EXCRETION

 

1.      PROCESS OF URINE FORMATION

Every day kidney produces 1 – 2 liters of urine.

The mechanism of urine formation involves three processes:

a)      Glomerular filtration

b)      Tubular reabsorption

c)       Tubular secretion

a) Glomerular filtration:

   -  Process by which the fluid along with the substances dissolved in it passes from the        

       glomerular capillaries in to the Bowman’s capsule is called glomerular filtration

  -   Blood cells and plasma proteins are not filtered

  -   Filtration occurs through filtering membrane which is made up of 3 layers – Fenestrated

       capillary endothelium, basement membrane & podocytes (epithelial cells of Bowman’s 

       capsule) with filtration slits

  -   The fluid which gets collected in the capsule is called filtrate

  -   The amount of filtrate formed per minute (GFR – Glomerular Filtration Rate) is 125ml.

  -   This is mainly determined by net filtration pressure which depends upon the Starling forces 

       acting across the filtering membrane – Glomerular capillary hydrostatic & colloidal osmotic pressure,

       Bowman’s capsule hydrostatic pressure

b) Tubular reabsorption:

    -     Passage of water and solutes from the filtered fluid in the kidney tubule into the blood is called

           tubular reabsorption

-          Solutes which are reabsorbed are nutrients (glucose & aminoacids), electrolytes (sodium, potassium & chloride) & ions such as bicarbonates.

-          Ihe modes of transport are both passive & active transport mechanisms

Reabsorption At Proximal Convoluted Tubule:

-          Majority of reabsorption takes place at PCT as the surface area is increased due to presence of brush border microvilli

-          65% of filtered water, sodium, chloride, potassium and other solutes

-          100% of glucose and aminoacids

-          Sodium is reabsorbed by secondary active transport along with substances like glucose and aminoacids

-          This is followed by osmosis of water into the blood. The reabsorption of water at PCT is called obligatory reabsorption of water

-          The reabsorption of water and sodium are exactly proportional. So the fluid which leaves the PCT is isotonic

          Reabsorption At Loop of Henle:

-          Loop of Henle consists of three segments -  Descending limb, thin ascending limb & thick ascending limb

-          About 20% of filtered sodium and chloride, 15% of filtered water and cations such as K⁺, Ca²⁺ and Mg²⁺ are reabsorbed in the Loop of Henle

-          In the descending limb, water absorption occurs passively because of hypertonic interstitial fluid in this part

-          The thin ascending limb is impermeable to water. Limited passive absorption of sodium and chloride occurs

-          Thick ascending limb is impermeable to water. 20% of filtered sodium and chloride and other cations are reabsorbed here by the following mechanisms:

- Sodium, potassium – 2 chloride symporter mediated active transport of sodium

           - Na⁺ - H⁺ antiporter mediated active reabsorption

           - Paracellular passive reabsorption of Na⁺, K⁺, Ca²⁺ & Mg⁺

-          Sodium reabsorbed here is the main driving force behind the countercurrent multiplier system which concentrates sodium and urea in medullary interstitium

-          As the reabsorption of solutes is not followed by water reabsorption, the fluid that leaves this segment is hypotonic compared to plasma. Hence this segment is called diluting segment

Reabsorption At Distal Convoluted Tubule (DCT) & Collecting Duct (CD)

-          Approximately 7% of filtered NaCl &  8 to 17% of water is resbsorbed

-          Late DCT & CD have two cell types -  “P” (Principal) cells & “I” (Intercalated) cells

-          Principal cells reabsorb Na⁺, Cl^- & H₂O

^-           Intercalated cells reabsorb K⁺

-          H₂O reabsorption by principal cells is influenced by the hormone ‘ADH’-Anti Diuretic Hormone. The reabsorption of water at this segment is called facultative  reabsorption of water.

^-           Na⁺ reabsorption by principal cells is influenced by the hormone aldosterone which is secreted from the adrenal cortex

c)Tubular Secretion:

    The substances which escape filtration are transported from peritubular capillary  in to the tubular fluid.

    This transport is called tubular secretion.

    Substances secreted:

^-           H⁺ ions, K⁺, NH₃^-

^-           Drugs

^-           Pencillin

^-           Creatinine

    Secretion at PCT:

^-           Organic anions & cations

^-           Exogenous organic compounds

^-           Certain drugs

^-           H⁺ & NH₃^- are secreted. H⁺   is secreted in exchange with Na⁺ absorption. This is mainly utilized for the absorption of  HCO₃^-

      Secretion at DCT & CD:

-          K⁺ secretion by principal cells takes place. This is increased by hormone aldosterone

-          H⁺ & NH₃^- are secreted. H⁺ secretion by intercalated cells is mainly responsible for acidification of urine