1. An osmoconformer is an organism whose body fluid composition is osmotically similar to its surroundings. An osmoregulator, on the other hand, maintains a relative constant internal osmotic environment.
2. A freshwater vertebrate has a higher concentration of salt than the environment (hyperosmotic), and therefore water tends to enter its body. To maintain proper body water levels, it must excrete water to prevent self-dilution.
3. A marine vertebrate has a lower salt concentration than its environment (hypoosmotic) by about one-third, so water tends to exit its body. It retains water to prevent dehydration.
4. The insect system uses an osmotic gradient to pull blood through the Malpighian tubules. Prior to excretion, the wastes are further concentrated through the selective reabsorption of water by the hindgut of the insect.
5. A nephron is the filtration-reabsorption device in the kidney. Bowman's capsule is located at the front of the nephron. The glomerulus is a fine network of capillaries located within Bowman's capsule. Filtration occurs as blood pressure forces fluid through the capillary walls, retaining proteins, cells, and large molecules and allowing water and small molecules to pass through to the interior of the capsule.
6. The kidney first evolved in freshwater fishes. Cartilaginous fishes use urea to elevate the osmotic concentration of their tissues. Mammals and birds produce urine hyperosmotic to their tissues.
7. Most salt and water is absorbed across the membranes of the proximal convoluted tubule. The remaining filtrate is isosmotic to blood plasma.
8. The descending arm of the Loop of Henle is most freely permeable to water. The base of the loop is freely permeable to salt, and the thick region of the ascending loop is where active transport of sodium ions occurs.
9. Freshwater fishes release a continuous dilute ammonia to the surroundings. Both fresh and salt water fishes release nitrogen from the gills, as well. Mammals excrete most of their nitrogenous wastes in the form of a concentrated urine (composed principally of a detoxified form of ammonia called urea). Reptiles and birds excrete a semi-solid, insoluble compound called uric acid-a necessity in organisms that have embryonic development within shells.
10. Antidiuretic hormone makes collecting ducts permeable to urea, which further allows for the reabsorption of water. Decreasing the levels of ADH means that ducts are not permeable to urea, which lowers the osmotic concentration in the medulla so that less water diffuses outward and is reabsorbed, and urine volume is increased. Aldosterone stimulates the kidneys to resorb salt, ultimately functioning in retention of water.