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RS 10 Mechanism of Urine Formation - Coggle Diagram

- - - - i.e. filtration of plasma from the glomerular
        capillaries into the renal tubules is the first step in the formation of urine.
    - - i.e. the return of needed solutes and water
        from the tubules into the blood, is the second step in the process of urine formation.
        
        It changes the composition of glomerular filtrate and reduces its volume markedly.
    - - involves net movement of water and unwanted
        solutes (that could not be filtered) from the blood into the tubules.
- - - - Characteristics of Filtration Membrane
        
        consists of three layers:
        
        capillary endothelium,
        glomerular basement membrane (GBM) and
        Bowman’s visceral epithelium (podocytes).
        Glomerular filtration occurs
        across this membrane.
        
        High permeability.
        
        The glomerular membrane is highly permeable to water and 100% dissolved substances because of its porous nature.
        
        Permeability selectivity.
        
        The filtration membrane exhibits a high degree of permeability selectivity based on two factors:
        
        Pore size.
        
        The capillary endothelial cells have pores that are 70–90
        nm in diameter,
        the glomerular basement membrane (GBM) has no
        pores but its permeability corresponds to pore size of 8 nm, and the
        podocytes form filtration slits which are approximately 25 nm wide.
        From this it is clear that the glomerular filtration barrier for solutes
        resides at the level of GBM. The permeability of the membrane to
        various neutral solutes depending on their effective molecular
        diameter is:
        
        Permeability selectivity.
        
        Molecules less than 4 nm in size are freely filtered.
        
        Molecules with diameter more than 8 nm are not filtered at all (i.e. zero permeability).
        
        Filtration of molecules having diameter between 4–8 nm is inversely proportional to their diameter.
        
        Electrical charge.
        
        It may however, be realized that permeability of
        the filtration membrane is not simply related to molecular size but
        also to its shape and electrical charge. The pores in the filtration
        membrane are negatively charged due to presence of glycoproteins
        rich in sialic acid. Thus, with the same molecular size, compared to
        anionic particles, there is, in order, increasing permeability for
        neutral and cationic particles. This explains why albumin (with a
        molecular diameter of 7 nm but a negative charge) is not filtered.
      - Composition of Glomerular Filtrate
        
        The unique characteristic features of the glomerular filtration membrane determine the composition of glomerular filtrate, in that it is like that of plasma except for the absence of proteins (colloids) and cells.
        
        Capillary permeability alterations in disease do not alter rate of filtration of diffusible molecules.
        
        Thus, transfer of such substances into glomerular filtrate depends only on GFR.
        
        Filtration membrane permeability alteration in diseases, however, may alter diffusibility of colloids and cells.
        
        For example, in a number of glomerular diseases, the negative charge on the filtration membrane is reduced because of immunological damage and inflammation.
        
        As a result filtration of proteins is increased, and albumin appears in the urine in significant amounts (albuminuria or proteinuria).
        
        It is important to note that normally the amount of proteins in the urine is less than 100 mg/dL, and most of this is not filtered but comes from the sheded tubular cells.
  - - - As calculated above,
        the normal glomerular filtration rate (GFR)
        in an average-sized man is about
        125 ml/min (range 90–140 ml/min).
        
        Its magnitude correlates fairly well with surface area,
        
        but values in women are 10% lower than those in men
        even after correction for surface area.
        
        Thus, in a 24 h period,
        as much as 180 L/day of plasma
        is filtered at the glomerulus.
        
        in one day
        kidneys filter an amount of fluid equal to
        
        4 times the total body water,
        
        15 times the ECF and
        
        60 times the plasma volume.
        
        Of the 180 L/day of glomerular filtrate
        which passes through the remaining part of the nephron,
        
        99% or more is reabsorbed and
        
        only 1% or less is excreted as urine.
        
        After age of 30 years,
        
        GFR declines with age.
        However, this decline in GFR usually does not adversely affect the kidney’s excretory function, or their ability to maintain fluid, electrolyte and acid–base balance.