human anatomy
The reproductive system
reproductive system
This system enables humans to reproduce and bear offspring. The ovum and sperm are the human reproductive cells.
The Female Reproductive System
The female reproductive system consists of both internal and external genitalia. The main components of the external system are the mons pubis, labia majora, labia minora, clitoris, bulbs of the vestibule, vulva vestibule, Bartholin’s glands, paraurethral glands, urethra, and vaginal orifice. The internal genitalia are located inside the pelvic cavity, and they consist of the vagina, uterus, ovaries and Fallopian tubes.
The vulva is the term that describes all of the structures that make up the female external genitalia. They are held by the perineum, which is the area of the body between the thighs, below the diaphragm. The mons pubis is a mass of subcutaneous adipose tissue shaped like an inverted triangle; it lies in front of the pubis symphysis, which is the secondary cartilaginous joint located between the left and right pubic bones.
The labia major are a pair of longitudinal skin folds covered with pubic hair that run from the mons pubis to the anus. The labia minora, meanwhile, are a pair of longitudinal skin folds that are thin and hairless. They are found between the labia major and enclose the vestibule. The labia minora function as protective structures, surrounding the clitoris and clitoral hood, urethral orifice, and vaginal orifice.
The clitoris is formed of erectile corpora cavernous tissue, which becomes engorged with blood when sexually stimulated. The vestibule contains the vaginal orifice, which is the opening of the female urethra, and the openings for the excretory ducts of the bartholin’s glands and paraurethral glands. The two bartholin’s glands are found on each side of the vestibule. They secrete lubricating mucus when a woman is sexually aroused. The paraurethral glands (also called Skene’s glands) are located on either side of the urethra. They secrete an antimicrobial substance that lubricates the urethral orifice. The bulbs of the vestibule are a pair of subcutaneous erectile soft tissues. They extend on each side of the vestibule and join in front of the urethral orifices.
The main component of the female’s internal genitalia is the vagina, which is an elastic, muscular tube with a soft, flexible lining. Various nerves provide sensation to the vagina, and glandular secretions help lubricate it. As the channel for childbirth, the vagina connects the uterus to the outside. The upper end of the vagina is attached to the cervix, which is the entrance to the uterus, while the vulva and labia form the entrance to the vagina. The vaginal orifice is partially covered with a membrane called the hymen, which can rupture during the first sexual encounter or due to trauma or injury.
The uterus is a thick-walled, hollow muscular organ located deep within the pelvic cavity that can expand to accommodate a growing fetus. It is connected at either side to a Fallopian tube, and the bottom is connected to the cervix, which protrudes into the vagina. The uterus is held in position within the pelvis by a series of ligaments: the broad ligament, round ligament, lateral ligament (or cardinal ligament), uterosacral ligament, and pubocervical ligament.
The Fallopian tubes (or uterine tubes) are paired J-shaped tubes that extend laterally from the uterus and open into the abdominal cavity near the ovaries. This distal part of each Fallopian tube is called the infundibulum, and it has fingerlike projections called fimbriae that extend over the ovaries. The ampullae are the longest and widest part of the Fallopian tubes, and they are where fertilization normally occurs. The isthmuses are the narrowest part of the Fallopian tubes, and their intramural sections communicate directly with the uterine cavity via the uterine Ostia.
The ovaries are bilateral gonads, and they release the ova (or eggs). They also have an endocrine function, secreting hormones. They are located in he ovarian fossa, adjacent to the uterus and below the Fallopian tubes.
The human ovary is not directly connected to its adjacent Fallopian tube. The fimbriae are an edge of tissue around the ostium (i.e., opening) of the Fallopian tubes. Only the longest fimbria connects a fallopian tube’s infundibulum to the relevant ovary. It is called the fimbria ovarica.
The pelvic floor muscles run from the coccyx 9or tailbone) to the pubis and acts like a hammock, supporting the uterus, bladder, and colon. The urethra, vagina, and anus all pass through an opening in these muscles.
The location of female reproductive system:
The female reproductive organs are all located within the pelvic cavity and are held in place by a series of ligaments. The broad ligament is a large mesentery (or sheet) that encloses the ovaries, Fallopian tubes, and uterus. It attaches to the sides and floor of the pelvic cavity, where it runs continuous with the parietal peritoneum. The mesosalpinx, fold within the broad ligament, forms the outer tissue layer that wraps around the fallopian tubes and part of the lining of the peritoneal cavity, which is the space between the parietal peritoneum (which surrounds the abdominal wall) and visceral peritoneum (which surrounds the internal organs). The mesovarium is the portion of the broad ligament that suspends the ovaries. Several other ligaments within the broad ligament’s mesentery sheet are connected to the ovaries and uterus.
Female Reproductive Organs:
Ampulla: longest and widest part of Fallopian tubes; fertilization usually occurs here.
Infundibulum: outermost portion of fallopian tubes; catches and channels released ova.
Fimbria: fingerlike projections at end of Fallopian tubes.
Myometrium: middle layer of uterine wall; induces uterine contractions.
Endometrium: mucous membrane lining the uterus.
Cervix: lower part of uterus.
Ovary: small, oval-shaped gland located on either side of uterus; produce ova and hormones.
Fallopian tube: serves as tunnel for ova to travel from ovaries to uterus.
Uterus: hollow, pear-shaped organ in which a fetus develops.
Perimetrium: outer layer of uterus.
Female External Genitalia:
Clitoral hood: fold of skin that covers clitoris.
Labia majora: encloses and protects external genitalia.
Vaginal orifice: canal that joins cervix to outside.
Bartholin’s glands: secretes mucus.
Labia minora: surrounds vaginal and urethral orifice.
Clitoris: a small sensitive protrusion; comparable to male penis.
The Male reproductive system
The male reproductive system is also part of the urinary system, since the male urinary and reproductive systems merge at the urethra. The male external genitalia includes the penis and scrotum. The internal genitalia includes the testes, epididymides, vans deferent, ejaculatory ducts, and the accessory glands, which are the prostate gland, seminal vesicles, and Cowper’s glands.
The penis is the male copulatory organ. It is composed of three cylinders that contain spongy erectile tissue, the corpus spongiosum and two corpora cavernous, which are encased in a sheath called the buck’s fascia (also called the deep fascia). The glans penis is the conical tip of the penis. It contains the opening of the urethra, which is called the urinary meatus (o external uretha orifice).
The scrotum is a saclike structure formed from the lower part of the abdominal wall. It hangs behind the penis and contains and protects three major structures: the testes, epididymides, and spermatic cords.
The testes (or testicles) hang outside the body, but they are part of the internal genitalia. They produce sperm and synthesize hormones. Sperm is produced through a process called spermatogenesis. Each testis is made of tightly coiled structures called seminiferous tubules and leydig cells.
The epididymides are situated at the head of each testis and function as storage reservoirs where sperm mature. These narrow, tightly coiled tubes that carry the sperm out if the testes. These tubes are extensions of the epididymides and form the main component of the spermatic cord. The vas deferent enlarge to form ampulla. Ejaculatory ducts, meanwhile, are slender tubes at the union of the ducts of the vas deferent and the seminal vesicles.
The accessory glands secrete fluids that make up semen. The prostate gland secretes prostatic fluid, which protects sperm and increases its lifespan and motility. The seminal vesicles (also called seminal glands) secrete a thick fluid that contains fructose, proteins, citric acid, inorganic phosphorous, potassium, and prostaglandins. The Cowper’s glands (also called bulbourethral glands) secrete fluids that help lubricate the glans penis when the penis is erect.
Circumcision: This procedure, usually performed on newborns, involves the removal of the foreskin from the penis. In the most common procedure, the foreskin is opened and separated from the glans penis, after which the foreskin is removed.
Circumcision may be done for religious, cultural, medical, or hygienic reasons. In teen and adult males, it might be necessary to perform a circumcision due to scar tissue, inflammation, or a tight foreskin.
The Location of the Male Reproductive System:
Unlike the female reproductive system, most of the male reproductive system is located outside of the body. The external structures of the male reproductive system are the penis, scrotum, and testes. The internal organs of the male reproductive system are called accessory organs. They include the vas deferent, seminal vesicles, prostate gland, and Cowper’s glands (also called bulbourethral glands).
Male Reproductive Organs:
Ejaculatory duct: fusion of the vas deferent and seminal vesicles; empties into urethra.
Seminal vesicle: saclike pouches that produce a sugar-rich fluid that helps sperm move and makes up most of the volume of ejaculatory fluid.
Bulbourethral gland: adds fluids to semen during ejaculation.
Epididymis: long, coiled tube that rests on each produced in the testes.
Testis: produce testosterone and sperm.
Glans penis: head of penis; unless circumcized, it is covered with a loose layer of skin called the foreskin.
The penis has a network of tissues, muscles, arteries, and veins that allow it to fill with blood and stiffen to create an erection. Erection is achieved and maintained by an inflow of arterial blood and an obstruction that prevents the venous from draining.
The seminiferous tubules are adjacent to the leydig cells (also called interstitial cells of leydig), which produce testosterone.
The menstrual cycle
The menstrual cycle involves two interacting reproductive organs: the ovaries and uterus. The ovarian cycle stimulates a series of changes in the ovarian and endocrine tissues that lead to the release of an ovum (or egg). The uterine cycle governs the preparation and maintenance of the uterine lining in anticipation of a possible pregnancy. The ovarian and uterine cycles happen concurrently.
The ovarian cycle has of three phases: follicular (or proliferative), ovulatory (ovulation), and luteal (or secretory). It begins and ends on the first day of menses (i.e., a woman’s period).
During the first half of the cycle - the follicular phase - the levels of two pituitary hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), slowly rise, triggering the growth of follicles on the surface of the ovaries, which prepares the ova for ovulation.
A follicle is a small sac in which an oocyte (or immature egg) development, which is called folliculigenesis. Primordial follicles are formed in the fetus’s ovaries within the first five months in utero. They then remain in the process of the first meiotic division and only develop into primary follicles at puberty. During puberty, a woman’s body contains roughly 400,00 mature follicles, each containing an immature oocyte.
During the follicular phase, a limited number of primordial follicles develop into primary follicles and eventually into secondary follicles. The growing follicles release estrogens and low levels of progesterone, which maintain the endometrium, facilitating fertilization. Secondary follicles are also marked by the appearance of a follicular cavity within the layer of granulose cells.Graafian follicles (also known as vesicular ovarian follicles) are mature follicles.
The follicular phase ends with ovulation, when a Graafian follicle ruptures and discharges an ovum. The ovum is then captured by the fimbriae on the fallopian tube, and from there it is taken to the uterine ampulla for possible fertilization.
In the luteal phase, the corpus lute forms on the ovary and secretes various hormones, most significantly progesterone, which prepares the endometrium for the implantation of an embryo and is necessary for a pregnancy to occur and continue.
The uterine cycle also occurs in three phases over approximately 28 days. During the proliferative phase, estrogen triggers the endometrium to begin to regrow and replace the blood vessels and glands that deteriorated at the end of the previous cycle. The secretory (or luteal) phase occurs at the same time as ovulation. The progesterone released from the corpus lute keeps the endometrium of the uterus prepared for pregnancy. The menstrual phase occurs if the ovum is not fertilized, and it is characterized by the shedding of the walls of the uterine cavity (i.e., menses).
The ovaries are the only organs in the human body that change size. From month to month, their length will fluctuate during the menstrual and uterine cycles. They also shrink during menopause.
Menarche: In girls, the first menstrual cycle (i.e., the first menstrual bleeding) is called menarche. This first period is usually painless and can often occur without warning. The average age varies, with genetics, diet, and overall health being key factors. In developed countries, most girls experience menarche between the ages of 10 and 16 years.
Ovaries are the only organs in the human body that change size. From month to month, their length will fluctuate during menstruation. They also shrink at menopause.
Menstrual Cycle Calendar: A woman’s menstrual cycle is counted from the first day of one period to the first day of the next. Length varies, with menstrual flow typically occurring every 21 to 35 days and lasting 2 to 7 days.
The ovarian and uterine cycles: The ovarian cycle has three phases: the follicular phase, ovulatory phase (i.e., ovulation), and luteal phase. The uterine cycle also has three phases: the secretory phase, when the uterine wall is shed; the proliferative phase, when a new uterine lining is formed; and the secretory phase, when pregnancy could occur.
Ovulation: The illustration at right shows the stages of ovulation and the follicular changes that happen at each step, from the primordial follicle to the secondary follicle and the Graafian follicle.
Sperm and ova
Gametes (or sex cells) are the human reproductive cells. Female Gametes are called ova (or egg cells), and male gametes are called sperm. The egg and sperm are haploid cells, meaning each carries only one copy of each chromosome. Upon fertilization, the zygote (i.e., the fertilized cell) is thereby given a complete set of chromosomes.
Sperm is the male reproductive cell, which is called a gamete. Human males produce motile sperm, specifically spermatozoa, which have a tail called a flagellum.
There are three phases in the development of spermatozoa. Spermatocygenesis is the first stage and involves the formation of diploid spermatocytes (meaning they contain 46 chromosomes, or one set from each parent) from immature sperm cells called spermatogonia. Spermatidogenesis is the next phase and its when diploid spermatocytes develop into haploid spermatids, meaning they now contain 23 chromosomes. Spermatogenesis is the final stage, when spermatids mature into spermatozoa.
The anatomy of the sperm includes a head, neck, midpiece (or body), and tail. The head contains a mass of chromatin and is surrounded by only a thin rim of cytoplasm. Above the nucleus lies a modified Golgi apparatus called the acrosome. It secretes an enzyme necessary for fertilization.
The sperm’s neck has proximal and distal centrioles, which are cylindrical organelles composed mainly of a protein called tubule. Centrioles enable sperm to form flagella and ova to develop into embryos after fertilization. Centrioles are also necessary for the first division of the ovum.
The middle section is the sperm’s powerhouse. It is composed of a mitochondrial sheath that surrounds an axial filament in cytoplasm. It provides the sperm’s motility.
The tail is the longest part of spermatozoa. It is composed of an axial filament surrounded by cytoplasm and a plasma membrane. The end piece consists of only a naked axial filament.
There are five stages of oogenesis, which is the production of ova ovum. During stage one the primordial follicle undergoes mitosis and produces two diploid oogonia. In stage two, each oogonium undergoes mitosis again and produces two diploid primary oocytes. During stage three, each primary oocyte starts to undergo meiosis, but cell division is block at the first meiotic prophase, and the follicle remains in a diploid state. These first three stages happen in utero, i.e., before birth. stage four takes place at the onset of puberty, when the meiotic block is removed and the primary oocyte completes its first meiotic division, producing a secondary oocyte and the first polar body. The secondary oocyte then proceeds to meiosis II and stops at metaphase II. At stage five, the secondary oocyte, producing the mature haploid gamete- the ovum.
An ovum differs little from an ordinary cell in appearance and structure, but the names used to identify its different parts are distinct. The cytoplasm is called ooplasm or yolk. The nucleus is the germinal vesicle, and the nucleolus is the germinal spot. The thick, transparent envelope around the ovum is the zona pellucida, and its outermost layer is called the corona radiata.
Sexual intercourse is the penetration of the penis into the vagina. Ejaculation is the discharge of semen from the penis. The seminal vesicle secretions contain fructose, which is the energy source for the sperm. The secretions from the prostate gland contain proteolytic enzymes, citric acid, acid phosphatase and lipids, which help semen liquefy once it’s inside the vagina.
The human female egg cell - the ovum - is one of the largest cells in the human body. It is about as wide as a single strand of hair and visible to the naked eye without magnification.
Anatomy of a Sperm:
Sperm and Aging: Sperm quality deteriorates with age. The amount of semen and sperm motility also decreases with age. Additionally, the number of sperm that carry an X chromosome seems to increase with age, so am older man may be more likely to father a male child. While testosterone supplementation can help relive some effects of advancing sperm production.
Acrosome: contains digestive enzymes.
Nucleus: contains 23 chromosomes.
Collar: contains mitochondria.
Flagellum: propels sperm.
Egg Fertilization:
Fertilization: when a sperm comes into contact with the surface of an ovum, it prompts the secondary oocyte to complete meiosis II, forming the ovum. Fertilization occurs when the nucleus of both a sperm and an ovum fuse to form a diploid cell, known as a zygote. This triggers an embryo to develop.
Cytoplasm: provides nutrients to ovum.
Acrosome: organelle that contains digestive enzymes.
Ovum nucleus: contains female genetic material.
Zona pellucida: translucent matrix that acts as a “docking site” for binding of spermatozoa.
Follicle: contains a single oocyte.
Sperm nucleus: contains male genetic material.
First polar body: results from meiosis.
pregnancy
A normal pregnancy lasts about 40 weeks, as measured from the last menstrual period to the time of delivery. This span is divided into three trimesters. Both the mother and fetus experience distant changes during this time. While this section focuses on the changes experienced by the mother, the following section outlines fetal development.
During the first trimester, many changes take place in a pregnant woman’s body. Human chorionic gonadotropin (HCG) and progesterone make the greats feel tender and swollen during to increased blood circulation and fat deposits. Women may experience marked fatigue and increased urination. Cardiac output also increases, pumping more blood to the uterus, which supplies the fetus. This results in the “pregnancy glow”. The cervix and vagina darken due to increased blood flow (Chadwick’s sign), and the vaginal portion of the cervix softens due to increased blood flow (Goodell’s sign).
The second trimester is heralded by the baby bump, as the uterus grows beyond the woman’s pelvis. The skin on the expanding belly may itch adits stretches. As the fetus gets bigger, the mother may also experience more back pain. Hyperpigmentation can also be seen in the body due to hormonal changes. The aureoles become darker, and the midline of the abdomen darkens. Melasma is a condition characterized by tan or brown patches on the face and is sometimes called the “mask of pregnancy”. By the second trimester, the fetus’s movements starting felt, which is often referred to as “quickening.”
The third trimester often includes the greatest weight gain during pregnancy. The navel can also become convex due to the expanding abdomen. The fetus’s weight may rest on the bladder, causing stress incontinence.The enlarged uterus pushes against the diaphragm and results in shortness of breath because the lungs have less room to completely expand. To counterbalance the stretching of the mother’s abdominal muscles, the pelvis tilts and the back arches to help maintain balance. This causes lower back pain, pain in the hips and pelvis, and affects a woman’s gait. There can also be pain in the round ligament of the uterus, and some women can experience colostrum leakage from the breasts. Braxton-Hicks contractions, a type of false labor pains, may occur as the woman approaches her due date.
Human chorionic gonadotropin (HCG), the hormone that tells the rest of the body that there is a baby on board, can be detected in urine seven to nine days after fertilization.
The Stages of Pregnancy:
The first trimester begins on the first day of the last menstrual period and lasts until the end of week 12.
Breasts: may feel tender and swollen.
Digestive system: may experience nausea, heartburn, constipation.
Abdomen: waistline begins to expand.
Vagina: lining becomes thicker and sensitive; a thin, white discharge may appear.
Bladder: more frequent urination.
The second trimester runs from week 13 through to week 28, roughly spanning the fourth, fifth, and sixth months of pregnancy.
Breasts: continue to enlarge and nipples may darken.
Back: may experience backaches.
Legs: may experience cramping, varicose veins.
Abdomen: continues to expand as uterus grows rapidly in size.
The third trimester is the final trimester and runs from weeks 29 to 40, roughly spanning months seven, eight, and nine.
Breasts: continue to enlarge and colostrum may leak from nipples; stretch marks may appear (also on stomach, thighs, and buttocks)
Arms: hair may start to grow on your arms, legs, and face.
Bladder: more frequent urination as pressure on the bladder increases.
Legs: more frequent cramping; swelling of the ankles (as well as the hands and face)
Skin: may feel hot, dry, and itchy; pigmentation may become more apparent.
Abdomen: continues to expand and drop as fetus moves into birth position; Braxton'-Hicks contractions may start.
Full term: If a baby is born from 39 weeks to 40 weeks and 6 days of pregnancy, it is considered to be full term. A baby is considered preterm if delivered before 37 weeks.
Fetal Development
A full-term pregnancy lasts 40 weeks as measured from the mother’s last menstrual period. This span can be divided into three trimesters, each featuring significant developments for both the mother and the fetus. While this section focuses on the changes happening to the fetus, the previous section outlined the changes that affect the mother.
The anatomical changes to the baby that take place during the first trimester can be grouped into two periods: Embryonic development occurs from conception to the eighth week, and fetal development occurs from the ninth week until birth.
Development is, however, a continuous process from conception onward. One major change to the zygote is the multiplication of cells. It becomes a two-cell embryo about 24 hours after insemination. It then takes 96 hours to become a morula, consisting of 16 blastomeres in a solid ball (the word morula actually comes from the Latin word for mulberry). As the outer cells compact, a fluid-filled cavity called the blastocoel develops inside the ball, converting the morula into a blastocyst. The inner cell mass then goes on to become an embryo.
If successful implantation occurs, it will have done so by day eight. The blastocoel is then converted into two cavities, the yolk sac and chorionic cavity, around day 13. Gastrulation occurs around day 16, with the establishment of the three germ layers: the endoderm, mesoderm, and ectoderm.
Neurogenesis begins around day 18, with the development of nervous tissues. Somitogenesis starts around day 20, as the muscles and skeleton begin to form. The heart is the first embryonic organ to form, and it starts beating around day 23.
By the fourth month of pregnancy, into the second trimester, eyebrows, eyelashes, fingernails, and the neck have all formed, and the kidneys start functioning and producing urine. At this stage, the fetus can swallow and hear. By the fifth month, the fetus has begun to sleep and wake in regular cycles. Lanugo (fine, soft hair) and vernix cases (a waxy coating) also cover the fetal skin. By the end of the sixth month, the eyes begin to open, and the lungs are completely formed though not functional.
By the seventh month, at the start of the third trimester, the fetus can stretch, kick, and respond to light and sound. By the eighth month the bones have started to harden, but the skull remains soft and flexible in order to make it easier for it to pass through the birth canal. The fetus also begins to quickly gain weight. In the ninth month, the fetus turns, assuming a head-down position, and the lungs have fully matured. The fetus continues to gain weight.
A baby born from 37 weeks to 38 weeks 6 days is considered to be early term; from 39 weeks to 40 weeks and 6 days, a baby is considered to be full term; from 41 weeks to 41 weeks and 6 days, a baby is considered to be late term; and from 42 weeks and beyond a baby is considered to be post term.
Key Development Stages:
A baby goes through numerous stages of development, beginning as a fertilized egg. The egg develops into a blastocyst, then an embryo, and then a fetus. Weeks 3 to 8 are called the embryonic stages, and 9 weeks to birth are the fetal stages. Weeks 0 to 8 can also be classified into 23 standardized Carnegie stages. These stages do not directly depend on either the age or size of the embryo but are instead based on the external and internal morphological development.
4 weeks: embryo is about 1/4 inch (63 mm) long, smaller than a grain of rice
10 weeks: fetus is around 1 1/4 inches (3cm) long
16 weeks: fetus is about 6 inches (15cm) long and weighs about 4 ounces (113 g)
20 weeks: fetus is about 10 inches (25 cm) long and weighs 1/2 to 1 pound (227-454 g )
Neck Development: The head and neck might be the most complicated anatomical region of the human body because of their complex and delicate architecture. Cells from all three embryonic layers - ectoderm, mesoderm, and endoderm - and cells from the neural crest contribute to the development occurs between the third and eighth week. The process involves a special intermediate structure, called the branchial arch.
Lanugo: is the first hair to be formed by the fetal hair follicles. It is thin, soft, unpigmented, and downy. It is normally shed before birth, at around eight months of gestation, but it can sometimes still be present at birth.
Childbirth
The pre-labor steps that signal the approach of childbirth include changes in the maternal cervix and the fetus dropping his or her head into the pelvic cavity. Labor is the full process of giving birth, from the start of contractions to actual delivery. The emergency of the baby and expulsion of the placenta mark the culmination of pregnancy.
By about week 36 of pregnancy, the fetus’s head descends into the pelvic cavity. This step is called engagement, and it signals that birth is imminent. The ideal position is called cephalic presentation, and it is characterized by a head-down approach into the birth canal, with the baby facing the back of the mother. When the baby’s buttocks or feet are positioned to be delivered first, it is called breech presentation.
The mother’s body also change as the fetus drops closer to the cervix. The cervix gradually softens, shortens, and becomes thinner, which is a process called effacement. Braxton-Hicks contractions, often called false labor, also help thin the cervix.
The next step toward childbirth is dilation, which is when the cervix gradually widens and opens. It is measured in centimeters (around 4 inches), the woman is ready to deliver the baby. If the amniotic membrane has not yet broken (commonly called the “water” breaking), the attending doctor, midwife, or nurse will rupture it. Contractions increase and grow stronger at this stage.
As the cervix dilates, the baby’s head descends through the pelvis. As the baby descends through the birth canal, they tuck their chin into the chest, which helps the head to pass through the pelvis. The doctor or midwife will sometimes need to perform an episiotomy, which involves cutting into the perineum to allow the head to pass. When the baby’s head appears at the vaginal opening, it is called crowning.
Once the head is delivered, the baby will rotate one quarter turn and be in line with the mother’s body, which is called external rotation. Next, the top shoulder is delivered under the pubic bone and is followed b the rest od the body. This is called expulsion. Postdelivery, the uterus mildly contracts, which helps seal bleeding blood vessels. The placenta also separates from thealso separates from the uterus and is expelled with a small gush of blood either unaided or by gently the doctor, midwife, or nurse pulling at the cord and pressing on the lower abdomen.
The umbilical cord is a flexible, cordlike structure that contains three blood vessels: two smaller arteries that carry blood to the placenta, and a larger vein that returns blood to the fetus. It attaches the fetus to the placenta during gestation. Postdelivery, the umbilical cord is cut, and the remainder that is attached to the baby eventually falls off, revealing the belly button.
The placenta is a flattened, circular temporary organ found in the uterus of pregnant women. The fetal part of the placenta is called the chorion, and the maternal component is called the decider basalts. The placenta keeps the fetal blood supply separate from the mother’s. The umbilical cord connects the placenta to the fetus, and through it the placenta provides gas exchange, nutrient uptake, waste elimination, and thermoregulation to the developing fetus via the mother’s blood supply. The maternal blood comes into direct contact with the fetal chorion, although no fluid is exchanged. Identical twins often share the same placenta but always have their own umbilical cords. Non-identical twins have separate placentas and umbilical cords.
Water birth is when a mother delivers her newborn baby in a pool of water at home during a home birth with the help of her partner and their midwife or doula.
Childhood and puberty
Childbirth is the period that spans infancy to puberty. During this time, children develop physical skills and cognitive abilities. Children grow quickly during infancy and more steadily during the remainder of childhood. Physical growths picks up speed again at adolescence. Puberty, meanwhile, is the process of hormonal and physical changes that happen in the human body, enabling reproduction.
Physical growth and mental development are two hallmarks of childhood. Physical growth depends on bone growth, especially the long bones. In a newborn, the bones in the head are still cartilaginous and soft, which allows the bone to grow longer. With age, through a process called ossification, bone replaces this cartilage. This process is completed at about 18 to 20 years of age, when a person attains their maximum physical growth.
In contrast to physical growth, mental development can continue throughout a person’s life. In a newborn, the skull has gaps and seams between the bones to accommodate an increase in brain size. By age 18, the cranial bones have completely fused, and the brain has attained its adult size.
And yet, mental development depends not just on brain size but also on synaptic connections. Early brain development is marked by an explosion of synapse formation between neurons. After about 2 to 3 years of age, the brain starts to remove synapses that it no longer needs, which is a process called synaptic pruning. The brain is plastic, meaning is can change and adapt, and neurons continue to make new connections throughout a person’s life. The process, however, slows after puberty.
During childhood, the levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are low. About a year before the first physical changes of puberty occur, the pituitary gland begins to release more FSH and and LH. FSH stimulates oogenesis in females and the onset of sperm production in males. LH, meanwhile, stimulates an increase in progesterone in females and testosterone production in males. As a result, the physical changes associated with puberty begin to develop.
Puberty: Hormonal changes in boys lead to gonadarche (testicular enlargement), pubarche (the appearance of pubic hair), spermarche (the first ejaculation), a growth spurt, wisdom tooth eruption, and the completion of growth by about age 20 years. In girls, hormonal changes lead to thelarche (breast development), pubarche, menarche (the first menstrual bleeding), a growth spurt, wisdom tooth eruption, and the completion of growth by about age 18 years.
Aging and death
Aging is the time-related deterioration of physiological function, both physical and mental. Aging can occur over different time spans for different individuals, with longevity often depending on heredity. Death is the permanent cessation of all vital functions essential for life.
Aging in humans is associated with a wide range of physiological changes that lead to a progressive deterioration of functional capabilities. Age-related changes to the cardiovascular system include the stiffening of blood vessels, which requires the heart to pump harder and may eventually lead to hypertension.
In the respiratory system, the lung parenchyma loses its supporting structure, which causes air spaces to dilate, making breathing harder. Lung function can also progressively decline as respiratory muscle strength decreases.
In the urinary system, the kidneys start losing nephrons, and blood vessels can harden. This reduces the rate of filtering, leading to waste accumulation. The bladder walls also lose elasticity, and the bladder muscles become weak, leading to incontinence.
Slow deterioration of different body functions can be considered as a descent towards death. Failure of one or more of these systems cam have a cascading effect of failure of all vital functions.