What are the components of the circulatory system?

A. initiate blood clotting: Platelets and coagulation factors initiate clotting; erythrocytes do not initiate clot formation.
B. transport nutrients: Plasma carries most nutrients (glucose, amino acids, lipids), not erythrocytes.
C. defend the body against pathogens: White blood cells (leukocytes) are the main defenders; erythrocytes do not have immune functions.
D. transport some carbon dioxide: Erythrocytes carry a portion of CO₂ (dissolved as bicarbonate via carbonic anhydrase inside RBCs and also some carbaminohemoglobin).
E. regulate erythropoiesis: Erythropoiesis is regulated mainly by erythropoietin (from kidneys) and iron/nutritional signals, not by erythrocytes themselves.

A. CP: Creatine phosphate (phosphocreatine) is an energy-reserve molecule in muscle that donates phosphate to ADP to regenerate ATP; it is not a second messenger in hormone signal transduction and would not appear as the intracellular signaling oval shown.

B. DPG: 2,3-diphosphoglycerate (2,3-DPG) is an RBC metabolite that modulates hemoglobin’s O₂ affinity; it is not produced as a cytoplasmic second messenger following receptor activation.

C. PEP: Phosphoenolpyruvate (PEP) is a glycolytic intermediate involved in metabolism, not a hormone-activated intracellular signaling molecule.

D. CAMP: Cyclic AMP (cAMP) is a common second messenger produced by adenylate cyclase after many peptide hormones bind cell-surface receptors; the diagram’s small oval (C) inside the cytoplasm leading to downstream effects (D) fits cAMP.