Tubular Reabsorption & Secretion

بسم الله الرحمن الرحيم

Dr.Mohammed Sharique Ahmed QuadriAssistant prof. Physiology

Al Maarefa College

Objectives

• Define tubular reabsorption, tubular secretion,& excretion.

• Explains the process of reabsorption and

secretion with example of glucose ,urea, PAH.

• Explain the concept of tubular maximum and renal threshold for Glucose.

Urinary excretion =

Glomerular filtration - Tubular reabsorption + Tubular secretion

WHERE & HOW

Tubular reabsorption and tubular secretion

• Reabsorption – return of most of the filtered water and many solutes to the bloodstream– About 99% of filtered water reabsorbed– Proximal convoluted tubule cells make largest

contribution– Both active and passive processes– Reabsorbed substances carried by the peritubular

capillaries to the venous system.– Tubular reabsorption is highly selective (unlike

filtration).

Tubular reabsorption and tubular secretion

• Secretion – transfer of material from blood into tubular fluid– Helps control blood pH– Helps eliminate substances from the body– Steps

• Simple diffusion from peritubular capillaries to interstitial fluid.

• Entry to tubular cell can be active or passive.• Exit from tubular cell to lumen can be active or

passive.

Tubular Secretion

Tubular secretion is important for: Disposing of substances not already in the filtrate

Eliminating undesirable substances such as urea and uric acid

Ridding the body of excess potassium ions

Controlling blood pH by secreting H+

Reabsorption and Secretion

• filtered load: The amount of a substance filtered into Bowman's space per unit time is called the

• Filtered load = GFR [P]x

Px = Concentration of substance in plasma • Excretion rate = V [U]x

[U]x = Concentration of substance in urineV= volume of urine

• Reabsorption/secretion rate = FL – Excretion rate

Reabsorption Routes

– Paracellular reabsorption (between the cells)

• Between adjacent tubule cells

• Tight junction do not completely seal off

interstitial fluid from tubule fluid

• Passive

– Trans cellular reabsorption (across the cell) • Through an individual cell

Reabsorption • Once the substance has moved pass the tubular epithelium cell

into the interstitial space, bulk flow then accounts for its movement back into the peritubular capillaries.

Reabsorption - Transport Mechanisms

• Transport mechanisms Reabsorption of Na+ especially important

– Primary active transport• Sodium-potassium pumps in basolateral membrane

only

– Secondary active transport• co-transport (glucose, amino acids)• counter-transport (K+, H+)

– Passive Reabsorption• Osmosis (H2O)• Electrostatic attraction (Cl-)

Sodium Reabsorption• Na+-K+ ATPase located at

the basolateral membrane of tubular cells

• Creating concentration gradient for Na+ to diffuse into the tubular cells from tubular lumen (diffusion).

• Keeps interstitial [Na+] high creating concentration gradient for Na+ to diffuse into blood (Bulk flow).

Glucose Reabsorption

• Na+-glucose co transporter in luminal membrane – called SGLT 2

• Proteins involves in facilitated diffusion of glucose at peritubilar membrane is GLUT 2

Additional Examples of Secondary Active Transport

Additional Examples of Secondary Active Transport

These substances include some amino acids, lactate, inorganic phosphate (Pi), H+, and Cl-.

Reabsorption Transport Maximum(Tmax)• Like transport systems elsewhere, renal active

transport systems have a maximal rate, or transport maximum (Tm), at which they can transport a particular solute.

• As the tubular load increases, all active transport systems becomes saturated, which creates a limit to the rate of substances transport.

• Therefore, excess of that substance is excreted.

Glucose Transport Maximum

Figure 27-4;Guyton and Hall

RENAL THRESHOLD FOR GLUCOSE• The is the plasma level at which the glucose first

appears in the urine . • The actual renal threshold is about 200 mg/dL of

arterial plasma, which corresponds to a venous level of about 180 mg/dL.

• This deviation is called splay. • What causes in splay ?

– TmG in all the tubules is not identical and – All the glucose were not removed from each tubule

when the amount filtered was below the TmG.

A uninephrectomized 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

50 100 150 200 250 300 350 Filtered Load of Glucose

(mg/min)

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

Passive Reabsorption

• Passive reabsorption depends on:– Electrical gradient (electrostatic attraction).

– Concentration gradient

– Membrane permeability

– Time available in the tubule for reabsorption

Sodium reabsorption makes both intracellular and extracellular fluid hypertonic to the tubular fluid.

Water follows with sodium into the peritubular capillaries.

Passive Reabsorption Secondary water Reabsorption via osmosis

H2O

Na+Na+

capillary Tubular cell Tubularlumen

Negative ions (Cl-) tend to follow with the positive sodium ions by electrostatic attraction.

Passive Reabsorption

Secondary ion reabsorption via electrostatic attraction

Na Na+

Cl-

capillary Tubular cell Tubularlumen

Urea–Example of Passive Reabsorption Na+ reabsorption

H2O reabsorption

Increase concentration of

urea in tubular fluid

Passive reabsorption of urea

Figure 27-5;Guyton and Hall

Mechanisms by which Water, Chloride, and Urea Reabsorption are Coupled withSodium Reabsorption

Mechanisms by which Water, Chloride, and Urea Reabsorption are Coupled withSodium Reabsorption

PAH –EXAMPLE OF SECRETION • PAH is an organic acid• Used for measurement of renal plasma flow • Both filtered and secreted • PAH transporters located in peritubular

membrane of proximal tubular cells.• There are parallel secretory mechanism for

secretion of organic bases like quinine and morphine

PAH Filtration, Secretion & Excretion

References

• Human physiology by Lauralee Sherwood, seventh edition

• Text book physiology by Guyton &Hall,11th edition

• Text book of physiology by Linda .s contanzo,third edition