15) Which of the below would have the lowest PO2? Alveolar air Venous blood Expired air Alveolar blood 16) ___________ lacks the ability to remember a pathogen or react differently to it in the future, whereas ___________ uses memory cells to adapt to a given pathogen and ward it off more easily in the future. Adaptive immunity; specific immunity Innate immunity; adaptive immunity A cytotoxic T cell; a natural killer cell Adaptive immunity; innate immunity 17) Steroid hormones are an example of lipid-soluble hormones and therefore they do not use receptors but instead bind directly to the plasma membrane. their receptors are found on the plasma membrane. they do not use receptors but instead bind directly to the nucleus. their receptors are found within the cell’s cytoplasm and/or nucleus.

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