Final answer:
False. Hormones do not need to be very concentrated in the blood to have an effect. They are present in low concentrations when they act on their target cells.
Step-by-step explanation:
False. In order to have an effect, hormones do not need to be very concentrated in the blood. In fact, hormones are present in low concentrations when they act on their target cells because they become diluted as they travel through the bloodstream. This is because hormones are chemical messengers produced by organs of the endocrine system, and they travel to their target cells through the blood.
Hormones are able to have their effects despite their low concentrations because they bind to specific receptor proteins on target cells. These receptor proteins are specific to the hormone, allowing the hormone to specifically target the cells that it needs to affect.
For example, adrenaline is a hormone produced by the adrenal glands on top of the kidneys. It has multiple effects on the heart, muscles, and airway. Even though adrenaline is present in low concentrations in the blood, it is still able to bind to specific receptors on target cells and cause changes within those cells, such as quickening the heart rate.