The Blood & Blood Vessels (Edexcel GCSE Biology: Combined 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

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

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

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

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

• 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 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

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