Final answer:
The osmolarity of the filtrate in the nephron varies across different segments. In Bowman's capsule, the osmolarity is about 300 mOsm/L. It then begins to decrease in the proximal convoluted tubule, increases in the descending limb of the loop of Henle.
Step-by-step explanation:
The nephron is a critical structure in the kidneys that helps maintain the body's fluid and electrolyte balance by filtering blood and forming urine. Across different parts of the nephron, the osmolarity of the filtrate changes as substances are reabsorbed or secreted. Below are the osmolarities at different points in the nephron:
Bowman's capsule - This is where filtration begins, and the osmolarity here is similar to plasma, generally around 300 mOsm/L. The osmolarity remains relatively constant since there's no absorption or secretion taking place.
Proximal convoluted tubule - Reabsorption of ions, water, nutrients, and other substances occurs here. The filtrate osmolarity begins to decrease as water is reabsorbed alongside solutes.
Descending loop of Henle - Water leaves the filtrate due to the high osmolarity of the surrounding interstitial fluid, leading to an increase in filtrate osmolarity as it moves down.
Ascending loop of Henle - This segment is impermeable to water, but actively transports Na+ and Cl- ions into the interstitial fluid, causing the filtrate to become more dilute, thus decreasing in osmolarity.
Distal convoluted tubule - Additional ions are secreted into the filtrate while reabsorption of other ions occurs, which may lead to slight changes in osmolarity, although the overall trend results in a further decrease in osmolarity since water can be reabsorbed in response to hormones if the body needs to conserve water.
Collecting ducts - The filtrate osmolarity can vary here, as this is a site of hormone-regulated reabsorption of water. The final osmolarity of urine can range widely, from 50 to 1200 mOsm/L or more, depending on the state of hydration.
Given these processes, let's correct the original list:
a) Bowman's capsule - The filtrate concentration does not decrease here; it remains isotonic with blood plasma at approximately 300 mOsm/L.
b) Proximal convoluted tubule - This is where the filtrate concentration begins to decrease as water and solutes are reabsorbed.
c) Loop of Henle - This includes both the descending limb (where filtrate concentration increases due to water leaving) and the ascending limb (where filtrate concentration decreases as Na+ and Cl- are removed).
d) Distal convoluted tubule - Here the filtrate concentration may decrease or remain relatively constant, depending on the levels of hormonal regulation.