Hello guys. Today we will be
discussing concentration of urine. What does it mean for a fluid to be
concentrated? To my understanding when a
fluid is said to be concentrated it means the fluid has more of solutes than
solvent, Now this is what the kidney does when the water content of the body is
reduced, it reabsorbs large amounts of water from the tubules than solutes and produces concentrated urine i.e. the urine now has more solutes than solvent.
This involves two processes: development and maintenance of a medullary gradient
by counter current mechanism and secretion of antidiuretic hormone (ADH) by the
posterior pituitary.
study the diagram below closely, it will aid your understanding.
The juxtamedullary nephrons are long
and penetrate deep into the medulla towards the renal sinus and because of this
fact they are involved in the concentration of urine. This nephrons have a loop
of Henle (LOH) which has a descending limb, hair pin bend and ascending limb,
so also the vasa recta( peritubular capillaries supplying the juxtamedullary
nephrons).The ascending limb of the LOH is supplied by the descending limb of
vasa recta; ascending limb of vasa recta supplies the descending limb of LOH.
glomerular filtrate enters the proximal tubules with an osmolarity of 300
mosmo/L and flows through the entire loop. The ascending limb of the loop of
henle is permeable to NaCl so it diffuses into the medulary interstisium from
here it diffuses into the descending limb vasa recta; blood flow in the vasa
recta is very sluggish and it flows in the opposite direction to the glomerular
filterate, By the blood time the blood gets to the ascending limb of the vasa
recta it is highly concentrated with NaCl this results in the diffusion of NaCl
into the medullary interstitium and then into the descending limb of the LOH,The
descending limb of LOH is highy permeable to water so water diffuses out into
the medullary interstituim and from here into the vasa recta which drains into
the renal vein, I.e. there is an exchange of NaCl and water between the
ascending limb of vasa recta and descending limb of LOH
The osmolarity of the medullary
interstitium increases as you go deep into the medulla because: more NaCl is
added into the fluid in the tubule from the glomerular filterate, a huge amount
of NaCl carried from the ascending limb of LOH is added to the descending limb
LOH, this results in gradual increase of the osmolarity from 300 -1200 mosm/L, this
forms the countercurrent multiplier by LOH. The vasa recta serves as reservoir
that holds water and NaCl that the LOH gives out and exchanges it for water or
NaCl when required by the LOH, now this is the countercurrent exchanger
mechanism by vasa recta. These two processes sum up the countercurrent
mechanism that develops and maintains the medullary gradient and
hyperosmolarity of the medullary interstitium. Note that the fluid following
out of the ascending limb of the LOH is dilute.
The fluid that gets to the distal
convoluted tubule is usually dilute with an osmolarity of about 100mosmo/L. In
the early part of this tubule more solutes are reabsorbed,water is not
reabsorbed here i.e. the fluid here becomes more dilute. In the presence of
high level of ADH large amount of water is being reabsorbed from the late part
of distal convoluted tubule and cortical collecting duct, urea is not
reabsorbed here so there is a buildup of urea concentration and by the time the
fluid gets into the medullary collecting duct it has high concentration of urea
which diffuses into the medullary interstium and then into the ascending limb
of LOH, the high levels of urea in the medullary interstitium contributes to
the hyperosmolarity of the medullary interstitim. The levels of ADH also causes
large amounts of water to diffuse from the medullary collecting duct into the
medullary interstitium until osmotic equilibrium is reached i.e. the osmolarity
of the medullary interstitim and the fluid in the collecting duct are equal and
this ranges between 1200 -1400 mosm/L.
The fluid then drains into large
collecting ducts then into the renal pelvis and passes into the bladder through
the ureter, when the bladder is full the micturition occurs.
I
hope guys understood everything. You may have to use your imagination to
picture how the solutes move in out of tubules. There won’t be quizzes today.
Don’t forget to comment. i love you all😙💕
