The Blood (Edexcel IGCSE Biology): Revision Note

Components of Blood

• 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

• 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

• Red blood cells are specialised cells which carry oxygen to respiring cells

• They are adapted for this function in three key ways:

  • Red blood cells are packed with haemoglobin, a protein that binds to oxygen to form oxyhaemoglobin

  • They have no nucleus which allows more space for haemoglobin to be packed in

  • Red blood cells have a biconcave disc shape, which means they are thinner in the middle than at the edges. This shape gives them a larger surface area to volume ratio, allowing oxygen to diffuse in and out quickly and efficiently.

Red blood cells