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School Biology notes: Human circulatory system: Part 4. What is in blood?

The human circulatory system Part 4. What substances are in the blood? What do they do? Explaining the functions of red blood cells, white blood cells, plasma & platelets

Doc Brown's GCSE level Biology exam study revision notes

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Index of notes on human circulatory system: heart, lungs & blood vessels

4. What substances are in the blood? What do they do?

Blood is considered a liquid tissue (unique in your body!)

Blood is a mixture of red blood cells, white blood cells, platelets, and plasma, which is a straw coloured fluid (when separated from red blood cells, mainly of water, but containing solutes (dissolved substances).

Know the structure and function of the following parts of the blood.

Red blood cells

Red blood cells carry oxygen from the lungs to every living cell of the body via the blood stream.

It's the red blood cells that give blood its red colour and unlike white blood cells, they have no nucleus.

Red blood cells are a squashed from both sides 'doughnut' shape (biconcave disc) to give them a large surface area for the iron containing haemoglobin molecules to capture the oxygen.

The large surface area gives efficient absorption of oxygen to combine with the haemoglobin molecules by the reversible reaction:

haemoglobin  +  oxygen  (c) doc b  oxyhaemoglobin (bright red pigment molecule)

The oxygen molecule is weakly bonded to the haemoglobin molecule by a reversible reaction, so it is easily released to be consumed in respiration.

A high oxygen concentration favours the formation of oxyhaemoglobin (reaction moves to right), and low oxygen concentration favours its dissociation to free oxygen (reaction moves to left) - this follows from Le Chatelier's Principle you learning in chemistry about reversible reactions and chemical equilibrium. See GCSE chemistry notes:

Reversible Reactions and Reversible reactions and chemical equilibrium

The red blood cells do not have a nucleus, allowing more space for haemoglobin molecules.

Note the four red blood cell adaptations:

(i) the haemoglobin molecule is adapted to carry the oxygen molecule O2 for cellular respiration in all the body's tissues, the complex formed is called oxyhaemoglobin,

(ii) the biconcave disc shape gives a large surface area / volume ratio for absorbing oxygen molecules - increases efficiency of diffusion of oxygen in and out of the red blood cells and reduces distance to centre of cell,

(iii) no nucleus - extra space for oxygen carrying haemoglobin molecules - nucleus not required,

(iv) red blood cells are very small and flexible and can easily pass through tiny capillaries.

In the lungs the oxygen molecule attaches itself to the iron atom at the centre of the large complex haemoglobin molecule to form oxyhaemoglobin.

The process is reversed in the body tissues to release oxygen for cell respiration (equation above).

If we don't have enough iron in our diet we can suffer from anaemia, a potentially serious condition where the blood can't carry enough oxygen needed for all the respiration demands of the cells.

White blood cells

White blood cells have a nucleus and are part of the body's immune system to fight diseases.

Some white cells can change shape to engulf and destroy potentially harmful microorganisms.

These cells are called phagocytes and their action is called phagocytosis.

Other types of white blood cells called lymphocytes and have variety of functions.

Type B lymphocytes produce antibodies (type of protein) to inhibit and fight the action of invasive microorganisms e.g. harmful bacteria.

Some white blood cells produce antitoxins to counteract the effect of toxins produced by microorganisms.

When your body is subjected to an infection, your white cells multiply to fight it - giving you a high white blood cell count.

Measurement of your white blood cell count is an important diagnostic indicator of the state of your body's defences.

Having a low white blood cell count increases your risk of infection, but having a very high white blood cell count may mean you have an infection or even a more serious condition like a blood cancer e.g. leukaemia.

For more on fighting pathogens-infections see

Keeping healthy - communicable diseases - fighting pathogen infections


Platelets do not have a nucleus.

Blood platelets are ever present small fragments of cells that help to clot blood and cover over an open wound in tissues e.g. cut in the skin.

This reduces blood loss from bleeding AND prevents potentially harmful bacteria (microorganisms) getting into your body via the blood system.

If this didn't happen, blood would just keep pouring out of your body!

If your platelet concentration is low you may suffer from excessive bleeding (blood loss) and bruising and require medical attention.

The liver is one of the most important sites for the assimilation of amino acids to synthesise many different proteins including plasma proteins, e.g. fibrinogen protein is essential for blood clot formation, wound healing, and blood vessel growth in response to injury, infection and inflammation..


Blood plasma is the straw coloured (pale yellow liquid fluid that carries everything in the blood, so its the major transportation fluid in the body.

Plasma looks straw coloured when separated from the red blood cells.

The constituents of blood can be separated in a centrifuge - high speed rotation of a container (of liquid mixture), that separates substances out by sedimentation, with the most dense material  moving the most outwards in the radial direction.

About 55% of blood is plasma, which itself is 90% water.

Blood plasma carries everything else needed for every cell in the body:

(i) red cells (oxygen), white cells (and the antibodies and antitoxins they produce) and platelets,

(ii) soluble nutrients e.g. amino acid and sugar products from the digestive system - the soluble products of digestion from the gut,

(iii) waste products (carbon dioxide from the organs to the lungs, urea from the liver to the kidneys where it is removed in urine),

(iv) proteins for enzymes and tissue growth

(v) control system hormones ('chemical messengers') from the glands to their target and activate the functions of organs.

Summary of learning objectives and key words or phrases

  • Know what substances are in blood and their function including red blood cells, white blood cells, plasma and formation and function of platelets.

  • Know and understand that blood is a tissue and consists of a fluid called plasma in which red blood cells, white blood cells, and platelets are suspended.

  • Blood is considered an irregular tissue because it doesn't physically support or connect things but it does involve groups of cells performing particular functions.

  • Plasma is pale straw-coloured liquid which transports everything around the body

  • What is the function of blood plasma? .... What does it do? ...

  •  Know and understand that blood plasma transports:

  • carbon dioxide from the organs to the lungs,

  • soluble products of digestion from the small intestine to other organs eg glucose, amino acids, mineral salt ions etc.

  • urea from the liver to the kidney prior to excretion in urine,

  • hormones which control the function of various organs and their associated chemical processes in the body,

  • and obviously, as already mentioned, red blood cells, white blood cells and platelets are constantly being carried around the body as well associated oxygen as oxyhaemoglobin and antibodies and antitoxins produced by the white blood cells.

  • Know and understand that red blood cells transport oxygen from the lungs to the organs.

  • What do red blood cells do? What is their function? Know that red blood cells have no nucleus but a large surface area to chemically absorb oxygen.

  • Know that red blood cells are packed with a red pigment called haemoglobin which readily combines with oxygen

  • Know that in the lungs haemoglobin combines with oxygen to form oxyhaemoglobin.

  • Know that in other organs and all body tissue oxyhaemoglobin splits up into haemoglobin and oxygen for cell respiration.

  • Know that white blood cells have a nucleus. What do white blood cells do? What is their function?

  • Know that white blood cells form part of the body’s defence system against microorganisms eg harmful bacteria.

  • White blood cells can attack and destroy harmful 'foreign' microorganisms.

  • White blood cells can produce antibodies to fight microorganisms.

  • White blood cells produce antitoxins to combat the effect of waste toxins produced by microorganisms.

  • Know that platelets are small fragments of cells.

  • Know that platelets have no nucleus.

  • Know and understand that platelets help blood to clot at the site of a wound and a lack of platelets is potentially dangerous from excessive bleeding and bruising.

  • The clotting action of platelets allows a skin to form over a wound which hardens into a scab and this prevents infection of the wound by harmful bacteria or any other microorganism.



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