Reabsorption occurs in the proximal and distal tubules. Approximately 99% of the water filtered by the glomeruli is reabsorbed. Many substances in the water are reabsorbed with active or passive transport. Active transport requires energy for movement of the substance across the membrane. Passive transport can be regarded as simple diffusion that does not require energy.

Important constituents of body fluids—substances such as glucose, amino acids, sodium, potassium, calcium, and magnesium—are almost entirely reabsorbed. Certain substances are reabsorbed in limited quantities, such as urea and phosphate, and consequently appear in the urine. In a healthy individual, creatinine is the only filtered substance not reabsorbed and entirely secreted, allowing creatinine to serve as an indicator of glomerular filtration ability. The last process in the formation of urine is secretion. Various substances, including hydrogen and potassium ions, are secreted directly into the tubular fluid through the epithelial cells that line the renal tubules. Secretion plays an important role in promoting the body’s acid-base balance.

The kidneys regulate blood volume through ADH, aldosterone, and the renin-angiotensin mechanism. When the circulating blood volume is excessive, the cardiac output and arterial pressure increases, resulting in greater pressure in the renal artery and the kidney afferent arteriole. Water and sodium are excreted. If the patient is hypovolemic, the kidneys reabsorb fluid to return the blood volume to normal limits.

The kidneys indirectly control the extracellular fluid volume, which results in a control of blood volume. The relative ratio of the extracellular fluid volume to blood volume depends on the physical properties of the circulation and of the interstitial spaces including compliance and dynamics. The kidney maintains the osmolality of the extracellular fluid mainly by regulating the extracellular sodium concentration. Extracellular sodium controls 90% to 95% of the effective osmotic pressure of extracellular fluid.³ The extracellular concentration of other electrolytes, including potassium, calcium, magnesium, and phosphate ions, is also under renal control.

Creatinine is considered a nonthreshold substance because it is excreted by the kidney in its entirety. High-threshold substances are almost entirely reabsorbed in the kidney. They are an important portion of the blood and are excreted only if they are in an excess concentration. High-threshold substances include glucose, potassium, calcium, and magnesium. Low-threshold substances such as urea, uric acid, and phosphates are only minimally reabsorbed by the kidney.

In consideration of the substances found in urine, knowledge of the characteristics of normal urine is useful. Normal urine should be clear and transparent with an amber color, a reflection of urobilin, a byproduct of bilirubin breakdown. Urine is usually acidic because of the excretion of sodium acid phosphate with a pH of approximately 6. The specific gravity is between 1.003 and 1.025. The volume of urine excreted every 24 hours by an adult is approximately 1500 mL.

The pH is an expression of the hydrogen ion concentration in the body fluids. Bicarbonate and carbon dioxide are also factors. The bicarbonate concentration is mainly under renal control, whereas the carbon dioxide is under respiratory control. Approximately twentyfold more bicarbonate than carbon dioxide is in the plasma, resulting in a 20:1 ratio. Thus, any change in the 20:1 ratio affects the pH. Any change that affects kidney function affects the bicarbonate portion of the ratio and becomes a metabolic problem. Conversely, any change in the function of the lungs, which affects the carbon dioxide portion of the ratio, is a respiratory problem.

If, for example, a large amount of a bicarbonate solution is rapidly infused into a patient and ventilation does not change (PCO₂ stays constant), the result is a higher value for the bicarbonate and no change in the PCO₂. The net result is a higher pH, which indicates alkalosis, in this case metabolic alkalosis. Conversely, if an acid is infused, the ratio becomes smaller and the pH falls, indicating acidosis, which is termed metabolic acidosis.

The kidneys regulate pH by increasing or decreasing the bicarbonate ion concentration in the blood and eliminating hydrogen ions. This regulation is done with a complex series of reactions, which begins with hydrogen ions being secreted into the tubular filtrate. Carbon dioxide, an end product of tubular cell metabolism, combines with water to form carbonic acid (H₂CO₃). The carbonic acid dissociates to form hydrogen and bicarbonate (HCO₃). The hydrogen ion is taken via active transport to the renal tubule and usually exchanges in the tubule with sodium. Via active transport, the sodium moves to the extracellular fluid where it combines with the bicarbonate that was reabsorbed into the extracellular fluid to form sodium bicarbonate (NaHCO₃). In the tubules, the hydrogen ion that was actively transported to the tubule combines with the filtrate bicarbonate to form carbonic acid. The carbonic acid dissociates to form carbon dioxide and water. The carbon dioxide is reabsorbed into the extracellular fluid and is eventually excreted by the lungs; the water is excreted as part of the urine.