Urine is formed in three steps: filtration, reabsorption, and secretion.Filtration involves the transfer of soluble components, such as water and waste, from the blood into the glomerulus. Reabsorption involves the absorption of molecules, ions, and water that are necessary for the body to maintain homeostasis from the glomerular filtrate back into the blood. Secretion involves the transfer of hydrogen ions, creatinine, drugs, and urea from the blood into the collecting duct, and is primarily made of water. Blood and glucose are not normally found in urine.

1. URINE FORMATION
BY
JYOTI REKHA PATRA

2. Urine Formation :-
• Urine is the liquid waste product of the human body.
• It contains urea, uric acid, salts, water and other waste products that
are the result of various metabolic processes occurring in the body. It
is formed in the primary excretory organs– the kidneys.
• The structural and functional unit of the kidneys is called the
nephrons. Millions of nephrons are involved in the process of urine
formation.
• The formation process occurs in 3 steps or phases:
• Glomerular Filtration
• Tubular Reabsorption
• Tubular Secretion

3. • Structure of a Nephron

4. • The nephron is the functional unit of a kidney. In fact, each
kidney consists of millions of nephrons. They all function
together to filter blood and expel waste products. It consists of
the following parts:
• Bowman’s capsule– It is the first part of the nephron. It is a
cup-shaped structure and receives the blood vessels.
Glomerular filtration occurs here. The blood cells and proteins
remain in the blood.
• Proximal ConvolutedTubule– The Bowman’s capsule extends
downwards to form the proximal tubule. Water and reusable
materials from the blood are now reabsorbed back into it.
• The loopof Henle– The proximal tubule leads to the formation
of a u-shaped loop called the Loop of Henle. It has three parts:
the descending limb, the u-shaped bend, and the ascending
limb. It is in this area in which urine becomes concentrated as
water is reabsorbed. The descending limb is permeable to
water whereas the ascending limb is impermeable to it.

5. • Distal ConvolutedTubule– The Loop of Henle leads into the distal
convoluted tubule. It is where the kidney hormones cause their effect.
• Collecting Duct– The DistalConvoluted Tubule of each nephron leads to
the collecting ducts. The collecting ducts together form the renal pelvis.
Through renal pelvis, the urine passes into the ureter and then into
the bladder.Once the blood is filtered in the renal corpuscle, the resultant
fluid is called the glomerular filtrate. This glomerular filtrate now passes
into the PCT.
• In the PCT, substances like NaCl, K+, water, glucose, and bicarbonate are
reabsorbed into the filtrate whereas urea, creatinine, uric acid are added to
the filtrate.
• From the PCT, the filtrate enters the U-shaped Loop of Henle where
reabsorption and secretion of water and various metabolites occurs. The
filtrate then passes into the DCT. From the DCT, the filtrate passes into the
collecting tubules, into the renal pelvis and the ureters as urine to be
stored int he urinary bladder.
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6. • Process of Urine Formation
• Glomerular Filtration
• This process occurs in the glomerular capillaries. The process of filtration leads to the
formation of an ultrafiltrate.The blood gushes into these capillaries with high
pressure and gets filtered across the thin capillary walls. The glomerular filtration rate
(GFR) is 125ml/minor 180 Litres/day.
• Tubular Reabsorption
• During glomerular filtration, all substances except blood cells and proteins are
pushed through the capillaries at high pressure. At the level of the Proximal
Convoluted Tubule(PCT), some of the substances from the filtrate are reabsorbed.
These include sodium chloride, potassium,glucose, amino acids, bicarbonate, and
75% of water.
• Tubular Secretion
• The peritubular capillaries that help in transporting the reabsorbed substances into
the bloodstream, also help in actively secreting substances like H+ ions, K+ ions.
Whenever excess K+ is secreted into the filtrate,Na+ ions are actively reabsorbed to
maintain the Na-K balance.
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7. • Compositionof Urine:-
• Urine is often light or pale yellow in colour and fresh urine has a slight ammoniacal smell. It is
often clear in turbidity with a pH of around 4-8.
• Chemical composition:Chemically, urine is composed mainly of urea, sodium chloride, potassium
ions, creatinine, ammoniaproducts, and some amount of protein, and other metabolites.
• Antidiuretichormone (ADH) :- Its also known as vasopressin, is secreted by the posterior
pituitary in response to dehydration or a drop in blood volume. It stimulates water retention by
making the kidney nephrons, or filtering units, more permeable to water. This goal is achieved by
increasing the number of water channels, or aquaporins, in the nephron's collecting duct. This
mechanism drives water reabsorption into the blood hence increasing blood volume and
pressure, and reestablishing homeostasis. Interestingly, once water gets reabsorbed, urine
output decreases and turns out more concentrated as it leaves the kidneys, and ultimately
the body.

8. • Aldosterone
• Also, in an effort to maintain blood volume and
pressure, aldosterone is another hormone that
works alongside ADH. Released by the adrenal
cortex, aldosterone is also known as the 'salt-
retaining hormone'. It stimulates sodium
retention and potassium excretion if sodium
levels get too low, or if potassium levels are
higher than normal. When the kidneys retain
sodium, water will follow. For this reason, the
end result of this hormone's action is an
increase in blood sodium concentration all while
promoting water retention. This will inevitably
increase blood volume and pressure.

9. • Atrial Natriuretic Peptide (ANP)
• In response to an increase in blood pressure, the heart secretes
an atrial natriuretic peptide (ANP) to lower blood pressure. ANP
promotes sodium and water excretion into the urine, therefore reducing
blood volume and pressure. Specifically, it inhibits the effects of the
two hormones discussed above, ADH and aldosterone.

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