Maintaining a Balance

PART 5: Regulation of Fluids





Temperature and Life

About Blood

Regulation of Fluids


Maintaining a stable water concentration Removal of wastes The role of kidneys Passive transport is not enough›
How active and passive transport work together in the kidney› The role of aldosterone and ADH Comparing renal dialysis with normal kidney function Compare urine
Conservation of water and concentration of nitrogenous waste Enantiostasis    

Maintaining a stable water concentration

Many metabolic reactions in cells can only occur in solution with water as the solvent. As most cell reactions are catalysed by enzymes the physical/chemical conditions within the cell need to be maintained within narrow limits.

Changes in water concentration can affect these conditions and affect cell processes.

Consequently, the water concentration also needs to be maintained within narrow limits.


Removal of wastes


Metabolic wastes include CO2 and nitrogenous wastes such as urea. If they accumulate in cells then they will alter the pH of the cell fluids. This will affect the function of enzymes and hence affect the normal metabolic activity of the cell.

Consequently, they need to be removed from the cell to allow the normal metabolic functions of the cell to continue.


The role of kidneys


Once metabolic wastes have been removed from cells they pass into blood. They need to be removed from blood so that the chemical conditions within blood remain stable.

Kidneys are the organs in fish and mammals that remove metabolic wastes from blood and also maintain the balance of water and soluble ions within blood.




Passive transport is not enough  


Diffusion and osmosis are both examples of passive transport, relying on random movements of molecules.

Diffusion is too slow for the normal functioning of the body and does not select for useful solutes.

Osmosis only deals with the movement of water and thus would only allow water to move out of the body, not the nitrogenous wastes.


How active and passive transport work together in the kidney 


Active transport involves using energy to move substances against a diffusion gradient. Passive transport involves movement of substances along a diffusion gradient and includes both diffusion and osmosis.

Both processes occur within the kidney. Water is reabsorbed from nephron tubules by osmosis. The concentration of ions such as Na+ and K+ are maintained by active transport with movement often against the concentration gradient within the kidneys.


How filtration and reabsorption in the kidney nephron regulate body fluid composition



Filtration of the blood occurs in Bowman's capsule where high blood pressure in the glomerulus forces all small molecules out of the blood into the capsule.

Water, urea, ions (Na+, K+, Cl- , Ca2+, HCO3- ), glucose, amino acids and vitamins are all small enough to be moved into the glomerular filtrate.

Blood cells and proteins are too large to be removed.

This filtering process is non-selective and therefore many valuable components of the blood must be recovered by reabsorption.


Reabsorption takes place selectively at various points along the proximal tubule, loop of Henle and distal tubule.

All glucose molecules, amino acids and most vitamins are recovered, although the kidneys do not regulate their concentrations.

The reabsorption of the ions Na+, K+, Cl - , Ca2+ and HCO3 - occurs at different rates depending on feedback from the body. In some cases, active transport is required.

Water is reabsorbed by osmosis (passive transport) in all parts of the tubule except the ascending loop of Henle. The amount of water reabsorbed depends on feedback from the hypothalamus. If no water were reabsorbed human would soon dehydrate, losing water at a rate of around 7.5 L per hour.

The chemical composition of the body fluids is precisely regulated by the control of solute reabsorption from the glomerular filtrate.


  The role of aldosterone and ADH

  Hormones assist the kidney in maintaining the appropriate water balance and salt balance.  


Aldosterone is a steroid hormone secreted by the adrenal gland. Its function is to regulate the transfer of sodium ions in the kidney.

When sodium levels are low, aldosterone is released into the blood causing more sodium to pass from the nephron to the blood.

Water then flows from the nephron into the blood by osmosis.

This maintains blood pressure in the body.

NOTE: Aldosterone acts on the distal tubule and the collecting tubule.

Addison's Disease

For some reason, some people's adrenal gland fail to produce sufficient aldosterone to maintain Na+ levels in blood. This is called Addison's disease.

Symptoms which include high urine output with a resulting low blood volume. Eventually, as blood pressure falls, this can result in heart failure.

A replacement hormone, fludrocortisone (Florinef), can be used used to treat this condition. This, together with a salt reduced diet, has the benefits of maintaining appropriate Na+ and hence water levels in the blood and normal blood pressure.

However, careful monitoring must be maintained to avoid fluid retention and high blood pressure.

more information...

ADH (Antidiuretic Hormone)

ADH Antidiuretic hormone or vasopressin) controls water reabsorption in the nephron.

When levels of fluid in the blood drop, ADH is released. This increases the permeability of the collecting tubules to water, increasing the amount of water reabsorbed from the urine into the blood.

The resulting urine is more concentrated.

When there is too much fluid in the blood, less ADH is released. This decreases the permeability of the collecting tubules to water decreasing the amount of water reabsorbed into the blood.

This results in a lower blood volume and larger quantities of more dilute urine.

NOTE: ADH acts on the collecting tubule only.


 Comparing renal dialysis with normal kidney function


The kidneys function as filters for the blood, removing products of amino acid breakdown. More than that, they serve to reclaim and regulate body water, maintain electrolyte balance, and ensure that the blood pH remains between 7.35 and 7.45. Without the function of the kidney, life is not possible.

Dialysis serves to replace some of the functions of the kidney. Since dialysis is not a constant ongoing process, it cannot serve as a constant monitor as do normal functioning kidneys, but it can eliminate nitrogenous wastes, extra water and maintain electrolyte levels and pH levels on an as needed basis.

In hemodialysis, a dialysis machine and a special filter called an artificial kidney, or a dialyzer, are used to clean your blood.


  The dialyzer, or filter, has two parts, one for your blood and one for a washing fluid called dialysate.  



Dialysate contains chemicals in amounts similar to normal blood. The composition of the fluid determines which solutes pass out of and which stay in the blood during dialysis The dialysis fluid must be a solution that closely matches the chemical composition of the blood.

A thin membrane separates these two parts. Blood cells, protein and other important things remain in your blood because they are too big to pass through the membrane.

Smaller waste products in the blood, such as urea, creatinine, potassium and extra fluid pass through the membrane and are washed away. Diffusion of substances necessary to the body (such as sodium chloride) is prevented by their presence in the dialysis fluid.

While the principle behind dialysis is simple, the technology needed to ensure that the procedure is safe can be complex.




...more information


Below is a table comparing (similarities and differences) the function of the kidney and renal dialysis.



Removes nitrogenous wastes
Maintains electrolyte and pH balance
Controls blood pressure by fluid and sodium removal
Continuous process 3-4 hours 3 days per week
Internally monitored with the assistance of hormones Externally monitored by medical specialists and computer controlled machines
100% of kidney function 10% of kidney function
Part of normal body processes

Potential problems include:
low blood pressure
nausea and vomiting
infection from catheters
blood clots
air bubbles


Comparing and explaining the differences in urine concentration between terrestrial mammals, marine fish and freshwater fish

  Water is an essential compound to all living things. Different organisms have different strategies in maintaining an internal water balance depending on the external environment in which they live.  

Terrestrial mammals live in an environment where water is not freely available. They lose water by breathing and regulate body temperature through perspiration. They also need a constant source of fresh drinking water to excrete accumulated salts and metabolic waste products.

To retain as much water as possible mammals have very efficient kidneys that reabsorb most of the water that passes through them producing concentrated urine.


Marine fish live in an environment where the external salt concentration is sea water is more than their internal salt concentration. This results in osmosis of water from the fish to the sea water. To maintain a water balance, marine fish drink copious amounts of water but produce small amounts of concentrated urine.


Freshwater fish have the opposite problem. Since their internal salt concentration is more than that of freshwater, osmosis of water occurs from the water to the fish. To cope with this freshwater fish produce copious amounts of dilute urine.


...more information

....but what about organisms living in esturarine environments?



Organism Urine Concentration Reason
Terrestrial mammal Concentrated To retain as much water as possible living in an environment where water is not freely available
Marine fish Concentrated To retain as much water as possible to cope with osmotic loss of water due to high salt concentration external environment
Freshwater fish Dilute To excrete as much water as possible due to osmotic intake of water due to low salt concentration of external environment

Conservation of water and concentration of nitrogenous waste


Terrestrial organisms live in environments where water is not always freely available. So they need mechanisms to conserve water while at the same timebe able to excrete nitrogenous waste.

If nitrogenous waste products are allowed to accumulate the lead to poisoning and death of the organism.

Nitrogenous wastes can be excreted as ammonia, urea or uric acid.


Ammonia is very toxic and must be removed immediately, either by diffusion or in very dilute urine. It is the waste product of most aquatic animals, including many fish and tadpoles. Ammonia is the immediate product of break down of amino acids — no energy is required to make it. It is highly soluble in water and diffuses rapidly across the cell membrane. However, it needs large quantities of water to be constantly and safely removed. Ammonia does not diffuse quickly in air.


Urea is toxic, but 10 000 times less toxic than ammonia, so it can be safely stored in the body for a limited time. It is the waste product of mammals, and some other terrestrial animals, but also of adult amphibians, sharks and some bony fish. It is made from amino acids but requires more steps and energy to make than does ammonia.

It is highly soluble in water, but being less toxic than ammonia, it can be stored in a more concentrated solution and so requires less water to remove than ammonia. It is a source of water loss for these species.

Uric Acid

Uric acid is less toxic than ammonia or urea, so can be safely stored in or on the body for extended periods of time. It is the waste product of terrestrial animals such as birds, many reptiles, insects and land snails. It is a more complex molecule than urea so it requires even more energy to produce. It is thousands of times less soluble than ammonia or urea and has low toxicity, which means that little water is expended to remove it. This is a great advantage for survival.

There is a fine balance between the use of water to remove nitrogenous wastes and conservation of water in the body. Australian terrestrial mammals that live in predominantly arid areas, such as the Bilby (Macrotus lagotus), must produce very concentrated urine and tolerate high levels of urea in their systems. Some insects excrete ammonia as a vapour across the body surface rather than as a solution of urine, an adaptation for conserving water. More commonly, uric acid is produced, which is a dry urate waste requiring no water to remove and with low toxicity so that it can be kept in the body for long periods of time.

Terrestrial or aquatic
Waste product
spinifex hopping mouse of Central Australia terrestrial very concentrated urine The animal lives in a very arid environment. It drinks very little water and excretes urea in a concentrated form, so that water can be conserved.
Euro, wallaroo
(Macropus robustus)
terrestrial concentrated urine Euros have a very efficient excretory system that recycles nitrogen and urea to make a very concentrated urine. This allows them to survive in very arid environments

Australian Insects

eg cicada, Bogong moth

terrestrial uric acid Insects are covered with a cuticle impervious to water. They conserve water by producing a dry paste of uric acid.





Organisms living in estuarine environments have additional problems in maintaining a stable water balance. In these environments the salinity of the water varies and estuarine organisms need additional adaptations to cope with this variation.


Enantiostasis is the maintenance of normal metabolic and physiological functioning, in the absence of homeostasis, in an organism experiencing variations in its environment.

All organisms living in an estuary experience large changes in salt concentration in their environment over a relatively short time span, with the tidal movement and mixing of fresh and salt water.

To maintain normal metabolic processes, estuarine organisms need adaptations to cope with the varying salinity.

One strategy to withstand such changes in salt concentration is to allow the body's osmotic pressure to vary with that of the environment. Organisms that do this, and therefore do not maintain homeostasis, are said to be osmoconformers. Most marine invertebrates are osmoconformers.

However, as the salt concentration of body fluids in an osmoconformer changes, various body functions are affected, such as the activity of enzymes. For normal functioning to be maintained, another body function must be changed in a way that compensates for the change in enzyme activity.

One example of enantiostasis is when a change in salt concentration in the body fluid, which reduces the efficiency of an enzyme, is compensated for by a change in pH, which increases the efficiency of the same enzyme.

In another strategy is to minimise the effects of salinity change, some shellfish burrow into the mud in an estuary.