Chapter 29: Homeostasis

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Chapter 29: Homeostasis

  • Homeostasis is the maintenance of a constant internal environment.

An important aspect of homeostasis is temperature regulation.

Temperature regulation in plants:
if a plant is too hot (e.g. during the summer), then it will increase transpiration in an effort to cool down.

Temperature regulation in animals:
Temperature regulation in animals is much more complex than in plants.
In terms of temperature regulation, animals can be divided into two groups:

  1. Endotherms: animals whose internal temperature remains constant despite environmental temperature changes; e.g. mammals and birds.
  2. Ectotherms: animals whose internal temperature changes with environmental temperature; e.g. insects, lizards, amphibians, fish.

1. Endotherms have a variety of mechanisms that enable them to maintain their body temperature:

When they are cold:

  • Shivering – rapid contraction and relaxation of skeletal muscles helps to generate heat.
  • Vasoconstriction – narrowing of the capillaries near the surface of the body directs blood to core areas helping to conserve heat.
  • Piloerection – piloerctor muscle contracts causing hair to stand up away from the skin, trapping a layer of air close to the skin.
  • Thyroxine secretion – thyroxine being a hormone that increases metabolism thereby generating heat.

When they are too hot:

  • Sweating – release of sweat onto the surface of the skin via sweat glands cools the body down through evaporation.
  • Vasodilation – widening of the capillaries close to the surface of the body thereby releasing heat and cooling the body down/
  • Rapid breathing – exchanges more air with the lungs helping to release heat from the body (more important in animals with fur).

2. Ectotherms’ activity levels depend directly on the temperature of their environment. If they need to warm themselves up, they will often sit in direct sunlight (e.g. lizards bathing in the sun).

pH regulation:
pH levels must be tightly regulated in all living organisms. This enables enzymes to function properly (see Chapter 9).
In animals blood pH must be kept at 7.4. The acidity of the blood is controlled by the lungs and the kidneys of animals.
Plants rely on the soil being the correct pH to enable full growth and reproduction.

Glucose level regulation:
Glucose levels in animals must be maintained at a concentration of approximately 1 g/L. If it drops too low, coma and death can result. If glucose levels rise too high it can cause damage to blood vessels and nerves. Glucose levels are controlled by two hormones: insulin (lowers blood glucose) and glucagon (raises blood glucose).

Osmoregulation:
Osmoregulation is the maintenance of the correct amount of water in the body. The kidneys will excrete excess water and conserve water if the body is lacking water.
In plants, the stomata of the leaves will close if there is not enough water in the plant.

Calcium level regulation:
Calcium is needed in animals for healthy bones and teeth and also muscle contraction. If calcium levels are low, the parathyroid glands in the neck secrete parathormone.