The urinary system consists of the kidneys, ureters, urinary bladder and urethra.

The paired kidneys (figure 1) are located retroperitoneally against the posterior body wall. They receive blood from the renal arteries, filter the blood and return it to the general circulation through the renal veins. Each kidney is surrounded by a tough fibrous capsule. Within the capsule, the kidney is divided into an outer cortex and an inner medulla. Each kidney is made up of about one million nephrons (figure 2). A nephron is the basic filtering unit of the kidney. It is made up of blood vessels and tubules which radiate outward from the hilum of the kidney, the medial notch where the major blood vessels and ureters enter the kidney.

Each renal artery enters its respective kidney at the renal hilus and divides into interlobar arteries. These arteries divide into smaller arcuate arteries and interlobular arteries. The smallest branches of the interlobular arteries are the afferent arterioles which carry blood into the glomerulus (figure 3), a knot of capillaries at the core of each nephron. The afferent arterioles are also lined with juxtaglomerular cells which help to control blood pressure.Each glomerulus is surrounded by a cup-shaped structure call Bowman's capsule.

The blood pressure in the glomerulus is high enough that about 10% of the blood fluid volume is squeezed out and absorbed by the Bowman's capsule. The remaining blood is carried out of the glomerulus through the efferent arteriole. The fluid absorbed into Bowman's capsule passes through a number of tubules and, as it does so, some constituents are reabsorbed into the blood, while some waste products in the blood are forced into the fluid. In order, from Bowman's capsule, the tubules of the nephron are called the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule and the collecting duct. The collecting tubules come together in bunches to form the renal papillae which empty into the renal pelvis (figure 4). In humans, the renal pelvis is actually divided into about a half dozen smaller areas, each called a calyx.

From the renal pelvis, the urine leaves the kidney by way of the ureters. The ureters pass down the posterior body wall and enter the floor of the urinary bladder (figure 5) in a region called the trigone (figure 6). The urinary bladder stores the urine. When a sufficient volume of fluid is accumulated, the bladder is reflexly stimulated to contract. This pressure drives the urine through the urinary sphincter and into the urethra (figure 7) from which it is voided.

English - Japanese Glossary

kidney: 腎臓 (jinzou); renal capsule: 腎被膜 (jinhimaku); renal cortex: 腎皮質 (jinhishitsu); renal medulla: 腎髄質 (jinzuishitsu); renal papilla: 腎乳頭 (jinnyuutou); nephron: ネフロン (nefuron); Bowman`s capsule: ボーマン嚢 (boumannou); proximal tubule: 近位 尿細管 (kininyousaikan); distal tubule: 遠位尿細管 (eninyousaikan); loop of Henle: ヘンレ系蹄 (henrekeitei); collecting duct: 集合管 (shuugoukan); renal artery: 腎動脈 (jindoumyaku); renal vein: 腎静脈 (jinjyoumyaku); interlobar artery: 葉間動脈 (youkandoumyaku); interlobar vein: 葉間静脈 (youkanjyoumyaku); interlobular artery: 小葉間動脈 (shouyoukandoumyaku); interlobular vein: 小葉間静脈 (shouyoukanjyoumyaku); glomerulus: 腎糸球 (jinshikyuu); afferent arteriole: 輸入動脈 (yunyuudoumyaku); juxtaglomerular cells: 傍糸球体細胞 (boushikyuutaisaibou); efferent arteriole: 輸出動脈 (yushutsudoumyaku); calyx: 腎杯 (jinpai); renal pelvis: 腎う (jinu); renal hilus: 腎門 (jinmon); ureter: 尿管 (nyoukan); bladder: ぼうこう (boukou); trigone: ぼうこう三角部 (boukousankakubu);urethra: 尿道 (nyoudou)