List three examples of substances that are actively secreted into the filtrate.

1. Tubular secretion disposes of unwanted solutes, eliminates solutes that were reabsorbed, rids the body of excess potassium ions, H+ ions (via active transport), rids the body of urea and uric acid that have been reabsorbed by passive processes, and controls blood pH. Tubular secretion moves materials from the blood of the peritubular capillaries through the tubule cells or from the tubule cells into the filtrate.

Excess hydrogen ions (H+) are combined with ammonia (NH3) to form ammonium ions (NH4+) and transported to the cells of the collecting ducts. Here the NH4+ dissociates back into ammonia and H+. Both are then secreted into the fluid within the collecting ducts (the protons by active transport).

2. The loop of Henle descends into the medulla of the kidney, where the osmotic gradient continues to
increase deeper into the medulla. This osmotic gradient is favorable for the movement of water, and because the descending limb is selectively permeable to water (via aquaporins), water moves out of the lumen and into the interstitial space. At the bottom of the loop of Henle, the urine reaches its maximum osmolarity. The tubule then ascends toward the cortex containing highly concentrated urine, moving through interstitial fluid that is less so than before. Along with passive diffusion of salts out of the lumen, the ascending limb (which is adjacent to the descending one) ACTIVELY pumps solutes (Sodium, potassium (K+) and chloride (Cl-) ions) from the lumen to the interstitial fluid, increasing the osmolarity of the interstitial fluid. The input of energy (active transport) allows for excessive salt pumping by the ascending limb. All together, this process creates a gradient within the medulla. Recall that the medullary thick ascending limb remains impermeable to water.

3. Glomerular filtration is the bulk flow of an essentially protein-free plasma from renal globerular capillaries into Bowman's capsule. This is the first step in the formation of urine. Glomerular filtration is caused by Starling force. The bulk flow of solutes (ions) into the capsule occurs by transporter proteins - this is not the same thing as a simple pore where things go from high concentration to lower concentration (diffusion).

Another way to look at this is as follows: Blood in the capillaries leading to the capsule has high pressure as it approaches the capillary bed, and this pressure is the major force pushing fluids from the blood into the nephron. Because continuous capillaries are permeable only to small molecules, plasma proteins and blood cells remain behind in the blood as fluids are pushed out, resulting in the blood having a higher osmotic.

Good luck!
Bio_man 8)

I think for # 3, you're better off looking at it the second way described by Bio_man. That's how I've always been taught, that is, that urine formation occurs as blood pressure forces filtrate from the glomerulus into the capsule. This bulk flow of fluids into the capsules of the nephrons in both kidneys creates about 180 L of filtrate per day! It's not so much that there is a higher concentration on the outside and its diffusion to the lower concentration; it's mainly due to the pressure. In diffusion, there is no pressure factor.

Here is a diagram depicting the nephron. Hope is summarizes what those two wrote.

Here's a slightly more detailed diagram:

Thank you!!!
This was soo helpful. =)

You're welcome! Click the Like button