diagram 1 : Cardiovascular System


Diagram 2 : Lymphatic Sytem

Diagram 2 : Lymphatic System

  • A circulatory system has three components
    1. Medium : fluid that flows in the system, carrying materials around the body
    2. Vessels : a system that carries the fluid
    3. Pump : keeps the fluid moving through the vessels
The Human Blood
  • A specialised tissue consisting of several types of cells suspended in a fluid medium called plasma
  • Composition of human blood :
    1. Plasma (55%)
    2. Cellular components (45%) : platelets, white blood cells and red blood cell

The components of blood


  • yellowish fluid in blood


  • composition may vary at different parts of the body
  • E.g : plasma in hepatic portal vein , which leads from ileum to the liver, is richer in nutrients than other blood vessels
  • about 90% of plasma is water in which a complex mixture of various substances is dissolved

Substances Transported by Plasma

Cellular Components of Blood

a) Erythrocytes (Red blood cells)


  • Biconcave disc shape :
    • provides a larger total surface area / volume (TSA/V) ratio than that of sphere and therefore increases the rate of gaseous exchange
    • makes the erythrocyte very flexible and allows it to squeeze through blood capillaries image008
  • Lack of nucleus in mature erythrocytes :
    • provides more space for haemoglobin to be packed into the cell
  • Formed in bone marrow
  • Limited lifespan about 120 days
  • Destroyed in the liver or spleen


Functions :

  • Erythrocytes transport oxygen from the lungs to all parts of the body
    • Haemoglobin combines with oxygen to form oxyhaemoglobin
  • 98% of oxygen is carried in the blood as oxyhaemoglobin; 2% dissolved in plasma
  • Erythrocytes carry carbon dioxide from the tissues to the lungs in the form of carbaminohaemoglobin
  • Erythrocytes also function as buffers and help to maintain the blood pH

b) Leucocytes (White blood cells)


  • are formed in the bone marrow and lymph nodes
  • bigger than erythrocytes ; fewer in number
  • each have a nucleus


  • Divided into :
    • Granulocytes 
      • have irregularly lobed nucleus and granular cytoplasm
      • Three types : neutrophils, eosinophils and basophils
    • Agranulocytes
      • have smooth rounded /  bean-shaped nucleus
      • Cytoplasm lacks granules
      • Two types: Lymphocytes and monocytes



c) Thrombocytes (Platelets)


  • irregularly shaped cell fragments
  • usually lacking nucleus
  • formed in bone marrow
  • have lifespan of 5 to 9 days
  • destroyed in the spleen and liver
  • play an important role in blood clotting










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