Page 102 - Science Class 07

Your blood travels through these blood vessels WBC
transpor ng oxygen, carbon dioxide, digested
food, hormones and even waste products. It is
amazing to see how transporta on in human
beings is carried out by the circulatory system,
with the heart and the vast network of blood
vessels.
BLOOD
Blood is red s ckle fluid. On average, an adult
man has about 5 litres of blood in his body.
Composi on of Blood
It is composed of a fluid por on plasma (50%
of the volume) and three types of corpuscles
(45-50% of the blood volume) namely RBC,
WBC and blood platelets.
Plasma
It mainly consists of light yellow fluid with 90-
92% of water, 6-9% of proteins and 1% of
dissolved substances. Plasma acts in blood
clo ng and in defense of the body against
diseases.
RBC
Red blood cells are also called erythrocytes.
These are disc-shaped cells produced in the
bone marrow. Red blood cells have no nucleus,
and their cytoplasm is filled with hemoglobin.
Haemoglobin : It combines with oxygen and
forms a compound called oxyhaemoglobin.
When oxyhaemoglobin reaches the cells and
ssues, it converts again into oxygen and
haemoglobin. The cells used this oxygen for
oxida on of food and haemoglobin becomes
free to carry more oxygen.
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White blood cells are referred to as leukocytes.
They are generally larger than red blood cells
and have clearly defined nuclei. They are also
produced in the bone marrow and have various
func ons in the body. Certain white blood cells
called lymphocytes are essen al components
of the immune system.
Platelets
These are small spherical bodies numbering
3,00,000 per cubic millimetre of blood. They
help in the forma on of a solid plug called clot
at the site of injury and prevent further loss of
blood.
When a man gets dengue fever the number
of platelets rapidly decreases from his
blood. Thus, some mes it becomes fatal. So,
the pa ent of dengue should be immedia-
tely admi ed in hospitals so that the
doctors could inject more blood in his body
and can give further treatment.
1. What is circulatory system?
2. Why is circulatory system so important?
3. What is blood?
4. Write the cons tuent of RBC.
5. What are platelets?
HEART—THE PUMPING STATION
The heart is a muscular organ located in the
chest cavity, between the lungs. It is posi oned
slightly towards the le in the thoracic region
and is enveloped by the pericardium. The
human heart is divided into four chambers –
Science-7