The Blood & Blood Vessels (Edexcel GCSE Biology: Combined Science)

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The 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, IGCSE & GCSE Biology revision notes

Blood micrograph

The components of the blood 1, downloadable IGCSE & GCSE Biology revision notes

Composition of human blood

Components of the Blood Table

Blood table, IGCSE & GCSE Biology revision notes

Red Blood Cells

  • 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 disk'  this shape gives them a large surface area to volume ratio to maximise diffusion of oxygen in and out

Red Blood Cells, IGCSE & GCSE Biology revision notes

Red blood cells

White Blood Cells

  • White blood cells are part of the body’s immune system, defending against infection by pathogenic microorganisms
  • There are two main types, lymphocytes and phagocytes
  • Lymphocytes:
    • Produce antibodies to destroy pathogenic cells and antitoxins to neutralise toxins released by pathogens
    • They can easily be recognised under the microscope by their large round nucleus which takes up nearly the whole cell and their clear, non-granular cytoplasm

  • Phagocytes:
    • Carry out phagocytosis by engulfing and digesting pathogens
    • Phagocytes have a sensitive cell surface membrane that can detect chemicals produced by pathogenic cells
    • Once they encounter the pathogenic cell, they will engulf it and release digestive enzymes to digest it
    • They can be easily recognised under the microscope by their multi-lobed nucleus and their granular cytoplasm

Phagocytosis, IGCSE & GCSE Biology revision notes

Phagocytosis

Platelets

  • Platelets are involved in helping the blood clot
  • Platelets are fragments of cells that are involved in blood clotting and forming scabs where the skin has been cut or punctured
    • When the skin is broken (i.e. there is a wound) platelets arrive to stop the bleeding
    • A series of reactions occur within the blood plasma
    • Platelets release chemicals that cause soluble fibrinogen proteins to convert into insoluble fibrin and form an insoluble mesh across the wound, trapping red blood cells and therefore forming a clot
    • The clot eventually dries and develops into a scab to protect the wound from bacteria entering

  • Blood clotting is important because:
    • Blood clotting prevents continued / significant blood loss from wounds
    • Scab formation seals the wound with an insoluble patch that prevents entry of microorganisms that could cause infection
    • It remains in place until new skin has grown underneath it, sealing the skin again

How the blood clots, IGCSE & GCSE Biology revision notes

How the blood clots

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 as hydrogencarbonate ions 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 - 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
    • 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

Blood Vessels

  • There are three main types of blood vessel:
    • Arteries
    • Veins
    • Capillaries

  • Smaller vessels that branch off from arteries are called  arterioles (small arteries) and those that branch into veins are called venules (small veins)
  • Each vessel has a particular function and is specifically adapted to carry out that function efficiently

Arteries

  • Key features:
    • Carry blood at high pressure away from the heart
    • Carry oxygenated blood (except the pulmonary artery)
    • Have thick muscular walls containing elastic fibres
    • Have a narrow lumen
    • Blood flows through at a fast speed

  • The structure of an artery is adapted to its function in the following ways
    • Thick muscular walls containing elastic fibres withstand the high pressure of blood and maintain the blood pressure as it recoils after the blood has passed through
    • A narrow lumen also helps to maintain high pressure

Veins

  • Key features:
    • Carry blood at low pressure towards the heart
    • Carry deoxygenated blood (other than the pulmonary vein)
    • Have thin walls
    • Have a large lumen
    • Contain valves
    • Blood flows through at a slow speed

  • The structure of a vein is adapted to its function in the following ways:
    • A large lumen reduces resistance to blood flow under low pressure

    • Valves prevent the backflow of blood as it is under low pressure

Comparing arteries and veins, IGCSE & GCSE Biology revision notes

Comparing the structure of arteries and veins

Capillaries

  • Key features:
    • Carry blood at low pressure within tissues
    • Carry both oxygenated and deoxygenated blood
    • Have walls that are one cell thick
    • Have ‘leaky’ walls
    • Speed of blood flow is slow

  • The structure of a capillary is adapted to its function in the following ways:
    • Capillaries have walls that are one cell thick (short diffusion distance) so substances can easily diffuse in and out of them
    • The ‘leaky’ walls allow blood plasma to leak out and form tissue fluid surrounding cells

Structure of a capillary, IGCSE & GCSE Biology revision notes

Structure of a capillary

Arterioles and venules

  • As arteries get further away from the heart, they divide more and get narrower
  • The narrow vessels that connect arteries to capillaries are called arterioles
  • Veins also get narrower the further away they are from the heart
  • The narrow vessels that connect capillaries to veins are called venules

The blood vessel network, IGCSE & GCSE Biology revision notes

The blood vessel network

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Lára

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.