The kidney
The kidney is a very important organ and is highly involved in homeostasis. The kidney processes the
plasma portion of the blood and ensure that substances that need to be excreted are (e.g. urea) and
also , it reabsorbs substances which are needed by the body(e.g. glucose). The kidney lies at the back
of the abdominal wall-but not in the abdominal cavity. It is often described as being retroperitoneal.
This means that it lies at the peritoneum, the lining of the abdominal cavity. A normal person
normally has two kidney but it is also possible to just live with one kidney. A person often doesn’t
know that they live with one kidney for their entire life unless a kidney disease occurs.
The functions of the kidney
There are many functions of the kidney. These include:1. The excretion of nitrogenous waste e.g. urea, uric acid, creatinine, even ammonia. These are
called nitrogenous wastes because they contain the element nitrogen
2. The control of pH
3. The control of ionic concentration( sometimes called the electrolyte concentration)
4. The control of blood pressure( Hydrostatic pressure)
5. Gluconeogenesis
6. The water potential of the blood
7. Has an endocrine function- secretes hormones into the blood.
The anatomy of the kidney
The kidney lies embedded in fat. This acts to insulate the kidney. It also has a fibrous capsule – made
out collagen. This fibrous capsule protects the kidney from damage. This image below shows a
longitudinal section of the kidney.
Pelvis: this is where the
(medullary) collecting duct collect
with the urine.
Ureter
Branch the
renal artery
Branch
of the
renal by
vein
Cortex: this contains the
glomerulus , bowman’s( renal
capsule) , the PCT and DCT.
The pyramids of the kidney: this is
the medulla of the kidney and
contains the loops of henle and the
collecting duct
The nephron
The nephron is the smallest unit of the kidney. It’s often described as being the functional unit of the
kidney as this is the structure that performs the entire functions of the kidney. The nephron is also
sometimes referred to as a kidney tubule. The majority of the kidney tubules are cortical. This
means that their loops of henle doesn’t plunge down deep into the medulla. The minority of the
kidney tubules are juxtamedullary. This means that their loops of henle do plunge deep down into
the medulla. This is responsible for the dilution of H2o( so it can be reabsorbed in the collecting
duct) . Also ion reabsorption in the medulla of the kidney. This places an important role in creation
of a hypertonic(hyperosmotic) region in the medulla. This hence allows water reabsorption via an
osmotic gradient, down its water potential. The image shown below shows an image of a simplified
kidney tubule.
Glomerulus( a
Afferent (vessel/ arteriole)knot of
this isn’t an artery but an
interconnected
arteriole
capillary loops)
Bowman’s (renal)
capsule
Proximal
convoluted
tubule(PCT)
Distal convoluted
tubule( DCT)
Efferent (vessel/arteriole)
Collecting duct
Capillaries( where substances being
reabsorbed enter)
Loop of henle( Henle is a
person’s name and the e
has an accent mark.
Kidney processes
The kidney performs these tasks many times during each days ensuring homeostasis is met
accordingly:1.
2.
3.
4.
5.
6.
Ultrafiltration/ glomerular filtration/ filtration
Selective reabsorption/ reabsorption
Secretion
Creation of a hypertonic region in the medulla of the kidney
Control of pH in the DCT
Water reabsorption in the descending limb and collecting ducts.
Ultrafiltration
This occurs in the renal corpuscle( glomerulus and the bowman’s capsule) of the kidney tubule. This
is where the plasma portion of the blood enters the kidney tubule. This process is effectively
inefficient as 20% of the plasma enters the kidney tubule at any point. However, this is backed up by
secretion.
There are many reasons why ultrafiltration occurs for. These are collectively known as starling forces.
These are:
1. The water potential of the fluid in the bowman’s capsule (glomerular filtrate)-actually zero
2. The water potential of the blood in the glomerulus.
3. The hydrostatic pressure ( blood pressure) in the glomerulus
4. The hydrostatic pressure in the bowman’s capsule
In summary, the plasma is able to pass through to the bowman’s capsule because of the net
glomerular filtration pressure. This is always positive under normal physiological control, however is
due to change at any point- mainly due to a fall in blood pressure.
Explanation:The hydrostatic pressure in the afferent vessel is higher than the pressure of the bowman’s capsule.
Also, the efferent vessel diameter is smaller than the afferent vessel, increasing the pressure inside
the glomerulus even more. So ,effectively the bowman’s capsule would be filled with plasma, down
a pressure gradient. However, this doesn’t happen. This is because of the water potential of the
glomerulus is low. This is because of the basement membrane acting as a filter and preventing
plasma proteins, blood cells out. This lowers the water potential drastically and so is favouring back
water by an osmotic gradient, so letting ions follow on down their concentration gradient. But the
hydrostatic pressure , as nearly always, in capillaries is the greater force and so facilitating the
filtration. Other factors which enhance this filtration is the leakiness of the capillaries. These
capillaries have large fenestrations (gaps) which means that it allows substances out from the blood
easily. Also the podocytes( the lining of the inside of the bowman’s capsule- epithelial tissue)
reduces the resistance to flow by it long foot processes- often called the foot process. The fluid that
enters the bowman’s capsule is now known as glomerular filtrate/ filtrate.
Selective reabsorption
The main role of the PCT is to reabsorb substances which are needed by the body e.g. glucose. The
cells lining the PCT, cuboidal epithelial cells, are highly adapted to the reabsorption by using a
mixture of facilitated diffusion and active transport. The initial problem is that the concentrations of
substances are the same as in the opposite side of the cuboidal epithelium. But this problem is
solved the (primary) active transport of sodium ions and potassium ions in the basolateral
membranes of these cells. The sodium ions are pumped into the cell and potassium ions are pumped
outside the cell coupling this to the ATP hydrolysis. So this pumps are called Na+ and k+ ATPase/ Na+
and k+ pumps. So reabsorption is an active process.
This image shows the cells lining the PCT
Process
These cells are highly adaptable at removing substances from the glomerular filtrate. This is because
the luminal membrane( the one facing the tubular lumen) has many microvilli. This increases the
surface area so that more proteins can become embedded onto the membranes and facilitate in the
reabsorption process. The sodium ion /potassium ion pump in the basolateral membranes set up a
concentration gradient where the Na+ concentration is relatively higher inside the cell rather than in
the lumen of the tubule. This, therefore allows reabsorption of Na+ by facilitated diffusion. However,
this can be only made possible if Na+ is attached to another molecule e.g. glucose. So there is no way
of reabsorbing Na+ without attaching the ion to another molecule. The proteins which allow this
transport are co-transporter proteins ( also called symporters- transport substances in the same
direction). So , the energy released when Na+ is reabsorbed is used to reabsorb other substances
against a concentration gradient so no ATP is needed as energy is released by Na+ reabsorption. This
is essentially the way in which all molecules are reabsorbed like amino acids and other organic
molecules. So the other molecules are absorbed by indirect/ secondary active transport.From all of
this reabsorption , you will get an osmotic gradient- so water is reabsorbed by osmosis down a water
potential gradient through water channel proteins called aquaporins. So, this increases the urea
concentration as water isn’t dissolving it- so urea is reabsorbed by simple/ passive diffusion through
the tight junctions between the cells. After this has all occurred, about 65% of water and all the
glucose, amino acids etc that are important to the body are reabsorbed and most of the sodium ions
will have been reabsorbed.
The concept of renal threshold
Sometimes, all substances cannot be reabsorbed. This is due that are many molecules present that
can be reabsorbed. This means that the substances will start to enter in the urine. This normally isn’t
the case. For example, glucose isn’t normally present in the urine, however, if glucose starts to
appear in the urine, it has gone above It’s renal threshold as it can’t be reabsorbed. The protein
carriers are filled completely. This normally happens in uncontrolled diabetes mellitus .
The role of the loop of henle in the reabsorption of water
The loop of henle lies deep in the medulla in most cases. This is responsible for reabsorbing
ions( sodium and chloride ions) into the medulla. This creates a hypertonic region in the medulla of
the kidney ( the interstitial fluid – essentially tissue fluid). This then allows water to be reabsorbed
in the collecting duct. That is if the duct is permeable under the influence of a peptide hormone
called anti-diuretic hormone (ADH). Sometimes known as vasopressin. The way in which the loop of
henle creates the hypertonic region is called the counter-current system/ hairpin counter-current
multiplier.
The process
Vasa recta:
capillary
Harpin bit
The glomerular filtrate is at normal isotonicity when it enters the descending limb- i.e. the solution is
isotonic with blood and so enters at 300 milliOsmolar( mOsm). mOsm is a unit that measures the
ionic concentration of a particular substance – the higher the value, the more ions it contains. The
ascending limb pumps sodium, potassium ions against a concentration gradient using ATP. This
lowers the water potential in the interstitial fluid but water can’t follow out as it is impermeable to
it. However, the descending limb is permeable to water but not ions, and so water will leave into the
medulla down its osmotic gradient. As glomerular filtrate is going down the descending limb, it
continues to lose water until the hairpin bit. This is because as you go down deep into the medulla,
it becomes even more hypertonic so water continues to go into the medulla as it is equalising
osmotic imbalances. As the glomerular filtrate is going into the ascending limb- it will lose ions but
not the water and so the water potential becomes higher. This is good because water can be
reabsorbed in the collecting duct if water if permeable down a water potential gradient. So the loop
of henle acts to firstly lower the water potential of the glomerular filtrate and then increase the
water potential drastically so that water can eventually can be reabsorbed in the collecting duct. This
couldn’t be achieved if there wasn’t an active transport process going on in the ascending limb and
secondly if the loop of henle wasn’t associated in a “loopy” way- the glomerular filtrate flows in
opposite directions( counter-current system) to maintain hypertonicity in the medulla and make the
glomerular filtrate hypotonic. Furthermore, the function of the water leaving the descending limb
isn’t really for reabsorption purposes but to increase the water potential back to normal as it goes
back to the normal blood circulation (illustrated by the image above). The capillaries it reabsorbs it
into is known as the vasa recta. This only acts to maintain the hypertonic region in the medulla of
the kidney. So it is a way of ensuring that the medulla is hypertonic.
Water reabsorption in the collecting duct(CD)
Under normal physiological conditions, the amount of water reabsorbed and the concentration of
the peptide hormone, ADH are at normal concentrations. And so not so much water is reabsorbed.
However, as the water potential of the blood starts to decrease- if this isn’t detected, a lot of water
will be excreted in the urine as water follows its water potential gradient. So – to prevent this from
occurring, osmoreceptors in the hypothalamus will detect this change. Blood flows past these cells
and a lowered water potential is thought to cause these cells to shrink and so these sensory
receptors activate the neurosecretory cells. These motor neurones will carry an action potential to
the synaptic bulb and cause the secretion of ADH into the blood where it will go in the systemic
circulation away from the posterior pituitary gland- so the pituitary gland functions as an endocrine
gland. ADH will bind to G-protein coupled receptor on the collecting duct cells (target cells) , where
it will activate adenyl cyclase, so converting ATP to cyclic AMP etc. to the point you get to the
insertion of aquaporins in the luminal membrane of these cells. So , therefore now you can get
water reabsorption through these channel proteins, in to the medulla ,down a water potential
gradient ,so resulting in smaller volume of concentrated urine. If the water potential of the blood
increases, the complete reverse will occur.
Physiological conditions associated with reabsorption in the CD
Sometimes, reabsorption of water in the collecting duct might not go how it is supposed to. This can
lead to a condition known as diabetes insipidus. This is when you are producing a lot of dilute urine
as you cannot reabsorb it in the collecting duct. This is termed diuresis. The one associated in the
collecting duct is called water diuresis. The causes of this vary and are usually for two reasons. One
of the reasons is because of the posterior pituitary gland isn’t secreting ADH- termed central
diabetes insipidus. And the other reason is because of the CD cells being unable to respond to the
ADH- termed nephrogenic diabetes insipidus. Diuresis can also be caused be uncontrolled diabetes
mellitus. This is because glucose becomes above its renal threshold and since glucose cannot be
reabsorbed anywhere else than the PCT- it will effectively mean that the hypertonic region created
in the medulla will be useless as not much water will become reabsorbed in the CD , even though if it
is under the influence of ADH. But this is not water diuresis- it is caused by the solutes rather than
the ADH being involved.
Final result: urine has been made now after this and now collects in the pelvis of the kidney. Now
the urine will go through the ureter and collect in the urinary bladder- and when ready the urine
goes through the urethra and is excreted.
Kidney failure
Kidney failure exists when the body can no longer produce urine. This means that metabolic waste
can build up in your body e.g. urea .Also, the person can no longer control the levels of water and
electrolytes in the body and therefore, this can generally makes a person feel very ill as well.
Kidney failure types
The two types of kidney failure and they are:1. Acute kidney failure
This is where the body experiences kidney failure as result of other illness already present in
a person. Furthermore, this can be caused by the inflammation of the glomeruli. This is
because of the damage of the basement membrane. This means that there will be a lack of
ultrafiltration. This means that blood cells (like erythrocytes and leucocytes) and plasma
proteins (like albumin and fibrinogen) can appear in the urine. Also because of this, the
water potential of the blood plasma increases, and therefore a person can have a lot of
water retention as oedema occurs (this is the swelling of tissues).
Another reason why acute kidney failure may happen is because of sepsis (inflammation of
blood and body tissues by bacteria)
2. Chronic Kidney failure.
This is when the body develops kidney failure over a period of time. The reasons why kidney
failure can be caused is because of hypertension (damage of the basement membrane),
diabetes mellitus (as the PCT can‘t reabsorb all the glucose, it’s above its renal threshold),
Inflammation of the glomeruli, viral infection and low blood pressure.
Dialysis is the separation of molecules by means of a partially permeable membrane. There are two
types:Peritoneal dialysis:
This is when the peritoneal membrane is used as the partially permeable membrane. The peritoneal
membrane is the tissue that lines the peritoneal cavity. The peritoneal cavity is where all the organs
lie and there is a fluid which bathes these organs
How it is done:A catheter (tube) is inserted into the peritoneal cavity. The dialysis fluid (dialysate) is a fluid which
has the correct concentrations of urea, glucose, a.a and so on. Anything in low conc. will diffuse into
the body and anything in higher conc. will diffuse into the dialysate. At the end, this dialysate will be
drained. The peritoneal membrane acts like the dialysis membrane in renal dialysis machine i.e.
partially permeable. It allows simple diffusion to occur and also osmosis
Advantages:1) More mobile, that means that the patient doesn’t have to sit around.
2) Less expensive
3) Less of a controlled diet ( so less swings in blood volume and concentrations) as it is
more often
4) Feel better as urea not allowed to accumulate so much
5) Psychologically better as patient not perceived as being seriously ill
Disadvantages:1) More chances of infection as a result of a pathogen entering the catheter.
2) Less efficient so it has to be done a number of times a day.
Haemodialysis:In this process, we need the use of a kidney machine.The artery and vein are sewn together to
create a fistula. The blood is taken into the machine and pumped to increase pressure so it can be
forced through the machine and it’s warmed to prevent any symptoms of hypothermia (because the
blood will cool significantly as it goes through the machine. When it goes back into the body it will
lower the core temperature. There is a partially permeable membrane which separates the blood
from the dialysate. Any excess electrolytes (e.g. Na+, Cl-) are removed by simple diffusion. Any
excess water is removed by osmosis. Plasma proteins and erythrocytes stay in as they are too large.
The dialysate and blood moving in different directions is referred as the counter-current system. This
increases the concentration gradient and prevent equilibrium to be reached. Heparin added to
prevent blood from clotting. There is air detection and filter before return.
Advantages of haemodialysis:1) More efficient
Disadvantages of haemodialysis:1)
2)
3)
4)
5)
More of a controlled diet as it is done more often
More expensive
More time-consuming
Large swings in blood composition and volume
Less free as you are stuck in a machine.
Kidney transplants
It’s the best way to treat kidney failure. The old kidneys are left in place unless they are likely to
cause infection or are cancerous. If they are cancerous i.e. contain a malignant tumour, metastasis
could occur. This means cancerous cells can break free from the tumour and spread round the body
via the blood. Tumours will then develop elsewhere. There is a great importance in the donor to be
of a close relative or someone that has a close tissue type. This is because the tissue type have to be
otherwise there maybe organ rejection as a result of an immune response. Sometimes, there is a
shortage of kidneys as result of not many people having the correct tissue type. This has led to
“organs for sale” .This is because not many people in the LEDC’s have money and therefore if they
patient and that particular donor are of the same tissue type, then they are likely to sell it. However,
a way of solving this is by xenotransplants- that is getting an organ from an animal. This is mostly
from pigs. On the other hand, there are many problems with this such as the moral or religious views.
Also, the organ of a pig may carry a virus which can cause a new disease in humans.