Calculation of reabsorption of filtrate volume
Reabsorption of the original filtrate volume occurs primarily in the proximal convoluted tubule. Here, around 65-75% of the original filtrate volume is reabsorbed back into the bloodstream. This reabsorption process involves the movement of essential substances such as glucose, amino acids, and ions from the tubular fluid back into the blood.

Volume of filtrate entering the Henle's loop
By the time the tubular fluid enters the Henle's loop, approximately 20-25% of the original filtrate volume has already been reabsorbed in the proximal convoluted tubule. This means that only about 75% of the original filtrate volume remains in the tubular fluid as it enters the loop of Henle.

Correct answer
Therefore, the correct answer to the question is option 'A', which indicates that around 75% of the original filtrate volume is reabsorbed by the time the tubular fluid enters the Henle's loop. This percentage reflects the significant reabsorption process that occurs in the proximal convoluted tubule, ensuring the conservation of essential substances and the maintenance of body homeostasis.

To determine the amount of the original filtrate volume that is reabsorbed by the time the tubular fluid enters the Henle's loop, we need to understand the process of filtration and reabsorption in the nephron.
1. Filtration: In the renal corpuscle (specifically the glomerulus), blood pressure forces water and small solutes out of the glomerular capillaries and into the Bowman's capsule, forming the filtrate.
2. Reabsorption: As the filtrate passes through the renal tubules, various substances are selectively reabsorbed by the surrounding tubule cells and returned to the bloodstream. This reabsorption process occurs in different segments of the tubules.
The Henle's loop is a part of the renal tubule, specifically the nephron loop, that consists of a descending limb and an ascending limb. The descending limb is permeable to water, allowing water reabsorption, while the ascending limb is impermeable to water but actively transports ions out of the tubule.
Based on these processes, we can determine that:
- The Henle's loop contributes to the reabsorption of water from the tubular fluid.
- The descending limb of the Henle's loop is responsible for reabsorbing water.
- The ascending limb of the Henle's loop is responsible for reabsorbing ions but not water.
Therefore, the amount of the original filtrate volume reabsorbed by the time the tubular fluid enters the Henle's loop is primarily due to the reabsorption of water in the descending limb. This reabsorption of water helps to concentrate the filtrate and conserve water in the body.
Based on this information, the answer to the given question is:
- The amount of the original filtrate volume reabsorbed by the time the tubular fluid enters the Henle's loop is approximately 75%.