11.1 Essential ideas

11.1.3a Introduction to osmoregulation

  • Osmoregulation is a homeostatic mechanism that keeps blood within a certain range of solute concentration.
  • Animals that maintain a constant internal solute concentration are osmoregulators. All terrestrial and freshwater animals, and most vertebrates, are osmoregulators.
  • Animals whose body fluids are isosmotic to the environment are osmoconformers. Some examples are sharks and rays.

Types of nitrogenous waste

  • All animals produce ammonia during metabolic processes. Ammonia increases the pH of cells, and is a toxic waste.
  • Some animals expend energy to process ammonia into less toxic nitrogenous wastes.
  • The type of nitrogenous waste in animals is correlated with habitat and evolutionary history.

Marine and



• Fish

• Most marine classes

Ammonia NH3

• High toxicity, low energy investment

• A gas that dissolves easily in water

• Diluted quickly by aquatic environments when released



• Mammals

CO (NH2)2

• Medium toxicity, medium energy investment

• Water-soluble; excreted with varying amounts of water depending on the internal conditions

• Birds

• Reptiles

• Insects

• Most terrestrial arthropods

Uric acid

• Low toxicity, high energy investment

• Insoluble, crystallises in water

• Advantages of insolubility:

- wastes produced by young accumulate inside of the egg but do not harm development

- water is conserved in the blood/haemolymph

Excretion and osmoregulation in insects: Malpighian tubules

  • Insects have an open circulation system. Blood and tissue fluid circulate together through the body cavity as a colourless liquid called haemolymph.
  • Malpighian tubules branch out into the body cavity and empty into the intestinal tract. Their main function is to remove uric acid from the haemolymph.

Figure 11.1.3a/a – Malpighian tubulesFigure 11.1.3a/a – Malpighian tubules

  • Uric acid it is actively transported into the lumen of the tubules, as are some ions, such as Na+ and K+.
  • Water is then absorbed into the tubules by osmosis.
  • Most of the water and salts are reabsorbed in the hindgut, leaving only concentrated uric acid and a small amount of water to be excreted.

Skill: Draw and label a diagram of the human kidney

Figure 11.1.3a/b – The kidneyFigure 11.1.3a/b – The kidney

  • Blood is brought to the kidney via the renal artery. Filtered blood is carried away from the kidney via the renal vein.
  • The composition of blood in the renal artery is different from that in the renal vein.

Blood in the renal artery

Blood in the renal vein

  • Nitrogenous wastes: urea or uric acid
  • No nitrogenous wastes
  • Variable water and salt content produced by metabolism
  • Water and solute concentration balanced by osmoregulation
  • Toxins and undigested soluble products such as antibiotics, drugs or food pigments
  • Toxins removed (at least partially)
    (see 11.2.3)
  • High oxygen, low carbon dioxide
  • High carbon dioxide, low oxygen

Skill: Annotate diagrams of the nephron

  • Blood is filtered by nephrons. Each human kidney has about one million nephrons.

Figure 11.1.3a/c – A nephron. You should be able to label and annotate the structures.Figure 11.1.3a/c – A nephron. You should be able to label and annotate the structures.


Globular mass of blood capillaries through which blood flows at very high pressure

Bowman’s capsule

Surrounds glomerulus and collects filtrate through porous wall (ultrafiltration)

Proximal convoluted tubule

Twisted tube in which active transport is used to remove useful substances from filtrate (selective reabsorption)

Loop of Henle

Long, narrow tube descending into the medulla and ascending into the cortex that maintains hypertonic conditions in the medulla

Distal convoluted tubule

Twisted tube in which adjustments are made to solute concentration of filtrate; anti-diuretic hormone (ADH ) acts at the end of the tubule

Collecting duct

Wide tube that collects filtrate from a number of nephrons and carries urine to renal pelvis; ADH acts on the entire length of the duct

Figure 11.1.3a/d – DolphinFigure 11.1.3a/d – Dolphin
Aquatic mammals excrete urea because of their evolutionary history: their ancestors were terrestrial mammals.

Key concept

All animals excrete nitrogenous waste products. Some animals also balance water and solute concentrations.

Key questions

  • Define osmoregulation.
  • Comment on the correlation between type of nitrogenous waste and habitat/evolutionary history.
  • Describe how osmoregulation is achieved in insects.
  • Explain the differences in composition between the blood in the renal artery and that in the renal vein.

Figure 11.1.3a/e – TadpoleFigure 11.1.3a/e – Tadpole
Amphibians are terrestrial for most of their life cycle, but tadpoles are aquatic. What type of nitrogenous wastes do frogs excrete at different stages of their life cycle?

Figure 11.1.3a/f – Marcello MalpighiFigure 11.1.3a/f – Marcello Malpighi
Marcello Malpighi (1628–94) was an Italian anatomist who contributed greatly to our understanding of the kidney and excretion in insects.

Course link

  • Uric acid is actively transported through the Malpighian tubule membrane. Review membrane structure and transport in 1.1.3 and 1.1.4.

Did you know?

  • Each human kidney has about one million nephrons.

Figure 11.1.3a/g – Pig kidneyFigure 11.1.3a/g – Pig kidney
Pig and human kidneys have a very similar size structure. Pigs are often used as model organisms for drug and clinical experimentation.

Language help

  • The liquid inside the nephron is called filtrate. Once this liquid reaches the collecting ducts, it is called urine.
  • Proximal and distal are anatomical directions. Proximal means ‘closer to’ (the Bowman’s capsule). Distal means ‘further away’.
  • Afferent and efferent mean ‘incoming’ and ‘outgoing’ respectively. Use this memory hint: afferent = arriving and efferent = exiting.

Figure 11.1.3a/h – Proximal distal anatomyFigure 11.1.3a/h – Proximal and distal anatomy