Kidney Excretion and Osmoregulation.

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  • Created on: 31-07-11 20:33

The kidney: excretion and osmoregulation
Kidneys have two main functions.
1. They are excretory organs, removing nitrogenous and other waste from the body.
2. They play an important part in maintaining a constant internal environment by helping to regulate pH, water and sodium ion
concentrations in the blood and tissues. This Factsheet will focus on the role of the kidney in excretion and osmoregulation.
Surplus nitrogen-containing compounds such as amino acids have to be
broken down in the body because they are toxic and are then excreted as
ammonia, urea or uric acid (Table 1).
Table 1. Nitrogenous excretory products
Collecting duct
Ascending limb
of loop of Henle
Descending limb
of loop of Henle
Figure 2. The kidney nephron
A- Ultrafiltration
B- Selective
C- Osmoregulation
Basic kidney structure
The basic structure of the mammalian kidney is shown in Figure 1.
Figure 1. Vertical section through mammalian kidney.
Excretory product Source
Urea Deamination of amino acids via the ornithine
cycle in the liver
Uric acid Deamination of purines (adenine and guanine)
Ammonia Deamination of amino acids. Ammonia is
secreted into the urine by cells in the kidney
Exam Hint - Don’t confuse urea and urine. Urea is made by the
deamination of amino acids in the liver. Urine is the fluid produced by
the kidneys.
Each kidney contains a million coiled tubes called nephrons and it is in the
nephron that urine formation occurs. Each nephron is divided into a number
of distinct regions with particular functions labelled A, B, C (Figure 2).
Protective capsule
of adipose tissue
(produces urine)
nephrons empty
to renal pelvis)
of nephrons and
collecting ducts
Renal vein
Renal artery
Renal pelvis
(cavity to
receive urine)
Ureter (to carry
urine to bladder)
Ammonia Urea Uric acid
Solubility Very high High Very low
Amount of water necessary
to remove from body
Very large Medium Very little
0 4 8
Some important biological properties of these three substances are
summarised in Table 2.
Table 2. Biological properties of excretory products.
From this you can work out that:
· Freshwater fish excrete ammonia. Although it is very poisonous, fish
are surrounded by large amounts of water so the ammonia can easily be
diluted to safe levels.
· Mammals excrete nitrogen mainly as urea. Urea requires more energy
in the form of ATP for its production but is much less toxic than
ammonia and fairly soluble. It therefore does not require large amounts
of water to remove it from the body.
· Birds excrete nitrogen mainly as uric acid. Flight demands a low body
mass. Removing nitrogenous waste as uric acid means that large amounts
of water are not required. Insects also excrete uric acid. As they are so
small, they are very prone to water loss so it is important that they do
not lose large amounts of water in excreting nitrogenous waste.
Molecules of ATP needed
High Medium…


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