Download Section B : Control of the Internal Environment 1. Structure of the

Section B : Control of the Internal Environment
1. Structure of the Human Urinary System
The human urinary system is composed of the kidneys, renal artery,
renal vein, ureter, bladder and urethra.
2. Role of the Mammalian Kidney
(a) Osmoregulation
Mammals need to constantly take in water to counteract that water lost
by sweat, breath, urine and faeces. To counteract this loss of water,
mammals gain water by eating, drinking and metabolic reactions
(respiration occurs in every cell of our bodies).
To remain healthy, the volume of water we lose everyday must be
replaced by an equal volume of water. If we do not maintain the balance
of water within the body, the major organs of our body will stop working
properly, eventually we would die.
Water gain
Water loss
1500 ml drinking 600 ml eating
500 ml sweat
450 ml breath
400 ml chemical reactions
1450 ml urine
100 ml faeces
i.e. 2500 ml water lost = 2500 ml water gained.
The kidneys, working closely with the brain, are responsible for
maintaining the delicate balance of water within the body. The regulation
of the quantity of water in the human body is known as osmoregulation.
The kidneys, together with the other organs of the urinary system, are
resposnible for osmoregualtion within a mammal's body. Depending on
how much water we gain in a 24 hour period, the brain will alter kidney
function to maintain the balance of water within the body (about 70% of
our body mass is water).
(b) Urea and Urine
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The body uses urine as a way of getting rid of nitrogenous waste.
Urine is a solution of urea.
Urea is produced by the body, as a waste product from breaking down
excess amino acids in the liver. Deamination of amino acids produces
urea.
Urea travels to the kidneys, dissolved in the blood. Once at the
kidneys, urea is filtered from the blood.
The kidneys dispose of the filtered urea as urine, a solution of urea.
(c) Structure and function of the kidneys
Structure: a longitudinal section through a kidney. (see your own notes)
Within the cortex / medulla area of a kidney are millions of functional
units, known as nephrons. Each of these very small structures perform
the primary role of the kidney – the production of urine.
A nephron is made up of several parts, each part fulfilling a different
role in urine production.
Structure of a nephron (see your own notes)
Function of the nephron:
Production of urine in the kidneys involves the filtration of the blood,
followed by reabsorption of useful subtances back into the blood from
the filtrate.
Filtration:
• Blood enters the kidney by the renal artery. The renal artery divides
thousands of times, until we are left with a single capillary to supply a
single glomerulus for each nephron.
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A glomerulus is a knot of capillaries, providing a very large surface
area for filtration of small soluble molecules from the blood into the
Bowman's Capsule.
Small soluble molecules such as glucose, salts, water and urea that
can pass through the capillary wall into the Bowman's Capsule.
Large molecules, such as plasma protein, and blood cells cannot move
through the walls of the capillaries.
Filtration will only occur at the glomerulus / Bowman's Capsule if the
blood present in the capillaries of the glomerulus is at a high enough
pressure:
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Blood arriving at the kidney comes through the renal artery, via
the aorta, straight from the heart. Hence, it is at a high
pressure.
A bottle neck is created, as the capillaries arriving at a
glomerulus are wider than those forming the glomerulus.
Therefore, the same volume is forced into a small space,
creating a higher pressure.
Reabsorption:
• As the filtrate produced at the glomerulus passes through the tubules
of the nephron, useful substances are reabsorbed back into the blood
stream.
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All glucose, some salts and a lot of water are reabsorbed as the
filtrate passes along the tubule of the nephron.
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No waste products, such as urea, are reabsorbed into the blood.
The cells of the kidney show selective reabsorption, to carry this out
they require energy which they get from respiration.
Over 99 % of the water filtered from the blood, at the glomerulus, is
returned to the blood before the filtrate enters the collecting duct.
The filtrate produced by each nephron collects at the pelvis of each
kidney. From here, the filtrate enters the ureter, before travelling
onto the bladder.
It is only once the filtrate enters the ureter that it is called urine.
Once in the bladder, urine is stored ready for expulsion from the body
along the urethra.
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The capillaries leaving each nephron contain purified blood.
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The capillaries join up together, eventually forming the renal vein.
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The renal vein leaves the kidney, containing only purified blood.
(d) Negative Feedback Control
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Changes in the water concentration of the blood are monitored by the
hypothalamus.
Osmoreceptors in the hypothalamus are stimulated by changes in
water concentrations of the blood.
If we decrease the volume of water we take in, or increase the amount
we sweat, the water concentration of our blood will decrease.
• A decrease in water concentration will trigger the release of
increased AntiDiuretic Hormone (ADH) by the pituitary gland.
• ADH increases the permeability of the kidney tubules and
collecting duct. This increase in permeability allows more water
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to be reabsorbed from the kidney tubules into the blood.
If more water is reabsorbed from the kidney tubules into the
blood, a smaller volume of more concentrated urine will be
produced.
If we increase the volume of water we take in, or decrease the amount
we sweat, the water concentration of our blood will increase.
• An increase in the water concentration of the blood will result in
less AntiDiuretic Hormone (ADH) will be released by the
pituitary gland.
• A decreased concentration of ADH will decrease the
permeability of the kidney tubules and collecting duct. This will
not allow less water to be reabsorbed from the kidney tubules
into the blood.
• If less water is reabsorbed from the kidney tubules into the
blood, a larger volume of more dilute urine will be produced.
(e) Osmoregulation in Marine and Freshwater Bony Fish
(i) Marine Bony Fish
• The tissues of a marine bony fish are hypotonic to the surrounding salt
water.
• Marine bony fish constantly face the risk of dehydration as water
moves from the fish to the surrounding water, by osmosis.
• During osmosis, water moves from an area of higher water
concentration (in the tissues of the fish) to an area of lower water
concentration (in the surrounding water) down a concentration
gradient.
• To counteract the excessive loss of water from its body, marine bony
fish:
• drink plenty of sea water.
• actively excrete salt from their tissues.
• produce small volumes of concentrated urine.
(ii) Freshwater Bony Fish
• The tissues of a freshwater bony fish are hypertonic to the
surrounding freshwater.
• Freshwater bony fish constantly face the problem of water moving into
their bodies from the surrounding freshwater by osmosis.
• During osmosis, water moves from an area of higher water
concentration (in the surrounding water) to an area of lower water
concentration (in the tissues of the fish) down a concentration
gradient.
• To counteract the excessive gain of water, by its body, freshwater
bony fish excrete large volumes of dilute urine.