Osmoregulation and the Evolution of the Kidney

Homeostatic mechanisms protect an animal's internal environment from harmful fluctuations. Since aqueous solutions are the chief environment for metabolic reactions they are also the target of homeostatic mechanisms.

Internal body fluids are compartmentalized and may be found as:

  • blood
  • interstitial fluid or
  • cytosol (cytoplasmic fluid

Water is essential for all life. Animals obtain water in several ways

  • drinking
  • included in solid food either free or as water of hydration
  • metabolically in condensation reactions

The uptake of water and its loss must balance. If animal cells take in too much water they will swell and eventually burst. Likewise if a cell loses more water than it takes in it will begin to shrivel ultimately leading to plasmolysis.


Water can pass through most membranes. It also moves down its concentration gradient. Thus water will move across a membrane from the hypotonic side to the hypertonic one spontaneously. This is called osmosis.

The difference in total solute concentrations between two compartments separated by a membrane results in a force or pressure called osmotic pressure.

The unit of measurement for osmolarity is milliosmoles per liter. Remember that normal human blood is about 300 mosm/L and seawater is 1000 mosm/L. Note that seawater is hyperosmotic to blood.

How Animals Cope with the Problem of Balancing Water Loss with Gain.

  1. Osmoconformers. Many marine animals are isotonic with their saltwater environment. Their body fluids are about 1000 mosm/L in concentration.
  2. Osmoregulators are animals which must constantly adjust their body fluids by active transportation. This is done by pumping ions in or out of their bodies.

    Note: water can not directly be transported or pumped across a cell membrane it can only follow its concentration gradient.

The ability to osmoregulate allows animals to live in fresh water, an environment deadly to osmoconformers who cannot stop the osmotic influx of water. Living on dry land also requires osmoregulation.

Examples of Maintaining Water Balance

Marine bony fish:
  1. constantly lose water to their environment by osmosis
  2. they compensate by drinking large amounts of seawater then pumping out excess salts, and excreting urine in relatively small quantities

Fresh water animals:

  1. constantly take in water because hypertonic body fluids cause water from the surroundings to enter the cell
  2. paramecium and other fresh water protozoans have special contractile vacuoles which bail out excess water
  3. freshwater fish excrete large amounts of very dilute urine and regain lost salts through food intake or by active transport from the surroundings.

Life in temporary waters

  • While dehydration dooms most animals some aquatic species especially invertebrates that live in temporary ponds or moist conditions can survive in a dormant state. This is called anhydrobiosis. Chemical adaptations include sugars which tenaciously hold on to the water that remains in the desiccated organism.

Terrestrial Animals

Adaptations for survival on dry land include:
  1. Surface coverings that prevent water loss
    1. the exoskeleton of insects are coated with wax effectively holds in water
    2. the scales of reptiles, feathers of birds, and hair of mammals insulate and trap moisture.
    3. the oily, keratinized skin composed of numerous layers of dead cells acts as a barrier
  2. Behavioral adaptations
    1. nocturnal activity when heat is reduced
    2. drinking and eating food which contains water
  3. Kidneys
    • by increasing the length of the loop of Henle and the brine concentration within the medulla of the kidney Kangaroo rats are able to survive almost entirely on metabolic water.

Osmoregulation - Some Evolutionary Steps

No matter what the environment osmoregulation depends upon active transport in the membranes of epithelial cells. This specialized epithelia which regulates solute movements is called transport epithelium.

Transport Epithelium is characterized by:

  • a single sheet of cells facing the external environment
  • cells which are connected to their neighbors by impermeable tight junctions forming a continuous barrier
  • a variety of membrane transport proteins which depending upon their orientation can shuttle the same ion or molecule into or out of the cell.

Examples of osmoregulatory organs

  • Protonephridia - flatworms have a simple tubular excretory system made of protonephridia. This tubular system takes in interstitial fluids by flame bulb units drawing in the body fluids, processing them and excreting a dilute nitrogen waste through openings called nephridiopores.
  • Metanephridia - earthworms use a tubular excretory system called the metanephridium. The earthworms closed circulatory system surrounds the excretory tubing within each segment. The formation of dilute urine allows copious amounts of water to leave the worm through an opening called a nephridiopore. This water loss helps compensate for the earthworm's respiratory system which takes in large amounts of oxygen saturated water through its moist skin.
  • Malpighian Tubules of insects. The tubular osmoregulatory system of insects is unique. These tubes absorb nitrogen wastes from the hemolymph. No capillaries are involved since insects have an open circulatory system. The malpighian tubules empty their contents into the anterior portion of the hindgut. Thus nitrogenous wastes are eliminated in nearly dry form along with the feces.

Nephron Structure and Function


Pressure filtration - Blood pressure forces small molecules from the glomerulus into Bowman's capsule. These molecules include water, glucose, amino acids, salts, and urea.

Selective reabsorbtion - Diffusion and active transport return molecules to blood at the proximal convoluted tubule. Molecules rapidly returned to the blood include water, glucose, amino acids, and various salt ions.

Tubular secretion - Active transport moves molecules from blood into the distal convoluted tubule or collecting duct. This steps helps to rid the blood of such wastes as uric acid, creatine, hydrogen ions, ammonia, and various foreign molecules such as penicillin.

Reabsorption of water - Along the length of the nephron and notably at the loop of Henle, water returns by osmosis following active transport of salt.

Excretion - Urine formation rids the body of metabolic wastes such as excess water, salts, urea, uric acid, ammonium, and creatine.


Further Information about the Kidney

Links to Information about the human kidney and how it works.


Modified May 20, 2003

To be continued