Urine Formation & Micturition Reflex

PHYSIOLOGY OF URINE FORMATION
Mechanism of urine formation mainly involves 3 processes:

1. Glomerular filtration
2. Tubular reabsorption
3. Tubular secretion
A) GLOMERULAR FILTRATION :
 Filtration of blood takes place through semipermeable walls of glomerulus and Bowman’s capsule.
 The filtered fluid is called glomerular filtrate and the fraction of blood plasma that becomes
glomerular filtrate is the filtration fraction.
 The daily volume of glomerular filtrate in adults is 150L (females) & 180L (males). But only 1-2L
is excreted as urine.
Filtration Membrane :
 Glomerular filtration membrane allows filtration of only water and small solutes while preventing
filtration of most plasma proteins, blood cells and platelets.
 Substances filtered from the blood cross three filtration barriers:
a) Glomerular endothelial cells -
 Leaky due to large fenestrations.
 Permits filtration of all solutes in blood plasma except blood cells and platelets
b) Basal lamina –
 Consists of minute collagen fibers and negatively charged glycoproteins.
 Prevent filtration of larger plasma proteins; allows passage of water and most small solutes
c) Filtration slit formed by a podocyte -
 Extending from each podocyte are foot like processes termed pedicels, which wrap around
glomerular capillaries.
 The spaces between pedicels are filtration slits.
 A thin membrane, the slit membrane, extends across each filtration slit; it permits the passage of
small molecules like water, glucose, vitamins, amino acids, very small plasma proteins, ammonia,
urea, and ions.
Principle of Filtration :
Glomerular filtration takes place due to the difference between blood pressure in glomerulus and
pressure of filtrate in glomerular capsule.
Net Filtration Pressure
 Responsible for the formation of filtrate and depends on 3 main pressures

1. Glomerular blood hydrostatic pressure (GBHP):
 Blood pressure of glomerular capillaries
 Promotes filtration, by forcing water and solutes in blood plasma through filtration membrane.
 55mmHg
2. Capsular hydrostatic pressure (CHP):
 Hydrostatic pressure exerted against filtration membrane by fluid already in capsular space & renal
tubule
 Opposes filtration
 15 mm Hg
3. Blood colloid osmotic pressure (BCOP):
 Pressure due to presence of proteins like albumins, globulins and fibrinogen in blood plasma
 Oppose filtration
 30 mm Hg
Glomerular Filtration Rate (GFR) :
 It is the amount of filtrate formed in both kidneys each minute.
 In males : 125 ml/min; In females : 105 ml/min
 A constant GFR is important in maintaining homeostasis.
 GFR is directly related to the pressures that determine net filtration pressure; any change in net
filtration pressure will affect GFR.
 Regulation of GFR
1. Renal auto regulation :
The kidneys themselves help maintain a constant renal blood flow and GFR despite normal, everyday
changes in blood pressure This capability is called renal autoregulation and consists of two
mechanisms — the myogenic mechanism and tubuloglomerular feedback.
i. Myogenic mechanism :
 The myogenic mechanism occurs when stretching triggers contraction of smooth muscle cells in
the walls of afferent arterioles.
 As blood pressure rises  GFR also rises because renal blood flow increases  stretches the walls
of the afferent arterioles In response, smooth muscle fibres in the wall of the afferent arteriole
contract  which narrows the arteriole’s lumen  renal blood flow decreases  thus reducing GFR
to its previous level.
 As BP decreases  Blood flow decreases  GFR decreases smooth muscle relaxed  dilates 
Increases blood flow & GFR
ii. Tubuloglomerular feedback:
 Here macula densa cells provide feedback to glomerulus.
 Increase in blood pressure  GFR increases  filtered fluid flows rapidly along renal tubules 
Unable to achieve sufficient reabsorption, mainly of NaCl and water  Macula densa cells act upon
juxtaglomerular apparatus  inhibit release of nitric oxide (vasodilator) Decreased levels of NO
constrict afferent arteriole  Reduce renal perfusion & GFR
(Vice versa happens when GFR decreases)
2.Hormonal regulation :
 GFR is regulated by Angiotensin II, ADH & Atrial natriuretic peptide (ANP)
 Angiotensin II is a potent vasoconstrictor  decrease renal perfusion decrease GFR
 Atrial natriuretic peptide: Increase in blood volume  Causes stretching of atria atrial cells
release ANP cause relaxation of mesangial cells  increases surface area available for filtration 
Increases GFR
 ADH regulates water reabsorption by increasing water permeability of principal cells in last part of
DCT and collecting ducts (by inserting aquaporins into these cells)
3. Neural Regulation:
 Mediated by release of nor adrenaline  constriction of afferent & efferent arterioles  reduce renal
blood flow reduces GFR
B) TUBULAR REABSORPTION & TUBULAR SECRETION:

 Reabsorption is the return of filtered water & most of filtered solutes back into bloodstream while
tubular secretion is transfer of materials from blood & tubule cells into glomerular filtrate.
 Solutes that are reabsorbed by both active and passive processes include – glucose, amino acids,
urea and ions – Na+, K+, Ca2+, Cl-, HCO3-, HPO42-.
 Secreted substances include – H+, K+, NH4+, creatinine and certain drugs like pencillin. Tubular
secretion helps to control blood pH and eliminate waste substances from body.
 Reabsorption takes place via paracellular or transcellular routes.
Site I - Proximal Convoluted Tubule :

 The largest amount of solute and water reabsorption from filtered fluid occurs in the proximal
convoluted tubules, which reabsorb
- 65% of the filtered water, Na+ and K+
- 100% of most filtered organic solutes such as glucose and amino acids
- 50% of the filtered Cl- and urea
- 80–90% of the filtered HCO3-
- Variable amount of the filtered Ca2+, Mg2+ and HPO42- (phosphate).
 Normally, filtered glucose, amino acids, lactic acid, water-soluble vitamins & other nutrients are
absorbed in the first half of PCT by Na+ symporters.
 Na+/H+ antiporters achieve Na+ reabsorption & return filtered HCO3- & water to the peritubular
capillaries. PCT cells continually produce the H+ needed to keep antiporters running by combining
CO2 with water to produce H2CO3 which dissociates into H+ & HCO3-.
 Diffusion of Cl- into interstitial fluid via paracellular route leaves interstitial fluid more negative than
tubular fluid. This electrical potential difference promotes passive paracellular reabsorption of
Na+, K+, Ca2+, Mg2+.
 Urea and ammonia in blood are both filtered at glomerulus and secreted by PCT cells into tubular
fluid. The NH3 quickly binds to H+ to become an ammonium ion (NH4+), which is transported through
Na+/NH4+ antiporters in apical membrane & secreted into tubular fluid.
 Urea is reabsorbed by passive diffusion.
Site II - Loop of Henle :

 The LOH reabsorbs :-
- 15% of the filtered water,
- 20–30% of the filtered Na+ and K+
- 35% of the filtered Cl-
- 10–20% of the filtered HCO3-
- variable amount of filtered Ca2+ and Mg2+
 The apical membranes of cells in the thick ascending limb of the nephron loop have Na+/K+/2Cl-
symporters that simultaneously reabsorb 1 Na+, 1 K+ and 2 Cl- from tubular fluid.
 Na+ that is actively transported into interstitial fluid diffuses into the vasa recta. Cl- moves through
leakage channels in the basolateral membrane into interstitial fluid and then into the vasa recta.
Because many K+ leakage channels are present in the apical membrane, most K+ brought in by the
symporters moves down its concentration gradient back into the tubular fluid  net effect –
reabsorption of Na+ and Cl- ; secretion of K+
 About 15% of the filtered water is reabsorbed in the descending limb of the nephron loop via
aquaporins, little or no water is reabsorbed in the ascending limb.
Site III - Distal Convoluted Tubule :
a. Early DCT :
 Reabsorbs about 10–15% of filtered water and 5% of filtered Na+, Cl-
 Reabsorption of Na+ and Cl- occurs via Na+/Cl- symporters in apical membrane and Na+/K+ pumps
and Cl- leakage channels in basolateral membrane.
 Parathyroid hormone also acts on early DCT to stimulate reabsorption of Ca2+
b. Late DCT & Collecting duct :

 Two different types of cells—principal cells and intercalated cells—are present at the late DCT
and throughout the collecting duct.
 The principal cells reabsorb Na+, water (only in presence of ADH) and secrete K+; while
intercalated cells reabsorb K+ and HCO3- and secrete H+.
Tubular secretion is important for –
 Disposing of substances like drugs and metabolites
 Eliminating undesirable substances or end products that have been reabsorbed by passive process
(Urea, Uric acid)
 Removal of excess K+
 Controlling blood pH - When pH drops acidic , renal tubule secrete more H+ into filtrate and generate
more HCO3 . When pH becomes alkaline, Cl- is reabsorbed instead of HCO3.
MICTURITION REFLEX
 Discharge of urine from the urinary bladder, called micturition, is also known as urination or
voiding
 Occurs under the control of ANS via combination of involuntary and voluntary muscle
contractions
When the volume of urine in the urinary bladder exceeds 200–400 mL
pressure within the bladder increases considerably
Activation of Stretch receptors in bladder wall - sense increase in bladder capacity
impulses propagate to the micturition center in sacral spinal cord segments S2,S3
Trigger a spinal reflex called Micturition reflex
 In this reflex arc, parasympathetic impulses from the micturition center propagate to the urinary
bladder wall and internal urethral sphincter  cause contraction of the detrusor muscle and
relaxation of the internal urethral sphincter muscle
 Simultaneously, the micturition center inhibits somatic motor neurons that innervate skeletal
muscle in the external urethral sphincter  relaxation of external urethral sphincter
On contraction of the urinary bladder wall and relaxation of the sphincters
Urination/ Micturition/ Voiding takes place
 External urethral sphincter is under voluntary control. Therefore individuals can urinate (ext.
urethral sphincter relax) / postpone the need.
 When postponed, capacity of bladder exceeds (700-800ml) over distension of bladder  painful
& micturition independent of the will of the person happens.

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PHYSIOLOGY OF URINE FORMATION

Mechanism of urine formation mainly involves 3 processes:

Net Filtration Pressure

 Responsible for the formation of filtrate and depends on 3 main pressures

B) TUBULAR REABSORPTION & TUBULAR SECRETION:

Site I - Proximal Convoluted Tubule :

Site II - Loop of Henle :

Site III - Distal Convoluted Tubule :

b. Late DCT & Collecting duct :

pressure within the bladder increases considerably

Activation of Stretch receptors in bladder wall - sense increase in bladder capacity

Urination/ Micturition/ Voiding takes place

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