A closed circulatory system usually consists of all of the following: pumping organ, veins, blood, arteries Many homeostatic mechanisms are regulated by biofeedback systems. When there is a change in the set point, the net effect of a negative feedback system is to set off mechanisms that will return the change to steady­state levels. B cells undergo clonal selection once activated. Cell­mediated immunity is directed by T cells, which react with antigen­MHC complexes. The flight muscles of insects contain more mitochondria than any known variety of muscle. Oxytocin stimulates uterine contractions Millions of sperm are released even though only one is needed for fertilization because Many sperm die as a result of the unfavorable pH in the vagina, Many sperm die by phagocytosis in the female tract, Many sperm are needed to penetrate the corona radiate, Many sperm lose their way because their movement is undirected. The first system to develop in the vertebrate embryo is the nervous system. The sympathetic nervous system is activated when there is a startling situation events of a second messenger system: neurotransmitter binds to its receptor, G protein is activated, cyclic AMP is produced, protein kinase is activated, phosphorylation takes place, protein closes potassium channel Ectothermic animals depend on heat from the external environment to adjust their body temperatures. In organogenesis, the ectoderm gives rise to sense organs. Homeostatic mechanisms are processes that maintain a