The Reproductive System

The human reproductive system uses organs and hormones to produce offspring. It supports reproduction and helps ensure the survival of the species. In humans, this system includes the gonads, accessory reproductive organs, and hormone signals that work together.

The gonads are the primary reproductive organs. In males, the testes produce spermatozoa and testosterone. Testosterone helps develop secondary male sex characteristics, including a deeper voice, larger muscles, facial and body hair...

The testes are the male gonads, also called testicles. They sit in the scrotum, a sac-like structure below the penis. Their location helps explain how the body protects them and keeps them at the right temperature for sperm production.

Each testis is wrapped by the tunica albuginea, a dense connective tissue layer. This layer gives the organ structure and protection. Outside it is the tunica vaginalis, a serous membrane that reduces friction with nearby structures.

The scrotum has its own...

The testes are built for sperm production and hormone release. A tough fibrous layer called the tunica albuginea covers each testis. It also sends septa, or fibrous partitions, inward to divide the tissue into lobules.

Each lobule contains 1 to 3 tightly coiled seminiferous tubules, where sperm are made. These tubules join into a network at the back of the testis called the rete testis. From there, the pathway continues through 15 to 20 efferent ductules to the epididymis.

Cells along the...

Sperm cells form in the seminiferous tubules of the testes through spermatogenesis. This process starts from undifferentiated stem cells and produces mature sperm cells that can fertilize an egg. It is essential for making functional sperm.

Spermatogenesis has three main stages: mitosis, meiosis, and spermiogenesis. During mitosis, spermatogonia, or stem cells, divide into two identical daughter cells called type A and type B spermatogonia. Type A spermatogonium stays in contact with the...

The sperm pathway in the male reproductive system includes the epididymis, vasa deferens, ejaculatory ducts, and urethra. These accessory ducts help sperm mature, move, and leave the body during ejaculation. They also help recycle damaged sperm and absorb fluid made by the seminiferous tubules.

The epididymis is a small, comma-shaped organ at the back of each testicle. It is about 4 cm long, but its tightly coiled tube is about 7 meters long when unraveled. The epididymis has three parts: the...

The penis is an external male reproductive organ that also helps with urination. It has three main regions: the glans penis, the body, and the root. Each region has a specific structure and role.

The glans penis, or head, is the end of the penis. It contains the external urethral orifice, which is the opening for urine and semen to leave the body. The glans is covered by the prepuce, or foreskin, and is especially sensitive.

The body of the penis, also called the shaft, extends from the glans.

Sperm move through a series of tubes from the testis to the urethra before ejaculation. Their journey begins in the seminiferous tubules, where Sertoli cells make fluid that helps push non-motile sperm forward. The sperm then pass through the straight tubules and into the rete testis, which serves as the first connected pathway in this transport route.

From the rete testis, sperm travel into the efferent ductules and then into the epididymis. The epididymis is where sperm mature. During this...

Sperm travel through a series of male reproductive ducts before they leave the body. These ducts include the epididymides, vasa deferentia, ejaculatory ducts, and urethra. They help sperm mature, store, and move from the testes to the urethra for ejaculation.

The epididymis is a small, comma-shaped organ at the back of each testicle. It has three parts: the head, body, and tail. The head connects to the efferent ductules of the testis, which carry sperm away from the testes.

The epididymis is...

Sperm move through semen, a fluid that supports their travel during ejaculation. A typical ejaculation releases about 2 to 5 milliliters of semen. This fluid contains mature sperm and secretions from accessory glands.

A mature sperm cell is about 60 micrometers long and has four main parts: the head, neck, midpiece, and tail. The head is flattened and tapered, and it is about 4 to 5 micrometers long. It contains a nucleus with condensed chromosomes and an acrosome, which is a cap-like...

Erection and semen release are part of the male sexual response. Sexual stimulation can come from physical touch, sight, or sound. These signals activate a parasympathetic reflex in the sacral region of the spinal cord. The reflex causes nitric oxide, or NO, to be released. NO widens the arterioles in the penis and increases blood flow to the erectile tissues, the corpora cavernosa and corpus spongiosum.

As these tissues fill with blood, the veins are compressed. This helps keep blood from...

Testosterone and male reproductive control depend on a hormone signal from the brain. The hypothalamus releases gonadotropin-releasing hormone, or GnRH. GnRH tells the pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

LH acts on Leydig cells in the testes. These cells make and release testosterone. At the same time, FSH works on Sertoli cells in the seminiferous tubules. Sertoli cells release androgen-binding protein, or ABP, which binds testosterone...

The vulva includes the external structures of the female reproductive system. These parts sit outside the vagina and form the outer opening area. The main structures include the mons pubis, labia, clitoris, hymen, vaginal opening, and Bartholin's glands.

The mons pubis is a cushion of fatty tissue that lies over the pubic bone. After puberty, this area usually develops hair. Behind it are the labia majora, which are the larger skin folds. They are usually covered with hair.

Inside the labia...

The vaginal canal is a muscular tube about 10 cm long. It serves as the entryway to the female reproductive system. It also allows menstrual flow and childbirth to pass through.

The inner wall of the vagina has folds called rugae. The top end is the fornix, which connects with the protruding cervical part of the uterus. The canal has three layers: an external fibrous layer, a middle muscular layer, and an inner lining with mucosal rugae. These features help the vagina stretch during sexual...

The ovaries are paired organs in the pelvic region. They are about the size of almonds and measure about 2 to 3 centimeters long. Each ovary is supported by the mesovarium, a peritoneal extension that also links it to the broad ligament. The suspensory ligament adds support and carries blood and lymphatic vessels. The ovarian ligament connects each ovary to the uterus.

The outer surface of the ovary has a layer of cuboidal cells. This layer forms a protective epithelium. Beneath it is the...

The ovarian cycle repeats each month and drives key changes in the menstrual cycle. It has two main phases: the follicular phase and the luteal phase. Ovulation, which is triggered by a surge in luteinizing hormone (LH), separates these phases.

The follicular phase is the more variable part of the cycle and averages about 14 days. A common cycle length is 28 days, but it can range from 21 to 40 days. Most of that variation comes from differences in the follicular phase and the timing of...

Oogenesis is the process of egg cell formation in the ovaries. These female gametes are essential for female fertility. The sequence starts during fetal development, when diploid oogonia in the developing ovaries divide by mitosis to form primary oocytes.

By birth, the primary oocytes have entered prophase I of meiosis. They then stop, or arrest, at this stage until puberty. Each primary oocyte is surrounded by pre-granulosa cells, which form a primordial follicle. By puberty, the ovaries...

Ovarian follicle development is the process that prepares an egg for ovulation in the ovary. It takes place in the ovarian cortex, where oogenesis, or egg development, occurs. This process is important for female reproductive health and begins before birth.

During fetal development, primordial follicles form. Each primordial follicle contains a primary oocyte in the center and a single layer of flat, squamous pre-granulosa cells around it. These follicles stay dormant in late prophase I of...

The ovarian cycle is controlled by hormones from the hypothalamus, pituitary gland, and ovaries. This hormone network is called the hypothalamic-pituitary-gonadal axis. It helps the body release a mature oocyte, or egg cell, for reproduction.

Before puberty, the hypothalamus releases gonadotropin-releasing hormone (GnRH) in low-frequency, low-amplitude pulses. Because the hypothalamic-pituitary-gonadal axis is still immature, estrogen levels stay low and a fully functional ovarian cycle does...

The uterine tubes, also called fallopian tubes, move the oocyte from the ovary to the uterus. Each tube is about 10 to 13 cm long. It is divided into the infundibulum, ampulla, isthmus, and interstitial part, which is also called the intramural segment.

The infundibulum has a funnel shape. It ends in finger-like fimbriae that reach toward the peritoneal cavity. During ovulation, these fimbriae extend over the ovaries and help capture the released oocyte.

The ampulla makes up more than half of...

The uterus and cervix are key parts of the female reproductive system. The uterus, also called the womb, is built to support implantation and the growth of an embryo. It is shaped like a hollow pear and sits between the urinary bladder and the rectum.

The uterus is held in place inside the pelvic cavity by paired broad ligaments on each side. It is also stabilized by three pairs of suspensory ligaments: the cardinal, uterosacral, and round ligaments. These structures help keep the uterus...

The uterus has three main histological layers: the perimetrium, myometrium, and endometrium. Each layer has a different structure and role in the pelvic cavity. Together, they make up the uterine wall.

The outermost layer is the perimetrium. It is a thin serous membrane. On the sides, it is connected with the broad ligament, which helps anchor the uterus in the pelvic cavity.

The thickest layer is the myometrium. It is made mostly of bundles of smooth muscle tissue. These muscle contractions...

The menstrual cycle is a monthly set of changes in the uterine endometrium, especially its functional layer called the stratum functionalis. It prepares the uterus for a possible pregnancy. In most cases, the cycle lasts about 21 to 35 days and averages 28 days. It also matches the ovarian cycle and is controlled by changing levels of the ovarian hormones estrogen and progesterone.

The first stage is the menstrual phase, which usually occurs from days 1 to 5. During this time, the stratum...

The menstrual phase is the first stage of the uterine cycle. It is counted as day one of the cycle and usually lasts about five days. During this time, the uterine lining sheds its functional layer, called the stratum functionalis.

If fertilization does not occur, the corpus luteum breaks down. This causes estrogen and progesterone levels to drop sharply. The hormone decrease leads to prostaglandin release, which makes the uterine spiral arterioles constrict.

When these small arteries narrow,...

The proliferative phase is the time when the endometrium rebuilds after menstruation. In a typical 28-day cycle, it lasts about 6 to 13 days. During this stage, estrogen from the developing ovarian follicle guides the repair and thickening of the uterine lining.

The stratum basale, or basal layer of the endometrium, is not affected by menstruation. This layer includes the basal parts of the uterine glands. Stem cells in the stratum basale divide by mitosis and regenerate the stratum...

The secretory phase is the part of the menstrual cycle when the uterus prepares for a possible embryo implantation. It usually runs from day 14 to day 28 in a typical 28-day cycle. This phase begins right after ovulation, when an egg has been released.

After ovulation, the corpus luteum forms. The corpus luteum is a temporary endocrine structure, which means it makes hormones for a short time. It releases progesterone and estrogens. These hormones cause the endometrial glands to grow and coil,...

Male reproductive health problems include testicular cancer, prostate disorders, and erectile dysfunction. These conditions can affect cancer risk, urination, and sexual function. Recognizing common signs can help with earlier medical care.

Testicular cancer is especially common in younger men, especially those ages 20 to 35. It often appears as a painless mass in the testicles. Some people may also notice a feeling of heaviness or a dull ache.

Prostate disorders are another major concern...

Female reproductive disorders include PMS, PMDD, endometriosis, and several cancers. These conditions can cause pain, bleeding changes, mood symptoms, and other health problems. Some are cyclical, while others can develop over time and need medical testing.

Premenstrual syndrome, or PMS, and premenstrual dysphoric disorder, or PMDD, are cyclical conditions that affect the days before menstruation. They can cause physical and emotional distress. Common symptoms include edema, mood swings, and...

The menstrual cycle is controlled by GnRH, FSH, LH, estrogen, and progesterone. GnRH, or gonadotrophin-releasing hormone, starts a pulsatile release pattern at puberty. This pattern tells the anterior pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

GnRH pulses change in speed and strength across the cycle. Faster pulses favor LH release, while slower pulses favor FSH release. FSH acts on granulosa cells in ovarian follicles and helps them multiply.

The female breast contains mammary glands that produce milk. It lies in front of the pectoralis major and serratus anterior muscles. A dense connective tissue layer, called fascia, links the breast to these muscles and helps support it.

The nipple is the pigmented projection where milk leaves the breast. Tiny openings in the nipple connect to lactiferous ducts, which carry milk outward. The areola is the darker circle of skin around the nipple. It looks rough because it contains modified...

Embryonic sex differentiation begins with a bipotential stage. At this point, the early embryo can form either male or female reproductive organs. During the fifth week of development, gonadal ridges grow from the intermediate mesoderm. Nearby, two duct systems are present: the mesonephric ducts, also called Wolffian ducts, and the paramesonephric ducts, also called Müllerian ducts.

These ducts form the starting point for the male and female reproductive systems. Both connect distally to the...

Puberty changes the body through hormone signals that prepare the body for reproduction. It usually begins between ages 8 and 13 in girls and 9 and 14 in boys, although genetics, environment, and overall health can affect timing. During this stage, secondary sexual characteristics develop and reproductive potential is reached.

Puberty starts with pulses of gonadotropin-releasing hormone, or GnRH, from the brain. These pulses trigger rises in luteinizing hormone (LH) and follicle-stimulating...

In vitro fertilization (IVF) is an assisted reproductive technology that helps eggs and sperm meet in a lab before an embryo is placed in the uterus. It is used for people and couples who are having trouble conceiving. The process begins with ovarian stimulation, which uses hormone medicines from a reproductive endocrinologist to help the ovaries produce multiple eggs instead of the single egg made in a natural cycle.

After the ovaries are stimulated, the next step is oocyte retrieval, or egg...

Hormonal changes after the reproductive years affect both women and men. In women, menopause marks the end of fertility and usually occurs between the fifth and sixth decade of life. In men, reproductive function declines more gradually, so fertility often lasts much longer.

In menopause, the ovarian follicle pool is exhausted. The ovaries become less responsive even when follicle stimulating hormone, or FSH, and luteinizing hormone, or LH, are high. Estrogen production then drops, which helps...

Contraceptives prevent pregnancy through surgery, hormones, barrier devices, and cycle tracking. Some methods stop sperm and egg from meeting, while others change ovulation or the uterine environment. The best choice depends on how the method works and how long it is meant to last.

Vasectomy is a surgical form of male sterilization. In this procedure, the vasa deferentia are cut and sealed so sperm cannot mix with semen during ejaculation. Vasectomy does not affect the testes’ ability to make...

Male fertility problems can come from hormone issues, sperm production problems, blocked sperm transport, or an unknown cause. These problems affect different steps in the reproductive process and can make it harder to achieve pregnancy.

Hormone balance is one major factor. Endocrine imbalance can happen with hypogonadism, Klinefelter syndrome, or pituitary disorders. These conditions can change hormone levels and lower sperm production.

Some causes involve the testes themselves. Testicular...

Female infertility is the inability to conceive after a year of regular, unprotected intercourse. It affects about 10–15% of couples worldwide. In many cases, the main problem is an ovulatory disorder, which keeps the ovary from releasing an egg.

Ovulatory disorders can take several forms. These include hypothalamic amenorrhea, polycystic ovarian syndrome (PCOS), premature ovarian failure, and hyperprolactinemic anovulation disorders. Hormonal imbalance can also play a role, especially when...

Sexually transmitted infections are caused by bacteria, viruses, or parasites and spread mainly through unsafe sexual contact. If they are not treated, they can lead to serious health problems. Many STIs also have few or no early symptoms, so infections can go unnoticed.

Chlamydia is caused by the bacterium Chlamydia trachomatis and is the most common STI in the United States. It lives inside human cells to reproduce. In men, it can cause urethritis, which may lead to discharge, burning during...