1. Physiology 441 The Renal System, Chp. 14
Text: Human Physiology (Sherwood), 6th Ed.
Julie Balch Samora, MPA, MPH
, Room 3145

2. Review How is glucose transported? What is the renal threshold?
Do we see this for passive transport?
Do the kidneys regulate glucose/ phosphate plasma concentration?

3. Review cont… What is the vascular sequence in the nephron?
What is the tubular sequence?

4. Review cont…
What happens if ↓ NaCl, ↓ ECF volume, and ↓ arterial blood pressure?

5. Tubular Secretion Additional mechanism to eliminate certain substances
Secretion of H+, NH3, K+, organic anions and cations
Acid and ammonia secretion imp. in acid-base balance
Ammonia is secreted during acidosis in order to buffer the secreted H+

6. H+ and NH3 secretion
Hydrogen and ammonia secretion aid with acid-base balance
H+ secretion provides a highly discriminating mechanism for varying the amt of H+ excreted in the urine, depending on the acidity of the body fluids
NH3 is secreted in pronounced acidosis to buffer the H+ secreted into the urine

7. K+ Secretion
K+ is actively reabsorbed in the proximal tubule, but is also actively secreted in the distal tubule.
Allows fine degree of control over plasma K+ concentration
Secretion of K+ is variable and subject to aldosterone control
Even slight fluctuations in ECF [K+] can alter nerve and muscle excitability

8. Organic anion and cation secretion
The proximal tubule contains 2 different carriers for secreting organic ions
These systems aid in secreting foreign organic substances
The liver helps this process by converting many foreign substances into anionic metabolites

9. Process of Secretion
Renal Processes 1

10. Urine excretion
The final quantity of urine formed averages about 1ml/min
This urine contains a high conc. of waste products & low or no conc. of substances needed by the body
Minimum volume of urine to eliminate wastes is 500 ml/day

11. Plasma Clearance The amount of plasma “cleared” of a substance
Plasma clearance for a substance that is FILTERED, but not REABSORBED or SECRETED == GFR == 125ml/min
EXAMPLE = INULIN

12. INULIN- Filtered, NOT reabsorbed, NOT secreted
Renal Processes 2

13. Plasma Clearance
Plasma clearance for a substance that is FILTERED AND REABSORBED, but NOT SECRETED < GFR
EXAMPLE = GLUCOSE, UREA
Clearance rate can be anywhere from 0 up to normal clearance (125 ml/min) depending on amount reabsorbed

14. Glucose/urea- Filtered and reabsorbed, NOT secreted
Renal Processes 2

15. Plasma Clearance
Plasma clearance for a substance that is FILTERED AND SECRETED > GFR
EXAMPLE = PAH (Para-aminohippuric acid)
Not only is the plasma that is filtered cleared of that substance, but additional amt cleared from plasma which was not filtered
Plasma clearance rate for PAH=RENAL PLASMA FLOW

16. Plasma clearance rate for PAH = Renal Plasma Flow
Renal Processes 2

17. Plasma Clearance Questions
What is the PC for a substance that is FILTERED AND REABSORBED, but NOT SECRETED?
What is the PC for a substance that is FILTERED AND SECRETED?
What is the PC for a substance that is FILTERED, but not REABSORBED or SECRETED?

18. Osmolarity
Measure of the concentration of individual solute particles dissolved in fluid
The higher the osmolarity, the higher the concentration of solutes
WATER moves by osmosis from an area of lower osmolarity (higher water conc) to an area of higher osmolarity (lower water conc) until the concentration difference is eliminated

19. Osmolarity
Isotonic- solution is the same conc. as normal body fluids (300 milliosmols/L)
Hypotonic- solution is more dilute than normal body fluids (< 300 milliosmols/L)
Hypertonic- solution is more concentrated than normal body fluids (> 300 milliosmols/L)

20. Question
How can the kidneys put out a concentrated urine when the body is dehydrated?
How can the kidneys excrete a dilute urine when the body is overhydrated?

21. Other nephrons emptying into the same collecting duct
Distal
tubule
Distal
tubule
Glomerulus
Proximal
tubule
Bowman’s
capsule
Proximal
tubule
Cortex
Medulla
Descending
limb of
loop of
Henle
Collecting
duct
Loop of Henle
Other nephrons emptying into
the same collecting duct
Vasa recta
Ascending
limb of
loop of
Henle
To renal
pelvis
Fig. 14-5, p. 505

22. Countercurrent System
There is a vertical osmotic gradient in the interstitial fluid of the medulla
This gradient, with the help of ADH (vasopressin) allows the kidney to vary the concentration of the urine depending on the body’s needs
Countercurrent multiplication- loops of Henle and Juxtamedullary nephrons
Countercurrent exchange- vasa recta of the juxtamedullary nephrons

23. Countercurrent System
At the end of the proximal tubule and beginning the loop of Henle, the filtrate is isotonic (300 mosm/L)
Na+ is actively reabsorbed against its concentration gradient in the proximal tubule, creating an osmotic gradient, pulling water across to maintain osmotic equilibrium

24. Countercurrent Multiplication
The loops of Henle of the juxtamedullary nephrons are responsible for establishing the concentration gradient in the renal medulla
Desc. limb is freely permeable to H2O, but NOT NaCl – so H2O goes into IF
The ascending limb actively transports out NaCl
Asc. Limb IMPERMEABLE to water

26. Medullary vertical osmotic gradient
There is a countercurrent flow produced by the close proximity of the two limbs
The ascending limb produces an interstitial fluid that becomes hypertonic to the ascending limb.
This interstitial fluid faces against the flow of fluid (countercurrent) in the descending limb, attracting the water by osmosis for reabsorption

27. Countercurrent Multiplication
The fluid in the descending limb becomes progressively more concentrated
At the bottom, its concentration is 1200 mosm/L!!
As the asc. Limb is impermeable to water, when NaCl is pumped out, water remains, causing the remaining liquid in the loop to become hypotonic (100 mosm/L)

28. Countercurrent Multiplication
Therefore, a vertical osmotic gradient varying from 300 to 1200 mosm/L exists in the medulla
The filtrate as it leaves the loop of Henle to enter the distal tubule is HYPOTONIC
Gets its name due to the flow in opposing directions through the two limbs and the stepwise multiplication of concentration due to transport and permeabilities

31. Countercurrent

32. Countercurrent exchange
The vasa recta are responsible for preserving the concentration gradient in the medulla that has been established by the loop of Henle
The blood in the vasa recta follows the same course as the loops of Henle of the juxtamedullary nephrons.
The vasa recta are exposed to the same gradient in the IF-blood concentration is similar

33. Countercurrent exchange
This process is a passive, osmotic exchange b/t the IF and two closely opposed vessels with fluid flowing in opposite directions

35. Importance of the Vertical Gradient
The collecting ducts are normally impermeable to water
However, if the body needs to conserve H2O, ADH (vasopressin) is secreted
ADH increases the permeability of the distal tubules and collecting tubules to H2O

36. ADH- Saving Water
Due to the vertical osmotic gradient, the tubular fluid loses more water into the IF, which eventually enters the plasma
At the end of the collecting tubule, it is possible to have urine concentrated to 1200 mosm/L (very small volume excreted)

37. Role of ADH
ADH is produced in the hypothalamus and stored in the posterior pituitary. The release of this substance signals the distal tubule and collecting duct, facilitating the reabsorption of water.
ADH works on tubule cells through a cyclic AMP mechanism.
During a water deficit, the secretion of ADH increases. This increases water reabsorption.
During an excess of water, the secretion of ADH decreases. Less water is reabsorbed. More is eliminated.

40. Elimination of excess H2O
If overhydrated, no ADH released, therefore distal tubules and collecting ducts remain impermeable to H2O
Even though water wants to leave (due to osmotic forces), it cannot
Therefore, a large volume of dilute urine is excreted

42. Urine Concentration

43. Summary
The medullary vertical osmotic gradient and ADH allow the kidney to excrete urine of varying concentrations
Without this osmotic gradient, we could not produce a concentrated urine, and would be unable to conserve water
We would likewise be unable to rid the body of excess water w/o this system