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No. 11 1. Introduction of the Urinary System 2. Kidneys 3. Ureters 4. Urinary Bladder 5. Urethra Introduction of the Urinary System If the cells of the body are to survive and carry out their functions effectively, they must be surrounded by a stable environment. The relatively constant state of the body’s internal environment is called homeostasis. To maintain homeostasis the concentrations of such substances as water, sodium, potassium, calcium, and hydrogen ions must remain relatively constant, as must the concentrations of a wide variety of cellular nutrients and products. Maintaining homeostasis involves most of the body systems. The digestive system, for example, supplies nutrients and also serves as a means of excreting some waste products. The lungs supply oxygen to the body and eliminate carbon dioxide and some water. The skin also plays a minor role in excretion-sweat, for example, contains small amounts of urea and ammonia. The kidneys, however, as the main excretory organs, are critically important in maintaining the balance of substances required for internal constancy. The kidneys eliminate from the body a variety of metabolic products, such as urea, uric acid, and creatinine. Further the kidneys conserve or excrete water and electrolytes as required so that the internal balance of these substances will be maintained. In fact, kidney malfunction can cause severe and even fatal problems as a result of upsets in fluid and electrolyte balance. To prevent death, it is necessary to replace the nonfunctioning kidneys with healthy ones by means of a kidney transplant or to remove potentially harmful substances regularly from the blood with an artificial kidney machine. 1. Components of the urinary system: Two kidneys, two ureters, one urinary bladder and a single urethra make up the urinary system. 2. Functions of the urinary system: Removing certain metabolic wastes and excess water from blood in the form of urine, maintaining the homeostasis of the body’s internal environments are the main functions of the urinary system. Urine drains out of each kidney, is conveyed through the ureter and is stored in the urinary bladder until it is expelled from the body through the urethra. Also the kidney has the endocrine function, producing erythropoietin, renin, and 1, 25-hydroxycholecalciferol. Section 1 The Kidneys Ⅰ. The Features of Kidneys The kidneys are a pair of beanshaped, reddish-brown organs. Each kidney has two extremities, two surfaces, and two borders. 1. Superior extremity and inferior extremity The superior extremity (upper pole) is broader and thinner than the inferior extremity (lower pole). 2. Anterior and posterior surfaces The anterior surface is slightly convex and the posterior surface contacting the posterior abdominal wall is plane. 3. Lateral border and medial border The lateral border is convex. On medial border of the kidney there is a vertical slit, the renal hilum, which transmits the renal vessels, nerves and a part of pelvis. The renal hilum leads into a central recess named the renal sinus, which is filled with the branches of the renal artery and vein, nerves, lymphatic vessels, minor renal calices, major renal calices, renal pelvis and adipose tissue. The structures which pass through the renal hilum are enclosed together by the connective tissue, to form the renal pedicle. From the front backward, the order of these structures in the renal pedicle is renal vein, renal artery, renal pelvis, and from superior to inferior is the renal artery, renal vein and renal pelvis. Ⅱ. The Location and Relations of Kidneys 1. The position The kidneys lie on the posterior abdominal wall one on each side of the vertebral column with its long axis almost parallel to the long axis of the body. The right kidney is lower than the left one. Left kidney: The superior extremity of the left kidney is at the level of the inferior border of the body of the eleventh thoracic vertebra, the inferior extremity of the left kidney is at the level with the intervertebral disc between the second and third lumbar vertebrae. Right kidney: The superior extremity of the right kidney is at the level with the superior border of the body of the twelfth thoracic vertebra, and the inferior extremity with the superior border of the body of the third lumbar vertebra. Renal hilum: The renal hilum is at the level of the first lumbar vertebra, 5 cm lateral to the midline of the body. Twelfth rib: The left twelfth rib is behind the middle part of the posterior surface of the left kidney and the right twelfth rib is behind the superior part of the posterior surface of the right kidney. Renal region: The area between the twelfth rib and the lateral border of the erector spinae is called the renal region in clinic. 2. The relations The upper poles of the kidneys are covered by the suprarenal glands. The renal pelvis is continuous with the ureter. The anterior surface of the upper part of left kidney is in contact with the stomach and spleen, the middle part with the tail of pancreas and splenic vessels, and the lower part with the coils of the jejunum and the left colic flexure. The anterior surface of the upper part of the right kidney is in contact with the right lobe of the liver, the medial border of the middle part with the descending part of duodenum, and the inferior part with the right colic flexure. Posteriorly, each kidney lies on muscles, i. e., the diaphragm above, the psoas major, the quadratus lumborum and the transversus abdominis below. Ⅲ. The Structure of Kidney On the coronal section of a kidney, the renal tissue is divided into two portions: the cortex and medulla. Ⅰ) The Renal Cortex The renal cortex lies immediately beneath the fibrous capsule, arch over the bases of the pyramids. It is rich in blood vessels and reddishbrown in colour. The parts from renal cortex dipping in between the pyramids are named the renal columns. The renal cortex is composed of renal glomeruli and renal tubules. Ⅱ) The Renal Medulla The renal medulla is deep to the cortex. It consists of a number of pale striated, conical masses, termed the renal pyramids. The bases of pyramids are directed toward the periphery of the kidney, while their apices converge towards the renal sinus, where they form prominent papillae projecting into the minor calices. Sometimes two or three apices of renal pyramids converge in one renal papilla. There are 7~12 renal papillae in each kidney. The foramina on their apices are called the papillary foramina. The urine formed in the kidney passes through these foramina into the lesser calices. There are 7~8 minor renal calices in the renal sinus. Each minor renal calyx is indented in a cup-shaped fashion, receiving from one to three papillae. Two or three minor renal calices converge into one major renal calyx. There are two or three major renal calices in each kidney. Finally the major renal calices join the renal pelvis, which is flat funnel-shaped. The renal pelvis becomes narrow at the renal hilum, then passes downward to continue with the ureter at the level of the inferior extremity of the kidney. Ⅳ. The Coverings of Kidneys Each kidney is surrounded by three layers of tissues. Ⅰ) The Fibrous Capsule The innermost layer, which covers the surface of the kidney, is the fibrous renal capsule. It can be stripped easily from a normal but can’t from a diseased kidney. Ⅱ) The Adipose Capsule The adipose capsule is outside the fibrous capsule and continues with the adipose tissue within the renal sinus. The amount of fat is very variable. Generally there are more adipose tissue along the borders of the kidney. Ⅲ) The Renal Fascia The adipose capsule is surrounded by the renal fascia which is divided into anterior and posterior layers to enclose the kidneys and suprarenal glands. The anterior layer of two sides is continued acrossing the median plane infront of the aorta and inferior vena cava. Whereas the posterior layer merges with the fascia of the psoas major and the quadratus lumborum. Both layers also merge with the connective tissue around the renal vessels. The two layers are fused each other above the suprarenal gland and continue with infradiaphragmatic fascia and transverse fascia. Supporting structures of the kidneys: The connective trabeculae from renal fascia cross the adipose capsule to the fibrous capsule to fix the kidney. Besides, the adipose capsule, the neighbouring organs, the renal vessels, peritoneum and pressure in the abdominal cavity play the role also in maintaining the normal location of the kidney. If these supporting structures are abnormal the kidney may descend to an abnormally low level. This downward displacement of the kidney is called nephroptosis. Ⅴ. Renal Vessels and segments The renal arteries, one on each side, arise directly from the aorta at the level of the 2nd lumbar vertebra. Each artery lies behind the corresponding renal vein, and at or just outside the hilum of kidney, it divides typically into anterior and posterior divisions. The anterior division passes in front of the pelvis and divides into four segmental arteries, while the posterior division passes behind the pelvis and becomes the posterior segmental artery. The second grade branches of renal artery are segmental arteries. The area which each segmental artery distributed is called a renal segment. Each kidney has five renal segments: ①Superior segment ②Superior anterior segment ③Inferior anterior segment ④Inferior segment ⑤Posterior segment. When one segmental artery is obstructed, the corresponding renal segment will be necrosis. This is because of no anastomosis between the segmental arteries. Section 2 The Ureters From the renal pelvis, urine is transported to the urinary bladder by ureters, one from each kidney. The ureters are two muscular tubes which convey the urine from the renal pelvis to the urinary bladder. The ureters descend between the parietal peritoneum and the body wall to the pelvic cavity, where they turn medially and enter the urinary bladder on its posterior lateral surfaces. Before opening into the baldder, the ureters travel obliquely through the bladder wall. As a result, contraction of the muscles of the bladder wall can compress the ureters and help prevent urine from flowing back into the ureters from the bladder. This occurs during bladder emptying. In effect, the muscles of the bladder wall act as sphincters on the ureters. Valve-like folds of the mucous membrane lining of the bladder cover the orifices of the ureters and assist in preventing urine from flowing back into the ureters during micturition. According to the course each ureter may be divided into three parts: Ⅰ. The Abdominal Part of the Ureter The ureter is continuous superiorly with the renal pelvis. A slight constriction may mark the site of transition. This part runs downwards and slightly medially in front of the psoas major and behind the peritoneum. At the superior pelvic aperture, the left ureter crosses through the terminal part of the left common iliac artery anteriorly, and the right ureter passes the beginning of the right external iliac artery anteriorly, to continue with the pelvic part. Ⅱ. The Pelvic Part of the Ureter It passes downwards along the lateral wall of the lesser pelvis, then turns medially at the level of the ischial spine to the base of the urinary bladder. At here the ductus deferens crosses it anteriorly to its medial side in the male. In the female the pelvic part of the ureter turns downwards, forwards and medially. At the level of ischial spine, 2 cm lateral to the cervix of uterus, it is crossed anteriorly by the uterine vessels. It then runs along the lateral side and the anterior surface of the vagina to the base of the urinary bladder. Ⅲ. The Intramural Part of the Ureter It passes obliquely through the wall of the urinary bladder for 1.5 cm long, before opening into the superolateral angle of the bladder. When the bladder is filled up, the intramural part serves as a valve to prevent the badckflow of urine. Constricted parts: The ureters are not uniform in calibere. Three constricted parts are the places where the renal stones stay usually. These constrictions are: ① Superior stricture: At the junction of the ureter and the renal pelvis, ② Middle stricture: At the point where ureter crosses the superior aperture of the lesser pelvis, ③ Inferior stricture: The intramural part. Section 3 The Urinary Bladder The urinary bladder is a hollow, muscular organ which stores the urine. It varies in size, shape, position and relations according to the amount of urine that it contains. In the adult the volume of the urinary bladder averages 300~500 ml, with the largest volume of 800 ml. In the babaies the volume is 1/10 of the adult’s volume. In old age the volume is volume is larger than in the adult, because the tension of the muscles of the urinary bladder is reduced. In female its volume is less than in the male. Ⅰ. The Features of Urinary Bladder The empty urinary bladder is in the form of a triangular pyramid. It may be divided into four portions: apex, body, fundus neck. The apex of bladder is directed ventrally toward the posterior surface of the upper part of the pubic symphysis. The fundus of bladder is triangular and directed posteriorly and inferiorly The body of bladder is between the apex and fundus. The neck of bladder is the lowest and also the most fixed part. It is pierced by the internal urethral orifice. In the male the neck rests upon the prostate gland. When the bladder is filled up with urine, it is oval in shape, the boundaries between the surfaces are not clear, and the mucosal folds become disappeared. Ⅱ. The Location and Relations of the Urinary Bladder The urinary bladder rests in front against the pubis symphysis. There are loose connective tissue and venous plexus between the bladder and the pubic symphysis. In males, the bladder is in front of the rectum. In females, it lies just anterior to the uterus and the superior portion of the vagina. The fundus of the bladder: In the male the fundus of bladder is anterior to the seminal vesicles, ampullae ductus deferentes and rectum. In the female it (the fundus) is attached posteriorly to the upper portion of the vagina and cervix of uterus by the loose connective tissue containing the venous plexus. The neck of the bladder: In the male the neck of bladder is firmly attached to the prostate gland, and in the female to the urogenital diaphragm. In the adult the empty urinary bladder is entirely within the lesser pelvis, but as it becomes distended it expands upwards and forwards into the abdominal cavity. The urinary bladder is a interperitoneal organ, only the superior surface and upper portion of the inferolateral surfaces are covered by peritoneum, which is reflected from the lateral wall of the pelvis and from the anterior abdominal wall just above the level of the pubic symphysis when the bladder is empty. As the bladder fills, the superior surface of the bladder enlarges and bulges upward into the abdominal cavity, the peritoneal covering is peeled off away from the lower part of the anterior abdominal wall, and the reflection of the peritoneum becomes higher. So that the bladder comes into direct contact with the anterior abdominal wall. Therefore the puncture of bladder can be performed just above the pubic symphysis without injuring the peritoneum. In the male the peritoneum covering on the superior surface of the bladder reflects to the rectum to form the rectovesical pouch. In the female it continues to the uterus to form the vesicouterine pouch. Ⅲ. The Interior Structure of the Urinary Bladder It is lined by mucous membrane. When empty, the mucous membrane lie in folds except over the trigone ofbladder where it is smooth. Trigone of bladder: The trigone of bladder is on the interior of the fundus of bladder. When the bladder is empty there are many folds on the mucosa of its internal surface, but on the fundus of bladder there is a smooth triangular area are called the trigone of bladder. The anteroinferior angle of the trigone is formed by the internal urethral orifice, the two posterolateral angles by the ureteric orifices. The area of this trigone is said to be susceptible to tuberculosis, tumor and inflammation. A ridge, the interureteric fold, connects the two ureteric orifices at the upper border of the trigone. Secton 4 The Urethra The urethra extends from the urinary bladder to the exterior. It is very different in males and females. The female urethra is a membranous canal, about 5 cm long, extending from the urinary bladder to the external urethral orifice in the vaginal vestibule. It is wider, shorter, straighter than the male urethra.