KIDNEY RLO 1 – function
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Kidney function
Glomerulus
The role of the kidneys is to control the
composition and volume of the blood by
eliminating water, salts and waste products in the
form of urine.
Bowman’s
Capsule
There are three physiological processes that
enable the blood to be filtered of waste material,
and that allow any beneficial substances to be
transferred back into the blood.
Proximal/
distal
tubules
These processes take place in the nephron and
are:
a Glomerular filtration
b Tubule reabsorption
c Tubule secretion
Fig 1. Processes involved in
urine formation
KIDNEY RLO 2 – function
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Glomerular filtration A
The first process by which the kidneys produce
urine is called glomerular filtration. Blood
enters the glomerulus under high pressure and
forces substances through pores in the blood
capillary endothelium into the capsule (Fig 2).
Approximately 1 litre of blood passes through
each kidney every minute.
Substances filter into the funnel-shaped
Bowman’s capsule which drains into the
proximal tubule. During filtration, roughly one
fifth of blood plasma is removed, consisting of
water, small proteins, salts (Na+, Cl-, K+),
glucose, and nitrogenous waste products such
as urea.
Afferent
arteriole
Glomerulus
Bowman’s
capsule
Fig 2. A section through
the glomerulus
Efferent
arteriole
KIDNEY RLO 2 – function
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Glomerular filtration B
Bowman’s capsule acts like a sieve allowing
small substances to be flow into the nephron,
whilst large molecules remain within the blood.
The high blood pressure of the glomerular
circulation forces small molecules across the
capillary wall and through pores in the
Bowman’s capsule membrane.
These substances drain from the Bowman’s
capsule into the poximal tubule for the next
step of the process.
Fig 3. Filtration between the glomerular capillary
and Bowman’s capsule
KIDNEY RLO 1 – function
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Tubule reabsorption A
After filtration, many substances that are
beneficial to the body are re-absorbed.
These are mainly salts (Na+, Cl-), amino
acids, glucose and water. Around 99% of
substances are reabsorbed.
Reabsorption occurs in the proximal
convoluted tubule and loop of Henle by
passive transport processes (processes that
don’t require ATP energy).
Beneficial substances are reabsorbed back
into the blood stream.
Proximal
convoluted
tubule
Loop of
Henle
The remaining waste substances continue
flowing around the tubule to the distal
convoluted tubule.
Fig 4. Tubule reabsorption
KIDNEY RLO 1 – function
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Tubule reabsorption B
Distal
convoluted
tubule
At the distal convoluted tubule, fine
adjustments to the composition of the urine
are made.
This depends on the salt and water status of
the body. If salt levels in the plasma are low,
the hormone aldosterone drives more
reabsorption of sodium. Less reabsorbtion
occurs when salt levels of the plasma are
high.
If the body is thirsty, antidiruectic hormone
(ADH) helps regulate the reabsorption of
water. In thirst, ADH increases the
permeability of the distal tubule and
collecting duct and increases water
resorption. In this situation, urine produced
will be low volume.
Fig 5. Tubule reabsorption
KIDNEY RLO 1 – function
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Tubule secretion
The final nephron process which is
involved in urine formation is tubule
secretion. Substances in the blood may
be secreted back into the renal tubules.
Some metabolic end products and
exogenous substances such as penicillin
and diuretic drugs are secreted from the
circulation into the tubules. In addition,
metabolic
substances
such
as
ammonium and hydrogen ions produced
in the tubule cells themselves may also
be transported into the lumen.
The process of tubular secretion is an
important step in controlling blood pH.
Fig 6. Tubule secretion
KIDNEY RLO 1 – function
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Urine formation
The end product of kidney filtration,
resorption and secretion is urine. Urine
comprises of 95% water containing
nitrogenous wastes in the form of urea
and uric acid, salts and other
endogenous and exogenous metabolites.
The kidneys provide the major route for
water excretion in the body (Fig)m with
other losses via the skin, lungs, faeces
and
sweat.
They are an important route for the
elimination of drugs and drug metabolites
from the body.
Fig 7. Average daily water intake and output