After reading this chapter, you should be able to:
Describe the morphology of a typical nephron and its blood supply.
Define autoregulation and list the major theories advanced to explain autoregulation in the kidneys.
Define glomerular filtration rate, describe how it can be measured, and list the major factors affecting it.
Outline tubular handling of Na+ and water.
Discuss tubular reabsorption and secretion of glucose and K+.
Describe how the countercurrent mechanism in the kidney operates to produce hypertonic or hypotonic urine.
List the major classes of diuretics; understand how each operates to increase urine flow.
Describe the voiding reflex.
The basic unit of the kidney is the nephron, made up of the renal tubule and its glomerulus. The size of the kidneys varies between species, as does the number of nephrons they contain. Each human kidney has approximately 1 million nephrons. The specific structures of the nephron are shown in diagrammatic manner in Figure 37–1.
Diagram of a nephron. The main histologic features of the cells that make up each portion of the tubule are also shown.
The glomerulus is formed by the invagination of a tuft of capillaries into the dilated, blind end of the nephron (Bowman’s capsule). The capillaries are supplied by an afferent arteriole and drained by the efferent arteriole, and it is from the glomerulus that the filtrate is formed. The diameter of the afferent arteriole is larger than the efferent arteriole. Two cellular layers separate the blood from the glomerular filtrate in Bowman’s capsule: the capillary endothelium and the specialized epithelium of the capsule. The endothelium of the glomerular capillaries is fenestrated with pores. The endothelium of the glomerular capillaries is completely surrounded by the glomerular basement membrane along with specialized cells called podocytes. Podocytes have numerous pseudopodia that interdigitate to form filtration slits along the capillary wall. The glomerular basement membrane, the basal lamina, does not contain visible gaps or pores. Stellate cells called mesangial cells are located between the basal lamina and the endothelium. Mesangial cells are contractile, play a role in the regulation of glomerular filtration, secrete the extracellular matrix, take up immune complexes, and are a target in glomerular disease.
Functionally, the glomerular membrane permits the free passage of neutral substances up to 4 nm in diameter and almost totally excludes those with diameters greater than 8 nm. However, the charge on molecules as well as their diameters affects their passage into Bowman’s capsule.
The proximal convoluted tubule is made up of a single layer of cells that interdigitate with one another and are united by apical tight junctions. The luminal edges of the cells have a striated brush border made up of many microvilli.