Structure and Function of Red Blood Cells and White Blood Cells

Bloods importance to human life has been recognised since pre-history,
acquiring mystic and religious significance through the ages.
Biologically, blood is a liquid tissue, transporting materials and
protecting us against disease. Suspended in the watery plasma are red
and white blood cells, serving different functions, but both of vital
importance to our bodies.

Red Blood Cells (RBC's or erythrocytes)

RBC's are responsible for carrying oxygen (O2) and removing carbon
dioxide (CO2) from about 30 trillion cells in the human body. In
normal blood RBC's account for about 45% of the total volume. On
average, we have about 5 million red cells per cubic millimetre of
blood.

[IMAGE]Produced in the red marrow of bones, RBC's arise from a single
type of cell, called a stem cell. During formation the nucleus is lost
and organelles degraded, allowing more internal space to be filled
with haemoglobin, the O2 carrying protein abundant in red cells.
Without a nucleus RBC's can never divide. Lacking in organelles, they
can only survive for about 120 days, before being 'eaten' by white
cells. Some 3 million RBC's die and are scavenged by the liver every
second.



Structure
=========

RBC's are biconcave shaped discs (as seen in this electron
micrograph), about 7.5µm diameter. The shape gives an increased
surface area, allowing the cell to contain more haemoglobin. The small
size and large surface area both increase the rate of diffusion of O2
and CO2.

The reddish colour of RBC's is derived from haemoglobin. The protein
globin (containing 4 polypeptide chains) combines with haem, which
contains an iron atom. One O2 molecule binds to each iron atom.
Therefore, each haemoglobin molecule can bind 4 O2 molecules.

[IMAGE]

Cell shape is maintained by a protein cytoskeleton. RBC's are very
flexible and can squeeze through minute gaps, such as capillary walls.
This can be seen in this electron micrograph.



Function
========

Haemoglobin in RBC's carries O2 from the lungs and transports it to
cells throughout the body. In a reversible process haemoglobin binds O2
in the lungs to form oxyhaemoglobin, which is taken to the cells where
it gives up it's O2.

Haemoglobin also carries CO2 from the cells, which is returned to the
lungs. 95% of CO2 generated is carried by RBC's (containing the enzyme
carbonic anhydrase to speed up the process). About 5% of CO2 is
dissolved in blood plasma. (A good thing too: if all CO2 were carried
this way blood pH would drop to an instantly fatal 4.5!).

It is essential that blood pH is maintained, and haemoglobin acts as a
powerful buffer in maintaining a pH of about 7.4.

White Blood Cells (WBC's or leucocytes)

Making up less than 1% of the total blood volume, WBC's like red cells
are mostly formed from stem cells in the bone marrow. They have a
defensive role in destroying invading organisms, and also assist the
removal of dead or damaged tissue cells.

White cells consist of lymphocytes and monocytes, with relatively
clear cytoplasm, and 3 types of granulocyte - neutrophils, eosinophils
and basophils, whose cytoplasm is filled with granules. Unlike RBC's,
WBC's contain nuclei, they are also much larger and colourless. They
use the blood to reach tissues and cells, where they migrate to
perform specific functions.


Granulocytes - Structure & Function
-----------------------------------

Neutrophils are the most abundant, forming 40-75% of WBC's. They have
segmented lobed nuclei. Their cytoplasm is granular, which are
responsible for the cells functions.

Neutrophils squeeze through capillary walls and into infected tissue
where they engulf invaders e.g. bacteria, by a process known as
phagocytosis (cell-eating).

[IMAGE]

Eosinophils (approx. 5% of WBC's) have a bi-lobed nucleus. They
increase in the presence of parasitic infection, and allergic states
e.g. asthma and hayfever. It is thought they neutralise the effect of
histamine.

Basophils(approx. 0.5% of WBC's) are the least common WBC, they
respond to various allergens, releasing their granular contents
containing histamine and other agents. This action can result in
hayfever, nettle rash, or most seriously anaphylactic shock.

[IMAGE]Lymphocytes and Monocytes (sometimes called agranulocytes) have
a mainly clear cytoplasm, and their nuclei are not multilobular.

Monocytesare large WBC's (12-20 µm) and account for 1-5% of total
white cells. They have a U-shaped nucleus, and their cytoplasm
contains lysosomal granules. Monocytes leave the blood and become
tissue macrophages, which remove dead and damaged cell debris, as well
as antigens, which enter the body; these include organisms, which
cause TB.

[IMAGE]Lymphocytes (app. 20-50% of WBC's) have a spherical nucleus and
shape, with little cytoplasm. They have a diameter of 6-18 µm, and
vary greatly in life span. They are the most numerous WBC in young
children.

The two major types of lymphocyte found in the blood are "B cells" and
"T cells". Both B and T cells are commonly found in the lymph nodes
and spleen. B cells are responsible for making antibodies. T cells
recruit other WBC's to sites of infection/tissue damage. They also
kill 'virus infected' cells and enhance the production of antibodies
by B cells.

The human immunodeficiency virus (HIV) binds to CD4 molecules on T
cells, invading and infecting these cells. Other T cells destroy these
infected cells, reducing their numbers. This makes the person
susceptible to bacterial and viral infections, as well as certain
cancers. Without modern drug treatment to support the body's immune
system, many patients die of advanced stage HIV (or AIDS), due to the
body's inability to fight infection.


Bibliography
------------

Bevan J. (1978) Anatomy and Physiology MITCHEL BEASLEY PUBLISHERS LTD

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