WATER BALANCE & OSMOREGULATION

Animals regulate both water content and over-all solute concentration, in addition to levels of specific solutes.

• concentration of solutes (over-all and specific) in body fluids is typically different from an animal's environment
• usually are NOT in equilibrium with the environment

Question: Why is it important to regulate the amount of water in the body?

Question: Why is it important to regulate the concentration of solutes in the body?

NITROGENOUS WASTE EXCRETION

Protein, nucleic acids constantly being broken down & rebuilt; nitrogenous waste products formed from break down

ammonia - final break down product; highly toxic

• excreted directly in most fish, other aquatic organisms
• diffuses easily out of gills into water

Question: Why does ammonia diffuse out of the body? Why does it diffuse out of the gills in particular?

urea - ammonia converted to this form; somewhat less toxic

• mammals, some fish, adult amphibians
• very soluble in water; excreted by kidney
• some animals' kidneys can concentrate it to a high degree and excrete in concentrated form in urine

Question: Why can't a terrestrial animal easily rid the body of ammonia by simple diffusion?

Question: Would you expect a desert mammal to produce urine with a high or low concentration of urea? Why?

uric acid - ammonia converted to uric acid; somewhat less toxic

• birds, reptiles
• low solubility in water
• lost in urine, excreted from cloaca as semi-solid paste as it precipitates out of solution
• also conserves water

Question: As fully terrestrial animals, birds and reptiles produce shelled eggs. What problems would an embryo enclosed in a shell encounter regarding waste excretion? Would ammonia work? Urea? Uric acid? Explain.

WATER BALANCE

To remain in water balance, water gain must exactly equal water loss

Aquatic animals - marine - read pages 939-mid941, fig 44.14

Most fish regulate body fluid concentration (~300 mOsm) at less than concentration of sea water (~1000 mOsm)

Question: In what net direction will water tend to move - into the fish or out of the fish? Why?

Question: In what net direction will salts tend to move - into the fish or out of the fish? Why?

Question: At what location in the body will most of this movement take place? Why?

Question: How can the fish recover the lost water? (There is only one way).

Question: How can the fish get rid of excess salt? Where is the best location to do this? Why?

Aquatic animals - fresh water

Most fish regulate body fluid concentration (~300 mOsm) at greater than concentration of fresh water (~30 mOsm)

• These fish have opposite problems of marine fish. Read text material p. 939 - mid941, fig. 44.14
  

Terrestrial animals

To remain in water balance, water gain must exactly equal water loss.

Question: What are the routes of water loss for a terrestrial animal?

Question: What are the routes of water gain for a terrestrial animal?

Question: What organisms produce metabolic water?

Question: For some animals, drinking water is unavailable, and there is very little water in the food, yet they function perfectly well. What must they be doing to survive in these conditions?

VERTEBRATE KIDNEYS

The kidney filters and processes blood at a high rate to remove wastes and excess, unwanted materials

• 20% of cardiac output of blood goes to kidneys (only 1% of body mass) (humans)
• 1000 to 2000 L of blood flows to kidneys daily in humans
• 180 L fluid per day are filtered by the kidneys in humans

Function of the nephron - functional unit of kidney - ~ million/kidney

glomerulus - knot of capillaries where blood brought in to be filtered

• more permeable than other capillaries of body
• blood forced through pores under pressure into Bowman's capsule

Bowmans's capsule - pores in wall that act as filtration device

• water, small solutes enter - this filtrate continues on through nephron
• cells, large molecules (proteins) stay behind in blood - continue into veins and back to heart

proximal tubule - active processing of filtrate

• reabsorption of valuable materials back into blood
• secretion of unwanted materials from blood into filtrate

Question: What are some examples of valuable materials that would be reabsorbed?

Question: What are some examples of unwanted materials that would be secreted?

loop of Henle - short to long loop of nephron

• sets up concentration gradient in kidney
• high concentration of solutes at bottom; very dilute at top

distal tubule - active processing of filtrate

• reabsorption of valuable materials back into blood
• secretion of unwanted materials from blood into filtrate

collecting duct - filtrate from several nephrons drains into one of thousands of collecting ducts

• permeability to water adjusts as needed to allow reabsorption of water, or excretion of water

Control of urine concentration/water loss

Normal set point ~ 300 mOsm; blood concentration monitored by osmoreceptors in hypothalamus

If too much water in body fluids/too dilute:

1. inhibition of release of ADH (anti-diuretic hormone) from hypothalamus and posterior pituitary

2. lack of ADH causes walls of collecting ducts to be impermeable to water

3. filtrate exits kidney in dilute state

Question: In this case, what will the volume and concentration of the urine be? Will this help rectify the original problem?

If not enough water in body fluids/ too concentrated:

1. stimulates release of ADH from hypothalamus and posterior pituitary

2. ADH causes walls of collecting ducts to be very permeable to water

Question: What will happen to the water in the filtrate in this situation? Why?

Question: In this case, what will the volume and concentration of the urine be? Will this help rectify the original problem?

Question: Ethanol inhibits the release of ADH. How will ingesting alcoholic beverages affect a person's water balance? Explain.