URINE FORMATION
Normally about 25% of cardiac output enters kidneys
Kidney excretes unwanted substances along with water from blood as urine
Normal urine output - 1-1.5 L/day
To produce urine , nephrons and collecting ducts perform 3 basic processes
1. Glomerular Filtration / Ultra filtration
2. Tubular reabsorption
3. Tubular secretion
1. GLOMERULAR FILTRATION:
Is a non selective process by which blood is filtered while passing through the glomerular
capillaries by filtration membrane
Filtration membrane is formed by glomerular capillary membrane , basement membrane
and visceral layer of Bowman’s capsule
When blood passes through glomerular capillaries, plasma is filtered into Bowman
capsule and this filtered fluid is called glomerular filterate
Glomerular filtration is also called ultrafiltration as even the minute particles of plasma
are filtered except large sized plasma proteins
Glomerular filtration depends on 3 main pressures:
A) Glomerular blood hydrostatic pressure – pressure in glomerular capillaries, which
promote filtration (about 55mm of Hg)
B) Capsular hydrostatic pressure – is the hydrostatic pressure exerted against filtration
membrane by fluid present in capsule and tubules , which opposes filtration ( about
15mm of Hg)
C) Blood colloid osmotic pressure (oncotic pressure)- is the pressure exerted by plasma
proteins in glomeruli which opposes filtration ( about 30 mm of Hg)
Therefore net filtration pressure (NFP) is ;
NFP = GBHP-CHP- OP
NFP = 55-15-30 =10 mm of Hg
This pressure causes blood plasma to filter from glomerulus to capsular space
Glomerular filtration rate (GFR) : - is defined as the volume of plasma filtered from
glomerular capillaries into Bowman’s capsule per unit time.
In adult males average 125 ml/min and in females 105ml/min
2. TUBULAR REABSORPTION:
is a selective process by which water, electrolyte and other substances are transported
from renal tubules back to blood
When the glomerular filterate flows through Proximal convoluted tubule about 99% of
filtered water is reabsorbed
The reabsorbed substances move into interstitial fluid of renal medulla and from there it
moves into peritubular capillaries
It uses both active and passive transport mechanism
Substances like Na, Ca, K, Phosphate, bicarbonates , glucose, amino acid are actively
reabsorbed against an electrochemical gradient
Urea and water are passively reabsorbed along the electrochemical gradient
Reabsorption occurs in all the tubular segments
Reabsorption in PCT:
88% of filterate is reabsorbed in proximal convoluted tubule (due to presence of brush
border cells)
Substances reabsorbed are:
glucose & amino acid- 100%
Na, & K- 65%
Cl- 50%
Bicarbonate- 80-90%
urea- 50%
water- 65%
Mg, Ca- variable amounts
Reabsorption in loop of henle:
In descending limb- 15% of water is reabsorbed
in ascending limb – water is not reabsorbed as it is impermeable to water
Other solutes reabsorbed are:
Na+, K+ – 20-30%
Cl- - 35%
Bicarbonates – 10-20%
Mg & Ca- Variable amount
Reabsorption in DCT and Collecting Duct :
Early distal tubule – reabsorbs Na⁺, Cl⁻ - 5%
- water – 10-15%
- depending on body needs Parathyroid hormone acts in this site
for Ca2+ absorption
Late Distal tubule and Collecting Duct- have 2 types of cells – Principal and Intercalated
cells
i. Principal cells- reabsorb Na⁺, Cl⁻ and water
ii. Intercalated cells- reabsorb K ⁺ and Bicarbonate
Its permeability to water is controlled by ADH.
Medullary collecting Duct- reabsorbs large amount of urea
3. TUBULAR SECRETION
Is the process by which substances are transported from blood into renal tubules
This process helps in getting rid of certain undesirable substances from blood
Helps in controlling pH of blood by secretion of hydrogen ions
Substances secreted in various parts of nephron are:
In PCT :- H⁺, ammonia, Urea, creatinine
In Henles loop :- Urea
In DCT and collecting Duct:-
the principal cells - secrete K
intercalated cells - secrete Hydrogen
By these 3 stages , filterate forms urine
MECHANISM OF URINE CONCENTRATION & DILUTION
Kidneys can produce dilute or concentrated urine depending on our fluid intake
When water content in the body increases, kidney excretes dilute urine which is achieved
by inhibition of ADH. So water reabsorption from tubules doesnot take place , making
the urine dilute
When water content in body decreases, concentrated urine is formed by developing a
medullary gradient and by action of ADH
ADH helps in formation of concentrated urine , based on presence of osmotic gradient of
solutes in the interstitial fluids of renal medulla , which is called the medullary gradient
Development and maintenance of medullary gradient is done by counter current system
(Henle’s loop and Vasarecta (capillaries surrounding the loop of Henle))
MICTURITION:
Discharge of urine from urinary bladder is called as micturition/ urination/ voiding
Urine is formed in the kidney and via ureters it reaches bladder where it is stored
The end part of ureters where it enters the bladder is compressed by detrusor muscles and
hence they are normally closed.
During peristalsis of ureter , urine enters bladder from ureter
The bladder also has smooth muscle called detrusor muscle
In its floor there is a small triangular area called trigone.
2 of its posterior corner contains ureteral opening and anterior corner has internal urethral
orifice which remains closed when detrusor is relaxed
Trigone is sensitive to expansion and once stretched to a certain degree, the urinary
bladder signals the brain of its need to empty.
The urethra contains a ring of voluntary muscles round its wall called external urethral
sphincter which can be closed voluntarily
Micturition reflex:
when volume of urine in bladder exceeds 200-400 ml
pressure within the bladder increases
stretch receptors in its wall sends impulse to micturition center in S2 and S3 spinal
segments via parasympathetic fibers
It sends parasympathetic impulses to bladder wall and internal sphincter
contraction of detrusor muscle and relaxation of internal urethral sphincter
simultaneously micturition center also inhibits somatic motor neurons innervating
external urethral sphincter causing it to relax
due to contraction of bladder and relaxation of sphincters urination takes place
This spinal reflex is controlled by fibers from cerebral cortex so that micturition can be
delayed or initiated voluntarily
In spinal transection cerebral control is removed and the bladder becomes automatic
In infants too the bladder is automatic because neurons to the external urethral sphincter
muscle are not completely developed
CHARACTERISTICS OF NORMAL URINE
Volume – 1 to 2 liters/day
Colour – yellow , straw colored to amber but varies with concentration and diet (yellow
color is because of urobilin and urochrome )
Turbidity – freshly voided urine is transparent but becomes turbid on standing
Odor – mildly aromatic but becomes ammonia like on standing
pH – 4.6 to 8.0
Specific gravity- 1.001 to 1.035
APPLIED ASPECTS:
1. POLYURIA
- Production of abnormally large volumes of dilute urine
- More than 2.5 to 3 litres / day in adults
2. OLIGURIA
- Production of abnormally small volumes of urine
- Less than 400 ml/day
3. ANURIA
- Is non passage of urine
- In practice defined as passage of less than 100 ml of urine in a day
