Be able to identify on drawings,
diagrams and images, the ureters, bladder and urethra.
How does the kidney filter out
the 'unwanted stuff' and reabsorb useful substances still in the
It's extremely complicated, and I
hope my 'simplified' diagram plus text explain how it all works!
The out structure of the kidney, the cortex and medulla, are not shown on the above diagram.
The renal cortex, also called the kidney cortex, is
a part of your kidney, which is itself part of the urinary tract.
The renal cortex (kidney cortex) is where the
filtering units of the kidney begin.
The renal cortex surrounds the inside of the organ,
which is called the medulla and the renal cortex is where the
nephrons, the blood-filtering units of the kidney begin.
What does the renal cortex do?
As part of your urinary tract, the renal cortex is involved in many
kidney functioning and not just in getting rid of body wastes.
Kidneys also control:
The balance of electrolytes and water,
your body’s blood pressure and amount of extracellular fluid
(outside the cells), red blood cell production, the
balance between acids and bases., Vitamin D production,
the balance between calcium and phosphate (a very busy organ!).
The part of the kidney that does
the actual separation is called a nephron - the filtration
unit and there are millions of them!.
Nephrons are the functional
units in the kidney, each consisting of a glomerulus,
Bowman's capsule and
its associated very long thin tubule (kidney tubule), through which the
glomerulus filtrate passes before emerging as urine.
Overall the nephron is like a
tubular membrane surrounded by tiny arteries of blood - into
which reabsorbed materials will pass. - one grey 'squiggly'
artery is shown on the diagram.
A glomerulus is a tiny
ball-shaped structure composed of capillary blood vessels
actively involved in the filtration of the blood to form
urine. The glomerulus is one of the key structures that make up
the nephron, the functional unit of the kidney.
Bowman's capsule is a
cup-like sack at the beginning of the tubular component of a
nephron in the mammalian kidney that performs the first step
in the filtration of blood to form urine. A glomerulus is
enclosed in the sac.
The parts of the nephron
after the renal artery are as follows (not shown in detail on
the above simplified diagram):
tiny arteries of the
glomerulus membrane sac ==> Bowman's capsule (filter) ==>
reabsorption kidney tubule sections (proximal convoluted loop ==> loop of Hele ==> distal
In the nephron loop
sections the selective reabsorption take place.
Animals inhabiting areas which suffer from drought
conditions have an extended Henle loop (part of a nephron)
to reabsorb more water.
How does the nephron complex
(i) The liquid part of the blood
from the renal artery, contains glucose, ions (e.g. sodium Na+
from salt - sodium chloride), water, urea and other
substances, is forced and filtered under high pressure through the glomerulus
into the tissue of the Bowman's capsule - this is where the main filtration
Larger molecules like
proteins or even larger blood cells cannot pass through the
membranes of the Bowman's capsule into the extended length of
the nephron - the residue remains in the bloodstream - the tiny
arteries of the glomerulus - this is the
The smaller molecules/ions
like water, sodium chloride, urea and sugars pass through
the membrane filters of the Bowman's capsule from the blood into
the nephron of the kidney.
(ii) As the filtered liquid flows
through the glomerulus of the nephron useful substances are
All the glucose is selectively
reabsorbed back into the bloodstream against the
concentration gradient - so energy is needed for 'active
transport', since spontaneous diffusion will not take place in
the desired direction for reabsorption.
Any residual ions
are also reabsorbed back into the bloodstream e.g.
sufficient salt that is needed, any excess is passed on and eventually
excreted in urine.
Water is reabsorbed, but
the amount dictated by the level of the
hormone (feedback system described in a later
section on this page).
Adaptation note: Animals
living in very dry regions, prone to drought, have longer
loops of Henle to reabsorb more water.
(iii) The residual waste substances
e.g. excess water, excess ions and urea,
which are not reabsorbed, flow out of the nephrons as urine, through
collecting ducts which all merge into the ureter and down into the
storage bladder. The urine is eventually excreted from
the bladder through the urethra.
(iv) Active transport moves substances against the
concentration gradient - opposing the normal diffusion direction.
Active transport requires a net energy input
from respiration and involves protein carrier molecules to transport material through a
membrane against the concentration gradient.
This happens in conveying glucose through the
membranes of the kidney tubules.