F214 The kidney

F214 The kidney excretion and osmoregulation

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B io Factsheet
September 1997 Number 1
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.
Excretion Basic kidney structure
Surplus nitrogen-containing compounds such as amino acids have to be The basic structure of the mammalian kidney is shown in Figure 1.
broken down in the body because they are toxic and are then excreted as
ammonia, urea or uric acid (Table 1). Figure 1. Vertical section through mammalian kidney.
Table 1. Nitrogenous excretory products
Cortex Protective capsule
Excretory product Source Medulla of adipose tissue
Urea Deamination of amino acids via the ornithine
cycle in the liver
Uric acid Deamination of purines (adenine and guanine) Nephron
(produces urine) Renal vein
Ammonia Deamination of amino acids. Ammonia is Renal artery
secreted into the urine by cells in the kidney Pyramid
(where Renal pelvis
tubule
nephrons empty (cavity to
to renal pelvis) receive urine)
Exam Hint - Don't confuse urea and urine. Urea is made by the Positions
deamination of amino acids in the liver. Urine is the fluid produced by of nephrons and
the kidneys. collecting ducts Ureter (to carry
urine to bladder)
Some important biological properties of these three substances are
summarised in Table 2.
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
Table 2. Biological properties of excretory products.
of distinct regions with particular functions labelled A, B, C (Figure 2).
Ammonia Urea Uric acid Figure 2. The kidney nephron
Solubility Very high High Very low
Amount of water necessary Very large Medium Very little
to remove from body Renal A
Toxicity corpuscle Distal
High Medium Low
convoluted
Molecules of ATP needed 0 4 8 B tubule
Proximal
convoluted
From this you can work out that:
tubule
· Freshwater fish excrete ammonia. Although it is very poisonous, fish C
are surrounded by large amounts of water so the ammonia can easily be Collecting duct
diluted to safe levels.
A- Ultrafiltration Ascending limb
· Mammals excrete nitrogen mainly as urea. Urea requires more energy
B- Selective of loop of Henle
in the form of ATP for its production but is much less toxic than
reabsorption
ammonia and fairly soluble. It therefore does not require large amounts Descending limb
C- Osmoregulation
of water to remove it from the body. of loop of Henle
· 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.
1

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The kidney: excretion and osmoregulation Bio Factsheet
A. Ultrafiltration (Figure 3).
The Bowman's capsule contains a dense capillary network called the Typical exam question
Ultrafiltration often comes up in application questions, i.e. those
glomerulus. Blood flows into these capillaries through a wide afferent
questions which test a candidate's ability to apply factual
arteiole and leaves through a narrower efferent arteriole. The blood pressure knowledge to new situations.…read more

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The kidney: excretion and osmoregulation Bio Factsheet
C. Osmoregulation (Figure 6)
The ability to produce concentrated urine is important in allowing terrestrial Figure 6. Distal convoluted
mammals to conserve water. The loop of Henle and the collecting duct tubule
form a system known as a countercurrent multiplier whose function is
to remove water from the fluid in the tubule and produce a concentrated 6
urine.
Cortex 5 Cortex
1. Na+ and Cl- ions are actively pumped out of the ascending limb.…read more

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The kidney: excretion and osmoregulation Bio Factsheet
Practice questions 4. The diagram represents part of a mammalian kidney nephron.
Semicolons indicate marking points.
1. The diagram shows a cell from the proximal convoluted tubule in the
nephron.…read more

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