the slitlike openings through which water leaves a shark's gills.

Water enters the gill chambers through the mouth or spiracles.  In the past, it was assumed that all sharks must swim to move water into their mouth and over their gills to respire.  We now know that sharks can respire by pumping water over their gills by opening and closing their mouths.  However, many sharks do have to swim continuously:   due to their low blood pressure, muscular contractions are needed to circulate their blood.

Blood in the gill filaments absorbs oxygen from the incoming water.

Gill rakers, cartilaginous projections on the gill support structure, protect the delicate gill filaments from particles in the water that might damage them.

Water exits through the gill slits.

A shark's delicate gill filaments are protected from particles in the water by gill rakers.