Some important terms Homeostasis :

1. Some important terms Homeostasis: • homeostasis = constancy • Blood and tissue fluid surrounding the cells is kept in remarkably constant in composition and temperature • this constancy of the internal environment exist in spite of the many changes in the external environment (temperature, food and water supply etc.) • the body compensate these changes by a series of homeostatic mechanisms, which frequently work on the same principle as a thermostat, that is by a negative feedback control.

2. Homeostasis: • nutrients above the requirement can be stored in the body and can be mobilised if the supply is not sufficient (energy from fat deposits, Ca from the bones, fat soluble vitamins from the liver etc.) • the ability of homeostasis declining with the age (Ca utilisation, mobilisation, osteoporosis) energy requirement Ca exretion Ca requirement (bones, cells etc.) energy intake fat pads Ca intake

3. Adaptation: • animals adapt to all kind of change • a food low in calcium causes the gut to absorb more calcium and the kidney to excrete less. • if salty foods are eaten, vasopressin will be secreted and less water excreted via urine and water intake will increase • adaptation is works under certain limits, if the deficiency is greater than the adaptation capacity of a certain nutrient, the so called deficiency symptoms will occur.

4. Turnover of the body’s constituents: All tissues of the body are constantly being broken down and rebuilt. Red blood cells for example have a life span of about 120 days. Most of the protein and iron from the old cells is reused but some is lost and this must be replaced by the diet. The same principle applies to bone, body fat (adipose tissue) and the rest of the body. Therefore food is always important in the maintenance of body structures.

5. Nutritional balance: • It refers to the relationship between loss or utilization of a nutrient and its gain from the diet. • If the energy eaten and the energy used by the body is equal the body’s energy stores of fat remain constant. If it is negative, with higher expenditure than intake, fat stores will be used up and weight lost, the reverse will occur in positive energy balance.

6. Nutritional balance: • For protein the term nitrogen balance is used. In an adult animal nitrogen intake from dietary protein will equal nitrogen loss from tissue breakdown plus any dietary surplus. In a growing animal or child, nitrogen balance will be positive, intake being greater than loss, as protein is used to form new tissue. Adults on the other hand would have a negative nitrogen balance, with increased tissue breakdown.