TRANSPORT IN MAN
In order to carry out various metabolic processes such as growth, coordination and
excretion, living organisms require nutrients and oxygen which are taken in from
the surrounding environment BUT are required by ALL CELLS IN THE BODY. For this
reason, living organisms, as we saw in plants, must have a transport system that
distributes nutrients and oxygen within their bodies. The transport system also
helps to remove metabolic wastes from the cells through the excretory system.
It will also help to distribute heat to all parts of the body. This transport system
in man is also called the blood circulatory system and it is made up of three
components as below;
- Blood; it is the transporting fuid that fows inside blood vessels
- Heart; it pumps blood into the blood vessels
- Blood vessels; these are tubes such as arteries, capillaries and veins
through which blood fows.
1. BLOOD
Blood is a specialized tissue which consists of cells suspended in a fuid medium
called plasma.
Components of blood
Blood is made up of the solid and liquid components. The solid components are;
A. Red blood cells
B. White blood cells
C. Platelets
The liquid component of blood is called;
D. plasma
A. RED BLOOD CELLS
In humans red blood cells are small cells which are biconcave in shape and
lack nuclei when mature. Red blood cells are also called erythrocytes.
The main function of red blood cells is to transport oxygen within the body.
Red blood cells are formed in the bone marrow of short bones such as
vertebrae. In embryos, red blood cells are formed in the liver and spleen.
Characteristics of red blood cells
a) They are biconcave in shape. This increases the surface area for
transportation of oxygen
b) They are small in size. This, together with their biconcave shape makes it
possible for them to squeeze through the narrow capillaries
c) They are numerous in number per unit volume. There are about 5 million
per mm3 of blood. This increases the surface area for transportation of
blood
d) They lack a nucleus when mature. This creates further space for packing
oxygen, therefore increasing surface area for transportation of oxygen
e) Red blood cells are packed with haemoglobin; the red pigment that gives
blood its characteristic colour. Haemoglobin is the protein component that
carries oxygen. Lack of nuclei actually makes more space for haemoglobin
f) Red blood cells also lack mitochondria; as a result they respire
anaerobically. Hence they do not use up the oxygen they carry.
g) Red blood cells have a short life span of approximately 120 days. This is
because they lack nuclei to control repair processes. The worn out red
blood cells are destroyed in the liver.
B. WHITE BLOOD CELLS
White blood cells are also called leucocytes. These cells are formed in the
bone marrow of long bones such as humerus. Some are formed in the lymph
nodes, thymus and spleen. White blood cells do not have a defnite shape.
Their main function is to destroy microorganisms such as bacteria that invade
the body. For this reason, white blood cells help fght diseases within the
body and increase body immunity.
Characteristics of white blood cells
a) They are much larger in size and fewer in number compared to
erythrocytes. There are about 7 thousand white leucocytes per mm 3 of
blood.
b) They have no defnite shape and they are said to be amoeboid in shape
This enables them to squeeze through narrow capillaries reach tissues and
sites of infection. The amoeboid shape also enables them to engulf
bacteria and other particles; hence destroying them
c) The white blood cells have nuclei but they lack haemoglobin
Types of white blood cells
There are two main types of leucocytes depending on whether they have
granules in their cytoplasm or not. These are
Granulocytes
Agranulocytes
Granulocytes – They are also called phagocytes. They have
granules in their cytoplasm. They make up about 70% of all white
blood cells. They fght disease-causing bacteria by engulfng and
digesting them
Agranulocytes – They possess no granular cytoplasm. There are two
types of agranulocytes;
Monocytes
Lymphocytes
(i) Monocytes are produced in the bone marrow and, like
granulocytes; they also destroy microorganisms by
engulfng and digesting them.
(ii) Lymphocytes are produced in the thymus gland and in
lymph nodes. The lymph nodes protect the body against
disease-causing microorganisms. Lymphocytes also
produce anti-toxins that neutralize the toxins produced
by disease-causing microorganisms in the body
C. PLATELETES
These are also called thrombocytes. They are cell fragments of larger cells
that lack nuclei. They are produced from special bone marrow cells. They are
smaller than erythrocytes and are about 250 000 platelets per 1mm 3 of
blood.
The main function of platelets is to aid in blood clotting. This reduces
excessive loss of blood in the event of an injury.
Characteristics of platelets
a) They are small in size with no defnite shape
b) Their life span is between 5 and 9 days after which they are destroyed by
the liver and spleen
c) They contain protein compounds such as thromboplastin (also called
thrombikinase) that are involved in the blood clotting process.
D. PLASMA
a) Water This is the major component of plasma constituting up to 90% of
its total volume. Water transports dissolved substances within the body.
Water is also involved in the regulation of blood pressure and blood
volume.
b) Plasma proteins These include enzymes, hormones, antibodies and
blood clotting factors such as fbrinogen and prothrombin
c) Food substances These include mineral ions such as sodium (Na +)
Potassium (K+) and Calcium (Ca+). End products of digestion such as
sugars (eg glucose) amino acids, fatty acids and glycerol are also present.
d) Waste products These include urea, ammonia, used up hormones and
dissolved carbon dioxide. These products are transported in plasma to
excretory organs for removal
e) Oxygen Oxygen is transported by red blood cells. Oxygen dissolves in
plasma and is picked up by the haemoglobin of red blood cells which
transports it to the tissues.
Serum is the blood plasma from which blood cells and plasma proteins
such as fbrinogen have been removed
