TUBULAR REABSORPTION
Dr. Shaikh Mujeeb AhmedAssistant ProfessorAlMaarefa College
URINARY BLOCK 313
ObjectivesDefine tubular secretionRole of tubular secretion in maintaining K+
conc. Mechanisms of tubular secretion.
• Three Basic Mechanisms (Renal Processes) Of Urine Formation include:1. Glomerular Filtration2. Tubular Reabsorption3. Tubular Secretion
URINE FORMATION
Tubular secretion
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• Secretion – transfer of material from blood into tubular fluid– Helps control blood pH– Helps eliminate substances from the body
Tubular Secretion
• First step is simple diffusion from peritubularcapillaries 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
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
Potassium handling by nephron(continued)
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 thus
principle cells increases 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
Tubular LumenPrincipal Cells
Cl -
H20 (+ ADH)
Aldosterone
K+
Na +
ATPK+
ATP
Na +
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 carriers1. For organic anions2. 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
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