when the small intestine is stimulated by the entranceof fats.THE URINARY SYSTEMLEARNING OBJECTIVE: Recall the partsof the urinary system and their function(s).The urinary system is the primary filtering systemof the body (fig. 1-55). This system is composed of twomain organs, the kidneys and urinary bladder. Thekidneys produce urine, which is drained from thekidneys by two tubes called ureters. Urine flows downboth ureters to the bladder. The urinary bladder is alarge reservoir where the urine is temporarily storedbefore excretion from the body. A tube called theurethra carries the urine from the bladder to theoutside of the body. All these parts, except the length ofthe urethra, are the same in both sexes.KIDNEYSThe importance of the kidney can be realized onlywhen its structure and functions are understood. Thebladder, ureters, and urethra store and pass theproducts of the kidneys.The kidneys are two large, bean-shaped organsdesigned to filter waste materials from the blood (figs.1-55 and 1-56). They also assist in controlling the rateof red blood cell formation, and in the regulation ofblood pressure, the absorption of calcium ions, and thevolume, composition, and pH of body fluids. Thekidneys are located in the upper posterior part of theabdominal cavity, one on each side of the spinalcolumn. The upper end of each kidney reaches abovethe level of the 12th rib. The suprarenal (adrenal) glandsits like a cap on top of each kidney. The kidneys areprotected by a considerable amount of fat andsupported by connective tissue and the peritoneum.Attached to the hollow side of each kidney is thedilated upper end of the ureter, forming the renalpelvis.StructureThe lateral surface of the kidneys is convex inshape, and the medial side is deeply concave. Themedial side of each kidney possesses a depression thatleads to a hollow chamber called the renal sinus (fig.1-55). The entrance of the renal sinus is referred to asthe hilum (fig. 1-55). Blood vessels, nerves, lymphaticvessels, and the ureters pass through the hilum.T h e s u p e r i o r e n d o f t h e u r e t e r f o r m s afunnel-shaped sac called the renal pelvis (fig. 1-56).The renal pelvis is divided into two or three tubes,called major calyces. The major calyces (sing. calyx)are further subdivided into minor calyces.There are groups of elevated projections in thewalls of the renal pelvis. These projections are calledrenal papillae. The renal papillae connect to the minorcalyces, through tiny openings in the minor calyces.The principal portion of the kidney is divided intotwo distinct regions: an inner medulla and outer cortex(fig. 1-56). The renal medulla is composed ofpyramid-shaped masses of tubes and tubules calledrenal pyramids. Renal pyramids drain the urine tothe renal pelvis. The renal cortex forms a shell overthe renal medulla. Renal cortex tissue dips down, likefingers, between the renal pyramids, and forms whatare called renal columns. The cortex possesses verysmall tubes associated with nephrons. Nephrons arethe functional units of the kidneys.RENAL BLOOD VESSELS.—The renal arterysupplies blood to the kidneys (fig. 1-56). The renalartery enters the kidneys through the hilum, and sendsoff branches to the renal pyramids. These arterialbranches are called interlobar arteries. At the borderbetween the medulla and cortex, the interlobar arteriesbranch to form the arciform arteries. The arciformarteries branch also and form the interlobulararteries.1-53HM3F0155RENALARTERYKIDNEYRENALSINUSAORTAURETERURETHRAURINARYBLADDERINFERIORVENACAVAHILUMRENALVEINFigure 1-55.—The urinary system.
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