Each nephron is supplied with blood by anafferent arteriole,an offshoot of the renal artery that branches and forms thecapillaries of the glomerulus. The capillaries converge as theyleave the glomerulus, forming anefferent arteriole. Branches ofthis vessel form theperitubular capillaries, which sur-round the proximal and distal tubules. Other branches extenddownward and form thevasa recta, hairpin-shaped capillar-ies that serve the renal medulla, including the long loop ofHenle of juxtamedullary nephrons.Bowman’scapsuleProximaltubulePeritubularcapillariesDescendinglimbLoop ofHenleVasarectaDistaltubuleEfferentarteriole fromglomerulusCollectingductAscendinglimbAfferent arteriolefrom renal arteryBranch ofrenal veinGlomerulusC O N C E P T C H E C K44.31.Compare and contrast the different ways that meta-bolic waste products enter the excretory systems offlatworms, earthworms, and insects.2.What is the function of the filtration step in excre-tory systems?3.Kidney failure is often treated by hemo-dialysis, in which blood diverted out of the body isfiltered and then allowed to flow on one side of asemipermeable membrane. Fluid called dialysateflows in the opposite direction on the other side ofthe membrane. In replacing the reabsorption and se-cretion of solutes in a functional kidney, the makeupof the starting dialysate is critical. What initial solutecomposition would work well?For suggested answers, see Appendix A.WHAT IF?C O N C E P T44.4The nephron is organized forstepwise processing of blood filtrateWe’ll continue our exploration of the nephron with a discus-sion of filtrate processing. We will then focus on howtubules, capillaries, and surrounding tissue function together.The porous capillaries and specialized cells of Bowman’scapsule are permeable to water and small solutes, but notblood cells or large molecules, such as plasma proteins.Thus, the filtrate produced in the capsule contains salts, glu-cose, amino acids, vitamins, nitrogenous wastes, and othersmall molecules. Because such molecules pass freely be-tween glomerular capillaries and Bowman’s capsule, theconcentrations of these substances in the initial filtrate arethe same as those in blood plasma.
