hi let's talk about the nephron the nephron is the functional unit of the
it comes in two basic varieties there are cortical nephrons cortical nephrons cortical nephrons sit
a little higher in the renal cortex and they're more ubiquitous about four fifths or 80% of the
nephrons in the kidney are cortical nephrons there are also juxta medullary nephrons juxta
medullary nephrons are the first ones that are being fed by afferent arterioles off of
the cortical radiant arteries these sit lower in the cortex and they have a very elaborate
nephron loop that goes very deep into the medulla juxta medullary nephrons are only about one fifth
of the total population of nephrons in the kidney but they are exceedingly good at pulling out more
water from the filtrate than our cortical nephrons and in other organisms especially desert-dwelling
organisms juxta medullary nephrons are much more frequent than they are in humans now when
we look at the individual parts of a nephron it's really quite simple so we have the renal corpuscle
the renal corpuscle consists of the renal or the Bowman's capsule or just the capsule that
surrounds the glomerulus and the glomerulus is a specialized bed of capillaries that converts
blood into filtrate that filtrate then travels through what's known as the proximal convoluted
tubules so it's a curlicue of bends and twists the proximal convoluted tubule is the primary
site of reabsorption for the nephron so there's a lot of materials that leave the filtrate and go
back into the cardiovascular system here there's also a minimal amount of secretion there from
the proximal convoluted tubule we move into the nephron loop the nephron loop has a descending
and an ascending limb the nephron loop is good for catching little last bits of water in the
filtrate that we want to reabsorb back into the cardiovascular system and then beyond the nephron
loop we have the distal convoluted tubules that feeds into the collecting duct the collecting
ducts are usually held in common between several nephrons and these descend through the cortex and
they end at the renal papilla and right at the renal papilla is the point of no return for when
filtrate really becomes urine so the vasculature of the nephron looks exceedingly complex but it
can actually be quite simple so recall coming around the other bend is the arcuate artery
and that spawns the cortical radiate artery and the cortical radiate artery gives rise to the
efferent arterioles those afferent arterioles feed into the glomerular so that's half the
battle they're coming off of the glomeruli are the efferent arterioles so these leave the glomeruli
and they then spawn what are known as peritubular capillaries these peritubular capillaries surround
the entirety of the proximal convoluted tubules anem loop and the distal convoluted tubule and
what's going to happen is there's going to be reabsorption of materials from the filtrate
back into the CVS and then secretion of materials from the CVS back into the filtrate now the one
difference for juxta medullary nephrons versus cortical nephrons is that coming off of that
efferent arteriole from the glomerulus not only do we have the peritubular capillaries but we
also have something known as the Vaes erecta the Vasa recta are these straight vessels that come
across and surround the nephron loop these vases erect are going to be very important for pulling
away water from the nephron loop and I'll show you how that's accomplished now following the
peritubular capillaries we have some pair two you leave annuals you could see some there and there
that are going to feed into the cortical radiate veins and then back into the arcuate veins those
arcuate veins are going back into inter lobar veins back into segmentals and then finally
back into renal veins now I'd like to take you step-by-step through the nephron and in what's
going on in each part of the nephron so let's start here at the the renal corpuscle so we've
got the renal capsule also known as Bowman's capsule and that's really going to surround
the glomerulus and it's going to be here that the process of filtration occurs filtration is
going to be very important because we're taking blood and we're turning it into filtrate and
you can think of filtrate as pre urine what's interesting is given the amount of blood flow
through the kidney there are approximately 1,500 to 2,000 liters of blood flowing through the
kidneys daily and that process of filtration at the capsule is able to reduce that blood down to
about 150 to 200 liters of filtrate now if you can imagine losing 150 to 200 liters of fluid daily
that would be very problematic for your survival you would you'd be desiccated in no time luckily
we have this process of tubular reabsorption where we pull materials out of the filtrate and into the
cardiovascular system so that we can reclaim a lot of that filtrate mostly water so the the nephron
has such a dynamic filter for the kidney is able to go from 150 to 200 liters of filtrate to about
1.5 to 2 liters of urine on a daily basis which is a phenomenal feat of physiology now from the
corpuscle we have the filtrate traveling along the proximal convoluted tubule or the PCT the proximal
convoluted tubule is significant because this is the principal site of reabsorption in the nephron
so in terms of the total amount of reabsorption 66 percent of various ions the salts that they form
in water being reabsorbed they're 100 percent of glucose and amino acids well that would be up to
the renal threshold of glucose which is about 160 to 180 milligrams per deciliter 80% of the
citrate the phosphate the potassium urea all of these things the lion chair being reabsorbed in
the proximal convoluted tubule things that are being secreted include bioactive substances so
pharmaceuticals medications are being moved from the cardiovascular system into the filtrate and
ammonia as a byproduct of metabolism is being moved into the filtrate from the cardiovascular
system now from here blood is going to travel down into the nephron loop now the nephron loop is also
referred to as the loop of Henle and what happens is filtrate descends and as filtrate descends
sodium passively moves out of the filtrate and into the medulla well water always is going to
follow sodium and so what we have is the outward flux of water from the filtrate now the Vasa recta
which are these vessels here pick up that water and they conduct it away from the medulla so that
water is now back in the cardiovascular system and it leaves the sodium and so what we have is
quite a buildup of sodium this becomes a richly hypertonic environment and whenever we have a
large amount of sodium building up places that makes it even easier for water to leave so more
water leaves water is picked up by the Vasa recta and conduct it away by the cardiovascular
system as that renal medulla becomes more than more hypertonic eventually that filtrate is
going to leave the nephron loop and go into the distal convoluted tubules in the distal
convoluted tubules we have a further refinement of both tubular secretion and reabsorption now
there's going to be two populations of cells that are going to be significant for this process the
first being intercalated cells intercalated cells are going to be of supreme importance to helping
maintain ph homeostasis because they're the ones that determine if bicarbonate ions and protons
either stay in the filtrate or are reabsorbed or are secreted into the filtrate and so depending on
the conditions in the blood these intercalated cells are able to either keep or secrete ions
to finally adjust pH the other population of cells our principal cells and principal
cells are going to be important for the final refinement of water reabsorption in the distal
convoluted tubule by two ways the first is by that renin-angiotensin-aldosterone or RA a pathway
that we discussed previously and what happens with these principle cells is that aldosterone
which is the final a in the RAA pathway causes the principal cells to move sodium out of the
filtrate and as a result of moving sodium out of the filtrate water is going to follow it there's
also a hormone called vasopressin or antidiuretic hormone that enables these principal cells to
push water out of the filtrate as well the primary site of reabsorption for water is going to be the
proximal convoluted tubule but the nephron loop and the distal convoluted tubules are also going
to be very helpful for refining that and then finally here in the distal convoluted tubules we
have the ability to retain calcium so parathyroid hormone secreted by the parathyroid gland is going
to cause the distal convoluted tubules to keep or reabsorb any calcium that might be
found in the filtrate at this and push it into the cardiovascular system so that we can maintain the
the body's supply of blood calcium so we've talked about the nephron the different types of nephron
and the parts of the nephron and what they're either able to keep through reabsorption or
secretion back into the filtrate which eventually is going to be excreted via the urine thank you