The nephron is the fundamental structural and functional unit of the kidney (see Fig. 20.3). Each human kidney contains approximately 2 million nephrons. Nephrons are responsible for the production of urine and correspond to the secretory part of other glands. The collecting ducts are responsible for the final concentration of the urine and are analogous to the ducts of exocrine glands that modify the concentration of the secretory product. Unlike the typical exocrine gland in which the secretory and duct portions arise from a single epithelial outgrowth, nephrons and their collecting tubules arise from separate primordia and only later become connected.
General Organization of the Nephron
The nephron consists of the renal corpuscle and a tubule system.
As stated previously, the renal corpuscle represents the beginning of the nephron. It consists of the glomerulus, a tuft of capillaries composed of 10 to 20 capillary loops, surrounded by a double-layered epithelial cup, the renal or Bowman’s capsule. Bowman’s capsule is the initial portion of the nephron, where blood flowing through the glomerular capillaries undergoes filtration to produce the glomerular ultrafiltrate. The glomerular capillaries are supplied by an afferent arteriole and are drained by an efferent arteriole that then branches, forming a new capillary network to supply the kidney tubules. The site where the afferent and efferent arterioles penetrate and exit from the parietal layer of Bowman’s capsule is called the vascular pole. Opposite this site is the urinary pole of the renal corpuscle, where the proximal convoluted tubule begins (see Fig. 20.7).
Continuing from Bowman’s capsule, the remaining parts of the nephron (the tubular parts) are as follows:  Proximal thick segment, consisting of the proximal con voluted tubule (pars convoluta) and the proximal straight tubule (pars recta) Thin segment, which constitutes the thin part of the loop of Henle  Distal thick segment, consisting of the distal straight tubule (pars recta) and the distal convoluted tubule (pars
The distal convoluted tubule connects to the collecting tubule, often through a connecting tubule, thus forming the uriniferous tubule (i.e., the nephron plus collecting tubule; see Fig. 20.3).
FIGURE 20.5 • Photomicrograph of fetal kidney. This photo-micrograph of an H&E–stained human fetal kidney shows the cortex, the medulla, and two associated pyramids. Note each surface convexity corresponds to a kidney lobe. During postnatal life the lobar convexities disappear and the kidney then exhibits a smooth surface. 30.
Tubes of the Nephron
The tubular segments of the nephron are named according to the course that they take (convoluted or straight), location (proximal or distal), and wall thickness (thick or thin).
Beginning from Bowman’s capsule, the sequential parts of the nephron consist of the following tubules:  Proximal convoluted tubule originates from the urinary pole of Bowman’s capsule. It follows a very tortuous or convoluted course and then enters the medullary ray to continue as the proximal straight tubule.
 Proximal straight tubule, commonly referred to as the thick descending limb of the loop of Henle, descends into the medulla.
 Thin descending limb is the continuation of the proximal straight tubule within the medulla. It makes a hairpin turn and returns toward the cortex.
 Thin ascending limb is the continuation of the thin descending limb after its hairpin turn.