preview summary:• There's a renal medulla and a renal cortex, the cortex tends to be outer region and the medulla is the more central region and the key structure within the renal medulla are the kidney nephron.
• Top left – this is what the kidney nephron looks like, it consists of a bowman...
There's a renal medulla and a renal cortex, the cortex tends to be outer region and the
medulla is the more central region and the key structure within the renal medulla are the
kidney nephron.
Top left – this is what the kidney nephron looks like, it consists of a bowman's capsule which
is this green region here. The glomerulus is kept within the bowman's capsule, and they
combine to form the filtering unit of the kidneys. You have the afferent arterial supplying
blood to the glomerulus, and you have the efferent taking it away, you'll get blood coming in
from the efferent arteriole into the glomerulus and it will filter toxins (things that the body
wants to get rid of) through the bowman's capsule and it will take it towards the proximal
tubule.
Top right – you have the afferent arteriole bringing blood in, anything that doesn't need to
be filtered by the kidneys leaves through the efferent arteriole. The glomerulus is supported
in this cup known as the bowman's capsule. Anything the body wants to get rid of it will filter
through here and it will go towards the proximal tubule.
Bottom left – you can see the proximal tubule and then you have this loop known as the
loop of Henle and its main role is to concentrate the filter. The counter current multiplier is a
process whereby the kidneys concentrate the urine. You have this descending limb going
down and the ascending limb going up towards the distal tubule. This loop pushes out salts
into the interstitial fluid and the water will follow by doing so it concentrates the urine.
What's physiologically noted is that animals living in particularly hot climates e.g., camels will
have very long loops of Henle because the longer the loop of Henle the more salts that they
can push out and the more water that they can reserve.
The distal tubule is kind of the business end of the nephron where it links into the collecting
duct and this collecting will eventually take you towards the renal pelvis and that's where
your urine will be excreted from. The distal tubule is key as will find out later on in the
lecture it's at this point where you can reabsorb some critical water if the body’s blood
pressure dips or if you have a decrease in extracellular fluid.
When you come across an exam question which talks about the kidney it's just nice to set
the scene by including some of this information.
Next, we will be looking at kidney function but also the structure of the glomerulus.
The kidney regulates extracellular fluid volume that’s one of its key roles essentially, it's
involved in a hydrogen ion homeostasis and by definition therefore the kidney plays a great
role in a pH balance. If the concentration of hydrogen ions was to increase your pH would
decrease, the blood would become more acidic and that can be very detrimental if you
consider its effects on the structure of proteins and enzymes. Even if hydrogen ion
concentration significantly decreases and the blood becomes more basic that's very
detrimental, you have the excretion of urea which is a waste product and there is a potential
for it to be used as a biomarker.
The kidneys produce erythropoietin which is a hormone which facilitates the production of
red blood cells and of course the red blood cells carry oxygen throughout the body so in
conditions when you're at high altitude where there is a lack of oxygen in the environment
erythropoietin production increases a compensatory mechanism to enhance oxygen uptake.
The kidneys are involved in vitamin D metabolism and vitamin D helps regulate the amount
of calcium and phosphate in the body so these are nutrients which are needed to keep
bones teeth and muscles healthy in general and a lack of vitamin D can lead to bone
, 4. Renal Function Tests
deformities such as rickets in children and can generally be a reason for pain felt in the in the
bones and conditions such as osteomaloxia.
The structure on the right is what a glomerulus looks like, so we have the basement
membrane, a capillary lumen and the urinary space. The basement membrane is very thin
and that serves its own purpose really because the glomerulus is actually involved in
facilitating the transfer of waste products through the bowman’s capsule and towards the
proximal tubule so it needs to be thin, you need these cells to allow passage of small solutes,
toxins whatever it may be that needs to go through.
Endothelial cells are very thin cells and then we have a large capillary lumen which is quite
beneficial, we have a large space which can then eventually filter stuff towards the tubules.
The mesangial cell and the phagocytic properties that they contain allow there to be an
element of immune function within the glomerulus itself. If you are going to be filtering
toxins and maybe waste products and it's a bacterium, you might find that it's quite useful
having immune cells present there to prevent any infection within the kidney itself.
Moving forward with their roles of the kidney you may come across aldosterone. When we
spoke about the kidney nephron towards the end, we mentioned briefly the distal tubule
and the collecting ducts. The collecting ducts which the end stage of the nephron have two
cells; the principal cells which dictate how much water will be reabsorbed and integrated
discs which determine the acid base balance.
If you have a drop in the sodium levels or extracellular fluid volume or for instance your
blood pressure decreases, this will be detected by the juxtaglomerular cells which secrete
renin. Renin acts as an enzyme to activate angiotensin into angiotensin 1 and then there's a
cascade of events where angiotensin 1 is converted to angiotensin 2 by the angiotensin
converting enzyme and angiotensin 2 is the main stimulus for the secretion of the hormone
aldosterone from the adrenal cortex. The adrenal glands sit on top of the kidneys so they're
also responsible for producing adrenaline, so it makes a lot of sense for these to produce
aldosterone and then affect kidney function because they're in close proximity with each
other. The little pathway that I just described is known as the renin angiotensin aldosterone
system (RAAS) and the net effect of aldosterone is to increase sodium reabsorption from the
kidney specifically the principal cells.
If there is a decrease in blood pressure via the actions of aldosterone the kidney reabsorbs
more of that sodium, sodium is a salt and once you reabsorb more sodium into the blood
water will follow suit via its osmotic potential, it likes to travel from high osmotic potential to
a low osmotic potential and you'll get that water being drawn in following the action or the
direction of sodium and that will increase blood pressure or it will correct the extracellular
fluid.
Vasopressin has a considerably important role in the function of the kidneys.
Vasopressin regulates tenacity body fluids; it is released from the posterior pituitary which is
a portion of the brain and it’s a response into hypertonicity and causes the kidneys to
reabsorb solute free water and return it to the tubules of the nephrons.
Vasopressin is involved when it wants to enhance water uptake in conditions where you
might be really dehydrated or there might even be some metabolic conditions going on
when you've had a loss of fluid, vasopressin is then released from the posterior pituitary,
and it facilitates the induction of these aquaporin water channels within the tubular lumen.
As water is flowing through the tubular lumen (4) these aquaporin channels allow water to
be reabsorbed back into the blood so if you have a drop in blood pressure then vasopressin
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