The life processes cannot continue in the body’s cells without sources of energy.

The blood carries glucose and oxygen around the body.

Energy is released from the glucose during metabolic oxidation and stored in ATP molecules.
This stored energy can then be retrieved when required by the life processes of the body.

Hormones
When the hormone epinephrine (Adrenalin) is secreted by the adrenal gland, it is delivered to all parts of the body by the cardiovascular system.

Among other effects, epinephrine increases the rate of metabolism of all cells of the body.

This helps to mobilize energy during a "fight-or-flight" stress reaction.

Fats
The body can use its stores of fat in when a lot of energy is required.

The lymphatic circulatory system picks up the end products of lipid (fat) digestion and carries them to the cardiovascular system.

This fat is generally deposited throughout the body, particularly the subcutaneous layer, as yellow fat.
In a rapid turnover, the high energy content of the fat is released for use throughout the body.

In infants, there is often brown fat at the junctions of the major blood vessels. In periods of high- energy requirements, this brown fat releases energy into the blood stream immediately.

Prioritisation of Blood Supply
Blood can be delivered to the body parts where it is most needed.

For example, when a specific portion of the cerebral cortex is active, more blood is delivered to that portion