return as straight vessels toward the base of the pyramid (see G3, Fig. 20.24). Thus, the efferent arterioles from the juxtamedullary glomeruli give rise to vasa recta involved in the countercurrent exchange system and their peritubular capillary network. These vessels are described in the explanation of the countercurrent exchange system (page 721). Generally, venous flow in the kidney follows a reverse course to arterial flow, with the veins running in parallel with the corresponding arteries (see Fig. 20.24). Thus,  Peritubular cortical capillaries drain into interlobular veins, which in turn drain into arcuate veins, interlobar veins, and the renal vein.
 The medullary vascular network drains into arcuate veins and so forth.  Peritubular capillaries near the kidney surface and capillaries of the capsule drain into stellate veins (so called for their pattern of distribution when viewed from the kidney surface), which drain into interlobular veins , and so forth.
The kidneys contain two major networks of lymphatic vessels. These networks are not usually visible in routine histologic sections but can be demonstrated by experimental methods. One network is located in the outer regions of the cortex and drains into larger lymphatic vessels in the capsule. The other network is located more deeply in the substance of the kidney and drains into large lymphatic vessels in the renal sinus. There are numerous anastomoses between the two lymphatic networks.
The fibers that form the renal plexus are derived mostly from the sympathetic division of the autonomic nervous system. They cause contraction of vascular smooth muscle and consequent vasoconstriction.  Constriction of the afferent arterioles to the glomeruli reduces the filtration rate and decreases the production of urine.  Constriction of the efferent arterioles from the glomeruli increases the filtration rate and increases the production of urine.  Loss of sympathetic innervation leads to increased urinary output.
It is evident, however, that the extrinsic nerve supply is not necessary for normal renal function. Although the nerve fibers to the kidney are cut during renal transplantation, transplanted kidneys subsequently function normally.
URETER, URINARY BLADDER, AND URETHRA
All excretory passages, except the urethra, have the same general organization.
On leaving the collecting ducts at the area cribrosa, the urine enters a series of structures that do not modify it but are specialized to store and pass the urine to the exterior of the body. The urine flows sequentially to a minor calyx, a major calyx, and the renal pelvis, and leaves each kidney through the ureter to the urinary bladder, where it is stored. The urine is finally voided through the urethra.
All of these excretory passages, except the urethra, have the same general structures, namely, a mucosa (lined by transitional epithelium), muscularis, and adventitia (or, in some regions, a serosa).
Transitional epithelium lines the calyces, ureters, bladder, and the initial segment of the urethra.
Transitional epithelium (urothelium) lines the excretory passages leading from the kidney. This stratified epithelium is essentially impermeable to salts and water. The epithelium begins in the minor calyces as two cell layers and increases to an apparent four to five layers in the ureter (Fig. 20.25) and as many as six or more layers in the empty bladder. However, chapter 20 Urinary System U R ETE R, U R I NARY B LADD E R, AN D U R ETH RA 723