Our sense organs 45

with a diameter of only 7 µm = 0.007 mm. The
tiny vessels in this clump (about 30 loops; see the
right hand part of the diagram on page 69) subsequently
combine to form the vas efferens
which carries the blood away. This is a marvellously
constructed system.
The tuft of blood vessels is enclosed in a doublewalled
container known as Bowman’s capsule,
whose diameter is only about 0.17 mm. The place
where the blood enters and leaves, is called the
vascular pole. The urinary pole, where a single
conduit emerges, is located at the other end of
1
Vas afferens, vas efferens (Latin vas = vessel;
affere = bring in; effere = take away): The vas afferens
leads from the interlobular arteries into the renal
glomerulus, and the vas efferens leads back out of
the glomerulus.
the capsule. The walls of the looped capillaries
act like a filter with extremely fine pores. These
allow water and other small molecules to be
squeezed out through them, but blood corpuscles
and large protein molecules cannot pass through.
The result of this ultra-filtration is that 180 litres
of “primary” urine is pressed from 1,700 litres of
blood during a 24 hour period, comprising a loss
of about 10 per cent by volume. But most of this
“loss” is recovered by means of the retro-absorption
taking place in the renal tubules, tiny tubes
which extend from the urinary poles of the capsules.
Water, glucose, and other substances are
recovered from the primary urine and returned to
the blood. A renal tubule starts with a convoluted
segment which joins directly to a straight seg-
Renal artery
Adrenal gland
Renal vein
Ureter
Aorta
Inferior vena cava
Renal cortex
Renal pyramid
Renal pelvis
68
Minor renal calyx
Renal capsule
Glomeruli in the cortex
Major renal calyx
Renal medulla