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Biology 2108 Lecture
Physiology:
Osmoregulation and Excretion

How is the excretory system related to cellular respiration?

amino acids
     l
     l ---> ammonia
    V
glucose + O2 -> CO2  + H2
O + useable energy



Why do animal need to be concerned with water loss if water is a by-product of cellular respiration?

So the excretory system is involved with:


Marine and freshwater organisms face the
opposite problem of water movement due to
osmotic differences between their internal
fluids and the external environment.










To reduce water loss through body surfaces, terrestrial vertebrates have keratin (a water-resistant, insoluble protein) in their epidermis, and arthropods have a waxy cuticle.

Some desert mammals are so efficient at water regulation that drinking is not required for survival; how then is water aquired?






Problems associated with water regulation are closely tied to the bodies production of nitrogenous waste produced by the breakdown and conversion of amino acids and nucleic acids. 

What kind of environments might select for an excretory system that would produce each of the above compounds?


Functions of the excretory system in humans:
-Removal of nitrogenous waste
-Water regulation

Three main concepts in the function of the nepheron (the functional unit of the kidney)

1) Pressure in the circulatory system crudely filters certain solutes and water from the capillaries (glomerulus) into the tubules of the nepheron.  20% of the blood flow is diverted to the kidneys (What percentage of blood flow is diverted to the lungs during on circulatory loop?).

What would this mean in terms of water loss if this filtrate was excreted from the body as is?
Would pressure filtration be very selective in terms of ions?



2) Both active transport and passive transport remove certain solutes from the filtrate.


3) The loop structure (loop of Henle) travels though concentration gradient resulting in osmotic re-absorption of water from the filtrate of the tubules, concentrating urine.  Blood flows in the capillaries (vasa recta) in the opposite direction of solute flow in the loop (countercurrent) also aiding in water retention by preventing loss of NaCl from the medulla.


The Loop of Henle allows filtrate to be concentrated twice.  The descending loop is permeable to water, thus concentrating filtrate.  Sodium chloride is remove in the ascending loop (at high energy cost); though concentration of the filtrate decreases, so has volume, so that even more water can be removed as filtrate descends in the collecting duct (4-fold concentration compared to concentration of the blood).




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