Human Blood
The average person has around 5L of blood constantly circulating through their body to
provide nutrients, hormones and oxygen, as well as removing waste products, maintaining
osmotic balance, and much more (1). Around half of the blood is made up of a yellow-ish
liquid called plasma, with 3 main types of cells - erythrocytes, leucocytes and platelets -
suspended in it, and many dissolved nutrients, gasses etc.
Despite containing over 90% water, plasma contains many different proteins and nutrients; it
is made up of ~70% plasma proteins, created in the liver and used primarily for repair of
damaged tissues, membranes and cells. Globulins, also known as immunoglobulins or
antibodies, are a type of plasma proteins used in the body’s defence against foreign
proteins, by binding to antigens on the surface of pathogens. Fibrinogen is another, helping
the blood to clot in order to seal blood vessels. The remaining 2-3% of the plasma is made
up of other substances including electrolytes, hormones, oxygen and carbon dioxide, and
waste products such as urea.
Erythrocytes, commonly known as red blood cells, are one type of cells which are
suspended in the plasma. These cells are well adapted for their primary function of carrying
oxygen around the body. Erythrocytes have a biconcave shape which increases the surface
area to volume ratio, meaning there is more surface area for oxygen to bind to, allows
oxygen to diffuse in more quickly, as well as making the cell flexible so it can fit through
narrow capillaries with sharp bends. The unique morphology of the cells arises from the
collapse of young cells, known as reticulocytes. After the collapse, the nucleus dissolves to
increase surface area to volume ratio and make space for more haemoglobin. Haemoglobin
is the protein which allows oxygen to bind to the cell and gives the red colouration, too.
Thrombocytes, also known as platelets, are cell fragments which help the blood to clot.
These fragments are disc-shaped and do not contain a nucleus. They occur at a ratio of
roughly 1:20 with red blood cells. When stimulated, they produce sticky, finger-like
projections and release clotting factors, allowing them to clump together at bleeding sites to
form a plug to seal the vessel. They can also send chemical messages to other
thrombocytes to form protein fibres, to help with clotting. These cells do not undergo many of
the regular cell functions, for example they do not reproduce as they are produced only by
the bone marrow.
Leucocytes, or white blood cells, make up only around 1% of blood volume, but play a vital
role in protecting the body from pathogens and disease, and removing dead and damaged
tissues (2). Most do this by engulfing the pathogens or tissues, in a process known as
phagocytosis. Leucocytes can be divided into two groups - granular and agranular.
Granulocytes have a multilobed nucleus and irregular cell membrane which allows the cell to
be flexible and change shape easily, which is essential in carrying out phagocytosis. They
also contain lysosomes - enzymes which break down engulfed pathogens/tissues and
produce antibodies and antitoxins - in their cytoplasm. The most numerous type of white
blood cell is the neutrophil, a type of granular leucocyte responsible for killing bacteria and
fungi. Other granulocytes include basophils - which secrete chemicals such as histamine to
alert the immune system when they detect pathogens in the blood - and eosinophils, which
kill parasites and cancer cells. Agranulocytes do not contain lysosomes in their cytoplasm
and have a large, spherical nucleus. One example of these are monocytes, which have a
longer lifespan than the average <1 day, and help to break down bacteria. Furthermore,
lymphocytes are agranular and produce antibodies, or immunoglobulins, to fight pathogens
and protect the body from infection.
Blood Tests
, The blood can indicate so much about a person’s health, making blood tests a valuable
diagnostic tool for a range of diseases. When taking a blood sample for testing, it is
important to minimise the risk of infection or spreading blood-borne diseases such as HIV,
so the injections site is wiped with antiseptic to prevent pathogens on the skin from entering
the bloodstream, then a sterile, single-use needle is used to collect the sample and
immediately disposed of in a sharps bin to prevent diseases from being passed on. After the
needle is removed, pressure must be applied to the area while the vessel seals to prevent
blood from leaking into the muscle and under the skin, and then covered with a plaster to
protect against pathogens.
Complete Blood Count
Once the sample has been taken and sent to the laboratory, there are many different tests
which may be carried out. A complete blood count (CBC) detects the sizes and numbers of
blood cells such as white blood cells and platelets, as well as the percentage of red blood
cells in a given volume (haematocrit) and haemoglobin (the iron-containing protein which
allows oxygen to bind to red blood cells). This test is carried out using a Coulter Counter, a
machine which passes the blood through a tube only one cell in aperture, where an electrical
current is passed through the cells, disrupting the current and producing counts and sizes of
cells. If an abnormal result is measured, the biomedical scientists will then need to examine
the blood manually. There are many different disorders which can be identified using a CBC,
making it a versatile test. Furthermore, it is mostly done by machines, making it relatively
cheap and efficient. The normal reference ranges for adults are shown in table 1 below.
Table 1: Normal CBC Reference Ranges for Adults (10)
Component Reference Range
Haemoglobin Males: 13.0 - 17.0 g/dL
Females: 11.5 - 15.5 g/dL
Haematocrit Males: 40 - 55%
Females: 36 - 48%
Platelet Count 150,000 - 400,000/mL
White Blood Cell Count 5,000 - 10,000/mL
Packed Cell Volume
Packed cell volume is a test to measure what percentage of the blood is made up of cells. A
normal result of around 45% means that there are 45mL of cells in 100mL of blood. A higher
than normal result indicates that the total blood volume is reduced, for example due to
dehydration. A lower than normal result, on the other hand, indicates anaemia, due to
decreased production or increased destruction of red blood cells, this can be due to many
disorders, for example Hodkin’s lymphoma. The test is carried out using a centrifuge to
separate the blood cells and plasma, and then the levels of each layer are measured - this is
completed using machinery and does not require scientists, making it a relatively
The average person has around 5L of blood constantly circulating through their body to
provide nutrients, hormones and oxygen, as well as removing waste products, maintaining
osmotic balance, and much more (1). Around half of the blood is made up of a yellow-ish
liquid called plasma, with 3 main types of cells - erythrocytes, leucocytes and platelets -
suspended in it, and many dissolved nutrients, gasses etc.
Despite containing over 90% water, plasma contains many different proteins and nutrients; it
is made up of ~70% plasma proteins, created in the liver and used primarily for repair of
damaged tissues, membranes and cells. Globulins, also known as immunoglobulins or
antibodies, are a type of plasma proteins used in the body’s defence against foreign
proteins, by binding to antigens on the surface of pathogens. Fibrinogen is another, helping
the blood to clot in order to seal blood vessels. The remaining 2-3% of the plasma is made
up of other substances including electrolytes, hormones, oxygen and carbon dioxide, and
waste products such as urea.
Erythrocytes, commonly known as red blood cells, are one type of cells which are
suspended in the plasma. These cells are well adapted for their primary function of carrying
oxygen around the body. Erythrocytes have a biconcave shape which increases the surface
area to volume ratio, meaning there is more surface area for oxygen to bind to, allows
oxygen to diffuse in more quickly, as well as making the cell flexible so it can fit through
narrow capillaries with sharp bends. The unique morphology of the cells arises from the
collapse of young cells, known as reticulocytes. After the collapse, the nucleus dissolves to
increase surface area to volume ratio and make space for more haemoglobin. Haemoglobin
is the protein which allows oxygen to bind to the cell and gives the red colouration, too.
Thrombocytes, also known as platelets, are cell fragments which help the blood to clot.
These fragments are disc-shaped and do not contain a nucleus. They occur at a ratio of
roughly 1:20 with red blood cells. When stimulated, they produce sticky, finger-like
projections and release clotting factors, allowing them to clump together at bleeding sites to
form a plug to seal the vessel. They can also send chemical messages to other
thrombocytes to form protein fibres, to help with clotting. These cells do not undergo many of
the regular cell functions, for example they do not reproduce as they are produced only by
the bone marrow.
Leucocytes, or white blood cells, make up only around 1% of blood volume, but play a vital
role in protecting the body from pathogens and disease, and removing dead and damaged
tissues (2). Most do this by engulfing the pathogens or tissues, in a process known as
phagocytosis. Leucocytes can be divided into two groups - granular and agranular.
Granulocytes have a multilobed nucleus and irregular cell membrane which allows the cell to
be flexible and change shape easily, which is essential in carrying out phagocytosis. They
also contain lysosomes - enzymes which break down engulfed pathogens/tissues and
produce antibodies and antitoxins - in their cytoplasm. The most numerous type of white
blood cell is the neutrophil, a type of granular leucocyte responsible for killing bacteria and
fungi. Other granulocytes include basophils - which secrete chemicals such as histamine to
alert the immune system when they detect pathogens in the blood - and eosinophils, which
kill parasites and cancer cells. Agranulocytes do not contain lysosomes in their cytoplasm
and have a large, spherical nucleus. One example of these are monocytes, which have a
longer lifespan than the average <1 day, and help to break down bacteria. Furthermore,
lymphocytes are agranular and produce antibodies, or immunoglobulins, to fight pathogens
and protect the body from infection.
Blood Tests
, The blood can indicate so much about a person’s health, making blood tests a valuable
diagnostic tool for a range of diseases. When taking a blood sample for testing, it is
important to minimise the risk of infection or spreading blood-borne diseases such as HIV,
so the injections site is wiped with antiseptic to prevent pathogens on the skin from entering
the bloodstream, then a sterile, single-use needle is used to collect the sample and
immediately disposed of in a sharps bin to prevent diseases from being passed on. After the
needle is removed, pressure must be applied to the area while the vessel seals to prevent
blood from leaking into the muscle and under the skin, and then covered with a plaster to
protect against pathogens.
Complete Blood Count
Once the sample has been taken and sent to the laboratory, there are many different tests
which may be carried out. A complete blood count (CBC) detects the sizes and numbers of
blood cells such as white blood cells and platelets, as well as the percentage of red blood
cells in a given volume (haematocrit) and haemoglobin (the iron-containing protein which
allows oxygen to bind to red blood cells). This test is carried out using a Coulter Counter, a
machine which passes the blood through a tube only one cell in aperture, where an electrical
current is passed through the cells, disrupting the current and producing counts and sizes of
cells. If an abnormal result is measured, the biomedical scientists will then need to examine
the blood manually. There are many different disorders which can be identified using a CBC,
making it a versatile test. Furthermore, it is mostly done by machines, making it relatively
cheap and efficient. The normal reference ranges for adults are shown in table 1 below.
Table 1: Normal CBC Reference Ranges for Adults (10)
Component Reference Range
Haemoglobin Males: 13.0 - 17.0 g/dL
Females: 11.5 - 15.5 g/dL
Haematocrit Males: 40 - 55%
Females: 36 - 48%
Platelet Count 150,000 - 400,000/mL
White Blood Cell Count 5,000 - 10,000/mL
Packed Cell Volume
Packed cell volume is a test to measure what percentage of the blood is made up of cells. A
normal result of around 45% means that there are 45mL of cells in 100mL of blood. A higher
than normal result indicates that the total blood volume is reduced, for example due to
dehydration. A lower than normal result, on the other hand, indicates anaemia, due to
decreased production or increased destruction of red blood cells, this can be due to many
disorders, for example Hodkin’s lymphoma. The test is carried out using a centrifuge to
separate the blood cells and plasma, and then the levels of each layer are measured - this is
completed using machinery and does not require scientists, making it a relatively
