Mechanism of urine formation or Process of urine
formation:
The process of formation of
Urine is called Uropoiesis. It takes place in nephron. Each
nephron is independent and Can Produce miniscule amount of urine. This process
gets completed in 3 steps; Ultrafiltration, selective reabsorption and tubular
secretion.
• Ultrafiltration: It is process in which blood is
filtered under pressure in malphigian body of nephron.
The main driving pressure for it is hydrostatic pressure
of blood in glomerulus (A) which is about (60-70) mmHg. However, this
pressure is opposed by osmotic pressure of plasma protein or Blood Colloidal
osmotic pressure (B) which is about 30mmHg and also opposed by capsular
hydrostatic pressure or hydrostatic pressure of nephric filtrate in
Cavity of Bowmans Capsule (C) which is about 20 mmHg.
Thus, the net filtration pressure (NFP) = A - (B+C)
= (60 to 70) - ( 30+20) = (60 to 70) - 50 = 10 to 20
Thus, 10 to 20 mmHg pressure is responsible for
Ultrafiltration.
During this process, blood has to pass through endothelium,
basement membrane and squamous Cells of visceral layer of Bowman's Capsule. out
of these 3 layers, basement membrane acts as filter.
The filtrate passes into Capsular space (Cavity of Bowman's
Capsule) and is Called nephric filtrate which is preliminary form of urine.
The nephric filtrate Contains all the substances in blood
except blood cells and plasma protein.
Nephric filtrate= Blood - (Blood Cells+ plasma protein) or
Nephric filtrate= plasma - plasma protein
About 125ml nephric filtrate is produced in a minute. Thus,
about 180 ltrs nephric filtrate is produced perday. This is Called glomerular
filtration rate(GFR).
• Selective reabsorption: out of 180 ltrs of nephric
filtrate only about 1.5 to 2 Ifrs get changed into Urine while other about 178
ltrs amount is reabsorbed in tubule. This process is Called Selective
reabsorption. All the useful substances present in nephric filtrate are
reabsorbed by this process.
In PCT : most of the substances are reabsorbed in
PCT. All most all glucose, amino acids, hormones & vitamins are reabsorbed.
About (70 to 80)% Na+, CI-, K+, H2O etc. are reabsorbed. some amount of urea is
also reabsorbed.
substances are reabsorbed by Active transport and passive
transport. Glucose, amino acids, vitamins, hormones, Na+ , K+ etc are actively
reabsorbed by using energy even against concentration gradient. water, CI- etc
are passively reabsorbed by diffusion.
Maximum reabsorption takes place in PCT.
In PCT , nephric filtrate is isotonic.
In DLOH: As the nephric filtrate moves down in DLOH,
water is reabsorbed as it is permeable to water. Thus, filtrate become more and
more hypertonic as it moves in DLOH.
In ALOH: As the nephric filtrate moves in ALOH,
Na+, CI- , k+ etc are actively
reabsorbed. Thus, filtrate become more and more hypotonic as it moves in ALOH.
At same level of DLOH and ALOH, filtrate is hypertonic in
DLOH.
In LOH, 25% of filtered ions and 20% of filtered water is reabsorbed.
Minimum reabsorption
takes place in ALOH.
• In DCT: Absorption of water and salts takes place
under the influence of hormones ADH and Aldosteron respectively. the absorption
depends upon amount of water and Salt present in body. thus, it is facultative
reabsorption.
In Collecting duct reabsorption of water takes Place and is
regulated by hormone ADH. At distal Part of Collecting duct reabsorption of
Urea abo takes place which is Secreted into tubule in thin part of ascending
limb of loop of Henle. This helps to increase the osmolarity of medulla and it
is Called Urea recycling.
• Tubular secretion: It is just reverse Process of selective reabsorption. In it; some harmful substances present in blood are secreted into tubule. It takes place in PCT and DCT along with selective reabsorption. The substances like
H+ ions, K+ ions, Hippuric acid, Creatinine , uric acid etc.
are passed into tubule.
In DCT, exchange of Na+ and K+ ions takes place in which Na+ ions in nephric filtrate is reabsorbed from tubule and K+ ions in blood Passes into tubule.
At the end of these 3 processes Urine is formed.