Insects can excrete nitrogenous wastes with very little loss of water. Therefore, some insect species can live in the driest habitats on Earth. The insect excretory system consists of Malpighian tubules. An individual insect has from two to more than a hundred of these tubules attached to the gut between the midgut and hindgut. The tubules are closed at the other end, and they project throughout the insect's body cavity (Figure 51.6). Insects have open circulatory systems and therefore cannot use filtration to produce an excretory fluid.
The cells of the Malpighian tubules actively transport uric acid, potassium ions, and sodium ions from the tissue fluid into the tubules. As these solutes are secreted into the tubules, water follows because of the difference in osmolarity. The walls of the Malpighian tubules have muscle cells that contract to help move the contents of the tubules toward the hindgut.
The tubule fluid changes in composition while it is in the hindgut. The contents of the hindgut are more acidic than the tubule fluid; as a result, uric acid becomes less soluble and precipitates out of solution as it approaches and enters the rectum. The epithelial cells of the hindgut and rectum actively transport sodium and potassium ions from the gut contents back into the tissue fluid. Because the uric acid molecules have precipitated out of solution, water is free to follow the resorbed salts back into the tissue fluid through osmosis. Remaining in the rectum are crystals of uric acid mixed with undigested food; this dry matter is what the insect eliminates. The Malpighian tubule system is a highly effective mechanism for excreting nitrogenous wastes and some salts without giving up a significant fraction of the animal's precious water supply.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.