Osmoregulation in Animals

Maintaining proper levels of water and electrolytes and removing nitrogenous waste are necessary for all animals to survive. Regulating water and electrolytes is known as osmoregulation, and eliminating nitrogenous waste is called excretion. These processes are closely related but have different names. Both of these processes contribute to homeostasis, which is essential for life. In vertebrates, the kidneys are responsible for both osmoregulation and excretion, while invertebrates have specialized organs for these functions.

Osmoregulation is the process of regulating the movement of solutes, which ultimately affects the movement of water through an osmosis process. Osmosis is a type of diffusion involving water’s movement across a semipermeable membrane, which allows water molecules to pass through but not solutes. This process occurs when two solutions separated by a semipermeable membrane have different concentrations of solutes or osmolarities. We measure the concentration of solutes in molarity, which represents the number of moles of solute per litre of solution, and we measure the osmolarity in milliosmoles per litre. When two solutions have the same osmolarity, they are termed isotonic. If one solution has a higher concentration of solutes, it becomes hypertonic, while the other solution with a lower concentration of solutes becomes hypotonic. When a semipermeable membrane separates these two solutions, water will move through osmosis from the hypotonic solution to the hypertonic one.

Osmoconformers and Osmoregulators:

Osmoconformers are animals that adjust the osmolarity of their body fluids to match that of the surrounding environment. This includes marine invertebrates, some freshwater invertebrates, and hagfish. These animals have a high tolerance for different osmotic environments. Osmoregulators, on the other hand, maintain a specific internal osmolarity that differs from their surroundings. Most aquatic invertebrates are strict or limited osmoregulators, with the exception of hagfish(Myxine sp., a marine cyclostome fish) and elasmobranchs (sharks and rays). Osmoregulators must either eliminate excess water or continuously take in water to maintain their internal osmotic balance, requiring energy to do so.

Osmoregulation in freshwater animals:

The osmolarity of freshwater is generally much less than 50 mOsmolL–1, while the freshwater vertebrates have blood osmolarity in the range of 200 to 300 mOsmolL–1. The body fluids of freshwater animals are generally hypertonic to their surrounding environment. The problems faced by these animals will be:

• Entry of excess water.
• Loss of body salt to the outside.

The freshwater animals prevent the net gain of water and a net loss of body salts by several means, such as:

• Freshwater animals (including freshwater fish) do not drink water to reduce the need to expel excess water.
• A specialised body covering minimises water uptake and salt loss (subcutaneous fat layer of scaleless fish and scales over the body of fish or crocodiles).
• Specialized cells, called ionocytes or chloride cells in the gill membrane of freshwater fish, can import Na+ and Cl– from the surrounding water (surrounding water has less than 1mm NaCl, and plasma concentration has more than 100 mM).

Osmoregulation in marine animals:

The osmolarity or concentration of dissolved particles of seawater is typically around 1000 mOsmol/L, while the osmolarity of human blood is around 300 mOsmol/L. This difference in concentration creates different osmoregulatory challenges for marine and freshwater environments. Marine fish, like hilsa, salmon, and others, have body fluids that are hypotonic to seawater, meaning they have a lower concentration of dissolved particles. As a result, these fish tend to lose water through their gills, mouth, and anus. To compensate for this water loss, they drink seawater, which leads to excess salt in their bodies. To remove this excess salt, marine fish have special cells called ionocytes or chloride cells in their gills that help to eliminate excess monovalent ions, or ions with a single charge, from their body fluids to the seawater.

Generally, faeces eliminate divalent cations or ions with two charges. Some fish, like hilsa and salmon, migrate between freshwater and seawater and use their gills to both drink seawater and excrete excess salt. Hormones also play a role in this process of switching between freshwater and seawater environments. In contrast to marine fish, the body fluids of marine invertebrates, such as ascidians and hagfish, are usually isosmotic to seawater, meaning they have the same concentration of dissolved particles. Elasmobranchs, like sharks and rays, and coelacanths retain special organic substances called osmolytes, such as urea and trimethylamine oxide, in their body fluids to raise their osmolarity and render them slightly hyperosmotic to seawater. However, these substances also help to lower the concentration of inorganic ions in the body fluids, making them hypotonic to seawater and reducing osmoregulatory challenges.

Osmoregulation in terrestrial animals:

Land animals are always subject to osmotic desiccation, like marine animals. Terrestrial air-breathing animals constantly lose water through their respiratory surfaces, which may even be fatal. Humans, for example, die if they lose around 12 per cent of their body water. Therefore, individuals must compensate for water loss by drinking and eating moist food. Animals utilise various means to minimise this water loss, such as the waxy coating of the exoskeleton of insects, the shell of land snails, and the multiple layers of dead, keratinised skin cells covering most terrestrial vertebrates. Kangaroo rats lose so little water that they can recover 90 per cent of the loss using metabolic water (water derived from different cellular metabolic processes).

Many desert animals are nocturnal to avoid the heat of daytime, a behavioural adaptation that minimizes dehydration. Camels produce dry faeces and concentrated urine. When water is not available, the camels do not produce urine, but store urea in tissues and solely depend on metabolic water. When water is available, they rehydrate themselves by drinking up to 80 litres of water in 10 minutes.

Conclusion:

Osmoregulation is the process by which an organism maintains the proper balance of water and electrolytes within its body. It is essential for the proper functioning of cells and organs and is crucial for the survival of most living beings. Osmoregulation involves the regulation of water intake and the excretion of water and electrolytes through various physiological processes. Animals typically achieve osmoregulation through the actions of the kidneys and specialized cells in the skin and gills. Overall, osmoregulation plays a vital role in the homeostasis of an organism and is essential for maintaining proper health and function