Biology  5/e   Raven/Johnson
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Membrane transport: The lipid bilayer membranes of cells are selectively permeable. Some materials can enter or leave the cell by simple diffusion, some by facilitated diffusion, and some are pumped against their concentration gradients by active transport. Facilitated diffusion and active transport require the use of specific transmembrane proteins and channels.

Countercurrent exchange: In a countercurrent exchange system, the exchange of a substance between A and B is maximized because A and B flow in opposite directions. Fish gills are extremely efficient at obtaining oxygen from the surrounding water because they are countercurrent systems: the water flows past them in the opposite direction of the blood flowing in them. The blood is able to pick up the maximum amount of oxygen possible from the surrounding water. The nephron tubules in the mammalian kidney also act as a countercurrent system, maximizing the amount of water that is reabsorbed from the urine, resulting in urine that is more concentrated than the blood plasma.

Hormones: Hormones are chemical messengers that are produced in one part of the organism's body and have their effect on target cells in a different part of the organism's body. They are transported in the circulatory system and tend to bring about slower, more long-term changes than do the electrical signals of the nervous system. Hormones are a key element in helping the body maintain homeostasis by using feedback systems and antagonistic controls.

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