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TUBULAR SECERETION. URINARY BLOCK 313. Dr. Shaikh Mujeeb Ahmed Assistant Professor AlMaarefa College. Objectives. Define tubular secretion Role of tubular secretion in maintaining K + conc. Mechanisms of tubular secretion. URINE FORMATION.

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tubular seceretion



Dr. ShaikhMujeeb Ahmed

Assistant Professor

AlMaarefa College

  • Define tubular secretion
  • Role of tubular secretion in maintaining K+ conc.
  • Mechanisms of tubular secretion.


  • Three Basic Mechanisms (Renal Processes) Of Urine Formation include:
    • Glomerular Filtration
    • Tubular Reabsorption
    • Tubular Secretion
tubular secretion
Tubular secretion
  • Secretion – transfer of material from blood into tubular fluid
    • Helps control blood pH
    • Helps eliminate substances from the body
tubular secretion1
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

Calculation of Tubular Secretion

Secretion = Excretion - Filtration

H+, K+,NH3

Organic acids

and bases

tubular secretion2
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+
tubular secretion3
Tubular Secretion
  • Most important substances secreted by the tubules:
    • H+
      • Important in regulating acid-base balance
      • Secreted in proximal, distal, and collecting tubules
    • K+
      • Keeps plasma K+ concentration at appropriate level to maintain normal membrane excitability in muscles and nerves
      • Secreted only in the distal and collecting tubules under control of aldosterone
    • Organic ions
      • Accomplish more efficient elimination of foreign organic compounds from the body
      • Secreted only in the proximal tubule
potassium balance
Potassium balance
  • 98% of K+ is in ICF & 2% in ECF
  • ICF = 150 m Eq/L & in ECF = 4.5 mEq/L
  • Balance → intake = out put
  • Maintenance of K balance is important in normal functioning of excitable tissue
importance of regulating plasma k concentration
Importance of regulating plasma K+ concentration
  • K+ plays a key role in the membrane potential of excitable tissues.
  • Both increase and decrease in plasma K+ can change intracellular to extracellular K+ conc. Gradient which can change the RMP.
  • Its impact on the heart – decreased cardiac excitability
  • Rise in ECF K+ conc. decreases excitability of the neurons & skeletal muscle cells.
  • Decrease in ECF K+ lead to skeletal muscle weakness, diarrhea and abdominal distension.

Potassium handling by nephron(continued)

  • Distal tubule & collecting ducts :
    • Responsible for adjustment ofK+excretion by either re absorption or secretion as dictated by need
      • α -Intercalated cells : absorption of potassium if person is on low K+ diet
      • Principle cells : if person on normal or high K+ diet potassium is excreted by principle cells
      • The magnitude of potassium excretion is variable depending on diet & several other factors for eg.aldosterone,acid base status ,flow rate etc
effect of h secretion on k secretion
Effect of H+ secretion on K+ secretion

During acidosis H+ secretion is increase lead to retention of K+.

factors affecting k secretion
Factors affecting K+secretion
  • Magnitude of K+ secretion is determined by the size of electrochemical gradient across luminal membrane
  • Diet:

High K+ diet concentration inside thus principle cells increases electrochemical gradient across membrane

factors affecting k secretion continued
Factors affecting K+secretion(continued)
  • Aldosterone :
    • Aldosterone Na+ re absorption by principle cell by inducing synthesis of luminal membrane Na+ channels & basolateral membrane Na+- K+ channel
    • more Na+ is pumped out of the cell simultaneously more K+ pumped into the cell
    • Thus increasing the electrochemical gradient for K+ across the luminal membrane that leads to increase K+ secretion
dual effect of aldosterone

Fall in Na+

- through RAAS

Increase in K+


Late Distal, Cortical and Medullary Collecting Tubules

Tubular Lumen

Principal Cells

H20 (+ ADH)

Na +

Na +







aldosterone actions on late distal cortical and medullary collecting tubules
Aldosterone Actions on Late Distal, Cortical and Medullary Collecting Tubules
  • Increases Na+ reabsorption -principal cells
  • Increases K+ secretion - principal cells
  • Increases H+ secretion - intercalated cells
relationship between na absorption k secretion
Relationship between Na+ absorption & K+ secretion
  • High Na+ diet:
    • more Na+ will be delivered to principle cells ,more Na+ is available for Na+- K+ATPase than more K+ is pumped into the cell which increases the driving force for K+ secretion
  • Diuretics :
    • loop & thiazide diuretics inhibit Na+ re absorption in part of tubule earlier to principle cells, so increases Na+ delivery to principle cells , more Na+ is reabsorbed & more K+ is excreted
organic anion and c ation secretion
Organic Anion and Cation secretion
  • Proximal tubule contains two types of secretory carriers
    • For organic anions
    • For organic cations
  • Organic ions such as Prostaglandin, epinephrine – after their action removed from blood
  • Non filterable organic ions also removed
  • Chemicals, food additives, non nutritive substances
  • Drugs – NSAID, antibiotics
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
urea uric acid
UREA & Uric acid
  • Urea is freely filtered – 50% reabsorbed in PCT.
  • Urate is freely filtered
  • In PCT there is reabsorption and secretion takes place.
  • In the initial & middle part of PCT reapsorption is more than secretion
  • In the distal portion of PCT moderate amount of urates are secreted.
  • Human physiology by Lauralee Sherwood, seventh edition
  • Text book of physiology by Linda .s contanzo,third edition
  • Text book physiology by Guyton &Hall,11th edition