Explain why the filtrate becomes hypotonic as it flows through the ascending limb of the nephron loop. Also explain why the filtrate at the bend of the nephron loop (and the interstitial fluid of the deep portions of the medulla) is hypertonic.

Verified step by step guidance

1. Understand the structure of the nephron loop, also known as the loop of Henle, which consists of a descending limb and an ascending limb.
2. Recognize that the descending limb is permeable to water but not to solutes, allowing water to leave the filtrate and enter the interstitial fluid, making the filtrate more concentrated as it descends.
3. Note that at the bend of the nephron loop, the filtrate is hypertonic due to the loss of water in the descending limb, and the surrounding interstitial fluid in the medulla is also hypertonic to facilitate this water movement.
4. Identify that the ascending limb is impermeable to water but actively transports ions such as Na+, K+, and Cl- out of the filtrate into the interstitial fluid, decreasing the filtrate's osmolarity.
5. Conclude that as the filtrate ascends, it becomes hypotonic because solutes are removed while water remains, resulting in a lower concentration of solutes in the filtrate compared to the surrounding interstitial fluid.

Key Concepts

Nephron Loop Function

The nephron loop, or loop of Henle, plays a crucial role in concentrating urine and regulating water and electrolyte balance. It consists of a descending limb that is permeable to water and an ascending limb that is impermeable to water but actively transports sodium and chloride ions out into the interstitial fluid, creating a concentration gradient essential for urine concentration.

Filtrate Osmolarity Changes

As the filtrate ascends the nephron loop, it becomes hypotonic due to the active transport of solutes from the ascending limb into the surrounding interstitial fluid. This process reduces the concentration of solutes in the filtrate, making it less concentrated compared to the surrounding fluid, which is critical for the kidney's ability to produce dilute urine.

Countercurrent Multiplier System

The countercurrent multiplier system refers to the mechanism by which the nephron loop establishes a hypertonic environment in the medulla. The active transport of ions in the ascending limb increases the osmolarity of the interstitial fluid, particularly in the deep medulla, while the descending limb allows water to exit, leading to a concentration gradient that facilitates water reabsorption in the collecting ducts.