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Osmoregulation and Excretion (ch. 44)

Osmosis and osmolarity
Osmosis is the tendency of a fluid to pass through a permeable membrane to go from areas of high concentration to areas of lower concentration. This process will result in equilibrium between the sides of the membrane. When organisms attempt to achieve homeostasis, they must go through osmoregulation. Osmoregulation is the term used to describe the balancing of water and solute concentrations. Surplus solutes and water are often disposed of be means of excretion. Some organisms, called osmoregulators, control their water gain and loss in hypoosmotic and hyperosmotic environments.

Osmoconformation vs. osmoregulation in marine vs. freshwater organisms
Different organisms conduct osmosis in different ways.
Freshwater fish gain water and loose salt, so osmosis must be used to keep salt and water balanced properly.
Largemouth Bass

Freshwater fish excrete large amounts of urine, which is much less concentrated than its body fluids.

Saltwater fish tend to loose water and take in salt. A saltwater fish does not excrete a large amount of urine,
and it is only marginally less concentrated than its body fluids.

Terrestrial vertibrates tend to lose water to the air, and take in salt, they excrete a moderate volume of urine and it is
more concentrated than their body fluids.

Anhydrobiosis, challenges of land dwelling

Anhydrobiosis is a type of cryptobiosis that occurs in situations of extreme desiccation. (very dry) This is a problem that the first land dwelling organisms faced. When organisms are underwater, they have no problem obtaining water, since they can absorb it from their environment. The first land organisms had to find ways of dealing with general lack of water. This required a very different strategy of osmosis, and explains the very different methods of excreting.

Role of transport epithelium and energetic of osmoregulation

The role of epithelial tissue is to regulate the solute movements for waste disposal and change in body fluids. This allows many organisms to perform feats of regulation that previously would have been thought to be impossible. An example of this, Albatross are capable of drinking seawater for their water they need. The Albatross uses its transport epithelium to carefully regulate its salt levels. Without this, the Albatross could not survive in its current conditions.

Forms of nitrogenous waste: what are they, who has them, and why?

Nitrogenous waste primarily appears in organisms as urea. Fishes turn excess nitrogenous bases into ammonia, which is then turned into urea by mammals and amphibians, and uric acid by birds. This nitrogenous waste is collected and confined into one place in the body(bladder for most organisms) and excreted.

Comparison of excretory systems: protonephridia, metanephridia, malpighian tubules, kidneys

Excretion is in most organisms involves a filter in which the blood passes through to remove waste products. These filtering systems are broken up into 4 catagories, protonephiridia, metanephridia, malpighian tubules, and kidneys. Protonephiridia are the filtration systems used by flatworms, which excrete a dilute filtrate rather than urine. Earthworms have metanephridia which are open-ended filters in each body segment that excretes urine. Insects have malpighian tubules, which use direct osmosis to remove nitrogenous waste. In vertebrates, kidneys function as osmoregulators as well as excretory systems.

A closer look at vertebrate kidneys and adaptations to diverse environments

Kidneys are unique, they act as osmoregulatros as well as excretory systems. Kidneys are made up of clusters of nephrons and collecting ducts. Each nephron has a cluster of blood vessels (known as the glomerulus) packed into a capsule. (known as the Bowman's Capsule). The blood that passes through these blood vessels is filtered for water and nitrogenous wastes. Water can than be reabsorbed if the body is short on water, leaving mostly urea. After filtration and re-absorption, the waste drains into collecting ducts. The urine then moves through the ureter to the bladder to be excreted.
Hormonal control of osmoregulation

Osmoregulation is controlled by the pituitary gland and several other glands. When osmolarity rises above a certain point, the pituitary gland releases antidiuretic hormone. (ADH) Antidiuretic hormone increases the permeability to water in the collecting ducts, causing more water to be flushed from the system via excretion. Blood pressure is measured by the juxtaglomerular apparatus.(JGA) When blood pressure drops below a certain point, it releases a hormone called renin, which causes more blood to be made. Angiotensin II is formed to serve as a counter-balance to renin, which raises blood pressure.