Thermoregulation and excretion

MDCAT Biology · Biology · Homeostasis. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Humans are endotherms: they make their own heat and keep the body core near 37 °C. The control centre is the hypothalamus. It detects the temperature of the blood, receives signals from the skin and switches on responses that reverse any change. This is negative feedback.

Animals cannot store excess amino acids. The liver removes the amino group (deamination), which forms ammonia. Ammonia is very toxic, so most land animals change it into a safer waste.

The kind of nitrogenous waste depends on how much water the animal can spare. In humans the liver turns ammonia into urea in the urea (ornithine) cycle.

Rules to remember

Too hot: vasodilation of skin arterioles, sweating (cooling by evaporation), hairs lie flat, lower metabolic rate
Too cold: vasoconstriction of skin arterioles, shivering (heat from skeletal muscles), hairs raised to trap air, more thyroxine and adrenaline raise the metabolic rate
Fever: pyrogens raise the set point of the hypothalamus
Ammonia: very toxic, very soluble, needs most water; aquatic animals such as bony fishes (ammonotelic)
Urea: less toxic, soluble, needs moderate water; mammals and adult amphibians (ureotelic)
Uric acid: least toxic, almost insoluble, needs least water; birds, reptiles and insects (uricotelic)
Urea cycle: ornithine → citrulline → arginine; arginase splits arginine into urea + ornithine. Overall: 2NH₃ + CO₂ → CO(NH₂)₂ + H₂O

Common mistake

Urea is made in the liver, not in the kidneys. The kidneys only remove urea from the blood and pass it out in the urine.

Worked example

A tadpole lives in a pond and excretes mainly ammonia. The adult frog spends much of its time on land and excretes mainly urea. Explain the change.

Ammonia is very toxic and must be diluted in a large volume of water.
A tadpole is surrounded by water, so ammonia simply diffuses away and costs no energy to make.
On land, water is limited, so the frog cannot flush ammonia out fast enough.
Urea is much less toxic and can be held in the body and excreted with far less water.

Answer: The frog changes to urea because it is less toxic and saves water on land; making it costs energy, which the tadpole does not need to spend.

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