Renal system

the nephron is the filtering unit of the kidney where urine is made. each nephron consists of

• the glomerulus - a clump of capillaries surrounded by a membrane called bowman's capsule
  
• a long coiled collecting renal tubule - formed from the bowman's membrane, it catches the filtrate siphoned from the glomerular capillaries and is where reabsorption of the fluid and electrolytes that the body needs takes place and turns the remainder of the filtrate into urine. notable landmarks along the tubule are the proximal tubule, loop of henle and distal tubule.
  
• peritubular capillaries - the capillaries that swirl and surround the entire length of the coiled collecting tubule where the chemical reabsorption of fluid and electrolytes is taking place.
  
• the collecting duct - this is where the end product, urine, emerges and moves on to the pyramids of the kidney
  
  • http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/image_preview.html?image=//media.tiscali.co.uk/images/feeds/hutchinson/ency/c01860.jpg&caption=the
    
  • http://www.visualhistology.com/visual_histology_atlas/vha_chpt15_the_urinary_system.html - bowman's capsule, renal tubule and collecting duct - peritubular capillaries surrounding the renal tubule are not shown
    
  • http://www.3dscience.com/3d_images/human_anatomy/urinary/nephron_glomerulus.php
    
  • http://www.siumed.edu/~dking2/crr/rnguide.htm - here's everything you want to know physiologically about nephrons
    

think of the nephron as a big factory with an assembly line coming into it (the renal artery). what the factory workers (nephrons) at the separating room (glomerulus) do is separate the blood cells from the serum of the blood and send the serum (filtrate) on for processing (renal tubule) where it is acted upon by an eager cells looking to retrieve back some of what they lost (fluids and electrolytes) at the separating room (peritubular capillaries) so they can continue on their way out the back door of the factory and get the heck out of dodge (the renal vein)!

if vasoconstriction is going on, not as much blood flows into the kidneys; if vasodilation is going on, a lot more blood flows to the kidneys. sodium is the most plentiful electrolyte in the body. sodium and water exist together. whatever is happening to sodium (loss or gain) is also happening to the body's water (loss or gain). your question is "does the body lose or gain sodium when there is vasoconstriction"? the question is really one of how is the sodium affected by vasoconstriction of the arteries? with a decreased gfr is the person losing or gaining body water? if they are retaining their water, which is generally a feature of chronic renal failure, then they are retaining sodium as well.