Physiology (5) 2/4/2018 Mohammad jaber
Slides are embedded in the sheet in bold, some figure are not included sorry for any mistake,, lets go.. There are two types of urine : 1-diluted urine ( Excess fluid in the body > less reabsorption > too much urine & less osmolarity (by reducing ADH secretion >> urine excreted with 50 mosm/l ). ) 2-concentrated urine (Less fluid in the body > increase reabsorption > less urine & more osmolarity (1200-1400 mosmole/l) ) 1. Dilute urine : Nephron function continuous reabs. Solutes while failing to reabsorbe water in distal tubule and coll. duct>> by reducing ADH secretion >> urine excreted with 50 mosm/l. If large excess of water in the body, kidneys excrete up to 20 L/day. ADH (VASOPRESSIN) Increase osmolarity increase ADH which increase permeability of distal tubules and collecting ducts to water which increase H2O absorption and decrease urine volume and vice versa 2. Concentrated urine : Maximum urine concentration 1200-1400 mosmole/l. Australian hopping mouse 10000mOsm/L. For concentrated urine need High ADH level & Hyperosmotic renal medulla Obligatory urine volume 70 Kgm human excrete 600 mosm/day >> minimal volume of urine = 600 mosm/day = 0.5 L/d 1200 mosm/l
now,how kidney form the diluted and concentrated form of urine?? **Proximal tubule: Reabsorption for water and solutes are equal proportion (Remain isosmotic) **Descending: H2O reabsorbed by osmosis & fluids reaches equilibrium with surrounding interstitium (Hyperteonic). **Ascending: especially thick, water impermeable, solutes heavily reabsorbed osmolarity about 100 mosm/l (Hypotonic). No effect of ADH **Early distal : same as above 50 mosm/l. **Late dist & coll duct: ADH effect in the proximal tubule, the osmolarity is the same from the beginning until the end of the proximal tubule. In the descending tubule, the water is reabsorbed,so the osmolarity will increase ( because the membrane is permeable to water ).* as far as you go deeper in the descending the osmolarity will increase. Like in the medullary nephron which goes deeper until the papilla. In the ascending tubule, the membrane is permeable to solutes and ions but not to water,so the osmolarity decreased. In distal tubule, it's not permeable to water until presence of ADH, especially in late distal,collecting tubule and collecting duct ( so in the absence of ADH, there's no water reabsorbed, so as you go from the distal tubule until collecting tubule the osmolarity will be decreased ) ** when there's shortage of water > the ADH increase > the absorbtion increase > the osmolarity increase.
So, as much as you have ADH, the possibility of concentrated urine is there, and vice versa. All of these called ( counter current mechanism or C.C. Multiplier ) where the osmolarity increase or decrease. In the descending, there's equilibrium between the tubule and interstitium, because of the permeability of water. But in the ascending, there's difference (gradient ) between interstitium and tubule that equal at least 200 mosm/l. The question here is, why there's equilibrium between inerstitium and descending?? or why the osmolarity not decreased in the interstitium due to reabsorbtion of water toward the interstitium????? The answer is that, as there's rebsorbtion of water from the descending, there's reabsorbtion of ions (NaCl) from the ascending tubule, that increase the osmolarity of the interstitium to become equal to the descending tubule.
**Student question, why there are differences in osmolarity in the interstitium itself?? The doctor answer, the interstitium dissected into segment which is parallel to each part of tubule. **Doctor question, in this case which part from the ascending is more permeable to solute (by imagination)?? The answer, the lower part, because the osmolarity in the interstitium is high in the lower part (900 mosm ) **Doctor question, which part of the descending is more permeable to water (by imagination )?? The answer, the upper part, because the osmolarity in the interstitium is less ( 300 mosm ).
The osmolarity in the interstitium and tubule, come from two parts : 1-Na (make 50% of osmolarity in the interstitium),, the permeability of Na is active process 2-urea ( make the other 50% of osmolarity ) the permeability is passive. it's a waste product that is secreted from collecting tubule ( but not all of it ), 50% filtrate from the collecting tubule toward the lower part of ascending and descending tubule, which involved in C.C. mechanism. As a quick recap ; More you go down in the tubule, the osmolarity increase,due to : 1-water permeability from the descending 2-ion permeability from the ascending into interstitium 3-urea **** the absorption of ions from the ascending is more than the descending.
The peritubular capillary which surround the medullary loop of henle, called vasa recta. ** the water and ions in the interstitium must be regulated, and shouldn't stay there to avoid their accumulation, so the water and solute regulated and removed from the interstitium by the vasa recta. **More pressure in the interstitium > more push toward the capillary, and vice versa. So as a summary, the vasa recta is not involved in creation of osmolarity,it's only keep it.
the red area, is the range between the maximal and minimal osmolarity at different tubule **more volume > less osmolarity, and vice versa. Some slides (123-125,137,139, 140, 143 ) are not involved please go back for them Good luck