Osmoregulation
Osmoregulation is one of the major processes by which animals maintain homeostasis of fluids. "Osmoregulation is the active regulation of osmotic pressure of body fluids to keep them from becoming too dilute or too concentrated" (Solomon et.al.). Animals in all environments (aquatic and terrestrial) must maintain the right concentration of solutes and amount of water in their body fluids; this involves excretion: getting rid of metabolic wastes and other substances such as hormones which would be toxic if allowed to accumulate in the blood via organs such as the skin and the kidneys; keeping the water and dissolved solutes in balanced is referred to as osmoregulation.
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2 Vertebrate excretory systems 3 References |
Forms of osmoregulation
Two major types of osmoregulation are osmoconformers and osmoregulators. Osmoconformers are animals that allow their body fluid concentration to vary with the environment. An example are marine fish. By drinking in sea water, and actively excreting salt out from the gills, the fish will gain salt as it produces an isotonic urine. Omoregulators are more common in the animal kingdom. Osmoregulators actively control salt concentrations despite the salt concentrations in the environment. An example are freshwater fish. The gills actively uptake salt from the environment. Water will diffuse into the fish where the fish will excrete a very hypotonic urine to expel all the excess water.
Kidneys play a very large role in human osmoregulation. Kidneys regulate the amount of water in urine waste. With the help of naturally producing hormones such as antidiuretic hormone, aldosterone, and angiotensin II, the human body can increase permeability of the collecting ducts in the kidney to reabsorb water and prevent it from being excreted.
A major way animals have evolved to osmoregulate is by controlling the amount of water excreted through the excretory system.
Vertebrate excretory systems
Waste products of nitrogen metabolism
Ammonia is a toxic by-product of protein metabolism and is generally converted to less toxic substances after it is produced then excreted; mammals convert ammonia to urea while birds and reptiles form uric acid to be excreted with other wastes via their cloacas.
How osmoregulation is achieved in vertebrates
Four processes occur:
- filtration - fluid portion of blood (plasma) is filtered from nephron (functional unit of vertebrate kidney) structure known as the glomerulus into Bowman's capsule or glomerular capsule (in the kidney's cortex) and flows down the proximal convoluted tubule to a "u-turn" called the Loop of Henle (loop of the nephron) in the medulla portion of the kidney.
- reabsorption - most of the viscous glomerular filtrate is returned to blood vessels which surround the convoluted tubules.
- secretion - the remaining fluid becomes urine which travels down collecting ducts to the medullary region of the kidney.
- excretion - the urine (in mammals) is stored in the urinary bladder and exits via the urethra; in other vertebrates the urine mixes with other wastes in the cloaca before leaving the body; ( frogs also have a urinary bladder).
References
- E. Solomon, L. Berg, D. Martin, Biology 6th edition. Brooks/Cole Publishing. 2002
