The urinary system consists of the kidneys, ureters, urinary bladder, and urethra, which filter blood and subsequently produce, transport, store, and intermittently excrete urine (liquid waste). The system is divisible into upper and lower parts. The upper part located within the abdomen and consists of the kidneys and a large portion of the ureters. The lower part constitutes the pelvic urinary organs, and includes the short portion of the ureters, the urinary bladder and the urethra.
The superior urinary organs (kidneys and ureters) and their vessels are primary retroperitoneal structures on the posterior abdominal wall, that is, they were originally formed as, and remain, retroperitoneal viscera. The superomedial aspect of each kidney normally contacts a suprarenal gland , which is enclosed in a fibrous capsule and a cushion of pararenal fat. This gland is referred to as the adrenal gland, and a weak fascial septum separates it from the kidneys so that they are not actually attached to each other. The suprarenal glands function as part of the endocrine system, secreting hormones like aldosterone. They completely separate in function from the kidneys and/or the urinary system.
Many harmful waste by-products that result from metabolism are removed from the blood stream through urine. Such harmful products include:
- urea and creatinine, which are end products of protein metabolism
- drugs or their breakdown products
- in diseased conditions, urine can contain glucose (as in diabetes mellitus), or proteins (in kidney disease), the excretion of which is normally prevented
- a considerable amount of water, the quantity of which is strictly controlled. It is greatest when there is heavy intake of water, and least when intake is low or when there is substantial water loss in some other way (for example by perspiration in hot weather). This enables the water content of blood plasma and tissues to remain fairly constant, thus maintaining homeostasis.
Urine production and the control of its composition is exclusively the function of the kidneys. The urinary bladder is responsible for storage of urine until it is voided. The ureters and urethra are simply passages for the transportation of urine into- and from the urinary bladder respectively. The two ureters and urethra form a trigone in the urinary bladder. This trigone indicates the three points of attachments of these passages to the urinary bladder.
Each of the kidneys has a convex lateral margin, and a concave medial margin where the renal sinus and renal pelvis are located. This indented medial margin gives the kidney a somewhat bean-shaped appearance. The ovoid kidneys filter water, salts, and wastes of protein metabolism from the blood, while returning nutrients and chemicals to the blood. They lie retroperitoneally on the posterior abdominal wall, one on each side of the vertebral column at the level of the T12 – L3 vertebrae. The right kidney lies slightly inferior to the left, probably owing to its relationship to the liver which lies just anterosuperior to it.
During life, the kidneys are reddish brown and measure approximately 10 cm in length, 5 cm in width, and 2.5 cm in thickness. Superiorly, the kidneys are associated with the diaphragm, which separates them from the pleural cavities and 12th pair of ribs . More inferiorly, the posterior surfaces of the kidney are related to the quadratus lumborum muscle. The subcostal nerve and vessels and the iliohypogastric and ilioinguinal nerves descend diagonally across the posterior surfaces of the kidneys. The liver, duodenum , and ascending colon are anterior to the right kidney. The right kidney is separated from the liver by the hepatorenal recess. The left kidney is related to the stomach, spleen, pancreas, jejunum, and descending colon.
At the concave medial margin of the kidney is a vertical cleft, the renal hilum, where the renal artery enters and renal vein and renal pelvis leave. The renal hilum is the entrance to a space within the kidney, the renal sinus, which is occupied by the renal pelvis, calices, vessels, and nerves and a variable amount of fat.
Each kidney has anterior and posterior surfaces, medial and lateral margins, and superior and inferior poles. The renal pelvis is the flattened funnel-shaped expansion of the superior end of the ureter. The apex of the renal pelvis is continuous with the ureter. The renal pelvis receives two or three major calices (calyces, singular calyx), each of which divides into two or three minor calices. Each minor calyx is indented by the renal papilla, the apex of the renal pyramid, from which the urine is collected and excreted. In living individuals, the renal pelvis and its calices are usually collapsed (empty). The pyramids and their associated cortex form the lobes of the kidney.
There are two ureters in total. Each one is a muscular duct of 25 – 30 cm in length with narrow lumen , that carries urine from the kidney to the urinary bladder and also connects the two structures. The ureters run inferiorly from the apex of the renal pelves (pleural for pelvis) at the hila (pleural for hilum) of the kidneys, passing over the pelvic brim at the bifurcation of the common iliac arteries. They then run along the lateral wall of the pelvis and enter the urinary bladder, forming the upper two points of the urinary bladder trigone. The abdominal parts of the ureters adhere closely to the parietal peritoneum and are retroperitoneal throughout their course.
The ureters are normally constricted to a variable degree in three places along their course from the kidneys to the urinary bladder:
- at the junction of the ureters and renal pelves
- where the ureters cross the brim of the pelvic inlet
- during their passage through the wall of the urinary bladder.
These constricted areas are the potential sites of obstruction by ureteric (kidney) stones.
Contractions of the urinary bladder musculature act as a sphincter preventing the reflux of urine into the ureters when the bladder contracts, increasing internal pressure during micturition. Urine is transported down the ureters by means of peristaltic contractions, a few drops being transported at intervals of 12 – 20 seconds.
Blood supply and lymphatics
The abdominal portion of the ureters are supplied by renal arteries, and are drained by renal veins into the renal and gonadal (testicular or ovarian) veins. The lymphatic vessels return lymph to the right or left lumbar (caval or aortic) lymph nodes and the common iliac lymph nodes.
Lymph drainage of the pelvic parts of the ureters is into the common, external, and internal iliac lymph nodes. The pelvic parts of the ureters are supplied by variable branches from the common iliac, internal iliac, and ovarian arteries.
The ureters are innervated by adjacent autonomic plexuses (renal, aortic, superior and inferior hypogastric). The ureters are superior to the pelvic pain line. Afferent (pain) fibres from the ureters follow sympathetic fibres in a retrograde direction to reach the spinal ganglia and spinal cord segments of T11 – L1 or L2. Ureteric pain is usually referred to the ipsilateral lower quadrant of the abdomen, especially to the groin (inguinal region).
The urinary bladder is a hollow viscus with strong muscular walls that is characterized by its distensibility. The walls of the bladder are composed chiefly of the detrusor muscle. In males, toward the neck of the bladder, fibres of the detrusor muscle form an involuntary sphincter called the internal urethral sphincter. This sphincter contracts during ejaculation to prevent retrograde ejaculation (ejaculation reflux) of semen into the bladder. Some fibres of the detrusor muscle run radially and assist in opening the internal urethral orifice. The ureteric orifices and the internal urethral orifice are at the angles of the trigone of the bladder.
At the end of urination (micturition) the bladder of a normal adult contains virtually no urine. When empty, the urinary bladder is somewhat tetrahedral and externally has an apex, body, fundus, and neck. The bladder’s 4 surfaces – a superior, 2 inferolateral, and a posterior, are most apparent when viewing an empty, contracted bladder that has been removed from a cadaver, when the bladder appears rather boat shaped. When the bladder is empty, the apex of the bladder points toward the superior edge of the pubic symphysis. The fundus is formed by the posterior wall of the bladder and lies opposite the apex. Between the apex and fundus is a major portion of the bladder, the body of the bladder, while the neck of the bladder is the region where the fundus and the inferolateral surfaces meet inferiorly.
The urinary bladder serves as a temporary reservoir for urine and varies in size, shape, position, and relationships according to its content and the state of neighboring viscera. When empty, the adult urinary bladder is located in the lesser pelvis, lying partially superior to and partially posterior to the pubic bones. In infants and young children, the urinary bladder is in the abdomen even when empty. The bladder usually enters the greater pelvis by 6 years of age; however, it is not located entirely within the lesser pelvis until after puberty. An empty bladder, in the adult, is almost entirely in the lesser pelvis. As the bladder fills, it enters the greater pelvis. In some individuals, a full bladder may ascend to the level of the umbilicus.
The bladder is held firmly within the extraperitoneal subcutaneous fatty tissue by the lateral ligaments of bladder and the tendinous arch of the pelvic fascia – especially its anterior components, the pubo-prostatic ligament in males and the pubovesical ligament in females.
The urinary bladder is separated from the pubic bones by the potential retropubic space (of Retzius) and lies mostly inferior to the peritoneum. It rests on the pubic bones and pubic symphysis anteriorly and the pelvic floor posteriorly.
Blood supply and lymphatics
The main arterial supply of the bladder are branches of the internal iliac arteries. It is drained by the internal iliac veins mainly, and in both males and females, lymphatic vessels from the superolateral aspects of the bladder pass to the external iliac lymph nodes. Lymph from other aspects of the bladder also drain into the internal iliac lymph nodes, and sacral or common iliac lymph nodes.
The bladder is innervated by sympathetic and parasympathetic fibres from various regions including the inferior thoracic and upper lumbar spinal cord levels.
The male urethra is a muscular tube (18-22 cm long) that conveys urine from the internal urethral orifice of the urinary bladder to the external urethra orifice, located at the tip of the glans penis in males. The urethra also provides an exit for semen (sperm and glandular secretions).
In the flaccid (none erect) state, the urethra has a double curvature. For descriptive purposes, the urethra is divided into four parts.
- The intramural (preprostatic) part (0.5 – 1.5 cm long)
- The prostatic urethra (3 – 4 cm long)
- Intermediate (membranous) part (1 – 1.5 cm long)
- The spongy urethra (approximately 15 cm long)
The intramural part of the urethra (preprostatic urethra) varies in diameter and length, depending on whether the bladder is filling (bladder neck is tonically contracted so the internal urethral orifice is small and high; the filling internal urethral orifice) or emptying (the neck is relaxed so the orifice is wide and low; the emptying internal urethral orifice).
The most prominent feature of the prostatic urethra is the urethral crest, which is a median ridge between bilateral grooves, then prostatic sinuses. The secretory ducts of the prostate, the prostatic ducts, open into the prostatic sinuses. The seminal colliculus is a rounded eminence in the middle of the urethral crest with a slit-like orifice that opens into a small cul-de-sac, the prostatic utricle. The prostatic utricle is the vestigial remnant of the embryonic uterovaginal canal, the surrounding walls of which, in the female, constitute the primordium of the uterus and a part of the vagina. The ejaculatory ducts open into the prostatic urethra via minute, slit-like openings located adjacent to and occasionally just within the orifice of the prostatic utricle. Thus urinary and reproductive tracts merge at this point.
Blood supply and lymphatics
The male urethra is chiefly supplied by prostatic branches of the inferior vesical and middle rectal arteries. It is drained by the prostatic venous plexus, while its lymphatic vessels pass mainly to the internal iliac lymph nodes; a few lymphatic vessels drain into the external iliac lymph nodes.
Its innervations are derived from the prostatic plexus (mixed sympathetic, parasympathetic, and visceral afferent fibers). The prostatic plexus is one of the pelvis plexuses (an inferior extension of the vesical plexus) arising as organ-specific extensions of the inferior hypogastric plexus.
The female urethra (approximately 4 cm long and 6 mm in diameter) passes anteroinferiorly from the internal urethral orifice of the urinary bladder, posterior and then inferior to the pubic symphysis, to the external urethra orifice. The musculature surrounding the internal urethral orifice of the female bladder is not organized into an internal sphincter. In the females, the external urethral orifice is located in the vestibule, directly anterior to the vaginal orifice.
The urethra lies anterior to the vagina (forming an elevation in the anterior vaginal wall); its axis is parallel to that of the vagina. The urethra passes with the vagina through the pelvic diaphragm, external urethral sphincter, and perineal membrane. Urethral glands are present, particularly in the superior part of the urethra. One group of glands on each side, the paraurethral glands, are homologues to the prostate. These glands have a common paraurethral duct, which opens (one on each side) near the external urethral orifice. The inferior half of the urethra is located in the perineum.
Blood supply and lymphatics
The female urethra is supplied by the internal pudendal and vaginal arteries. Its venous return follow the course of the arteries and have similar names. Most lymphatic vessels from the urethra pass to the sacral and internal iliac lymph nodes, but a few vessels from the distal urethra drain into the inguinal lymph nodes.
The nerves to the urethra arise from the vesical (nerve) plexus and then pudendal nerve. The pattern is similar to that in the male given the absence of a prostatic plexus and an internal urethral sphincter. Visceral afferents from most of the urethra run in the pelvic splanchnic nerves, but the termination receives somatic afferents from the pudendal nerve. Both the visceral and the somatic afferent fibers extend from the cell bodies in S2-S4 spinal ganglia.