The filtration fraction in uriniferous tubule of a healthy person is:

Question

Accepted Answer

The correct option is (2): 0.2

The filtration fraction (FF) is a critical concept in renal physiology, representing the proportion of the renal plasma flow (RPF) that is filtered by the glomerulus to form the primary filtrate. Mathematically, the filtration fraction is defined as the ratio of the Glomerular Filtration Rate (GFR) to the Renal Plasma Flow (RPF): FF = GFR / RPF. In a healthy adult, the typical GFR is approximately 125 milliliters per minute (mL/min), which is the volume of fluid filtered from the blood into the Bowman's capsule each minute. Concurrently, the RPF, which is the volume of plasma delivered to the kidneys per minute, is typically about 625 mL/min. By substituting these physiological values into the formula, we find that the filtration fraction is 125 mL/min divided by 625 mL/min, which precisely equals 0.2. This means that, on average, 20% of the plasma that flows through the glomerulus is filtered into the renal tubules. This fraction is tightly regulated by the kidney and is a key indicator of its overall health and function, as it influences the oncotic pressure in the peritubular capillaries, thereby affecting tubular reabsorption. A filtration fraction of 0.2 is thus the standard, expected value for a healthy human being.

The incorrect option is (1): 0.002

A value of 0.002 for the filtration fraction would suggest an extreme and physiologically impossible scenario. If the FF were 0.002, it would mean that only 0.2% of the plasma flowing into the kidney is being filtered. Given a normal Renal Plasma Flow of 625 mL/min, this would result in an absurdly low GFR of only 1.25 mL/min (0.002 * 625). Such a drastically reduced GFR would be indicative of severe, life-threatening kidney failure, likely resulting from profound hypotension or acute glomerular obstruction, and is certainly not characteristic of a healthy person. This value is several orders of magnitude smaller than the functional requirement for waste excretion and fluid balance.

The incorrect option is (3): 0.02

A filtration fraction of 0.02 implies that only 2% of the plasma is being filtered. While this is less extreme than option (1), it still represents a highly abnormal and dysfunctional state. This value would result in a GFR of about 12.5 mL/min (0.02 * 625), which is less than 10% of the normal GFR. This significant reduction would lead to major clinical issues, including the retention of metabolic waste products and a severe disruption of fluid and electrolyte balance. The kidney would be unable to effectively clear waste from the body, leading to uremia. Therefore, a filtration fraction of 0.02 is too low to sustain normal kidney function and cannot be the value for a healthy individual.

The incorrect option is (4): 2

A filtration fraction of 2 is biologically impossible and fundamentally contradicts the definition of the ratio. Since the filtration fraction is defined as GFR/RPF, a value of 2 would imply that the Glomerular Filtration Rate is twice the Renal Plasma Flow (GFR = 2 $\times$ RPF). This would mean that the kidney is filtering more fluid from the plasma than the total volume of plasma entering the kidney. Filtration is a process where only a *fraction* of the incoming blood plasma is forced out through the glomerular capillaries, and this fraction can never physically exceed 1, or 100%. Therefore, any value greater than 1 must be physiologically incorrect in this context.