when the small intestine is stimulated by the entrance of fats. THE URINARY SYSTEM LEARNING OBJECTIVE:  Recall the parts of the urinary system and their function(s). The urinary system is the primary filtering system of the body (fig. 1-55). This system is composed of two main organs, the  kidneys  and  urinary bladder. The kidneys   produce   urine,   which   is   drained   from   the kidneys by two tubes called ureters. Urine flows down both  ureters  to  the  bladder.  The  urinary  bladder  is  a large reservoir where the urine is temporarily stored before   excretion   from   the   body.   A  tube   called   the urethra   carries   the   urine   from   the   bladder   to   the outside of the body. All these parts, except the length of the urethra, are the same in both sexes. KIDNEYS The importance of the kidney can be realized only when its structure and functions are understood. The bladder,   ureters,   and   urethra   store   and   pass   the products of the kidneys. The  kidneys  are  two  large,  bean-shaped  organs designed to filter waste materials from the blood (figs. 1-55 and 1-56). They also assist in controlling the rate of  red  blood  cell  formation,  and  in  the  regulation  of blood pressure, the absorption of calcium ions, and the volume,   composition,   and   pH   of   body   fluids.   The kidneys are located in the upper posterior part of the abdominal   cavity,   one   on   each   side   of   the   spinal column. The upper end of each kidney reaches above the level of the 12th rib. The suprarenal (adrenal) gland sits like a cap on top of each kidney. The kidneys are protected   by   a   considerable   amount   of   fat   and supported  by  connective  tissue  and  the  peritoneum. Attached  to  the  hollow  side  of  each  kidney  is  the dilated  upper  end  of  the  ureter,  forming  the   renal pelvis. Structure The  lateral  surface  of  the  kidneys  is  convex  in shape,  and  the  medial  side  is  deeply  concave.  The medial side of each kidney possesses a depression that leads to a hollow chamber called the  renal sinus  (fig. 1-55). The entrance of the renal sinus is referred to as the hilum (fig. 1-55). Blood vessels, nerves, lymphatic vessels, 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   a funnel-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  the walls of the renal pelvis. These projections are called renal papillae. The renal papillae connect to the minor calyces, through tiny openings in the minor calyces. The principal portion of the kidney is divided into two distinct regions: an inner medulla and outer cortex (fig.   1-56).   The   renal   medulla   is   composed   of pyramid-shaped  masses  of  tubes  and  tubules  called renal pyramids.  Renal pyramids  drain the urine to the renal pelvis. The  renal cortex  forms a shell over the renal medulla. Renal cortex tissue dips down, like fingers, between the renal pyramids, and forms what are called  renal columns. The cortex possesses very small tubes associated with  nephrons. Nephrons are the functional units of the kidneys. RENAL BLOOD VESSELS.—The renal artery supplies  blood  to  the  kidneys  (fig.  1-56).  The  renal artery enters the kidneys through the hilum, and sends off   branches   to   the   renal   pyramids.   These   arterial branches are called  interlobar arteries. At the border between the medulla and cortex, the interlobar arteries branch  to  form  the  arciform  arteries.  The  arciform arteries   branch   also   and   form   the    interlobular arteries. 1-53 HM3F0155 RENAL ARTERY KIDNEY RENAL SINUS AORTA URETER URETHRA URINARY BLADDER INFERIOR VENA CAVA HILUM RENAL VEIN Figure 1-55.—The urinary system.