Site author Richard Steane
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Blood consists of a liquid (55% by volume) and solid particles (45%), including several types of cells (also called "corpuscles" in older books).
The liquid is called > plasma . This is mainly water, but it has many things dissolved in it. These substances are either needed by cells of the body, or surplus to their requirements, but tissue fluid derived from blood either brings or takes them away, in solution. It also protects all the body's cells from damage due to osmosis.

List some types of substances that may be in solution in it.

> digested food
> hormones
> waste substances

For each type, you should be able to name a specific substance.

> glucose, amino-acids etc
> testosterone, insulin, ADH etc
> CO2, urea

Red blood cells

Mouseover the diagram of a red blood cell alongside to label its parts. erythrocyte

The main type of cells in blood are very numerous; so much so that they give the blood its colour, although they look rather pale when seen in a thin layer.

The diagram of a red cell is magnified about 6000 times (on the printed page - although it may depend on your system).

There are about 5 million red cells in 1 mm3 of blood - which about 50 times bigger than a single drop!
1000 mm3 make 1 cm3 (1 ml), & 1000 cm3 make 1 dm3 (1 litre).

There are, on average, 5.5 litres of blood in the human body.

Using this information, answer these questions:

What is the actual diameter of a red cell? > (46/6000mm) 0.008 mm. (=8Ám - micrometres)

How many red cells are there in your body (approximately)?

- This one may make you look in the manual of your calculator!

> 5 x 1000 x 5 x 1 000 000 = 2.5 x1013
25 000 000 000 000 {Calculator may say 2.5 [E] 13}
Compared with ordinary cells, red cells have something missing, and something that no other cells of the body possess.

What is missing >nucleus and what is unique to them? > haemoglobin

What is the main function of red cells?

>transport of oxygen

What can you find out from the colour of blood?

> whether it is oxygenated (bright red) or deoxygenated (dull red)

Because it consists of many similar cells, blood can also be called a >tissue.

While any nursing student should be able to answer these questions about human blood, a nurse practioner (NP) is able to provide a wide range of health care services. Any nurse who is interested in a graduate degree in advanced practice nursing can find more information from the many nurse practitioner programs online today.

White cells

(not called white blood cells, because they often leave the blood). There are not so many white cells as red cells; they are in fact about 600 times fewer. However they can leave the blood system and move towards micro-organisms (mostly bacteria and viruses) which are causing infections in any part of the body. There are several types of white cells, but they all share the function of fighting diseases, and we make use of them in vaccination to give us immunity against diseases.

This white cell is called a phagocyte, and its function is to engulf and destroy invading microorganisms. animation white blood cell This white cell is called a lymphocyte, and its function is to produce antibodies against invading microorganisms.

Lymphocyte List several ways that white cells can protect the body against these diseases.

> engulf bacteria

> make antibodies

> destroy toxins


These are not actually cells, but parts of cells; they have a role in starting the clotting of blood when a blood vessel is damaged. (see section below) This is usually a natural defence reaction when the body is wounded in some way, leading to a clot at the exposed surface of the body, which hardens into a scab, under which the process of healing may take place.

In other circumstances, however, a similar reaction may also occur, producing a circulating clot called a "thrombosis" which may be life-threatening in its own right if it blocks an important blood vessel inside the body (see later). This is the cause of heart attacks and strokes (prevention of blood flow in part of brain or heart). Some of these may be due to our lifestyle, including diet and stress. For example, in "hardening of the arteries", blood vessels are partly blocked by the buildup of cholesterol.

Blood as a transport system

The plasma carries several different substances in solution around the body.

Substance Origin Destination
carbon dioxide body lungs
digested food gut liver & rest of body
wastes (e.g. urea) liver kidneys
hormones endocrine glands "where needed" & rest of body
Plasma proteins
- normally remain in blood
  Outline of function
albumin liver keeping blood osmotic pressure and pH constant, and assisting in transport and release of other soluble components
fibrinogen liver complex reaction when wound occurs resulting in formation of insoluble fibrin, which is the basis of a clot which seals the wound
globulins (antibodies) lymphatic system when an infection occurs: stick to invading organisms and inactivate them

Red cells are specialised to carry one substance in particular, (actually not strictly in solution).

Substance Origin Destination
oxygen lungs rest/all of body

The clotting process

Blood is usually a liquid as it is being pumped around the body by the heart, but under some circumstances, it needs to solidify and form a clot. Clot The normal function of this clotting process is to prevent excessive loss of blood due to accidental damage to blood vessels. It is also important that microorganisms (bacteria, viruses etc) do not enter the body through the wound, so a temporary covering layer is required. This is achieved by a series of complicated chemical reactions, each dependent on the previous ones, which result in the insoluble protein fibrin being produced at the site of the wound.
Clot reactions
These fibres of fibrin then trap red blood cells and platelets, forming a blood clot, which dries and hardens into a scab beneath which the removal of damaged tissue and repair can occur. Eventually the edges of the wound are filled by replacement tissue and if this is visible, a scar may result.

Transport of gases by blood

Both oxygen and carbon dioxide dissolve to a limited extent in the water of the blood plasma, but there are special mechanisms which make the transport more efficient.

In the lungs, oxygen passes from the air which has been breathed in and into the liquid lining the alveoli, where it dissolves. This dissolved oxygen passes by diffusion through the wall of the alveoli and the surrounding capillary network, into the blood plasma. From here, oxygen passes into the cytoplasm of the red blood cells where it reacts with the red pigment haemoglobin.

the top reaction takes place in the lungs
oxygen + haemoglobin (reversible reaction) oxy-haemoglobin
the bottom reaction takes place in the tissues

Using this oxy-haemoglobin inside red blood cells, blood is thus able to carry much more oxygen than if it were simply dissolved in water.

The blood is pumped to other parts of the body, such as tissues like the brain and active muscle which need oxygen supplied to them for respiration. Here the oxy-haemoglobin is easily broken down, releasing oxygen again. This leaves the haemoglobin inside the red blood cells, ready to return to the lungs to collect more oxygen.

Carbon dioxide is carried in solution in the blood plasma, but in a slightly different way. First of all it dissolves in tissue fluid surrounding respiring tissue, but when it diffuses into the blood plasma it reacts with water to form carbonic acid which decomposes to form the hydrogencarbonate ion together with the hydrogen ion. The hydrogencarbonate ion also used to be called the bicarbonate ion.
carbon dioxide + water (reversible reaction) carbonic acid (reversible reaction) hydrogencarbonate ion + hydrogen ion

The chemical equation for this is:
CO2 + H2O (reversible reaction) H2CO3 (reversible reaction) HCO3- + H+

The hydrogen ion tends to make the blood more acidic, but this is partly neutralised by plasma proteins and haemoglobin.
Once again, the reaction is reversed in the lungs so carbon dioxide gas is released, to be excreted when breathing out.

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