Renal Physiology Tubular Transport & Clearance Quiz -12

The kidneys handle substances in the blood through three main processes: filtration at the glomerulus, reabsorption from the tubules back into the blood, and secretion from the blood into the tubules. By comparing how much of a substance is filtered and how much finally appears in the urine, it is possible to determine whether that substance is reabsorbed or secreted by the renal tubules. The filtered load of a substance is the amount that enters the renal tubules from the blood each minute. It is calculated by multiplying the glomerular filtration rate (GFR) by the plasma concentration of the substance. The excretion rate is the amount of the substance leaving the body in urine per minute and is calculated by multiplying the urine concentration by the urine flow rate. If the excretion rate is less than the filtered load, some of the substance must have been taken back into the blood, indicating net tubular reabsorption. If the excretion rate is greater than the filtered load, the extra amount must have been added to the tubular fluid by the tubules, indicating net tubular secretion. When the excretion rate equals the filtered load, the substance is handled only by filtration. This concept can be illustrated using sodium. In the example provided, a large amount of sodium is filtered into the tubules each minute because both GFR and plasma sodium concentration are high. Only a very small amount of sodium appears in the urine. The large difference between filtered sodium and excreted sodium represents extensive tubular reabsorption, which is essential for maintaining body fluid balance. Clearance values also help in understanding tubular handling. Inulin is used as a reference substance because it is freely filtered and neither reabsorbed nor secreted. Its clearance represents the GFR. If another substance has a clearance equal to inulin, it is only filtered. A clearance lower than inulin indicates tubular reabsorption, while a clearance higher than inulin indicates tubular secretion. For example, glucose has a clearance close to zero because it is almost completely reabsorbed. Sodium and chloride have very low clearance values, showing that most of the filtered amount is reabsorbed. Creatinine has a clearance slightly higher than inulin because a small amount is secreted by the tubules. These principles provide a simple way to understand how the kidneys regulate the composition of body fluids.