Perhaps one of the most evocative signs of sudden ill health or damage to the human body is the sight of red streams flowing from a fresh skin cut. Yet this very action, as we shall learn in a moment is just part of the body's many defence mechanisms. In this article, we shall look at key aspects of normal healthy human blood, and with the aid of small images, learn how to identify a few of the more serious diseases of the blood system.
Although a serious topic of study, this short article is intended for both students of Biology and general public interest. It is designed to be fairly light and easy to read without the reader requiring previous in-depth knowledge of Biology.
Blood consists mostly of red blood cells with a smaller number
of white blood cells. Several different
staining methods are used to make
the cells more readily seen under an optical microscope. In Fig.1x
you can see a sample of human blood cells. These have been
stained to make red blood cells appear as pink or red, and white
blood cells appear as dark blue or purple. A quick glance at
Fig.1x should demonstrate that all the cells visible are in fact
red blood cells. It should be remembered that these colours have
been produced artificially.
A red blood cell has no nucleus. In healthy blood, cells will appear as regular shaped biconcave disks. However, at lower powers of magnification they will appear as small red discs with most cells a lighter colour towards there centres; an area where each cell is less dense due its biconcave structure. It is difficult to see their solid shapes even at high powers in an optical microscope. A Scanning Electron Microscope (SEM) can greatly increase magnification and show the true structure of individual red blood cells. At 5000x magnification, they look like small round cushions or pillows - the sort that you may have on your chairs or sofa at home. Red blood cells are called erythrocytes.
Size, functionality, and other properties
An average red blood cell is typically 7-8 microns in diameter,
which interestingly - is about the size of the internal diameter
of human blood capillaries. Fortunately, the red cell membrane
(thin outer skin of a cell) is very pliable allowing each cell to
squeeze through blood capillaries with smaller diameters than the
cell itself. In a healthy adult each red blood cell has a life
span of about 12 weeks after which time it is destroyed either by
the spleen or the liver where some of the constituent parts are
recovered and recycled while useless parts are ultimately
excreted. The rate of destruction is quite incredible with
between 2-10 million individual cells being destroyed and
replaced every second. The reason why this range is so wide (2 -
10 million) is due to the variable conditions where humans
breathe: the lower amount of oxygen in the atmosphere, the
greater the rate of red cell production. By the same process, if
oxygen content in the atmosphere is high, then the production of
cells is reduced.
There are approximately 5 million red cells per cubic millimetre of blood in an average adult. Red blood cells are responsible for the transportation of oxygen to areas in the body with low concentration of this life-giving gas. This is accomplished by the presence of haemoglobin in the cell which combines with oxygen to form oxyhaemoglobin when passing through oxygen-concentrated regions. This reversible process results with oxygen being released when the cell is in an area with low oxygen concentration.
Carbon monoxide poisoning
The red cell haemogoblin can combine with oxygen as we have seen
above. However, the ferrous irons in haemoglobin are attracted,
and more readily combine with, gas molecules of carbon monoxide
(by several hundred times) than with those of oxygen. Should this
occur, the red blood cell is incapable of transporting oxygen,
effectively disabling the oxygen transport mechanism if
sufficient cells come into contact with carbon-monoxide rich
regions. Needless to say - the lungs represent the major site
where red blood cells will be exposed to gases which they may
combine with. When a person is in a carbon monoxide enriched
environment, the human lungs will inhale a higher content of this
gas; which will be subsequently taken-up by the red blood cells.
Carbon monoxide is one of the gases discharged from the exhaust system of petrol-fuelled motor cars!
Note: In the following images 1a to 1d, red blood cells may appear as coloured white or light grey if you are using a web-browser in 256 colour mode only. This article has been so formatted as to appear colour-correct when the images are viewed as 24 bit files (64000 colour mode in Windows).
In between the red cells are small irregularly shaped cell
fragments enclosed within their own membrane. These are
called 
platelets (stained
purple in Fig.1b). They are fragments of cytoplasm derived
from larger cells (megakaryocytes) found in bone marrow. There
are approx. 250,000 platelets per cubic millimetre of blood.
Their function is involved with initiating the blood-clotting
mechanism - a highly sophisticated and complex process involving
at least 12 clotting factors. Such a complex mechanism which
proves so effective in preventing blood loss, and reducing
infection, is also involved in preventing clotting in
undamaged blood vessels. The blood-clotting process is too
complex to cover within the scope of this article.
Several different types of white blood cells can be found between the red cells and platelets. In the following images these have all been stained artificially dark blue or purple. They are larger than erythrocytes (red blood cells) but populate the blood system in much less denser quantities, typically 7000 cells per cubic millimetre of blood. Their function is to help protect the body from disease and infection. Unlike red cells, they are all nucleated (contain nuclei), yet despite this, live for only a few days in the bloodstream. White cells can be classified as belonging to 2 main groups - agranulocytes and the granulocytes. Several types from these two groups are described below.
Granulocytes: Polymorphonuclear leucocytes
These cells appear as twisted-shaped purple masses (artificially
stained) and are aptly named Polymorphonuclear cells, which 
literally means 'many-shaped nucleus'. Each
cell contains granular cytoplasm and a lobed nucleus; although
basophils may not all have lobed nuclei! Granulocytes (Fig.1a)
are capable of amoeboid movement. They make up around 72% of of
the total white cell count and consist of 3 main types:
neutrophils - constituting 70% of white cell count, eosinophils -
1.5%, and basophils - 0.5% of white cells. Neutrophils
move to infected parts of the body to engulf and consume invading
bacteria. Eosinophils are thought to have anti-histamine
properties and their population in the bloodstream is governed by
hormones produced by the adrenal cortex. They also are able to
ingest bacteria but normally do so at a lesser extent than
neutrophils. Basophils contain heparin, histamine and
serotinin. Where they have not been discharged from the cell,
small granules are usually visible around the nucleus.
Agranulocytes: mononuclear leucocytes
Two main types exist: monocytes and lymphocytes. They constitute
28% of the total white cell count in proportions of 4% and 24%
respectively. 
The cells are
mononuclear with either a kidney bean-shaped nucleus as in
monocytes (Fig.1d), or a round nucleus -
lymphocytes. 
Monocytes Fig.1c
often behave in a similar way to neutrophils by travelling to
inflamed areas of the body to ingest bacteria and larger foreign
particles. Lymphocytes are of special significance as they are
involved with the body's immune defence mechanism and either help
mediate or initiate antibody production. They are also part of
the body's own tumour-cell killing system; they can be very
long-lived - surviving up to 10 years in humans. Both types of
agranulocytes have non-granular cytoplasm.
Both red blood cells (erythrocytes) and white blood cells
(leucocytes) are manufactured in bone marrow. Fig.2 shows
a tiny section from a human rib. The hard bone is stained blue
and 
the bone marrow,
consisting of blood cells, is stained reddish orange. Although
difficult to identify at this low level of magnification, both
red and while blood cells are being created along with platelets,
and are in various stages of development. It should be noted that
lymphocytes, the white cells involved with antibody production,
may be manufactured in lymphoid tissue and the spleen as well as
in bone marrow, whereas all the other red cells, white cells, and
platelets are made in the bone marrow only.
Blood obviously plays a vital role in maintaining good health and life itself in humans. When such a system malfunctions or is invaded by disease, serious consequences will ensue. Sometime in the future, we will examine a few of the things that can affect the human bloodstream and damage healthy blood cells: blood cancer, sleeping sickness, and malaria.
An Advanced Atlas of Histology by W.H. Freeman and Brian
Bracegirdle
Published by Heinemann Educational Books Ltd.
Halley Court, Jordan Hill, Oxford OX2 8EJ. England. (First
published 1976, reprinted 1979, 1980, 1985, 1990).
ISBN 0 435 60317 5
Highly recommended for pictorial content.
Biological Science 1&2 (2nd Edition) by N.P.O. Green, G.W.
Stout, D.J. Taylor. Editor: R.Soper
Published by the Press Syndicate of the University of Cambridge
1984, 1990
The Pitt Building, Trumptington Street, Cambridge CB2 1RP.
England.
ISBN 0 521 38380 3
Highly recommended for all Biology Students for both editorial
and image content.
Tissue Structures by Alan Potter: Highly recommended
concise introduction to human tissue for school students.
*Video* (c) Brunel
Microscopes Ltd. 05/93
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