17.1.1 Nephron Functioning

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Transcript 17.1.1 Nephron Functioning

Maintaining a Balance
Topic 17: Nephron Functioning
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
 Explain how the process of filtration and reabsorption in the
mammalian nephron regulate the body fluid composition
The basic functional unit
within the kidney is a
microscopic tubule called
the nephron, the
smallest structural part
of the kidney that is
capable of producing
urine. Each kidney
contains millions of these
tiny units, which coil and
twist across both the
cortex and medulla.
A nephron consists of four
functional parts:
1. Bowman’s capsule
2. Proximal (first) convoluted
3. Loop of Henle
4. Distal (second) convoluted
tubule which leads into a
collecting duct
Bowman’s Capsule
Bowman’s capsule occurs at one end and is an enlarged part of
the nephron tubule. It is a double-walled sac, indented on one
side to accommodate a spherical network of blood capillaries
called the glomerulus.
Bowman’s Capsule
A useful analogy is to think of the Bowman’s capsule as a baseball
mitt – double-walled, hollow (where your hand fits inside) and
curved around to accommodate a baseball – in close contact
with and partly surrounded by the Bowman’s capsule.
Bowman’s Capsule
The hollow part of the
Bowman’s capsule is filled with
fluid called glomerular filtrate.
This fluid continues its flow
along the length of the
nephron. As it flows along the
nephron, the chemical
composition of the fluid is
adjusted. Various substances are
removed or added to obtain the
final product – urine.
Nephron Functioning
Three main processes that lead to urine formation occur in the
nephron. These are:
 Filtration
 reabsorption
 secretion
The renal artery that enters the kidney branches into numerous
smaller and smaller vessels, each terminating in a globular
network of capillaries – the glomerulus. The filtration of blood
takes place at the surface between the glomerulus and the inner
lining of each Bowman’s capsule.
A high pressure system exists in
the blood flowing through the
glomerulus, created partly by size
of the blood vessels entering and
leaving – the vessel entering the
capillary network is slightly larger
than that existing. Substances
within the blood that are small
enough are squeezed through the
capillary wall under pressure.
They pass through the cellular
layer lining the Bowman’s capsule
and move into the lumen.
Blood cells and proteins are retained in the blood, while large
volumes of water pass through, carrying dissolved substances
including amino acids, glucose, salts (ions), nitrogenous wastes
and other toxic molecules. Once inside the Bowman’s capsule,
this fluid is termed glomerular filtrate.
The process of filtration separates substances from the blood
based on their size. It does not take into account whether they
are wastes to be excreted or nutrients that are still required in
the body. Glomerular filtrate is therefore not the final fluid
excreted – its composition is adjusted as it flows along the
remainder of the nephron by two processes.
Tubular Reabsorption:
 Substances that the body need are reabsorbed from the filtrate
into the kidney cells or bloodstream so that they are not lost
with urine.
Tubular secretion:
 Additional wastes that may still be in the bloodstream (those
that were not squeezed out of the blood under the high
pressure of filtration) must be added (secreted) into the fluid.
The chemical composition of body fluids is precisely regulated
by selective reabsorption of certain solutes from the glomerular
filtrate at various points along the nephron – the proximal
tubule, loop of Henle and distal tubule.
The filtrate contains molecules
that the body needs (eg: amino
acids, glucose, certain ions and
some water) and so they are
actively reabsorbed from the
nephron and passed back into
the interstitial fluid and blood
capillaries surrounding the
nephron, a process called
tubular reabsorption.
These capillaries join up
to form larger vessels
which drain into the
renal vein, where they
are carried from the
kidney back into the
general circulation.
All nutrients are reabsorbed from the filtrate and varying
quantities of inorganic ions are reabsorbed, depending on the
particular requirements of the body at that time.
Solute Reabsorption
All amino acids, glucose and
varying quantities of ions such
as Na⁺, K⁺, Cl⁻, Ca²⁺, and
HCO₃⁻ and some vitamins are
reabsorbed. The differing rate of
reabsorption of particular ions
depend on feedback from the
body. All solutes that are
reabsorbed from the nephron
move by means of active
transport and facilitated
Solute Reabsorption
Glomerular filtrate also contains a relatively high concentration
of dissolved urea and other wastes, most of which are not
Water Reabsorption
As the solutes are actively
reabsorbed, water follows by
the passive process of osmosis.
An enormous quantity of
water is reabsorbed by
osmosis – approximately 99%
of the huge volume of filtrate
that passes into the Bowman’s
capsule is reabsorbed along
the length of the nephron and
only 1% is actually excreted as
Water Reabsorption
The membranes of the cells
lining the nephron can change
their permeability to water and
ions, thus regulating the
amount of these substances that
are reabsorbed. Hormones
control these changes in
membrane permeability.
Parts of the Nephron Involved
The loop of Henle descends into the medulla and then ascends
up towards the cortex, leading into the distal tubule. In the
ascending limb of the loop of Henle, a large number of ions (Na
in particular) are actively pumped out into the interstitial fluid in
the medulla, since the membranes are permeable to salts but
impermeable to water.
Parts of the Nephron Involved
Some urea may move by
diffusion out of the collecting
tubule and into the
surrounding interstitial fluid.
These solutes which
accumulate in the medulla
draw water by osmosis.
Parts of the Nephron Involved
Some water moves from the
descending limb of the loop of
Henle and a large amount of
water moves from the collecting
tubules, both of which have
membranes that are permeable to
water, allowing it to move
passively by osmosis into the
surrounding tissue of the medulla.
Energy is used to ensure that urea
does not return to the capillaries,
so this is not true tubular
Tubular Secretion
Tubular secretion is the third
process that contributes to
urine formation in the
nephron. It involves the
removal of toxic substances
from the blood capillaries and
tissues and their active
secretion into the nephron.
Tubular Secretion
Metabolic wastes such as urea,
uric acid, ammonia and
hydrogen ions are secreted into
the fluid within the nephron,
along with drugs such as
penicillin, saccharin and
morphine. Movement of urea
and ammonia is mainly by
means of diffusion.
Tubular Secretion
Urea may be recycled in
the nephron to help move
water by osmosis – some
urea may move from the
collecting tubules into the
interstitial fluid to help
draw water out of the loop
of Henle. It diffuses back
into the descending limb of
the loop of Henle so it may
be continuously recycled if
the body needs to reabsorb
-Students to complete 17.1.7 Nephron Worksheet