3.1 Homeostasis in Living Things
Homeostasis is the regulation of the internal environment in an organism to maintain optimal conditions for survival, despite changes in the external environment.
Importance of Homeostasis
- Ensures body temperature, water, pH, and blood sugar levels remain within normal ranges.
- Provides optimal conditions for cellular enzyme activities and metabolic processes.
Homeostasis in Humans
1. Regulation of Body Temperature
- When Body Temperature Rises (Hot Environment / Exercise):
- Blood vessels dilate (vasodilation) to allow more blood to flow near the skin surface, increasing heat loss by radiation.
- Sweat glands produce sweat; evaporation of sweat absorbs heat and cools the skin.
- Erector muscles relax, laying hair flat to allow heat to escape.
- Metabolic rate decreases and physical activity slows down.
- When Body Temperature Drops (Cold Environment):
- Blood vessels constrict (vasoconstriction) to minimize blood flow near the skin surface and reduce heat loss.
- Sweat glands produce little to no sweat.
- Erector muscles contract, pulling hair upright to trap an insulating layer of warm air.
- Skeletal muscles contract and relax rapidly (shivering) to generate thermal energy.
- Metabolic rate increases.
2. Regulation of Water Content
- When Water Level Decreases (Dehydration / Excessive Sweating):
- Brain triggers the release of anti-diuretic hormone (ADH).
- Kidneys reabsorb more water, producing a smaller volume of concentrated (darker) urine.
- Thirst sensation is stimulated to encourage water intake.
- When Water Level Increases (Excessive Fluid Intake):
- Brain reduces the release of anti-diuretic hormone (ADH).
- Kidneys reabsorb less water, producing a larger volume of dilute (lighter) urine.
Homeostasis in Animals
- Dogs: Pant and stick their tongue out to evaporate saliva and lower body temperature.
- Cats: Lick their fur to allow saliva evaporation for cooling.
- Reptiles (Lizards): Ectothermic (cold-blooded); bask in the sun to absorb heat when cold, and retreat into shade when hot.
- Snails and Bees: Produce mucus or evaporate water in hives to regulate humidity and temperature.
Homeostasis in Plants
Plants regulate water levels primarily through transpiration, where water evaporates through open stomata on the leaves.
- Transpiration cools the plant on hot days and creates a suction force that pulls water and minerals from the roots up to the leaves.
- High Water Supply / Light: Guard cells absorb water, become turgid, and bend outward, opening the stomata for transpiration and gas exchange.
- Low Water Supply / Excessive Heat: Guard cells lose water, become flaccid, and close the stomata to prevent excessive water loss and dehydration.