Formation of Urine:
In this article we will discuss about the formation of the Urine: Description & Steps of Urine formation.
(2) Steps of the urine formation
(3) Ultra filtration
(4) Glomerular filtration
(5) Selection Reabsorption
(6) Tubular secretion
Urine is pale yellow colour aqueous fluid produce by kidney, containing urea and unwanted organic and inorganic substance.
Man excrete 1litre to 1.8litre of urine in 24hours. However the volume of the urine increase and decrease depend upon intake of water atmospheric temperatures, diet, health etc.
Steps of the Urine Formation:-
Formation of urine is highly physically and complex process. It involves in the following three steps :-
* Ultra Filtration
This is the first steps of urine formation.
The filtration of blood through the membrane of Bowman’s capsule and Glomerular is called Ultra Filtration or Glomerular filtration.
As the afferent atrocious of Glomerular capillaries are wider than that of efferent atrocious . Each Glomerular receive blood from Afferent atrocious and discharge it’s blood into the efferent atrocious.
As a result there is built of pressure at the capillaries network called Glomerular hydrostatic pressure (GHP). This pressure is about +60mm Hg.
Two other pressure called internal pressure of renal tubular and blood colloidal osmotic pressure (BCOP) of plasma protein oppose the Glomerular hydrostatic pressure.
The blood colloidal osmotic pressure is about 32mm Hg and the capsular Hydrostatic Pressure (HP) is about 18mm Hg.
There force focusing filtration (mm Hg).
Glomerular hydrostatic pressure= 60.
Bowman’s capsule colloidal osmotic pressure=0.
Total – 60+0= 60
Forces oppose in filtration (mmHg)
Capsule Hydrostatic pressure= 18.
Blood colloidal osmotic pressure=32.
Totall – 18+32=50.
There for, net filtration pressure =
60mmHg – 50mmHg= 10mmHg.
Some of the value can change mainly under different physiological condition where as other as others are after is decrease state.
Composition of Glomerular filtration:-
It contain glucose, amino acid, vitamins, urea, uric acid, creatine, water, hormones and some abnormal constituents.
The volume of the Glomerular filtration 180litre per 24hours but the volume of urine is only 9 to 18 litre.
The Glomerular filtration contain many substance necessary for normal metabolism such as water glucose amino acids, electrophiles etc as well as substances to be excreted and removed such as urea, creatinine, toxic substances etc. So the reabsorption of selected material in real tubule is called selective reabsorption.
Reabsorption in the proximal convoluted tubules:-
About 90% water and glucose is reabsorbed in the PCT. About 80% of Na, vitamins, chlorides, Ca are reabsorbs into the surrounding blood vessels.
Reabsorption in the Descending limb:-
Water is reabsorbs in the descending limb. So that the solute concentration of Glomerular filtration is increase and it become hypertonic to blood plasma.
Reabsorption in the ascending limbs:-
As the ascending limb is impermeable to water, so there is no water release takes place in the ascending limb but absorption of sodium ion, mg, urea, into the fluid takes place. Thus the filtration of ascending limb become hypertonic.
Reabsorption in the distal convoluted tubule:-
In the distal convoluted tube active reabsorption of Nacl takes place under the influence of hormones aldosterone.
Reabsorption of water takes place under the antidiuretic APH the filtration become isotonic to blood plasma.
Reabsorption in the collecting tubule:-
Further reabsorption of water from the filtrate makes it more and more concentric. This filtrate now called urine which is hypertonic to blood plasma.
Counter current multiplier system and concentration of urine:-
This theory was proposed by swirz and bray. It explain the formation of concentrated urine. This system operate in the hair pin like loop of hence. The descending is permeable to water but impermeable to solutes.
The ascending limb impermeable to water but permeable to solutes. The ascending limb and ascending run parallel and opposite.
The flow of filtration in one limb is opposite direction to the movement of filtration the other limb.
The ascending limb transfer chloride and sodium into the intensities fluid while some ions diffuse into the ascending limb.
The net result is that sodium chloride remain in the intensities space and there by cause to diffuse out to concentration urine. This process help in the concentration of urine and conservation of H20.
Secretion of some substance by the wall of renal tubule into the Glomerular filtration is called tubular secretion.
The substance remove by this process include potassium, H+, ammonia, penicillin, creatinine, uric acid, drugs and toxic substances.
The objective of this process is to excrete toxic produced out by the body.
Release of Urine:-
The act of emptying the urinary bladder is called micturition. Urine is formed in the kidney. It passes through the ureter into the urinary bladder.
A desire for passing urine is failed when at least 400ml of urine is collected in the urinary bladder.
It put pressure in the urinary bladder. Micturition is the reflex action and it’s both volume tary and involuntary in nature.
The urinary bladder is supplies with pelvic sacral and hypogastric nerves cerebral center of cerebral cortex act through parasympathetic fibre to initiative micturition and the urine flow out brought urethra.