The main functions of kidneys include maintenance of the water and mineral balance, constant level of pH in the extracellular compartment; elimination of the toxic and waste products of metabolism. These functions are fulfilled by way of glomerular filtratuion, tubular reabsorption and secretion, or, in general, urine production.
The reduced glomerular filtration rate may result from the following circumstances:
• a decrease in the blood flow and effective filtration pressure in the glomerular capillaries (shock, hypovolemia, heart failure);
• an increase in the hydrostatic pressure in the tubules (obstruction of the urinary tract);
• an increase in the colloid-osmotic presure of blood (concentration of blood during dehydration of the body);
• a decrease in the glomerular filter permeability (glomerulosclerosis);
• a decrease in the surface area of the glomerular filtration (nephrons damage).
The impairment of the tubular reabsorption may be caused by exogenous or endogenous toxins, inflammatory mediators, ischemia, overload of the transport systems, acquired or hereditary insufficiency of hormones, receptors, enzymes, or protein transporters involved in the transfer of substances across the tubular epithelium.
Kidney failure may manifest in five main syndromes:
• acute renal failure;
• subacute renal failure;
• chronic renal failure;
• nephritic syndrome;
• nephrotic syndrome.