Extracellular fluid (ecf): fluid outside the cell: plasma: fluid within the blood vessels, interstitial fluid: fluid surrounding the cells. Diffusion: move molecules down their gradient: depends only on kinetics, simple diffusion: passive, no atp, small molecules. Ex) oxygen: facilitated diffusion: needs carrier, no atp, big molecules. Needs some form of energy: primary active transport: directly uses atp, moves particles against the conc. Gradient: secondary active transport: indirectly uses atp. Cotransport (symport): both molecules go into or out of the cell, in one direction. Countertransport (antiport): one molecule goes in while the other goes out. *clicker question: sglt1 is a secondary active co-transporter which means: Na+ and glucose both go down their conc. Na+ goes down and glu goes against its conc. Na+ goes against and glu goes down its conc. Negative feedback: goes against the direction of the stimulus: sweating when hot. Positive feedback: goes in the same direction of the stimulus: pregnancy contractions.