Download TUBULAR REABSORPTION

TUBULAR REABSORPTION
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
• Three Basic Mechanisms (Renal Processes) Of
Urine Formation include:
1. Glomerular Filtration
2. Tubular Reabsorption
3. Tubular Secretion
Tubular secretion
• Secretion – transfer of material from blood
into tubular fluid
– Helps control blood pH
– Helps eliminate substances from the body
4
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 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 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
 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
• 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
Potassium handling by nephron(continued)
 Distal tubule & collecting ducts :
 Responsible for adjustment of K+ 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
During acidosis
H+ secretion is
increase lead
to retention of
K+.
Principle cells in Late DCT & CT
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
principle cells increases
thus
electrochemical
gradient across
membrane
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
Principal Cells
Tubular Lumen
H20 (+ ADH)
Na +
Na +
K+
ATP
K+
Cl -
Aldosterone
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
 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 Cation secretion
• Proximal tubule contains two types of secretory
carriers
1. For organic anions
2. 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 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.
References
• 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