Human Anatomy Lesson 12
Lesson 12 - Pelvic Organs
The pelvis and perineum house organs associated with the gastrointestinal (GI), urinary, and reproductive systems. In this lesson, I will cover the organs and structures associated with these systems, and briefly consider the functions carried out by various structures.You will learn a lot more about each of these systems when you take physiology.
Learning Objectives - By the end of this lesson, you should be able to ...
- List and describe the organs of the GI tract, urinary tract, and reproductive system
- Enumerate differences between male and female organs
- Track the course of semen through the male reproductive system
- Track the implantation of an egg into the uterus and describe how spermatids reach the egg
- Distinguish between primary and accessory structures in the male and female reproductive systems.
The distal end of the GI tract is situated in the pelvis and pierces the perineum. The organs at the distal end of the GI tract are the rectum and anal canal.
- Rectum: continuous with the sigmoid colon above (at the level of the S3 vertebra) and the anal canal below. The rectum follows the concave contour of the sacrum, and the anorectal junction is "flexed" where the puborectalis muscle loops back upon itself around the rectum.The lower part off the rectum is called the rectal ampulla.
- Anal canal: begins at the terminal end of the rectal ampulla where it narrows at the pelvic floor, passes through the perineum, and terminates at the anus. The anal canal is surrounded by internal sphincters and external sphincters. The upper part of the anal canal is lined by anal sinuses, anal columns, and anal valves which, together, form a circle called the pectinate line, a structure which marks the approximate position of the anal membrane in the fetus. The anal pecten, made up of stratified squamous epithelium, is a transition zone inferior to the pectinate line, and it ends inferiorly at the white line, which marks contact between the anal pecten and skin.
The urinary system includes the kidneys, ureters, bladder, and urethra. The latter three organs are in the pelvis, at least in part, and form the distal end of the urinary tract.
- Ureters: pass from the renal pelvis of the kidneys through the thorax and abdomen and into the pelvis through the pelvic inlet. The ureters run anterior to the bifurcation of the common iliac artery to join the base of the bladder. The distal parts of the ureters are in the pelvic cavity.
- Bladder: the bladder is a sac that is shaped like a three-sided pyramid that is located in the pelvic cavity. When full, the bladder expands into the abdomen. The bladder is connected to the anterior body wall via the median umbilical ligament, and its base faces posteriorly and inferiorly. The ureters enter the bladder at the upper corners of the base. The neck of the bladder surrounds the origin of the urethra, and is anchored to the posterior aspect of the pubic bone by the pubovesical ligament in females and the puboprostatic ligament in males.
- Urethra: runs from the base of the bladder through the perineum. Its path differs significantly in males and females.
- In females, the urethra is about 4 cm long, and passes through the deep perineal pouch and perineal membrane, opening into the vestibule that lies between the labia minora. The urethral opening is anterior to the vaginal opening, and bound to the anterior surface of the vagina. Two small paraurethral glands (Skene's glands) open into the lower end of the urethra. Skene's glands release female ejaculate, and research suggests that variation in size and presence of Skene's glands may be related to differences in the ability of women to achieve orgasm.
- In males, the urethra is about 20 cm long, and has two bends (when the penis is flaccid). The male urethra is commonly split into 4 parts: (1) preprostatic part, which is at the neck of the bladder. This part of the urethra runs through the internal urethral sphincter, which is made up of smooth muscle. The (2) prostatic part runs through the prostate and leads to the (3) membranous part, which runs through the deep perineal pouch and perineal membrane. The external urethral sphincter in the deep perineal pouch surrounds the urethra. At this point, the urethra bends, and the bulbourethral duct from the bulbourethral gland disperses fluid into the urethra. The urethra then moves through the corpus spongiosum of the penis and is called the (4) spongy part, where there is a second bend, then terminates at the external urethral orifice.
Male Reproductive System
The male reproductive system is made up of the testis, epididymis, ductus deferens, ejaculatory duct, urethra, penis, and 3 accessory glands (prostate, seminal vesicles, bulbourethral glands).
- Testes: develop high in the abdominal wall and then descend through the inguinal canal in the anterior abdominal wall to reach the scrotum, which is an outpouching of the perineum. The ductus deferens (formerly known as the vas deferens) and spermatic cord are carried along with the testes as they descend during embryonic development. Each testis is covered on its anterior and lateral side by a tunica vaginalis, which is a closed sac of peritoneum that is the remnant of visceral and parietal peritoneum from the abdominal cavity. The tunica vaginalis covers seminiferous tubules, which are directly covered by tunica albuginea. The seminiferous tubules produce spermatozoa, and lead to straight tubules, which connect with rete testis, which gives off 12-20 efferent ductules.
- Epididymis: formed by the 12-20 efferent ductules, which pierce the tunica albuginea.
- Ductus deferens: transmits spermatozoa from the tail of the epididymis to the ejaculatory duct in the pelvic cavity. Spermatozoa produced in the seminiferous tubules therefore travels through the straight tubules, rete testis, efferent ductules, and epididymis to reach the ductus deferens. The ductus deferens runs through the spermatic cord, the inguinal canal, bends medially around the lateral side of the inferior epigastric artery, crosses the external iliac artery and vein, and descends medially on the pelvic wall deep to the peritoneum, crosses the ureter posterior to the bladder, and joins the seminal vesicle to form the ejaculatory duct, which penetrates the prostate to connect to the prostatic urethra.
- Seminal vesicle: secretes fluid which contributes to the volume of the semen.
- Prostate: contributes to the formation of semen during ejaculation. The prostate is surrounded by the bladder, rectum, and pubic symphysis.
- Bulbourethral glands: located in the deep perineal pouch. These glands open into the bulb of the spongy urethra at the root of the penis, and contribute to lubrication of the urethra and pre-ejaculatory emission from the penis.
Female Reproductive System
The main components of the female reproductive system are the ovaries, uterus, vagina, clitoris, and greater vestibular glands.
- Ovaries: like testes, the ovaries develop high in the abdominal wall and then descend. They do not migrate into the perineum, but instead assume a position on the lateral wall of the pelvic cavity. Mature eggs are ovulated into the peritoneal cavity, and normally into the uterine tubes. The ovaries are suspended by mesovarium, which is a posterior extension of broad ligament.
- Uterus: positioned between the bladder and rectum. The uterus is composed of a body (with fundus) and cervix, which joins the vagina inferiorly. Uterine (Fallopian) tubes connect the body of the uterus to the ovaries. The uterus and cervix are angled relative to the vagina, so that the cervix protrudes into the vagina.
- Uterine tubes: composed of fimbriae, infundibulum, ampulla, and isthmus, working from distal to proximal. Eggs released into the uterine tube are usually fertilized in the ampulla before migrating into the body of the uterus.
- Vagina: fibromuscular tube that extends from the perineum through the pelvic floor. Semen is deposited into the vaginal vault.
- Broad ligament: different sections of the broad ligament support the female reproductive organs. As noted previously, the mesovarium supports the ovaries; the round ligament attaches the uterus to the body wall; mesometrium supports the uterus; and mesosalpinx supports the uterine tube.
- Human Anatomy Lesson 13
This lesson covers the superficial tissues of the head, including the muscles of facial expression and the nerves, arteries, veins, and glands.
- Human Anatomy Lesson 14
This lesson covers the skull, which is the bone that surrounds the sensory organs in the head.
- Human Anatomy Lesson 15
This lesson covers the 12 cranial nerves, including their origin, route of passage through the skull base, and the things that they innervate.
- Human Anatomy Lesson 16
This lesson covers the brain and its meninges and vasculature.
- Human Anatomy Lesson 17
This lesson covers the temperomandibular joint and the structures in the temporal and infratemporal fossae.
- Human Anatomy Lesson 18
This lesson covers the anatomy of the orbit, eye, and ear. It is the 18th lesson in a comprehensive undergraduate Human Anatomy course.
© 2014 Robert McCarthy