Unit 13: Thermoregulation and Osmoregulation — Long Questions

Homeostasis is maintained through the collaboration of three essential elements:
1. Receptors (sensors): Specialized structures - organs, tissues, cells or subcellular components - that detect changes in the internal or external environment and transmit this information to the control centre. For example, receptor cells in the skin detect changes in temperature, and receptor cells in blood vessels detect changes in blood pressure.
2. Control centre: Receives information from receptors, processes it, and sends appropriate messages to effectors so that the internal environment stays within a narrow range of set points optimal for body function. The brain, spinal cord and endocrine glands act as control centres.
3. Effectors: The parts of the body - muscles and glands - that respond when they receive a message from the control centre. Muscles respond by contracting while glands release secretions, restoring the internal environment to its set point. For example, when body temperature drops below the set point, effectors in the skin and muscles cause shivering and vasoconstriction, which increase heat production and raise body temperature back to the set point.

Feedback mechanisms are the foundation of homeostasis: they are regulatory processes in which the output of a system is used to control the system's own behaviour. There are two types:

1. Negative feedback: The output of the system reduces the activity of the system; it is the most common form of regulation and includes body temperature regulation, blood glucose regulation, and hormone secretion. For example, if body temperature exceeds the set point of 37°C, thermoreceptors in the hypothalamus detect the change and the control centre sends messages to effectors such as sweat glands. Sweating lowers the temperature, and this drop in temperature (the output) negatively influences the thermoreceptors, so they stop sending signals - the response switches itself off once the set point is restored.

2. Positive feedback: The output of the system increases the activity of the system, enhancing the original stimulus; it is less common and occurs in processes needing rapid, significant change, such as blood clotting and childbirth. For example, during childbirth, uterine contractions send nerve impulses to the hypothalamus, which signals the pituitary gland to release oxytocin; oxytocin stimulates further contractions, sending more impulses and causing more oxytocin release. This loop continues, intensifying, until the baby is born.

Unregulated positive feedback can cause health problems, since there is no self-correcting "off switch": e.g., in severe infection, cytokines released by immune cells further stimulate the immune response, and if not properly regulated this escalates into excessive inflammation and tissue damage.

1. Freshwater habitats: Freshwater animals are hypertonic to their environment (higher solute concentration than the surrounding water). Water continuously enters their bodies by osmosis, especially across permeable surfaces like gills, while essential salts and ions such as Na⁺ and Cl⁻ diffuse out into the surrounding water. To osmoregulate, they prevent excess water entry and eliminate the water that does enter by producing large volumes of dilute urine, and they actively transport salts back into the body to compensate for losses. Unlike marine animals, they rarely drink water.

2. Marine water habitats: Most marine animals are hypotonic to seawater (environment has higher solute concentration). Water constantly leaves their bodies by osmosis through gills and skin, salts diffuse in from the environment, and they take in extra salt by drinking seawater. To compensate, they drink seawater continuously, conserve water by producing small volumes of highly concentrated urine, and remove excess salts using specialized chloride cells in the gills (active, ATP-driven transport) as well as through the kidneys. Some marine fishes also have rectal glands that excrete excess salts into the digestive tract to be eliminated with the faeces.

3. Terrestrial habitats: Terrestrial animals lose water through evaporation from respiratory surfaces, through the skin, and in waste excretion, so intake (drinking, food, metabolic water) must balance this loss. They have anatomical adaptations such as relatively impermeable outer coverings - a waxy chitin exoskeleton in insects, waterproof keratinized scales in reptiles, and dead keratinized skin with oil-secreting glands in mammals and birds - to reduce evaporative loss. Their kidneys conserve water, and they convert ammonia into less water-demanding urea or uric acid for excretion. Behavioural adaptations also help, e.g., many desert animals remain underground during the day and emerge at cooler, more humid night, and some animals like the Kangaroo Rat can survive on metabolic water alone.

The main nitrogenous wastes - ammonia, urea and uric acid - are excreted in forms suited to how much water an animal's habitat allows it to use.

1. Ammonia (NH₃): Formed when an amino group (-NH₂) is removed from amino acids/nucleic acids and combines with H⁺ in the liver. It is highly toxic and must be excreted rapidly and dilutely - one gram needs about 500 mL of water to reach a nontoxic level. Only animals with plentiful water, such as most fishes, protozoans, sponges and coelenterates, can afford this; marine animals excrete it directly into the surrounding water. Animals excreting ammonia as their chief nitrogenous waste are ammonotelic.

2. Urea (CO(NH₂)₂): Terrestrial animals, facing the challenge of conserving water, cannot excrete ammonia directly. They convert ammonia into urea in the liver and excrete it via the kidneys. Urea is less toxic than ammonia and needs only about 50 mL of water per gram for safe dilution. Animals excreting urea as their major waste are ureotelic. (Humans, being ureotelic, also excrete small amounts of uric acid, a by-product of purine breakdown; excess uric acid can crystallize in joints and tissues, causing gout.)

3. Uric acid (C₅H₄N₄O₃): Animals under extreme water-conservation pressure - insects, desert animals, birds and most reptiles - cannot even afford to excrete urea. Using considerable energy, they convert ammonia into uric acid, which is even less toxic than urea, poorly soluble in water, and precipitates out; it needs only about 1 mL of water per gram for excretion. Animals excreting uric acid as their main nitrogenous waste are uricotelic.

Thus, the nature of nitrogenous waste excreted correlates directly with water availability in an animal's habitat: plentiful water (freshwater/aquatic) favours ammonia, moderate water availability (terrestrial, with access to water) favours urea, and extreme water scarcity (desert, flight, egg-laying) favours uric acid.

On the basis of the source of body heat, animals are classified into:

1. Ectotherms: Animals that gain most of their body heat from external sources, so their internal temperature generally fluctuates with the surroundings. Most invertebrates, and among vertebrates, fishes, amphibians and reptiles, are ectotherms. Their metabolic rate is very low, so they cannot generate enough heat internally and rely on behavioural thermoregulation: basking in the sun (radiation) or sitting on warm rocks (conduction) when cold, seeking shade or burrowing into cool earth when hot, and becoming dormant in very cold climates.

2. Endotherms: Animals that generate the majority of their body heat through internal metabolism, maintaining a relatively constant body temperature often higher than the environment. Birds and mammals are endotherms. When cold, they may shiver (muscle contraction) or burn brown adipose tissue for heat, and most have insulation such as hair, feathers or fat to retain heat.

On the basis of the ability to thermoregulate, animals are classified into:

1. Poikilotherms: Animals unable to maintain their body temperature within narrow limits; their body temperature varies considerably, usually matching the environment. All invertebrates, and among vertebrates, fishes, amphibians and reptiles, are poikilotherms. This allows them to survive on comparatively little food, since they do not need to "fuel" a constant high body temperature, enabling survival in nutrient-poor environments where a homeotherm would starve.

2. Homeotherms: Animals that maintain a relatively constant internal body temperature regardless of environmental fluctuations, using the hypothalamus in the brain as a "thermostat" to trigger cooling or heating responses. Mammals and birds are homeotherms; for instance, humans maintain their internal temperature at roughly 37°C whether in very cold or very hot environments.

Humans are endothermic homeotherms and regulate body temperature through a negative feedback mechanism. Thermoreceptors in the skin detect a rise or fall in body temperature and send signals to the control centre, the hypothalamus (which also contains thermoreceptors). The hypothalamus sends signals to muscles and glands (effectors), producing corrective responses, and also stimulates higher brain centres so we "feel" warmth or coldness and can take voluntary actions such as bathing or switching on a fan or heater.

Responses to overheating (hyperthermia), when body temperature rises above 37°C:

• Vasodilation: expansion of blood capillaries just beneath the epidermis, increasing blood flow to the skin so more heat is lost from the blood.
• Sweating: evaporation of sweat carries heat away from the body, producing a cooling effect.
• Decreased thyroxine secretion: the thyroid gland may reduce thyroxine output, lowering the basal metabolic rate and thus reducing internal heat production.

Responses to cooling (hypothermia), when body temperature falls below 37°C:

• Vasoconstriction: narrowing of blood vessels reduces blood flow to the skin, reducing heat loss.
• Shivering: spasmodic contraction of skeletal muscles generates heat, raising body temperature.
• Goosebumps: tiny muscles at the base of hair follicles contract, erecting the hairs to trap insulating air.
• Increased metabolic rate: produces more heat to raise body temperature back to the set point.