Blood Composition (OCR GCSE Biology A (Gateway))

Red Blood Cells

Blood as a transport system

• The role of blood in the body is to act as a transport system carrying useful substances to every cell of the body, and carrying harmful waste substances to other organs for processing and/or excretion

  • It also plays a vital role in transferring heat from “active” organs to cooler parts of the body (such as the extremities – hands and feet)

    • Blood consists predominantly of red blood cells 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

    • Less than 1% of blood contains white blood cells and platelets

Blood micrograph

Composition of human blood

Red blood cells carry oxygen

• Red blood cells are specialised cells that carry oxygen from the lungs to cells throughout the body

• The structure of red blood cells are adapted for this function in 3 key ways

  • Red blood cells are small with a 'biconcave disk' shape which gives them a large surface area to volume ratio to maximise diffusion of oxygen in and out

  • 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

  • Their small size and flexibility allows them to pass through capillaries

Red blood cells

Blood 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:

  • Red blood cells, white blood cells and platelets

  • Water

  • Carbon dioxide - the waste product of cellular respiration

  • Digested food and mineral ions - absorbed from the small intestine and delivered to cells around the body

  • Urea - the waste substance produced in the breakdown of proteins by the liver. Urea is 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

  • Antibodies - which are special proteins to help fight infection

  • Heat energy - created in respiration (an exothermic reaction), heat energy is transferred to cooler parts of the body or to the skin where heat can be lost