The posterior male urethra forms from the urogenital sinus (see the image below). This sinus derives from the endoderm-derived cloaca, which is separated from the anorectal canal by the growth of the urorectal septum in the fourth week of gestation. The spongy urethra is formed after the seventh week by tubularization of the urethral folds along the urethral groove under the influence of dihydrotestosterone. The most distal portion of the urethra is likely formed by invagination of an epithelial tag at the distal end of the genital tubercle. [1]
Embryologic development of pendulous urethra.
The male urethra originates at the bladder neck and terminates at the urethral meatus on the glans penis. It is roughly 15-25 cm long in the adult and forms an "S" curve when viewed from a median sagittal plane in an upright, flaccid position (see the image below). The male urethra is often divided into 3 segments on the basis of its investing structures: prostatic urethra, membranous urethra, and spongy (or penile) urethra.
Male urethra and its segments.
Other systems for naming the parts of the urethra have been described. The urethra can be separated simply into anterior and posterior urethra. The spongy urethra can be subdivided into fossa navicularis, pendulous urethra, and bulbous (bulbar) urethra. Finally, the bladder neck, or preprostatic urethra, can be classified as a distinct part of the urethra (see the image below).
Posterior wall of urethra.
The prostatic urethra is the portion of the urethra that traverses the prostate. It originates in the region of the bladder neck, courses roughly 2.5 cm inferiorly, and terminates at the membranous urethra. It lies in a retropubic location and is bordered superiorly by the bladder and supported inferiorly by the sphincter urethrae externus muscle and the perineal membrane (formerly called the urogenital diaphragm). It is invested in the prostate, a glandular and fibrostromal organ that secretes seminal fluids and has clinical relevance.
The urethra runs through the prostate eccentrically, with most of the prostatic tissue in a posterior and inferior location. The prostatic urethra is surrounded by an inner circular layer and an outer longitudinal layer of smooth muscle. The urethra forms an angle of roughly 45º (range, 0-90º) at the midpoint of the prostatic urethra. The segment proximal to this location is surrounded by the involuntary internal sphincter. It is also the area most commonly affected by benign prostatic hyperplasia (BPH).
The posterior wall of the prostatic urethra contains the urethral crest, which is bordered laterally by prostatic sinuses, into which the prostatic glands drain. The most prominent aspect of this crest is the seminal colliculus, or verumontanum, where the paired ejaculatory ducts and the opening of the prostatic utricle (a small midline paramesonephric duct remnant) meet the lumen of the urethra. The seminal colliculus has no functional significance but is a crucial landmark in urethroscopy and transurethral surgery.
The shortest and least distensible portion of the urethra is the membranous urethra. This region spans from the apex of the prostate to the bulb of the penis. It is invested in the external urethral sphincter muscle and the perineal membrane.
The external sphincter is related anteriorly to the dorsal venous complex and is connected to the puboprostatic ligaments and the suspensory ligament of the penis. The external urethral sphincter muscle and the perineal membrane fix the urethra firmly to the ischial rami and inferior pubic rami, rendering this portion of the urethra susceptible to disruption with pelvic fracture.
The spongy urethra is the region that spans the corpus spongiosum of the penis. It is divided into the pendulous urethra and the bulbous (or bulbar) urethra. The pendulous urethra is invested in the corpus spongiosum of the penis in the pendulous portion of the penis. The urethra is located concentrically within the corpus spongiosum.
In the distal urethra lies the fossa navicularis, a small dilation of the urethra just proximal to the urethral meatus. The meatus is a slitlike orifice with its long axis in a midline sagittal plane. The urethral meatus is slightly ventral to the tip of the penis. The bulbous urethra is invested in the bulb of the penis, the portion of corpus spongiosum that lies between the split corpora cavernosa in the superficial perineal space.
Bulbourethral (Cowper) glands, a male homologue of the greater vestibular (Bartholin) glands, originate in the external urethral sphincter muscle but terminate in ducts that empty into the bulbous urethra. The spongy urethra lies closer to the dorsum of the penis in the bulb.
The prostatic urethra is supplied by the inferior vesical artery, which branches to penetrate the prostate and the bladder neck in superolateral positions. The bulbourethral artery supplies the membranous and bulbar urethra, whereas the pendulous urethra is supplied by the deep penile artery, a branch of the internal pudendal artery. In general, venous drainage mirrors the arterial supply. The prostatic and membranous urethra drain to the obturator and internal iliac nodes.
Lymphatic drainage from the spongy urethra drains to the deep and superficial inguinal nodes. [2]
The urinary system's function is to filter blood and create urine as a waste by-product. The organs of the urinary system include the kidneys, renal pelvis, ureters, bladder and urethra.
The body takes nutrients from food and converts them to energy. After the body has taken the food components that it needs, waste products are left behind in the bowel and in the blood.
The kidney and urinary systems help the body to eliminate liquid waste called urea, and to keep chemicals, such as potassium and sodium, and water in balance. Urea is produced when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. Urea is carried in the bloodstream to the kidneys, where it is removed along with water and other wastes in the form of urine.
Other important functions of the kidneys include blood pressure regulation and the production of erythropoietin, which controls red blood cell production in the bone marrow. Kidneys also regulate the acid-base balance and conserve fluids.
Kidney and urinary system parts and their functions
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Two kidneys. This pair of purplish-brown organs is located below the ribs toward the middle of the back. Their function is to:
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Remove waste products and drugs from the body
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Balance the body's fluids
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Release hormones to regulate blood pressure
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Control production of red blood cells
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The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries, called a glomerulus, and a small tube called a renal tubule. Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney.
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Two ureters. These narrow tubes carry urine from the kidneys to the bladder. Muscles in the ureter walls continually tighten and relax forcing urine downward, away from the kidneys. If urine backs up, or is allowed to stand still, a kidney infection can develop. About every 10 to 15 seconds, small amounts of urine are emptied into the bladder from the ureters.
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Bladder. This triangle-shaped, hollow organ is located in the lower abdomen. It is held in place by ligaments that are attached to other organs and the pelvic bones. The bladder's walls relax and expand to store urine, and contract and flatten to empty urine through the urethra. The typical healthy adult bladder can store up to two cups of urine for two to five hours.
Upon examination, specific "landmarks" are used to describe the location of any irregularities in the bladder. These are:
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Trigone: a triangle-shaped region near the junction of the urethra and the bladder
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Right and left lateral walls: walls on either side of the trigone
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Posterior wall: back wall
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Dome: roof of the bladder
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Two sphincter muscles. These circular muscles help keep urine from leaking by closing tightly like a rubber band around the opening of the bladder.
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Nerves in the bladder. The nerves alert a person when it is time to urinate, or empty the bladder.
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Urethra. This tube allows urine to pass outside the body. The brain signals the bladder muscles to tighten, which squeezes urine out of the bladder. At the same time, the brain signals the sphincter muscles to relax to let urine exit the bladder through the urethra. When all the signals occur in the correct order, normal urination occurs.
Facts about urine
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Normal, healthy urine is a pale straw or transparent yellow color.
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Darker yellow or honey colored urine means you need more water.
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A darker, brownish color may indicate a liver problem or severe dehydration.
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Pinkish or red urine may mean blood in the urine.