The Blood (Edexcel IGCSE Biology: Double Science)

• Blood consists of red blood cells, white blood cells, platelets and plasma
• Over half of the volume of the blood is made up of plasma
• The majority of the other half is made up of red blood cells
• The remaining fraction consists of white blood cells and platelets

Blood micrograph

Composition of human blood

Components of the Blood Table

• Plasma is a straw coloured liquid which the other components of the blood are suspended within
• Plasma is important for the transport of many substances including:
  • Carbon dioxide - the waste product of respiration, dissolved in the plasma and transported from respiring cells to the lungs
  • Digested food and mineral ions - dissolved particles absorbed from the small intestine and delivered to requiring cells around the body
  • Urea - urea is a waste substance dissolved in the plasma and transported to the kidneys
  • Hormones - chemical messengers released into the blood from the endocrine organs (glands) and delivered to target tissues/organs of the body
  • Heat energy - heat energy (created in respiration) is transferred to cooler parts of the body or to the skin where heat can be lost

• Red blood cells are specialised cells which carry oxygen to respiring cells
• They are adapted for this function in 3 key ways
  • They are full of haemoglobin, a protein that binds to oxygen to form oxyhaemoglobin
  • They have no nucleus which allows more space for haemoglobin to be packed in
  • The shape of a red blood cell is described as being a 'biconcave disc'  this shape gives them a large surface area to volume ratio to maximise diffusion of oxygen in and out

Red blood cells