The kidney controls blood pressure by expelling surplus sodium from our bodies.
The kidney can correct any imbalances by removing excess acid (hydrogen ion) or bases (bicarbonate) in the urine and restoring the bicarbonate concentration in the blood to normal. Kidneys regulate acid – base loads by adjusting hydrogen ion (H+) secretion and bicarbonate (HCO3 ) filtration in response to elevated carbon dioxide (CO2) or HCO3. Tubular secretion is a process where it secretes some unwanted components from the blood into the lumen of the nephron. Some substances are secreted from the plasma into the lumen by the cells of the nephron. Examples of such substances are ammonia (NH3). As in reabsorption, there are transporters on the cells that can move these specific substances into the lumen.
The third process by which the kidneys clean blood (regulating its composition and volume) is called tubular secretion and involves substances being added to the tubular fluid. This removes excessive quantities of certain dissolved substances from the body, and also maintains the blood at a normal healthy pH (which is typically in the range pH 7.35 to pH 7.45).
Now let's put all of these processes – filtration, reabsorption and secretion – together to understand how the kidneys maintain a constant composition of the blood. The kidney while removing wastes like urea from the blood also regulates its composition, i.e., the percentage of water and salts. This function is called osmoregulation – it implies the regulation of osmotic pressure of the blood. Let's say that you decide to eat several bags of salty (NaCl) potato chips at one sitting. The Na will be absorbed into your blood by your intestines, increasing the concentration of Na in your blood. The increased Na in the blood will be filtered into the nephron. While the Na transporters will attempt to reabsorb all of the filtered Na, it is likely that the amount will exceed their ability.