Adrenaline and blood glucose concentration

In a threatening or stressful situation, the hormone adrenaline helps prepare the body for a fight-or-flight response. One of its effects is to increase blood glucose concentration. This makes more glucose available in the blood as the body responds.

Adrenaline also increases heart rate. The increase in heart rate and the rise in blood glucose concentration are separate effects of the same hormone: the rise in blood glucose is not caused by the faster heart rate.

Negative feedback in homeostasis

An internal condition can change as the body or its surroundings change. Negative feedback helps keep that condition close to a normal target value, called its set point.

When the condition moves away from its set point, the change is detected and triggers responses that oppose the change. If the condition rises above its set point, the responses act to lower it. If it falls below its set point, the responses act to raise it.

As the condition returns towards the set point, the difference from the target becomes smaller. The stimulus for the responses therefore diminishes, so the responses decrease rather than continuing at the same strength.

Negative feedback maintains relative stability, not a condition fixed at exactly one value. The condition can vary around its set point, while changes away from it trigger responses that counteract them.

Adrenaline and the circulatory response to stress

During fear or stress, the adrenal glands release the hormone adrenaline into the blood. Adrenaline increases the heart rate, so the heart beats faster and helps deliver more oxygen and glucose to the brain and muscles.

The brain and muscles need oxygen and glucose for respiration, which releases energy for activity. The increased supply supports the rapid activity needed to confront a threat or escape from it. This is called the fight-or-flight response.

Thyroxine

The thyroid gland produces thyroxine, a hormone that stimulates the body's basal metabolic rate. This is the rate at which the body uses energy at rest. Thyroxine also has important roles in growth and development.

Control of thyroxine levels

The pituitary gland releases thyroid-stimulating hormone (TSH), which stimulates the thyroid gland to produce thyroxine. Changes in thyroxine levels affect how much TSH is released:

• When thyroxine levels rise, TSH release decreases. The thyroid receives less stimulation and produces less thyroxine.
• When thyroxine levels fall, TSH release increases. The thyroid receives more stimulation and produces more thyroxine.

This is negative feedback: a change in thyroxine level causes a response that opposes the change. It helps keep thyroxine levels relatively stable.