1

Basic Mechanisms of Urine Formation

Filtration, secretion, reabsorption and excretion.

How do we determine these rates?

Master formula

Renal Physiology- Chapter 27

2

For any substance,

The rate of = rate of + rate of – rate of excretion filtration secretion reabsorption

Renal Equations (for lab exercise)

Other terms that are used to express these ideas:

•For “Rate of excretion” we often use the term “urinary output”

•For “Rate of filtration,” when referring to filtered fluid, we often use the term GFR

if referring to filtered solute, we use Tubular Load

3

Calculating Tubular Load of a Substance (any solute, “s”)

At the glomerulus, fluid and solutes are constantly being filtered and enter the tubule.

GFR is the term for the volume of plasma fluid filtered each minute.

Once in the tubule, it is “tubular fluid” and no longer plasma, but not yet urine

Tubular Load (TLs): the amount of any substance (s) entering the tubule, each minute.

TL depends on two things: the plasma concentration and the rate of filtration of that solute

TL s = Ps x GFR

4

Tubular Filtrate Resembles Plasma

By far, filtered fluid is mainly NaCl and water.

It contains other salts and electrolytes, amino acids, small sugars, vitamins and other small molecules, such as wastes.

Na+ and Cl- are present in such large amts; they are over 99% reabsorbed along the length of the tubule. (Why? Remember that plasma volume affects blood pressure)

Reabsorption of other solutes, like amino acids and sugars, is often linked to the transport of Na+

5

Tubular Lumen

Tubular Cells

Luminal MembraneNote: this side contains microvilli in PCT.

Basolateral Membranes

Filtrate arriving from Bowman’s Capsule

Peritubular Capillaries

The PCT has an extensively amplified apical membrane called the brush border and its basolateral membrane is highly invaginated and contains many mitochondria

6

Tubular ReabsorptionTransport: Active (primary or secondary) and Diffusion

Many different solutes are reabsorbed by various transport methods

As a result of solute reabsorption, osmosis will occur

As a result, water movement from the tubule will affect the gradients of other solutes still in the tubule, and if they are permeable, their reabsorption.

Figure 27-1;Guyton and Hall

7

Primary Active Transport of Na+3 modes of net reabsorption of sodium

Sodium/Potassium ATPase

Facilitated diffusion/ Secondary active transport on apical membrane

Simple diffusionBulk flow into

peritubular capillary

Figure 27-2;Guyton and Hall

8Figure 27-3;Guyton and Hall

Mechanisms of Secondary Active Transport

Across the luminal membrane:

Co-transport of Na+ Glucose Na+ Amino Acids. Na+ H+ Counter-transport

(Exchange)

• Across the basolateral membranes:glucose and amino acids diffuse out to peritubular capillaries (they are permeable there) but Na must be actively transported out of the cell by Na/K pump.

9

Transport Maximum

Some substances have a maximum rate of tubular transport due to saturation of carriers, limited ATP, etc.

• Transport Maximum (Tm): Once the transport maximum is reached for all nephrons, further increases in tubular load are not reabsorbed, but are then excreted.

• Threshold is the plasma concentration at which tubular load just exceeds the transport maximum (Tm) for reabsorption, where below threshold all solute molecules are reabsorbed, and above threshold, some solutes are not

Examples: glucose, amino acids, phosphate, sulfate

Figure 27-4;Guyton and Hall

10

1

2

3

4

5

=solute= transporter

Transport maximum is reached when carriers are saturated.

5/min

11

1

2

3

4

5

=solute= transporter

Saturation is reached, Maximum speed, Tm

Excretion

5/min

12

A patient with uncontrolled diabetes has aGFR of 90 ml/min, a plasma glucose of 200 mg/dl (2mg/ml), and a transport max (Tm) shown in the figure. What is the glucose excretion for this patient?

.

Reabsorbed

Excreted

TransportMaximum(150 mg/min)

Threshold

250

200

150

100

50

0

Glu

cose

(m

g/m

in)

a. 0 mg/minb. 30 mg/minc. 60 mg/mind. 90 mg/mine. 120 mg/min

50 100 150 200 250 300 350Filtered Load of Glucose

(mg/min)Copyright © 2006 by Elsevier, Inc.

13

Answer: Filt Glu = Reabs Glu = Excret Glu =

.

Reabsorbed

Excreted

Threshold

250

200

150

100

50

0

Glu

cose

(m

g/m

in)

Filtered Load of Glucose(mg/min)

a. 0 mg/minb. 30 mg/minc. 60 mg/mind. 90 mg/mine. 120 mg/min

GFR = 90 ml/min PGlu = 2 mg/mlTmax = 150 mg/min

50 100 150 200 250 300 350

TransportMaximum(150 mg/min)

(GFR x PGlu) = (90 x 2) = 180 mg/min Tmax = 150 mg/min

30 mg/min

Copyright © 2006 by Elsevier, Inc.

14

Interstitial Fluid

TubularLumen

Tubular Cells

Make sure you understand that Reabsorption of Water and Solutes is Coupled to Na+ Reabsorption

in the PCT

Na +

K+ATP

Na +

K+ATP Na +

- 3 mV0 mv

H20

Cl-

Urea

Na +

H +

glucose, amino acidsNa +

- 70 mV

Copyright © 2006 by Elsevier, Inc.

15Figure 27-7;Guyton and Hall

Changes in Concentration in Proximal Tubule

Special feature of PCT: is freely permeable to water

As a result of solute reabsorption, osmosis will occur

Isosmotic reabsorption…what does that mean? Define. (300mOsm)

As a result, water movement from the tubule will affect the gradients of other solutes still in the tubule, and if they are permeable, their reabsorption.

~100% Glucose and Amino Acids 67% of Filtered Sodium 65% of Filtered Water

What happens to a solute concentration in the tubule if it is reabsorbed more than water? or less than water?

16

Before we move onto the other nephron segments be mindful of the following:

The kidneys must be able to excrete urine that is either hypo-osmotic or hyper-osmotic with respect to bodily fluids

This means that the varying osmolality requires that solute be separated from water at some point along the nephron!

The loop of Henle, in particular the thick ascending limb, is the MAJOR site where solute and water are separated.

Thus the excretion of both dilute and concentrated urine requires normal function of the loop of Henle

17

LOH- Thin Descending limb

permeable to water AQP-1 water channels ??? of filtered water

reabsorbed here No active sodium

transport minimal permeability to

sodium and urea (simple diffusion

INTO the tubule only -110% of urea)

18

Thick- Ascending limb—Diluting Segment!!!!

Impermeable to water About ??? of sodium

reabsorption Fluid leaving thick

ascending limb is hypo-osmotic

Actively pumps sodium out of tubule to surrounding interstitial fluid (Na+/K+ ATPase)Na+/2 Cl-/K+ co-transporter on luminal side

Na+/H+ counter-transport (H+ secretion)Also, Ca+2, HCO3-, Mg+2, K+, and Na+ paracellularly due to positive net charge in lumen from backflow of K+

19Figure 27-9;Guyton and Hall

Sodium Chloride and Potassium Transport in Thick Ascending Loop of Henle

20

Look at properties of cells along tubule:

Cells of tubules are NOT permeable to water. Water can’t go in or out.

Cells of tubules actively reabsorb Na+ and Cl-(out of tubule and into surrounding area). Salt is removed but NOT water.

Interstitial space becomes highly concentrated!

This makes filtrate more dilute and osmolality decreases.

400

21

Purpose of the LOH- Counter Current Multiplier

to create an osmotic gradient deep in medulla of kidney, not for its own benefit, but to benefit the collecting duct that sits adjacent to it.

Creates “salt gradient.” If Loop is disabled, then collecting

duct adjacent to it cannot give concentrated urine.

Concentration and volume of urine is determined by concentration gradient produced in Loop of Henle and by the presence of certain hormones.

Urine concentration can then range from 50 mOsmolal to 1400 mOsmolal.

Let’s write in the osmolalities for the filtrate in the descending and ascending tubules

300

700

1000

12000

300

700

1000

22

Factors That Contribute to Buildup of Solute in Renal Medulla -Countercurrent Multiplier

Active transport of Na+, Cl-, K+ and other ions from thick ascending loop of Henle into medullary interstitium

Active transport of ions from medullary collecting ducts into interstitium

Passive diffusion of urea from medullary collecting ducts into interstitium

Diffusion of only small amounts of water into medullary interstitium– most absorbed in PCT.

“sluggish blood”

23Figure 27-11; Guyton and Hall

Characteristics of Early and Late Distal Tubules and Collecting Tubules Characteristics of Early and Late Distal Tubules and Collecting Tubules

• not permeable to H2O• not permeable to ureaJuxtaglomerular apparatus

• permeability to H2O depends on hormones• not permeable to urea

24

DCT- early

associated with Juxtaglomerular apparatus (helps in tubuloglomerular feedback mechanism for GFR) Mesangial cells: smooth muscle

like properties, structural support, phagocytic activity, secrete prostaglandins

Granular cells of the afferent arteriole- makes renin

Macula densa of DCT- chemoreceptors

Functionally similar to thick ascending loop

Not permeable to water (still diluting segment) nor urea

Active reabsorption of Na+, Cl-, K+, Mg++

Thiazide diuretics affect Na/Cl co-transporter

25

Late DCT, connecting tubule

Principal cells: what do they do?

No urea permeabilityK+ sparing diuretics

work here Antagonists to

aldosterone binding sites Sodium channel blockers

(reduces K+)Water reabsorption

dependent on hormones

26

Tubular LumenIntercalated Cells

Na +

ATP

Cl -

K+H+

ATP

H20 (depends on

hormones)

H +

ATPK+

ATP

ATP

K+

Copyright © 2006 by Elsevier, Inc.

Intercalated Cells what do they do?

27

Medullary Collecting Ducts-

Permeable to urea goes back to ALOH

Can reabsorb more water (ADH dependent)- to be discussed later!

important for determining final urine output

Can secrete hydrogen ions.

28

Concentration of DifferentSubstances in Tubular System

• Concentrations of solutes depend on relative reabsorption of the solutes compared to water.

• if water is reabsorbed to a greater extent than the solute, the solute will become more concentrated in the tubule (e.g., creatinine, inulin)

• if water is reabsorbed to a lesser extent than the solute, the solute will become less concentrated in the tubule (e.g., glucose, amino acids) Figure 27-14;

Guyton and Hall

29Figure 27-15; Guyton and Hall

Peritubular Capillary Net reabsorption forces

30

Determinants of PeritubularCapillary Hydrostatic Pressure

Peritubular Capillary

Glomerular CapillaryRa Re

ArterialPressure

Arterial Pressure PcRa Pc

Re Pc

31

We’ve covered filtration and reabsorption….Now, it’s time for Tubular Secretion

First step is simple diffusion from peritubular capillaries to interstitial fluid

Enter to tubular cell can be active or passive

Exit from tubular cell to lumen can be active or passive

Examples: potassium, hydrogen, organic acids, organic bases, NH3

H+, K+, NH3

Organic acids and bases

Secretion = Excretion – Filtration+ reabsorption (0)