Uterus

The uterus is the organ in which the fertilized ovum normally becomes embedded and in which the developing organism grows and is nourished until its birth. The cavity of the uterus and that of the vagina below it together form the “birth canal”, through which the fetus passes at the end of its period of gestation. The uterine tubes open into the upper part of the uterine tubes of the uterine cavity.

The Greek word for uterus is hystera. Its combining form, hyster-, is used in words such as hysterogram and hysterectomy.

The uterus varies in shape, size, location, and structure. These variations are dependent upon age and upon other circumstances, such as pregnancy.

In the nulliparous woman, the walls of the uterus are thick and muscular. The entire organ is shaped like an inverted pear, and its narrow end, which is directed downward and backward, forms an angle of slightly more than 90 degrees with the vagina ( angle of anteversion ). The uterus lies within the pelvis, and its long axis is approximately in the axis of the upper pelvic aperture. It does not usually lie exactly in the median plane, but is inclined to one side or the other, usually to the right. Commonly it is also slightly twisted. Its position is not fixed, however, and readily changes with the degree of fullness of the bladder, which is below and in front, and with the degree of fullness of the bladder, which is below and in front, and with the degree of fullness of the rectum, which is above and behind. The uterus is about seven and a half centimeters long, five centimeters wide in its upper part, and two and a half centimeters thick. It is subdivided into a fundus, a body, an isthmus, and a cervix.

   

Parts and Relations

The fundus is the rounded part of the uterus that lies above and in front of the plane of the openings of the uterine tubes. The body is the main part of the uterus, and it extends downward and backward to a constriction, the isthmus. It can be palpated bimanually. It has two surfaces, and two borders, or margins. The vesical surface is separated from the urinary bladder in front and below by the uterovesical pouch. The intestinal surface is separated from the sigmoid colon above and behind by the rectouterine pouch, which usually contains some coils of the ileum. The left and right margins are related to the respective broad ligaments, and to the structures contained between the two layers of each ligament.

The isthmus is the constricted part of the uterus and is about 1 cm or less in length. During pregnancy, it becomes taken up by the body and is therefore often referred to by obstetricians as the “lower uterine segment”. The fetal membranes, however, do not usually become firmly attached to it. It resembles the body histologically but shows some differences in its musculature, epithelium, and number of glands. The changes that it undergoes during menstruation are not as marked as those in the body.

 

The cervix extends downward and backward from the isthmus to the opening within the vagina. It is the least freely movable part of the uterus and is divided into two parts by the anterior wall of the vagina, through which it passes. The supravaginal part is separated from the urinary bladder in front by loose connective tissue, and from the rectum behind by the recto-uterine pouch. It is related laterally to the ureter and uterine artery. The vaginal part extends into the vagina. Its cavity communicates with that of the vagina by means of the ostium of the uterus ( formerly called external os or orifice ). This opening is a short depressed slit in the nillipara, but in women who have borne children it is larger and more irregular in outline. The ostium has anterior and posterior lips, which usually reach the posterior wall of the vagina.

The cavity of the uterus is wide above at the entrance of the uterine tubes, but it gradually decreases in width as it extends downward to the isthmus. It is very narrow in sagittal section, because the anterior and posterior walls are almost in contact.

The canal of the cervix is narrower at its ends than in its middle. A vertical fold located on its anterior wall and another on its posterior wall. Palmate folds radiate obliquely from these in such a way that those on the anterior wall do not oppose those on the posterior wall. Instead, they fit each other so as to close the canal. They tend to disappear after pregnancy. The cavity of the uterus and the canal of the cervix can be viewed radiographically after the introduction through the vagina of a suitable radiopaque material (hysterosalpingography).


Position

In the adult, the entire uterus is usually anteverted. In this position, it extends forward and upward from the upper end of the vagina at an angle of about 90 degrees. The uterus is generally anteflexed also, that is, the body is bent downward at its junction with the isthmus. These positions are readily altered, especially during distention of the urinary bladder or the intestine. When the bladder is full, the uterus extends upward and backward (retroversion).

   

Attachments and Peritoneal Relations

The uterus gains much of its support by its direct attachment to the vagina. Indirect attachment to nearby structures, such as the rectum, urinary bladder, pelvic diaphragm, and bony pelvis, also help to support it.

The peritoneum is reflected from the posterior aspect of the bladder to the isthmus of the uterus and the passes upward on the vesical surface of the body. This reflection forms the uterovesical pouch. After passing around the fundus of the uterus, the peritoneum passes downward on the intestinal surface of the body, and on the back of the cervix and the upper part of the vagina, from which it is reflected onto the front of the rectum. The recess formed by this reflection is the rectouterine pouch.

The broad ligament is formed at the lateral margin of the uterus by the two layers of peritoneum that cover the vesical and intestinal surfaces. It extends to the lateral wall of the pelvis. The two layers are continuous with each other above, where they enclose they the uterine tube. They are close to each other near the uterus, but they diverge laterally and below. The anterior layer passes forward to become continuous with the peritoneum covering the floor and lateral wall of the pelvis. The posterior layer extends backward from the cervix of the uterus as the rectouterine fold. This fold forms the lateral boundary of the rectouterine pouch, and , after passing along the side of the rectum, reaches the posterior wall of the pelvis. The plane of the broad ligament varies with the position of the uterus.

The mesosalpinx is the part of the broad ligament between the uterine tube and the line along which the broad ligament is drawn out to form the mesovarium. In addition to branches of the ovarian and uterine vessels, it contains two structures, called the epoophoron and the paraoophoron. The mesometrium is the part of the broad ligament below the mesosalpinx and mesovarium.

The epoophoron consists of a duct, which runs parallel to and below the tube, and tubules, which run upward from the region of the ovary to join the duct at a right angle. It is the remains of a part of the mesonephric duct and some of its tubules. The paraoophoron lies medial to the epoophoron and is a group of very small tubules. It usually cannot be recognized grossly in the adult. Both structures are important only in that cysts sometimes arise in them.

The broad ligament encloses between its two layers some loose connective tissue and smooth muscle, collectively called the parametrium. Where the two layers are close together ( near the uterus and near the uterine tube ), the parametrium is not abundant, but laterally and below, where the layers diverge, it increases in amount. The broad ligament also encloses the uterine tube, the ovarian ligament, part of the round ligament, the uterine artery and vein, the uterovaginal plexus of nerves, and a part of the ureter.

The round ligament is a narrow, flat band of fibrous tissue that is attached to the uterus just below and in front of the entrance of the uterine tube. It contains some smooth muscle near this attachment. After passing laterally and forward across the umbilical artery and external iliac vessels, it hooks around the inferior epigastric artery. It then traverses the inguinal canal and becomes lost in the subcutaneous tissue of the labium majus. In the fetus, a tubular process of peritoneum, the processus vaginalis peritonei, accompanies the round ligament into the inguinal canal. This prolongation occasionally remains in the adult.

The visceral pelvic fascia at the side of the cervix and vagina is considerably thickened and contains numerous smooth muscle fibres. Part of this thickening passes laterally to merge with the upper fascia of the pelvic diaphragm and is called the lateral (or transverse) cervical cardinal ligament. The uterine artery runs on its upper aspect. The rest of this thickening passes backward in the recto-uterine fold and is attached to the front of the sacrum. This is the uterosacral ligament, and it can be palpated per rectum.

   

Changes with age

At birth, the uterus reaches above the level of the pelvic inlet. The cervix is larger than the body, and the palmate folds extend into the upper part of the uterine cavity. The difference between the axis of the uterus and that of the vagina is relatively small. The growth of the uterus is slow until puberty, when it grows rapidly until its adult size and shape are reached. After the menopause, the uterus becomes smaller, more fibrous, and paler in colour.

   

changes during pregnancy and post-parturition

The size of the uterus increases tremendously during pregnancy. The fundus rises above the level of the pubic symphysis in the third month. It reaches the supracristal plane in the sixth month and the level of the xiphisternal joint in the eighth month. It descends slightly in the ninth month, when the maximal circumference of the fetal head becomes engaged below the pelvic inlet. During this increase in size of the uterus, there is also a large increase in its weight, and the walls of the uterus become thinner.

After parturition, the uterus undergoes a process called involution. It gradually becomes reduced in size and weight until, after six to eight weeks, it reaches its resting state, in which it is about 1 cm larger in all dimensions than it was before pregnancy. It is also slightly heavier, its cavity is somewhat larger, and the lips of the opening into the vagina are irregular in outline.

   

Structure

The uterus has three layers, a mucosa, a muscular coat, and a serosa, details of which are covered in histology.

   

Blood supply

The uterine arteries provide the main blood supply to the uterus. Each artery passes medially on the upper aspect of the lateral cervical ligament. As it approaches the cervix, it gives off a branch the supplies the cervix and the upper part of the vagina and it then turn upward to run between the layers of the broad ligament, near the lateral margins of the body. As it ascends, it sends branches to both surfaces of the body. The uterine arteries become greatly enlarged during pregnancy and are tortuous after parturition.

The blood is returned from the uterus by way of a venous plexus that follows the uterine artery. An important anastomisis between the portal and systemic venous systems is formed by veins that run below the rectouterine pouch and connect the uterine venous plexus with the superior rectal vein.

   

Lymphatic drainage

The lymphatic vessels from the fundus and upper part of the body drain into lumbar (or aortic) nodes, those from the lower part of the body into the external iliac nodes, and those from the cervix into the external iliac, internal iliac, and sacral nodes. Some vessels from the region of the uterus near the entrance of the uterine tube pas with the round ligament and drain into the superficial inguinal nodes.

   

Nerve supply

The uterus receives autonomic and sensory fibres by way of the uterovaginal plexuses, which run along the uterine arteries. The uterus is painless to most stimuli, but pain may be felt when the cervix is grasped with a forceps or is dilated. Some uterine disorders are painful, however, and pelvic pain may be felt in some phases of the menstrual cycle. There is some evidence that fibres concerned ascend and enter the spinal cord by way of the lumbar splanchnic nerves. Resection of the superior hypogastric plexus has been performed to alleviate severe pain of this kind.


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Chapter 36: Female Reproductive System and the Contents of the Female Pelvis