Which of the following is a system of organs that collects oxygen from the external environment and transports it to the bloodstream?

a. endocrine system
b. immune system
c. integumentary system
d. respiratory system

D

Explanation:

Respiration is the process of taking in oxygen and release of Carbondioxide in the lungs(breathing),the absorption of the oxygen in the alveolar, into the blood stream for distribution to tissues , the combustion of the metabolites(glucose,amino acids, and fatty acids and glycerol)with the inhaled oxygen to give energy as ATPS and water vapour.

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.