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GCSE Biology Revision Notes
B.5.Blood
BLOOD COMPOSITION AND FUNCTION
INTRODUCTION
Blood consists of several kinds of cells suspended in a liquid matrix called plasma. The
cellular elements (cells and cell fragments), occupy about 45% of the volume of blood.
THE 4 FUNCTIONS OF BLOOD
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Transport – delivering oxygen and nutrients (e.g. amino acids, glucose) to cells
around the body.
Defence – white blood cells form the major part of the immune system.
Hormone transport – hormones are transported around the body in the blood stream
where they can influence particular target cells (e.g. insulin affecting glucose stores
in the liver).
Waste disposal – waste products such as urea and carbon dioxide are transported in
the blood to excretory organs (the kidney for urea, the lungs for carbon dioxide).
PLASMA
Blood plasma is about 90% water. Among its many solutes are dissolved ions. The kidney
maintains ion concentrations at precise levels, an example of homeostasis.
Another important class of solutes are the plasma proteins. The various types of plasma
proteins have specific functions. Some, the antibodies, help combat viruses and other
foreign agents that invade the body. Others are involved with forming blood clots such as
fibrinogen.
Plasma also contains a wide variety of substances in transit from one part of the body to
another, including nutrients (glucose, amino acids), metabolic wastes (carbon dioxide, urea),
respiratory gases (oxygen) and hormones.
Q: Can you state the name of the fluid component of blood and explain its function?
CELLULAR COMPONENTS
Suspended in blood plasma are two classes of cells: red blood cells, which transport oxygen,
and white blood cells, which function in defence.
A third component, platelets, are fragments of cells that are involved in the clotting process.
Red Blood cells
Red blood cells (RBCs) are by far the most numerous blood cells. Their main function of O2
transport depends on rapid diffusion of O2 across their plasma membranes.
Human RBCs are small disks (about 7–8.5 μm in diameter) that are biconcave in shape – this
means they have a dimple on each side of the disk.
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GCSE Biology Revision Notes
B.5.Blood
The biconcave shape provides a large surface area, resulting in a rapid rate of diffusion of
oxygen into or out of the RBC.
RBCs lack nuclei, an unusual characteristic that leaves more space for haemoglobin, the
iron–containing protein that reversibly binds to oxygen.
Note that your text book says that they have “thin cell surface membranes” that increases
the rate of diffusion of gases; in fact all cells have membranes of the same thickness and so I
would advise against using this as an adaptation.
However, the disk-shape of the cell does mean that the whole cell is very thin, which
decreases the distance that oxygen has to diffuse to reach the haemoglobin in the centre of
the cell, increasing the rate of diffusion.
Q: Can you list the adaptations of red blood cells and explain how each feature enables
them to carry out their function?
White Blood Cells
The function of white blood cells is to fight infections. Although there are a variety of
different types of white blood cells, at GCSE the two you need to be familiar with are
phagocytes and lymphocytes.
Phagocytes engulf and digest bacteria and debris from the body′s own dead cells in a
process called phagocytosis.
Lymphocytes produce antibodies.
See separate notes on the Immune System for further details.
Platelets
Platelets are fragments of cells about 2–3 μm in diameter. They have no nuclei and originate
as pinched–off cytoplasmic fragments of large cells in the bone marrow. Platelets then enter
the blood and function in the important process of blood clotting.
See separate notes on the Immune System for further details on clotting.
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