Our sense organs 45

The kidneys
– marvels of filtration
The human kidney, weighing between 120 and
160 g, is a complex physico-chemical factory
which is essential for purifying the blood (Diagram
p 68). The two kidneys are located on either
side of the backbone in the region of the loins.
They control the fluid balance of the body and
keep constant the composition and volume of the
extra-cellular liquid in which all the cells of the
body are bathed. In this way the functioning of all
the cells of the body is optimised. When there is
too much liquid or too much of dissolved substances,
the kidneys ensure that excesses are eliminated.
If there is a shortage of water, the kidneys
reduce the excretion of water without affecting
the essential elimination of metabolic end-products.
All these processes are carried out in such
a way that the kidneys control the water and salt
content of the blood and excrete the waste products
of protein metabolism (urea) and purine
metabolism (uric acid), as well as any toxins.
For this purpose, blood flows through the kidneys
at an astounding rate – 1.2 litres of blood each
minute, which is four times their own weight. The
specific perfusion of an organ is a term indicating
the amount of blood flowing through it per
minute in proportion to its weight. This amounts
to (1,200 cm 3 blood/min)/(280 g kidney weight) =
4.3 cm 3 /(g x min). This is appreciably higher than
for the other major organs like the brain, liver, or
heart muscle. Every day 1,700 litres of blood flow
through the kidneys. This is more than twenty
times the entire body weight and 340 times the
total amount of blood in the body, which is about
5 litres. The quantity of blood passing through
the kidneys in one hour is 15 times the entire volume
of the blood in the circulatory system.
There are about 2.5 million renal glomeruli (Latin
singular glomerulus = the diminutive of glomus
= cluster; plural: glomeruli) which, together with
a similar quantity of renal tubules (having a total
length of nearly 100 km), contribute to a cleverly
designed physical filtering process, actually
a molecular sieve.
The volume of liquid filtered by the glomeruli per
unit of time provides a measure of the excretory
capacity of the kidneys. This is called the glomerular
filtration rate (GFR) which is about 120 cm 3
per minute. This means that approximately 180
litres, which is 60 times as much as the volume of
all the blood plasma (the liquid part of the blood,
being about three litres), are filtered and purified
every day. The total volume of extra-cellular fluid
in the body (including blood) is about 14 litres, so
even this amount passes 13 times per day
through the kidney filter. The renal glomeruli thus
excrete 180 litres of ultra-filtrate (primary urine)
per day. This high volume is required to properly
excrete metabolic products through the capillary
walls. But if this enormous quantity of urine were
passed directly, the water loss would be horrendous.
This volume of urine would fill 18 buckets
a day, and we would never be able to leave the
toilet. What’s more, we would have to drink a
similar quantity of water each day.
However, the all-wise Creator set up an ingenious
principle whereby more than 99 per cent of
the water and the major portion of other vital
substances are returned to the body. As the filtrate
travels further along the renal tubules (diagram
page 69), water, glucose, and sodium chloride
are recovered and returned to the blood. The
recovery ratio is about 100:1, so that only
between 1 and 1.6 litres of urine are eliminated,
depending on the quantity of ingested liquid and
on other water losses (e. g. sweat).
In cross-section (diagram p 69), the kidney displays
an outer granular cortex and an inner, radially
striped medullary layer. The nephron (Greek
nephron = kidney) is the smallest functional
renal unit. It comprises the glomerulus, together
with the blood-transporting arterioles, the renal
tubule, and the papillary duct (left part of the
diagram on page 68). Granular in appearance, the
renal glomeruli are an extremely well-designed
apparatus. The vas afferens 1 (diameter between
20 and 50 µm), conveying the incoming blood,
branches into a bundle of very fine capillaries
67