how does the kidney regulate the constancy of the internal environment?

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in health the kidney functions to maintain within limits,the volume of water in the extracellular fluid,the concentration of the electrolytes in extracellular fluid the osmolality of the extracellular fluid,and the concentration of hydrogen ion in the extracellular fluid.the kidney is the site for the excretionof the waste metabolism,particularly those of protein catabolism.under certain abnormal conditions,the kidney produces vasoexcitatory subtances or their precursors.at the present time this is thought to revolve around the jux-taglomercular apparatus and the renin-angiotensin system.the kidneys secrete a subtance that influences the maturation of red blood cells.although the kidneys are frequently referred to as organs of excretion,the above list of their function indicates the inadequacy of this concept of renal function.the primary function of the kidney is to regulate the volume and composition of the extracellular fluid;their excretory function is incidental to this regulatory function.the kidney is a remarkable organ.which each heartbeat approximately one fourth of the blood pumped by the heart circulates through the kidney.each day 180 liters of glomercular filtrate are formed.in health 99 per cent of the water in the glomercular filtrate is returned to the extracellular fluid.other subtances are either absorbed or secreted is needed to maintain constancy of the fluid environment of the cells.depending on intake or lossesfrom the body,the quantity of sodium excreted in the urine varies from negligible to considerable amounts. the volume of extracellular fluid is directly related to this renalcontrol of soduim balance.the kidney closely regulates the volume and concentration of urine in relation to the quantity of water and solutes added to extracellular fluid and edogenous sources.