WATER REABSORPTION

 

WATER REABSORPTION

Amount of filtrate formed = 125 ml/minute or 180 lt/day

Amount of filtrate reabsorbed = 124 ml / minute or 178.5 lt/day

Amount of fluid excreted in urine = 1.5 lt /day

Reabsorption at PCT (65% of filtered fluid)

      Mechanism: Pumping of sodium out of tubular epithelial cells by Na⁺-K⁺ ATPase pump

                                          ↓

                          Passive diffusion of Na⁺ along with other solutes

                                           ↓

                          Hypoosmolarity of tubular fluid

                                            ↓

                          Osmosis of water into the cells

                    (through water cannels called “aquaporins”)

           This type of osmosis of water in PCT is called “obligatory water reabsorption”

Reabsorption at LH: About 15% of filtered fluid is absorbed at the thick ascending limb of

                                    Loop of Henle

        Mechanism: Diffusion independent of solute reabsorption

Reabsorption at DCT & CD: (5% at DCT & 14.7% at CD)

       Mechanism: Osmosis of water through aquaporins is influenced by the hormone ADH (Anti

                             Diuretic Hormone) secreted from posterior pituitary. This type of water

                             reabsorption at DCT & CD under the influence of  ADH is called

                            “ facultative water reabsorption”.

1.      PROTEINURIA

Presence of protein in urine more than the usual amount (100 mg/dl) is called proteinuria

Most common protein found is albumin. So the defect is commonly called albuminuria

Cause:

-          Usually the proteins are not filtered. As they are negatively charged, they are repelled by

negative charges at the pores of glomerular capillary wall

-          In cases of renal diseases like nephritis, the negative charges are dissipated.

-          The permeability of the glomerulus to protein is increased.

Effects:

-          Loss of protein from plasma leads to hypoproteinemia

-          Hypoproteinemia leads to decreased colloidal osmotic pressure

-          Decreased colloidal osmotic pressure à decreased plasma volume & edema

Orthostatic proteinuria:

      Proteinuria in standing position

2.      AUTOREGULATION

Definition: Ability of the kidneys to regulate their own blood flow inspite of the changes in  

                   systemic blood pressure is called autoregulation

-          Seen between a pressure range of 90 – 120 mmHg

-          Seen even after cutting of renal nerves & in an isolated kidney perfused with isotonic saline

Mechanisms:

a)      Myogenic theory

b)      Tubuloglomerular feedback

Myogenic theory:

         Increase in blood pressure à stretching of smooth muscle of afferent arteriole à

         contraction of smooth muscle à vasoconstriction à decrease in blood flow

Tubuloglomerular feedback (also called as chloride feedback theory):

        Mechanism: Increase in blood pressure à  Increased renal blood flow à Increased GFR                                    

                              à increased chloride concentration at macula densa à increased

                              absorption of  chloride at macula densa à increased absorption of chloride

                              at macula densa à release of adenosine by JG apparatus à constriction of

                              afferent arteriole & contraction of messangial cells à decrease in RBF & 

                              GFR

                             Decrease in in blood pressure à  Decreased renal blood flow à Decreased

                             GFR à decreased chloride concentration at macula densa

 

 

 

                              Vasoconstrictor mechanism                                 Vasodilator mechanism

                   (production of angiotensin II & activation of              (Release of dopamine & NO