Our sense organs 45

The blood
– a universal transport
medium
The blood (Latin sanguis, Greek haima) is the
great transport medium of the body. While circulating,
this liquid 1 performs several important
functions:
1 Respiration: The blood carries oxygen from the
lungs to all body tissues for them to utilise. At
the same time carbon dioxide is carried from the
tissues back to the lungs where it is exhaled. This
vital function is mainly performed by the red
blood cells.
2 Nourishment: The cells of the body continually
require energy and raw materials, and the blood
plays an all-important part by transporting nutrients
to the individual cells of the body. The products
resulting from the digestion of food are
absorbed by the blood from the gut (mainly the
small intestine). The length of the small intestine
is between five and seven metres, and its total
surface area exceeds that of the lungs. If one
were to iron out all its protuberances, its area
would be larger than a tennis court. Nutrients,
dissolved in water, are collected by the blood
from the small intestine and carried to the liver
along the portal vein (vena portae). Consisting of
a patchwork of about 100,000 hexagonal pieces
called lobules (hepatic lobule, Greek hepar =
1 Measures of volume: Many types of volume are
mentioned in this chapter, and the following conversion
table may be useful in this respect:
1 cubic metre = 1 m 3 = 1000 dm 3 = 1000 l
1 cubic decimetre = 1 dm 3 = 1 litre
1 cubic centimetre = 1 cm 3 = 1000 mm 3 = 1 millilitre
1 cubic millimetre = 1 mm 3 = 10 9 µm 3
1 cubic micrometre = 1 µm 3 = 1 thousand millionth
of a mm 3
1 litre = 1 l = 1 dm 3 = 1000 cm 3 = 1000 ml = 100 cl
1 centilitre = 1 cl = 10 ml
1 millilitre = 1 ml = 1 cm 3 = 1000 µl
1 microlitre = 1 µl = 1 mm 3
liver), the liver is the largest internal organ. The
basic metabolism of the body takes place in the
liver. Employing unique and wonderful processes,
the cells of the lobules chemically convert sugar,
fats, proteins, and other nutrients. The products
are either stored, re-used, or released. About
30 per cent of the blood leaving the heart passes
through the liver, and from there the nutrients
are distributed throughout the body.
The blood plays another important role in nutrition:
Superfluous nutrients are stored in certain
“depots” and transported to active cells when
required.
3 Excretory function: The end products of cellular
metabolism diffuse into the interstitial spaces
between the cells and from there to the blood.
These chemicals are then filtered out by the
excretory organs, mainly the kidneys. (Excretion =
the removal or elimination of metabolic products
(excreta) which cannot be used by the body, like
urine and sweat; in contrast, the process of secretion
produces substances which the body needs.)
4 The control of concentrations: The cells of the
body can only function optimally when the prevailing
internal conditions are within certain limits
(eg. the concentration of dissolved substances
should be constant, as well as the temperature
and the acid-base ratio). Certain organs continually
monitor these blood values and correct them
when necessary. The composition of the intercellular
liquids is kept in equilibrium by interchanges
with the blood.
5 Temperature control: At the normal temperature
of the body, 37 °C, all processes function
optimally. The organs generate heat, but the
main component of the blood, water, has a high
specific heat value, so that the heat capacity of
blood is also high. Furthermore, since the blood
flows continually, the transfer of heat is much
faster than in the case of a non-moving liquid.
On the one hand, the blood absorbs any excess
heat, which is then dissipated over the surface
of the body and through the lungs. On the other
hand, circulation of the blood ensures that all
parts of the body receive the necessary heat.
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