Homeostasis

Three words frame the unit. Osmoregulation is the regulation, between an organism and its environment, of solute and the gain and loss of water. Excretion is the elimination of nitrogenous waste. Thermoregulation is holding internal temperature within a tolerable range. Water, solutes and temperature are the internal components most exposed to external change, which is why the rest of the chapter is arranged around them.

The thermostat is the hypothalamus, working to a set point of $37^\circ\text{C}$, and that account is Punjab material the Sindh book does not carry. Above the set point, warm thermoreceptors signal for increased blood flow to the skin and for sweat gland activation, and the sweat is evaporated for cooling. Below it, cold receptors signal the hypothalamus to inhibit heat loss and switch on heat conservation: constriction of the superficial vessels, shivering and non-shivering mechanisms. Fever uses the same machine differently, since pyrogens displace the set point above $37^\circ\text{C}$.

Sweating works because of a property of water. Turning a gram of sweat into vapour takes about $2260\ \text{J g}^{-1}$ out of the skin, which is why evaporative cooling is so effective, and a plant does the same without sweat glands, using transpiration as the way it stays cool. On the water side, only 1 to 2 per cent of absorbed water is used and the rest lost as vapour, pulled up the xylem by cohesion and adhesion. Heat-shock proteins stop enzymes denaturing in the heat.