Our sense organs 45

6 Transportation of hormones: The blood also
conveys the body’s own chemicals, which are collected
from their points of origin or their places
of storage. Various hormones (Greek horman =
drive, excite) are secreted by the endocrine
glands (Greek éndon = inside, krinein = separate,
excrete). Many vital processes are controlled by
special substances in the blood, known as hormones.
They are produced in certain tissues or
glands, taken up by the blood, and transported to
other specific tissues or organs, often located at
considerable distances. There they perform functions
vital to the organism, although they appear
only in minute quantities in the blood.
Some hormones are:
Adrenalin – secreted when the sympathetic nerves
are excited, causing blood vessels to contract.
Renin and angiotensin – control of blood pressure.
Histamine – plays an important role in antigen/
antibody reactions.
Somatotropin – growth hormone
Insulin – controls the processing of the glucose
absorbed in the intestine, as well as that synthesised
by the body itself.
Male and female sexual hormones.
Cortisone – controls (among other things) the
immune response.
Thyroid hormones – control heat and metabolism.
7 Defence: The leucocytes (white blood cells) and
certain chemicals (antibodies) protect the body
against toxins and intruding micro-organisms.
8 Coagulation: Blood coagulation affords a vitally
important protection against loss of blood; it
also repairs damaged blood vessels.
Obviously, the blood carries out vitally important
functions. It is the raison d’être as well as the
handmaiden of the circulatory system. It supplies
each and every cell with fuel (obtained from the
nutrients we ingest), with oxygen, with vitamins,
with hormones, and with warmth. Metabolic
products and excess heat are also carried away
from every cell. Blood flows ceaselessly throughout
our lifetime. It does not collect at specific
points, but flows in a never-ending loop through
the circulatory system. Filling and emptying itself
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every second with blood, the heart is the centre
of this system.
Composition of the blood: Blood plasma (the
liquid component) comprises 56 % of the blood,
and solid particles (the blood corpuscles) make
up the other 44 %. Three types of corpuscles can
be distinguished:
Red blood corpuscles
– Red blood cells (= erythrocytes; Greek erythrós
= red; kytos = hollowed, convex).
Quantity: 4.5 – 5 million per mm 3 blood
– White blood corpuscles (leucocytes; Greek
leukós = bright, shiny, white).
Quantity: 4,000 – 10,000 per mm 3 blood
Three types can be distinguished: lymphocytes
(30 %), granulocytes (66 %), and monocytes
(4 %)
– Platelets (thrombocytes; Greek thrómbos =
coagulated blood)
Quantity: 150,000 – 350,000 per mm 3 blood
Red blood corpuscles: Did you know that every
drop of blood contains 250 million red cells?
During its average lifespan of 120 days each of
these highly specialised cells performs an
extremely important task: it absorbs oxygen
175,000 times, and discharges carbon dioxide the
same number of times. Its diameter is close to
one thousandth of a millimetre. If all 25 million