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