We begin by revisiting the structural features of the insect renal system. We can take a look at the figures from the previous lecture. This figure shows Malpighian tubules arising from the alimentary canal. Again, these are thin, close-ended tubes floating free in the hemocoel where they are continuously bathed in hemolymph. Small tracheae run along the length of the tubes, which supply oxygen to mitochondria located in muscles and in Malpighian tubules.
Malpighian tubules is isomotic to the hemolymph, but of different composition.
Hemolymph is generally high in sodium, chloride and amino acids, while the intratubular fluid is high in potassium. The general expression is that potassium is actively pumped from hemolymph to tubule lumens and water passively follows the ion movement. Recall there are homogeneous and heterogeneous Malpighian tubules. They differ in resorptive properties, but the general patterns apply to both types of Malpighian tubules. The flow of water into Malpighian tubules increases local hydrostatic pressure within the tubules, resulting in a bulk flow of fluid.