11.3 The kidney and osmoregulation

1. Cells lining the outside of the Malphigian tubules actively transport ions and uric acid from the hemolymph into the tubules' lumens.

This means that water is drawn out of the hemolymph into the lumens by osmosis.

2. The tubules empty into the gut.

3. In the hindgut water and salts are reabsorbed. Nitrogenous waste is excreted within feces.

1. Fenestrations in the capillary walls allow fluid to pass through but not blood cells

2. The basement membrane's glycoproteins allow everything through except plasma proteins due to their size and charges

3. The small gaps between the podocytes' foot processes stop small molecules from being filtered out of the blood in the glomerulus.

Sodium ions: These are moved out of the filtrate to the outside of the tubule by pump proteins' active transport. The Na+ ions then diffuse to the peritubular capillaries.

Chlorine ions: The above action created a charge gradient so the Cl- ions are attracted from the filtrate to space outside the tubule.

Glucose: Like chlorine ions, glucose reabsorption works in tandem with Na+ absorption. Co-transporter proteins co-transport glucose to the outside of the tubule. When the Na+ ions move down the concentration gradient, energy is provided for glucose to move at the same time to fluid outside the tubule.

Water: Pumping solutes out of the filtrate to the space outside the tubule creates an osmotic gradient so water is reabsorbed by osmosis.

1. Energy is expended by the wall cells of the ascending limb to pump Na+ ions from the filtrate to the interstitial fluid. 

Therefore the interstitial fluid is hypertonic to the filtrate, but water stays inside the filtrate as the ascending limb is impermealbe to water.

300 mOsm + 200 mOsm = 500 mOsm

2. Fluid moves down the water-permeable descending limb, so water moves out by osmosis into the interstitial fluid. 

The interstitial fluid is now hypotonic to the filtrate. Eventually so much water is drawn out of the descending limb that the interstitial fluid and the filtrate are isotonic, at 500 mOsm.

3. Protein pumps in the ascending limb pump out even more Na+ ions and raise the concentration of the interstitial fluid to 700 mOsm. 

Filtrate in the descending limb reaches 700 mOsm.

This continues until a maximum of 1,200 mOsm is reached in the interstitial fluid.

Little water is reabsorbed, so a large volume of urine with a low solute concentration is released.

Therefore the solute concentration of the blood is increased.

The hypothalamus signals to the pituitary gland to secrete ADH.

The ADH makes the walls of the distal convoluted tubule and the collecting duct more water permeable.

All the way down the collecting duct most of the water is reabsorbed via osmosis, thanks to the high solute concentration in the medulla.

A small volume of urine with a high solute concentration is produced.

The solute concentration of the blood is decreased