URINE FORMATION//MECHANISM OF URINE FORMATION//URINE FORMATION STEPS//counter current mechanism//

Formation of Urine

The formation of urine involves three main steps like Glomerular filtration, Reabsorption and Secretion. These steps occur in different parts of nephron (functional unit of kidney).

1.     Glomerular filtration:

The first step of urine formation is the blood filtration. It is carried out by the glomerulus, called Glomerular filtration or Ultra filtration.

Kidney filtered about 1100-1200ml blood per minute.

The glomerular capillary blood pressure causes blood filtration through layers. These are as follows

a.     Endothelium of glomerular blood vessels

b.     A basement memberane

c.      The epithelium of  Bowman’s capsule

The epithelial cells of  Bowman’s capsule called Podocytes are arranged in such a manner so that some minute spaces occurs between them called filtration slits or slit pores.

In glomerular capillaries, high pressure is responsible for filtering a larger amount of water, glucose, Na+, vitamins, amino acid and harmful substances like urea, uric acid, ammonium salts.

But the RBC, WBC and plasma protein are unable to pass out due to high molecular weight.

That is why this process of filtration through glomeular capillaries in the Bowman’s capsule is called Ultra filtration and the filterate is called glomerular filtrate.

2.    Selective reabsorption:

It is the second step of formation of urine form filtrate.

 

99% of the material in the filtrate is reabsorped by the renal tubules (PCT, Loop of Henle, DCT and collecting duct) this process is called Reabsorption.

The main function of reabsorption is to maintain homeostasis so that volume, electrolytic balance, composition, pH and osmotic pressure of blood are kept constant.

The tubular epithelial cells in different segments of nephron perform reabsorption  passive transport or active transport.

Water and urea are reabsorbed by passive transport and glucose and amino acid are reabsorbed by active transport and reabsorption of Na ⁺, occurs by passive and active transport.

·        Reabsorption in Proximal Convoluted Tublue (PCT):

The process of reabsorption mostly takes place in PCT.

All of the essential nutrients, 70-80% of electrolyte and water are reabsorbed in it.

It is also helps in the absorption of  HCO₃^-  from the filtrate.

·        Reabsorption in  Loop of Henle:

Reabsorption in loop of  Henle is minimum but it plays an important role in maintaining the high osmolraity of medullary interstitial fluid.

The descending limb of loop of Henle reabsorbed water but sodium and other electrolytes are not reabsorbed.

 The ascending limb of loop of Henle actively reabsorbed K⁺, Cl^-, and Na^+.

·        Reabsorption in  Distal Convoluted Tubule (DCT):

Reabsorption of Na⁺, water, HCO3^- takes place in it.

Selective secretion of hydrogen ion, Cl^-, NH₃, K⁺ also takes place here.

·        Collecting Duct:

large amount of  water, some urea  and  Na⁺  reabsorbed.

3.    Tubular Secretion:

It is also an important step in formation of urine.

It helps in the maintenance of ionic and acid- base balance of body  fluids by removing chemicals like K⁺, H⁺,NH4⁺ ions, foreign bodies, and molecules ( medicines).

As the result of these three processes the waste are changed into urine.

Diagrammatic representation of Reabsorption and Secretion of major   substances at different parts of the Nephron

Characteristics of Urine:

An adult man normally passes about 1-1.5 liter of urine per day

Composition:

Urine contains, water 95%, salts 20%, urea 2.6%, uric acid 0.3%, traces of creatinine, creatine, ammonia etc.

Colour :

Pale yellow colour of urine due to the presence of pigment Urochrome formed by the breaking down of haemoglobin.

pH:

pH ranges from 4.5 -8.2, average pH is slightly acidic  i.e., 6.0.

Odour:

Unpleasant, if allowed to stand imparts strong smell like, ammonia.

Counter Current Mechanism 

Higher vertebrates such as mammals, birds and man have the ability to produce concentrated urine.

This can be done by a special mechanism called counter current mechanism.

The counter current mechanism is a mechanism in which the exchange of two fluids can take place from one direction to another with their concentrations.

Henle’s loop and vasa recta play an important role in this mechanism.

The flow of filtrate in the limbs of henle’s loop is in opposite directions forms a counter current.

The flow of blood within the two limbs of vasa recta also occurs in the counter current pattern.

The gradient of increasing hyperosmolarity of medullary interstitium (1200- 1450mosmol/L) is maintained by a counter current mechanism and the proximity between the henle’s loop and vasa recta.

This gradient mainly caused by the NaCl and Urea.

The transport of these substances is facilitated by the special arrangement of henle’s loop and vasa recta.

NaCl is transported by the ascending limb of loop of Henle which is exchanged with the descending limb of vasa recta.

NaCl is returned to the medullary interstitium by the ascending part of vasa recta.

Small amount of urea enter the thin segments of the ascending limb of loop of Henle  and back in medullary intersititium by collecting duct.

The counter current mechanism helps in the maintencnce of concentration gradient in the medullary interstitium.

Presence of such type of gradient helps in an easy passage of water from the collecting duct resulting in the formation of concentrated Urine which is nearly four times concentrated than the initial filtrate formed.

 

 

                     Diagrammatic representation of counter current mechanism