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TUBULAR SECRETION
Lecture-5Dr. Zahoor
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Objectives – Tubular SecretionDefine tubular secretionRole of tubular secretion in
maintaining K+ conc. Mechanisms of tubular secretion.
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• Three Basic Mechanisms (Renal Processes) Of Urine Formation include:1. Glomerular Filtration2. Tubular Reabsorption3. Tubular Secretion
URINE FORMATION
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TUBULAR SECRETION
Tubular Secretion is transfer of substances from the peritubular capillaries (capillaries surrounding the tubule) into the tubular lumen.
It helps to eliminate the compounds from the body
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Tubular Secretion
• First step is simple diffusion from peritubularcapillaries to interstitial fluid
• Entry from interstitial fluid 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
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Calculation of Tubular Secretion
Secretion = Excretion - Filtration
H+, K+, NH3
Organic acids and bases
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Tubular Secretion
Tubular secretion is important for: Disposing of substances not already in the filtrate Eliminating undesirable substances such as urea and uric acid
Getting rid of excess potassium ions
Controlling blood pH by secreting H+
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TUBULAR SECRETION
The most important substances secreted are
- Hydrogen ion - Potassium ion - Para Amino Hippuric acid ( PAH) - Organic anion and cations - Drugs e.g. penicillin, aspirin,
Cemitidine - Hormones e.g. erythropoietin,
renin
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TUBULAR SECRETION
Hydrogen Ion Secretion: Hydrogen Ion Secretion is important in acid
base balance Hydrogen ion is secreted in the tubular lumen
and eliminated from the body in the urine H+ ion is secreted in proximal, distal, and
collecting tubule
We will discuss H+ ion secretion in detail with acid base balance.
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TUBULAR SECRETION
Potassium Ion Secretion: K+ ion secretion is controlled by
aldosterone 98% of K+ is intracellular (ICF – K =
140meq/l) ECF – K+ = 4meq/l Slight changes in ECF – K+ can affect
the membrane excitability, therefore, plasma K+ concentration is tightly controlled by the kidneys
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TUBULAR SECRETION OF K+
K+ is actively reabsorbed in PCT Na+ is actively reabsorbed and K+ is
actively secreted by principal cells in the distal and collecting tubules and controlled by Aldosterone.
Intercalated cells in DCT and CT actively secrete H+ and K+ ion
Intercalated cells work for acid base balance
Potassium handling by nephron
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MECHANISM OF K+ SECRETION
K+ is secreted in the principal cell of DCT and CT. It is coupled to Na+ reabsorption by energy dependent basolateral Na+ - K+ pump
14K+ ion secretion
Principle cells in Late DCT & CT
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CONTROL OF K+ SECRETION
Most important control mechanism for K+ secretion is by aldosterone
Aldosterone causes Na+ reabsorption and K+ secretion by principal cells
Increased K+ concentration directly stimulates the adrenal cortex to increase aldosterone secretion
Decreased K+ concentration in plasma – causes decreased aldosterone secretion
NOTE – Aldosterone secretion by Na+ is through Renin-Angiotensin Aldosterone mechanism
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Control Of Aldosterone Secretion By K+ and Na+
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Sodium Reabsorption
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Mechanism of Aldosterone Action
Aldosterone : Aldosterone causes Na+ re absorption and K+ secretion
by principle cell
Aldosterone causes increased Na+ entry through luminal membrane into principal cells
Aldosterone causes Na+ pumping out of principal cells by Na+-K+ pump into interstial fluid at basolateral membrane
Aldosterone causes K+ entry into the cell by Na+ - K+ pump, it increases the concentration of K+ in the cell, which is driving force for K+ secretion (passively)
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Changes in Distal K+ Secretion Causes of
increased Distal K+ Secretion
- High K+ diet- Hyperaldosteronis
m- Alkalosis- Thiazide diuretics- Loop diuretics
Causes of decreased Distal K+ Secretion
- Low K+ diet- Hypoaldosteronism- Acidosis- K+ - sparing
diureticsNOTEHyperaldosteronism – Hypokalemia Hypoaldosteronism – Hyperkalemia
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EFFECT OF H+ SECRETION ON K+ SECRETION
During acid base regulation, intercalated cells in DCT secrete either K+ or H+
When there is acidosis, H+ ions secretion is increased and K+ secretion is decreased
This decreased secretion of K+ leads to K+ retention in the body fluids, therefore, in acidosis, there is hyperkalemia {increased K+}
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IMPORTANCE OF REGULATING PLASMA K+ CONCENTRATION
K+ plays important role in membrane potential
K+ changes in ECF, when there is increase or decrease in K+ level, it can affect the heart and can cause decreased cardiac excitability, decrease conduction, cardiac arrhythmia, fibrillation
K+ changes not only affects cardiac muscle but it also affects skeletal muscle and nerve conduction
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EFFECT OF DECREASED K+
Decreased K+ causes hyperpolarization, which decreases the excitability of the nerve
In Skeletal muscle, K+ depletion causes muscle weakness
K+ depletion causes abdominal distension due to smooth muscle dysfunction
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.
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Thank you