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Nephrons
8
...
8
...
The beginning of the tubule lies in the renal cortex and is expanded into a cupshaped capsule – Bowman’s capsule – which encloses a small network of
capillaries called the glomerulus
...
Capillaries surround the tubule until it joins the collecting duct which leads to
the renal pelvis (centre) of the kidney (k Box 8
...
Box 8
...
The kidneys retain their capacity
to maintain normal function
except at times of stress, e
...
infection
...
8
...
Box 8
...
The ultrafiltrate is therefore devoid of
proteins, blood cells or platelets
...
Less than
1% of the water and solutes in filtrate
are excreted because of reabsorption
by other segments of the nephron
...
Box 8
...
This is the
reason why urine should not normally
contain any significant amounts of
protein
...
afferent arteriole
efferent arteriole
Functions of the nephrons
As blood passes through the kidneys, three key processes take place in the nephrons
that result in urine:
• ultrafiltration of the blood in the glomerulus that filters out urea, salt, water,
glucose and other small particles from the blood (k Box 8
...
g
...
8
...
1 Ultrafiltration
Ultrafiltration (glomerular filtration) occurs in the glomerular capillaries, which
act as a semipermeable membrane (k Box 8
...
The efferent arteriole carrying
blood away from Bowman’s capsule has a smaller diameter than the afferent
arteriole, making the resistance in the efferent arteriole higher than that in the
afferent arteriole (k Fig
...
4)
...
g
...
The fluid filtered in this way is called glomerular
filtrate
...
• Hydrostatic pressure in glomerular capillaries tends to push water and solutes
into Bowman’s capsule (k Fig
...
4: red arrows)
• Forces that oppose glomerular filtration are:
– colloid osmotic pressure of blood which tends to attract fluid back into blood
(k Fig
...
4: blue arrows)
– hydrostatic pressure of the filtrate already in Bowman’s capsule (k Fig
...
4:
yellow arrows)
...
There is a natural decline in glomerular filtration as people get
older
...
2
...
8
...
Reabsorption allows many useful solutes that have been filtered out from the blood
in Bowman’s capsule, to return to the blood in different segments of the nephron (k
Fig
...
5)
...
8
...
Á The descending limb of the loop of Henle is permeable to water but
impermeable to sodium chloride
...
 The ascending limb of the loop of Henle actively transports sodium chloride
back into the blood
...
Overall, relatively more sodium chloride than
water has been reabsorbed by the loop, making the filtrate very dilute when it
enters the distal convoluted tubule
...
100
4
distal
convoluted
tubule
300
CORTEX
MEDULLA
300
300
1
proximal
convoluted
tubule
2
descending
limb of the
loop of Henle
100
Blood from
efferent
arteriole
DCT and
collecting
ducts
NaCl
200
H2O
ADH regulates
permeability
400
NaCl
200
H2 O
H2O
3
ascending
limb of the
loop of Henle
ascending
limb of
vasa recta
descending
limb of vasa
recta
400
H2 O
urea
NaCl
H2 O
900
700
600
NaCl
urea
1200
Vasa recta
1200
Fig
...
5 Variation in reabsorption of solutes from filtrate in different segments of
the nephron
...
6)
...
8
...
The luminal membrane is selectively permeable and determines which
substances from the filtrate are reabsorbed and by what method:
• sodium ions (Na+) are actively transported (k Fig
...
3), a process which requires
energy provided by ATP
...
5
...
2
...
Box 8
...
g
...
The epithelium of the proximal tubule and loop of Henle is impermeable to the
nitrogenous waste molecule urea so it becomes increasingly more concentrated as
it flows along towards the distal tubule (k Box 8
...
Some urea is reabsorbed from the collecting ducts
...
8
...
Process of reabsorption in the loop of Henle
By the time filtrate enters the loop of Henle, it is reduced in volume when compared
with the filtrate in Bowman’s capsule
...
8
...
Since the kidney contains millions of nephrons, the net effect of selective permeability
in this segment is to create an osmotic gradient from the cortical region to the
medullary region of the kidney (k Box 8
...
Box 8
...
Taking it Further
Find out more about movement
of substances across membranes in
online Chapter 20
...
The permeability of the distal tubule to:
• water depends on the action of antidiuretic hormone (ADH) (k 8
...
4)
• sodium and potassium ions depends on the activity of the hormone aldosterone (k
8
...
5)
• urea is excreted in urine but some diffuses passively back into the bloodstream
...
2
...
Normally only a few
substances are added to the final urine in this way – hydrogen and potassium ions,
nitrogenous wastes (ammonia, urea, uric acid and creatinine) and some drugs, e
...
penicillin
...
3
This process is part of the mechanism that homeostatically regulates the pH of
blood
...
8)
...
8 Acid–base homeostasis
...
Potassium ions (K+) are normally in low concentrations in plasma, and are secreted
into final urine under the influence of the hormone aldosterone
(k 8
...
They are actively transported from blood into filtrate in the distal tubule
and collecting duct in exchange for sodium ions (Na+)
...
However, the majority of drugs and
medications in blood are bound to plasma proteins, so active secretion against a
concentration gradient takes place in the distal part of the nephron (k Fig
...
5) (k
Box 8
...
8
...
4 Antidiuretic hormone and water excretion
When the amount of water in the blood falls below a set level, e
...
dehydration,
vomiting or excessive sweating, the hypothalamus sends a message to the pituitary
gland which releases ADH (k 11
...
1)
...
3
...
The osmolarity
difference that was set up in the the loop of Henle (k Fig
...
5) means that, in the
presence of ADH, more water is reabsorbed into the collecting duct and a small
volume of very concentrated urine is excreted
...
g
...
Without ADH the kidney tubules do not reabsorb as much
water, and large volumes of dilute urine are produced
...
10)
...
9 A range of factors affect the
rate of the secretion of drugs into
urine, e
...
plasma concentration of
the substance, pH of urine,
presence or absence of kidney
disease
...
Box 8
...
Alcohol can decrease the amount of
ADH produced, which results in a
greater volume of more dilute urine,
and can lead to dehydration
...
It increases the amount of ADH
which reduces the amount of urine
produced, so fluid does not leave
the body
...
Diabetes insipidus
Diabetes insipidus is a clinical condition caused by a deficiency of ADH
(central DI) or a lack of renal response to ADH (nephrogenic DI) which
results in the production of large volumes of urine and causes excessive
thirst
...
3)