After the human body oxidizes nutritional matter, it starts the process of eliminating metabolic waste, excreting it in various forms. Excretion by urination, defecation or perspiration rids the body of unwanted solid and liquid toxic wastes. By doing so the body prevents accumulation of waste and subsequent poisoning. Kidneys, one each on both sides of the backbone (inner to 11th and the 12th rib) in the abdominal cavity, are essential to body’s excretory needs. Kidneys tirelessly filter blood, discharge undesired chemicals, selectively reabsorbing vital constituents for conservation and maintenance of pH equilibrium in body fluids. Kidneys play a major role in maintaining homeostatic state of blood pressure and blood volume as also production of RBC.
Like muscle tissues catabolize proteins for energy, proteins are also synthesised in other tissues resulting in formation of amino acids and metabolic waste, ammonia. Some of the ammonia produced as a result of catabolism gets used up to synthesise nitrogen molecules such as nucleotides. Excess ammonia must be converted into a nontoxic compound that can function as an amino group carrier in the blood to transport ammonia from the tissues to the liver where the ammonia will be converted into urea. Ammonia being toxic to several body tissues gets converted to urea for harmless transportation to kidneys and subsequent excretion. It may be noted here that its only the liver and the kidneys which can convert ammonia into urea.
In a normal human being, kidneys filter around 50 grams of urea per day of which 25 to 40 grams is excreted with urine and the rest is reabsorbed in the blood stream. All glucose, all amino acids and 85% of mineral ions are absorbed in active transportation from filtrate to the tissue fluid. Then they diffuse into blood capillaries. 80% of water is reabsorbed in blood by osmosis.
Surprisingly some urea too gets reabsorbed by the blood by diffusion (urea being an uncharged molecule, passes through membranes by lipid diffusion). Urea diffuses down its concentration gradient until the concentrations of urea in the filtrate and blood are equal. So in each passing cycle through the kidneys, half of the urea is removed from the blood and half remains in the blood to be pumped back to the body.
The widely accepted school of thought justifying reabsorption of urea is that consequent to continuous acid loss in urine, the alkali part fails to keep pace being not so abundant than acids. As a result hydrogen ions have to be continuously replenished to maintain acid/ alkali ratio or the pH value of urine. The daily acid load is constituted by one’s diet comprising primarily of foods containing acid and the production of acid as a result of metabolic activity in different organs of the body. The intake of alkali containing foods and the production of alkali as a result of metabolism offsets the daily acid load but the net effect is daily addition of acid to the body that must be buffered and excreted to maintain acid base balance.
The reabsorption of urea, after having been excreted once, is a reconconciliarry action, where it diffuses itself across renal tubules to combine with hydrogen ions and prepares itself for for re circulation maintaining pH balance of urine.
The process of buffering of the daily nonvolatile acid load becomes the key factor capable of disturbing pH equilibrium in urine excretion. Buffering minimizes the effect that strong acids such as HCl would have on the pH. Since the minimum attainable urine pH is 4.0 to 4.5, the options are limited to countering acid loss by regenerating hydrogen ions which is accomplished by reabsorption of urea, thus restoring alkali balance in urine.