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Hormones are the secondary messengers that are secreted in smaller amounts in the body. The role of hormones is very much essential to the human body. Ghrelin is the hormone that is secreted during the appetite condition. It is otherwise known as the hunger hormone.

Blood is the circulating fluid carrying the essential components like nutrients, gases, and hormones to the target site. High or low levels of hormones in the blood influence the organ and overall body functioning.

The given conditions are as follows.

The ghrelin concentration is given as $1.3×{10}^{-10}$M, which is present in one litre of blood.

To calculate the number of molecules, the given concentration has to be multiplied by Avogadro’s number.

$$\begin{gathered} \text{Total no. of molecules =}1.3×{10}^{-10}×\frac{6.023×{10}^{23}\text{molecules}}{\text{mol}} \\                                               ≈7.8×{10}^{13}\text{molecules}. \end{gathered}$$

Hence, the number of molecules of ghrelin present in one litre of blood is$≈7.8×{10}^{13}$ molecules.