Water Balance in Animals

Ch 10 p325-330

Key Questions

• How do humans maintain water levels in the blood?– What structures (organs and glands), hormones

and processes are involved?• How do fish maintain water balance?– Freshwater vs salt-water

• How do birds maintain water balance?• How do reptiles maintain water balance?• How do amphibians maintain water balance?

Assumed knowledge

• In unit 1, the Kidneys, which are very much involved in maintaining water balance are investigated in detail.

• If you are interested see powerpoint on the kidney.

Why is water important

• Water is a significant component of blood.

• Decrease water blood pressure decreases• Increase water blood pressure increases

• Remember blood pressure is under homeostatic control.

The role of the kidneys

• The general roles of the kidneys are to:– Remove nitrogenous wastes from the blood– Control the concentration of nitrogenous wastes

in urine by diluting it with water

Nitrogenous wastes (in humans)

• Nitrogenous wastes begin as ammonia (NH3)• The liver breaks down amino acids (produced

from the breakdown of proteins) into ammonia.

• Ammonia is then broken down into urea which passes into the bloodstream and onto the kidneys.

Nitrogenous wastes

• Ammonia is very toxic • Urea is less toxic for most organisms both have to be diluted with water.– How much water can be lost in diluting waste?

• Some organisms can excrete ammonia (e.g. Freshwater fish) because they have a lot of water to continually drink.

Uric acid

• Urea can be further broken down into uric acid but this process requires significant energy.

• Uric acid is least toxic (compared to ammonia and urea)

• Therefore, animals with minimal water availability (e.g desert animals), will produce uric acid because as it is not toxic very little water is required to dilute it.

The BilbyProduces highly concentrated urine

The TarkawarraThe desert hopping mouse produces very concentrated urine – as little as one drop a day – to conserve water

A trade-off

• The body faces an ongoing trade-off:

How much water can be spared from the blood to dilute urine?

How much can blood pressure (related to loss of water in blood) be allowed to drop?

Antidiuretic Hormone

• Antidiuretic hormone (ADH) - aka vasopressin.• ADH is central in the kidneys reabsorption of

water.• Dehydration ADH will be sent to the kidneys more water will be reabsorbed through osmosis back into the bloodstream

(urine becomes more concentrated)

ADH and the pituitary gland

The pathway for ADH (Vasopressin)

• Osmoreceptors in the hypothalamus detect the concentration of solutes in the blood(by default the concentration of water –the solvent - in blood)

• If water concentration is low Neurosecretory cells in the hypothalamus produces ADH

ADH – From the hypothalamus to pituitary

ADH is transported to the posterior pituitary through the axons of neurosecretory cells.(nerve cells which produce hormones) ADH is released into the blood and travels to the kidneys

Negative Feedback

1. More ADH in kidneys more water reabsorbed less solute concentrated in blood reduced ADH secretion

2. High solute concentration in blood Thirsty Drink water Less solute concentration in blood

Renin

• Renin – a hormone produced in the kidney, important in maintain blood pressure.

Renin

• Dehydration Blood volume drops (less water in blood)Pressure sensitive detectors in kidney detect dropRenin is secreted Renin is transported to the adrenal cortex (adrenal glands) Aldosterone is secreted into blood More water is reabsorbed in the kidneys Blood pressure increases

Maintaining water balanceFigure 10.36 from Nature

Water balance in Amoeba

• Single celled organisms (unicellular).

• Protozoa• Water enters Amoeba

through osmosis.• Excess water is collected

in the contractile vacuole.• When there is sufficient

water they expel it across the membrane.

Fish: Freshwater and saltwater

• A few terms (not assessed) relating to solutions (or water surronding a fish)Hypotonic — a dilute solution, with a higher water concentration.Isotonic — a solution with exactly the same water concentration. No net movement of water.Hypertonic — a concentrated solution, with a lower water concentration.

Freshwater fish

• Freshwater fish are surrounded by freshwater and water is moving down the concentration gradient into the fish. Therefore:– Do not need to drink– Are at risk of losing salts (ions)– Gills are permeable to water– Kidneys reabsorb sodium and chloride ions– Highly dilute urine containing ammonia

Saltwater fish

• Saltwater fish face the problem of having little freshwater available to them. Freshwater moves down the concentration gradient out of the fish. Therefore:– They drink a lot of saltwater– Gills are impermeable to water– Excrete very little urine– Gills actively secrete sodium and chloride ions

Table 10.4 Water regulation in freshwater and saltwater fish

Seabirds

• All birds excrete uric acid.

• Seabirds have the problem of taking in too much salt and their blood has a high concentration of salts. They have salt excreting glands above their eyes.

Reptiles

• Aquatic reptiles – lots of water available Excrete waste as ammonia or urea

• Terrestrial reptiles – need to conserve water Excrete waste as uric acid or urate salts highly insoluble – can be exreted in solid form

• Some reptiles – salt secreting glands in nasal cavities – to get rid of salt.

Amphibians

• Live in fresh water and therefore the concentration of ions (e.g. Na+ and Cl-) is higher in the frog than outside. Problem – water constantly diffuses in through osmosis. Problem – like with freshwater fish - loss of these ions due to diffusion down the concentration gradient.

Amphibians

• Produce large quantities of dilute urine.• Actively transport (requiring energy) of

sodium and chloride ions into the body from the water.