Anatomy and Development of the Urinary Tract

Original article by Tom Leach | Last updated on 28/6/2014
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Summary

  • The kidneys develop from metanephros.
  • The bladder and urethra develop from the urogenital sinus
  • The prostate develops from an outgrowth of the urethral epithelium
 

Ureters

These enter the pelvis at the pelvic inlet, and run just anterior to the bifurcation of the common iliac artery. They are about 25cm in length.
From here, the ureters carry along down the pelvic wall to join the ‘base’ of the bladder .
 

Bladder

This is the most anterior thing in the pelvic cavity.
As it fills, it expands upwards into the abdomen.
 
When empty the bladder is shaped like a 3-sided pyramid that has tipped over and is layed on one of its sides.  
 
The apex of the bladder points anteriorly towards the top of the pubic symphysis. At the apex, is a ligament called the median umbilical ligament and this connects the apex of the bladder to the umbilicus. It is a remnant of urachus which formed the bladder.
 
The base of the bladder is triangular in shape and points posterioinferiorly. The two ureters enter the bladder at the tips of this triangle, and the urethra leaves at the other tip.
The epithelium of the base of the bladder is smooth and firmly attached to the underlying smooth muscle. This is in contrast to the rest of the bladder, where the mucosa is folded, and loosely attached to the smooth muscle.
 
The neck of the bladder is the most inferior and the most fixed part of the bladder. It is held in place by two fibromuscular bands that connect it to the posterior aspect of the pubic bone. These bands are called the pubovesical ligaments in women, and the puboprostatic ligaments in men.
 
The position of the bladder changes with child development. At birth, the bladder is almost entirely abdominal. With age, and especially during puberty, the bladder moves downwards until it assumes its adult position after puberty.
The bladder and urethra are supplied by the internal iliac artery.
 

Urethra

Women

The urethra is about 4 cm long, and it passes posteriorly downwards. The inferior surface of the urethra is bound to the anterior aspect of the vagina.
At the lower end of the urethra are two glands called Skene’s glands. These drain via two little ducts that open out on either side of the urethral orifice.
 These glands are the female ‘equivalent’ of the prostate gland, and at first glance they appear to have no function. In some women they are not present. Some people believe that they are the ‘G-spot’. They can be felt through the anterior surface of the vagina. The glands have been shown to release a secretion when mechanically stimulated. This secretion is similar in composition to that of the prostate gland. The secretion seen in ‘female ejaculation’ is from these glands.  
 

Men

The urethra is about 20cm long, and it has two bends along its course. During erection, the second bend is not present. Also note how the first bend does not occur until after the urethra has passed through the deep perineal pouch and perineal membrane.
The urethra in men is divided into four parts:
  1. Preprostatic part – this is 1cm long and extends from the bladder to the prostate. It is associated with a ‘cuff’ of circular muscle fibres – the internal urethral sphincter. This sphincter closes during ejaculation, and this prevents movement of semen into the bladder during ejaculation.
  2. Prostatic Part – this is 3-4cm long and is surrounded by the prostate. In this region, the urethra is marked by a longitudinal fold along the midline known as the urethral crest. The depression on each side of this crest is called the urethral sinus and it is here than the prostate empties its secretion.                                                              
    1. Halfway along the prostate is a small blind opening called the prostatic utricle – and it is though that this is the equivalent of the uterus in men. Just below the prostatic utricle there are two openings that are the openings of the ejaculatory ducts. So, the connection of the reproductive and urinary tracts occurs in the prostatic urethra in men.
  3. Membranous part – this is narrow and it passes through the deep perineal pouch. During transit through the deep perineal pouch, in both men and women, the urethra is surrounded by skeletal musclewhich forms the external urethral sphincter.
  4. Spongy urethra – this is inside the corpus spongiosum of the penis. At the base of the penis it is enlarged to form a blulb, and it does so again, this time at the other end of the penis to form the navicular fossa.  
 

Kidneys

  • These lie retroperitoneally at the level of L1-L2
  • Usually, they are supplied by a single artery, but in some people there are multiple arteries.
  • The renal vein drains the kidney into the inferior vena cava.
  • Renal lymphatics drain into the para-aortic nodes.
 

Male Reproductive system

Testes

  • These originally develop high on the posterior abdominal wall, and then descend (normally before birth) down through the inguinal canal, and into the anterior abdominal cavity and finally into the scrotum. During their descent, the testes carry with them their blood and lymphatic supplies, as well as the vas deferens.
    • Therefore, the lymphatic drainage of the tests is to para-aortic nodes and not to inguinal or pelvic lymph nodes.
  • The tests are ellipsoid in shape. Each testicle is enclosed in a fibrous pouch that is continuous with the anterior abdominal wall. The spermatic cord is the fibrous material that connects the covering of the testes with the anterior abdominal wall.
  • The sides and anterior aspect of each testicle is covered by a peritoneal sac called the tunica vaginalis. During development, this sac was continuous with the abdominal cavity, but this closes off, leaving a fibrous remnant at the top of the testicle.
  • Each testicle is composed of seminiferous tubules that are surrounded by connective tissue called the tunica albuginea. The seminiferous tubules are the site of sperm production.
  • At the end of the seminiferous tubules is a section of tube known as the straight tubule. This connects to a collecting chamber known as the rete testis. Then, efferent tubules (about 12-20 of these), link the rete testis to the epididymis, where the sperm mature.
  • The epididymis is a single, long, coiled duct that runs along the posterolateral side of the testicle. The epididymis can be divided into the head of epididymis, and the true epididymis. The true epididymis itself can be divided into a body and a tail.
    •  The end of the epididymis is continuous with the vas deferens.
    •  Sperm are stored in the epididymis until ejaculation
 

Ductus deferens (vas deferens)

  • This is a long muscular tube the transports sperm from the epididymis to the ejaculatory ducts in the pelvis. It ascends from the scrotum as part of the spermatic cord, and passes through the inguinal canal.
  • After this is turn medially and crosses the external and internal iliac veins at the pelvic inlet, to enter the pelvic cavity
  • It descends medially along the pelvic wall, and crosses over the ureter just where the ureter empties into the bladder. It then carries on along the base of the bladder, almost at the midline, before ti is finally joined by the duct of the seminal vesicle, to form the ejaculatory duct.
 

Seminal vesicle

These are accessory glands of the male reproductive system. There are two of them found between the bladder and rectum. They run parallel to the ductus deferens at the base of the bladder.
The secretions of the seminal vesicle contribute significantly to the volume of the semen.
The glands are about 10cm, but are coiled up so that they only appear about 5cm long.
About 60% of semen volume can be accounted for by seminal vesicle secretion. However, most of the initial ejaculatory volume comes from sperm and prostatic zinc. So it is unclear exactly what seminal vesicle fluid does.
Some argue that it is an evolutionary remnant – for example in rats, the seminal vesicle secretes a spermicidal ‘plug’ that means any further sperm entering the female will have less chance of causing fertilization.
 
The secretions contain proteins, enzymes, vitamin C, mucous and other nutrients – and in the lab it has been shown that this can act as a nutrient supply for sperm – however in real situations it is unlikely any sperm will come into contact with the seminal vesicle fluid within the female.
 

The Prostate

This is a single gland that surrounds the urethra. It lies immediately below the bladder, anterior to the rectum, and posterior to the pubic symphysis
The prostate secretes about 10-30% of semen volume in the form of a slightly alkaline fluid that helps neutralise the acidity of the vagina.
Sperm ejaculated with prostatic fluid have a much better chance of survival, and much greater motility than sperm ejaculated with seminal vesicular fluid.
The prostate also contains smooth muscles that help expel the semen during ejaculation.  
The prostate is actually made up of loads of tiny little glands  that all empty individually into the urethra.
 

Bulbourethral glands

There are two of these that lie in the deep perineal pouch.
They secrete lubricant into the urethra, and also usually secrete just before ejaculation.