Conservation of  Salt and Water

      
       

 

          
       

Glomerular Filtration

Renal blood flow is 25% of cardiac output (1200 ml/minute). Of this, renal plasma flow is about 660ml/minute, and 120ml/minute is filtered out of the blood and into the nephron. Ultimately approximately 1.2ml of this fluid is excreted as urine (1% of filtered load). The major determinants of GFR are 1. Renal blood flow and renal perfusion pressure. 2. The hydrostatic pressure difference between the tubule and the capillaries. 3. The surface area available for ultrafiltration (see figure 1).

 The rate at which fluid is filtered by the glomerulus is the glomerular filtration rate (GFR).

Filtration and filtration pressure

Proximal Tubule

In the proximal tubule, 2/3 of filtered sodium, water and chloride are reabsorbed along with most of the filtered glucose, amino acids, bicarbonate and vitamins. The mechanism of reabsorption is for sodium to be actively pumped out of the tubule, and water to follow passively.

Loop of Henle

There are two parts to the loop, a thin descending limb and a thick ascending limb. The purpose of this anatomical structure is to allow both concentration and dilution of the urine, by way of the influence of  hormones on distal structures. A sodium-potassium-chloride (1Na+:1K+:2Cl-) pump actively extracts these electrolytes from the tubular fluid in the thick ascending limb. The latter is impermeable to water, but the descending limb is permeable. Sodium and chloride are actively pumped out of the fluid, such that the fluid arriving at the distal tubule is hypotonic (dilute), the presence of these electrolytes in the interstitial tissue increases tissue osmolality, and water flows along the osmotic gradient from the descending limb into the interstitium (figure 4). The fluid left in the tubule becomes hyperosmolar (in relation to normal plasma).

Distal Tubule and Collecting Ducts

The result of this “countercurrent multiplier” system is threefold:

  1. The high concentration of sodium and chloride (and urea) in the medullary interstitium makes this part of the kidney hyperosmolar.

  2. Fluid delivered to the distal convoluted tubule is hypotonic. So, as this fluid passes down through this tubule and the collecting duct, it is exposed to very high osmolar pressures in the surrounding tissues. If the patient is dehydrated, the pituitary gland produces ADH (antidiuretic hormone/vasopressin), which makes the collecting ducts permeable to water, and water is rapidly reabsorbed along the concentration gradient. If not, a dilute urine is excreted.

  3. Extracellular fluid volume depends on the amount of sodium in the body, so it is essential that the kidney is capable of conserving sodium. If the extracellular volume drops, a complex series of neurohormonal interactions lead to the release of aldosterone, which makes the collecting ducts permeable to sodium, which is are absorbed.

          
                   
       

         
     

       
       

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