The fight or flight response is an instinct in mammals that prepares the body to either run or fight for life when a potentially dangerous situation is detected.

The autonomic nervous system detects a threat and communicates with the hypothalamus.

The hypothalamus then communicates with the sympathetic nervous system and the adrenal-cortical system, using neuronal pathways and hormones in the bloodstream to initiate body reactions.

Adrenaline and noradrenaline, secreted by the adrenal medulla, cause several changes in the body, such as increasing the heart rate.

Adrenocorticotropic hormone, secreted from the pituitary gland, stimulates the adrenal cortex to release other hormones that prepare the body to deal with the threat.

1. Increased heart rate - pumps more oxygenated blood around the body
2. Dilation of pupils - allows more light to be taken in for better vision
3. Constriction of arterioles in skin - sends more blood to the major muscles, brain, and heart
4. Increased blood glucose level - allows more respiration
5. Relaxation of airway muscles - more oxygen enters the lungs
6. Shutdown of certain organ systems like digestion - preserves energy for other processes

Adrenaline triggers the liver cells to undergo glycogenolysis, breaking down glycogen into glucose.

This causes glucose to be released into the bloodstream, allowing the rate of respiration to increase and providing more energy for muscle contraction.

Adrenaline binds with receptors on liver cell-surface membranes. This triggers a chain reaction inside the cells that eventually leads to the conversion of glycogen into glucose.

The second messenger model of hormone action involves a hormone (the first messenger) triggering the formation of a second messenger (cAMP) inside the cell, which activates enzymes to carry out a function.

It is the process where one hormone molecule can cause many cAMP molecules to be formed, and at each stage in the cascade that follows, the number of molecules involved increases.

1. Adrenaline binds to complementary receptor on the cell-surface membrane of a liver cell
2. The binding of adrenaline causes the protein to change shape, activating a G protein
3. This activates the enzyme adenylyl cyclase
4. The activated adenyl cyclase converts ATP into cAMP
5. cAMP acts as a second messenger, binding to and activating many protein kinases via phosphorylation, amplifying the signal from adrenaline
6. Protein kinases activate enzymes that catalyse the breakdown of glycogen into glucose

Glucose moves out of liver cells by facilitated diffusion and into the blood through channel proteins.

This increases the blood glucose concentration so that more glucose can be delivered to body cells for respiration.