Chapter 41: Human Reproductive System
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The male reproductive system
- Penis: muscular organ through which urine and semen travel. It can also fill with blood, becoming erect. This occurs during sexual arousal, enabling the penis to be inserted into the vagina.
- Sperm ducts: there are two sperm ducts that carry sperm from the testes to the urethra.
- Prostate gland: exocrine gland located just underneath the bladder. It secretes fluid contributing to semen.
- Cowper’s glands: pair of exocrine glands that also secrete fluid contributing to semen.
- Seminal vesicles: pair of exocrine glands that also secrete fluid contributing to semen.
- Epididymis: sperm storage organs that are located on top of each testis. Sperm mature in the epididymis before being released during sexual intercourse. If sperm is not released, it is broken down and absorbed by the cells of the epididymis.
- Testes: pair of exocrine and endocrine glands located outside the body in a sac called the scrotum. They are responsible for producing sperm (exocrine secretion) and the hormone testosterone (endocrine secretion). Testosterone is produced by the testes in response to luteinising hormone (LH, which is secreted by the pituitary).
- Scrotum: pouch in which each testis are located. They hold the testes outside the body as sperm production requires a temperature of 35 ºC.
The testes produce sperm in response to the hormone follicle-stimulating hormone (FSH, which is secreted by the pituitary).
Sperm are produced by meiosis (see Chapter 13: The Cell Cycle).
Sperm are highly specialised cells containing three parts: head, midpiece and tail.
- Head: contains the nucleus with 23 chromosomes (human haploid number) and an acrosome – needed to penetrate the egg cell at fertilisation.
- Midpiece: contains many mitochondria that produce energy needed for movement of the sperm cell towards the egg cell.
- Tail: responsible for propelling the sperm cell.
The female reproductive system
- Vulva: consists of a number of external genital organs, forming the entrance to the vagina and function in sexual arousal.
- Vagina: muscular organ approximately 7 cm long. It is also known as the birth canal and receives the penis during sexual intercourse.
- Cervix: located at the top of the vagina – it is the junction between the vagina and uterus.
- Uterus (womb): muscular organ responsible for holding the developing embryo/foetus/baby during pregnancy. The internal lining of the uterus is called the endometrium and undergoes a series of changes throughout the female’s 28 day menstrual cycle.
- Fallopian tubes: also known as the oviducts, they form the tubes that carry the egg/fertilised egg from the ovaries to the uterus.
- Ovaries: two exocrine and endocrine organs located within the abdomen at the end of the fallopian tubes. They release one egg (exocrine secretion) per menstrual cycle and the hormones progesterone and oestrogen (endocrine secretion) directly into the bloodstream.
Egg cell development
- Eggs are present in the female ovaries from birth.
- Eggs begin to be ovulated (released from the ovary) at puberty in the female (about the age of 13).
- Usually only one egg is released (ovulated) per 28 days (the length of the female menstrual cycle).
Role of meiosis in gamete formation
- Meiosis is a nuclear division leading to four daughter cells each with half the chromosomes as the parent cell.
Meiosis is required for gamete formation so that following fertilisation, the new individual will have the correct number of chromosomes in its cells. Meiosis also creates variation in the gametes, so that no two gametes are the same genetically, and therefore, no two offspring are the same (unless they resulted from the same zygote).
Secondary Sexual Characteristics
- Secondary sexual characteristics are those features that distinguish males from females, but are not part of the reproductive system.
Secondary sexual characteristics in males include:
- Facial hair
- Pubic hair
- Enlarged larynx
- Broad shoulders and chest
- Large musculature
Secondary sexual characteristics in females include:
- Pubic hair
- Wide hips
Roles of the sex hormones
- Testosterone: produced in the male body by the testes in response to the pituitary hormone luteinising hormone (LH). It functions in maintaining male secondary sexual characteristics and plays a role in the production of sperm.
- Oestrogen: produced in the female body by the ovaries in response to follicle stimulating hormone (FSH). It functions in the formation and maintenance of female secondary sexual characteristics and the repair of the endometrium during the menstrual cycle.
- Progesterone: produced in the female body by the ovaries following ovulation. It functions in maintaining the endometrium during the menstrual cycle and during pregnancy.
Menstrual Cycle – detailed study and hormonal control
- The female menstrual cycle is a series of changes that occur in the female reproductive tract over the period of 28 days.
- It begins in the female at puberty and continues every 28 days until menopause (approximately age 45)
- It can be divided into three separate phases:
1. Follicular phase and menses
3. Luteal phase
1. Follicular phase and menses:
- This phase begins with menses – which is the shedding of the lining of the uterus (endometrium) – usually occurring over the first five days of the cycle.
- It is caused by low levels of oestrogen and progesterone.
- Low levels of the female sex hormones removes the inhibition of these hormones on the pituitary and therefore, follicle-stimulating hormone (FSH) levels begin to increase once again.
- As FSH increases, new follicles begin to form within the ovaries and they start secreting oestrogen.
- Eventually, one of these follicles becomes dominant and is called the Graafian follicle.
- The Graafian follicle continues to grow from days 6-13, increasing in size and secreting increasing amounts of oestrogen.
- Increasing amounts of oestrogen helps to repair the lining of the uterus.
- Increasing amounts of oestrogen also inhibit FSH secretion from the pituitary, ensuring no further follicles develop during the cycle.
- Oestrogen levels reach a critical level in the blood stream just before ovulation (approximately around day 14).
- This critical level of oestrogen stimulates the pituitary to secrete a burst or surge of luteinising hormone (LH).
- The surge in LH levels causes ovulation – the release of an egg cell from the ovary into the fallopian tube.
3. Luteal phase:
- Once the Graafian follicle has released its egg, it becomes the corpus luteum or yellow body and oestrogen levels drop.
- The corpus luteum secretes increasing amounts of progesterone.
- Progesterone maintains the endometrium and thickens it further in readiness for a potential pregnancy.
- Towards the end of the luteal phase, progesterone and oestrogen levels drop.
Endometriosis is where the cells lining the uterus (endometrium) move and grow outside the uterus.
- Pain in the pelvic area and infertility.
- Pain medication, hormonal drugs to treat any hormonal imbalance, or surgery in severe cases.
Fibroids are benign tumours that grow in the muscular wall of the uterus.
- Pain and heavy menstrual bleeding.
- Pain medication, ultrasound, or surgery in severe cases.
Stages of sexual intercourse:
- In males: blood flows into the penis, which becomes erect so that it can be inserted into the vagina.
- In females: secretions from the vagina and blood flowing into the vagina causing it to elongate.
- Increases in heart rate and breathing rate occur in both sexes.
Copulation is the insertion of the erect penis into the vagina.
Orgasm occurs when sexual arousal reaches a maximum (climax).
- Orgasm in males is accompanied by ejaculation.
Ejaculation is the release of semen from the penis during sexual intercourse.
Fertilisation is the fusion of a sperm cell with an egg cell to form a diploid zygote.
Survival time of sperm and egg cells:
Sperm survive anywhere from 0 to 7 days in the female reproductive tract.
Egg cells survive up to 48 hours after ovulation.
Location of fertilisation:
Fertilisation always occurs in the fallopian tubes.
The fertile period of the menstrual cycle is the time at which the female is most likely to become pregnant.
Implantation and placenta formation:
Implantation is the embedding of the embryo into the lining of the uterus.
- Composed of uterine and embryonic tissue.
- Fully formed and functional after 3 months.
- Attached to the foetus via the umbilical cord.
Functions of the placenta:
- Secretes progesterone, taking over from the corpus luteum.
- Allows nutrients, water, oxygen antibodies, drugs and hormones to pass from the mother’s bloodstream to the baby’s bloodstream.
- Allows wastes produced by the baby, such as carbon dioxide and urea, to pass to the mother’s bloodstream for excretion.
- Keeps mother’s blood separate from baby’s blood (as they could be two different blood groups).
Development of the embryo
- At fertilisation, the zygote starts to divide by mitosis
- A ball of cells called the morula is formed.
- The morula moves down the fallopian tube towards the uterus.
The morula is a ball of undifferentiated cells that forms as a result of mitosis.
- Mitosis continues in the morula which then becomes a blastocyst around day 7.
A blastocyst is a fluid-filled sac containing an inner cell mass that gives rise to the embryo.
- Mitosis continues in the blastocyst producing an inner cell mass with three layers:
- The ectoderm gives rise to the skin and nervous system.
- The mesoderm gives rise to the musculoskeletal system, kidneys, lungs and heart.
- The endoderm gives rise to the liver, pancreas and the inner linings of the breathing, digestive and excretory systems.
- A protective sac, called the amnion, forms around the developing embryo.
- Amniotic fluid fills the sac and acts as a shock absorber to protect the developing embryo.
- At the end of the eighth week of pregnancy all the major internal organs have formed and the embryo is known called a foetus.
- At the 12th week (3 months) the placenta is fully formed and functional (see above).
- As pregnancy progresses, the foetus increases in size and the internal organs mature in readiness for life outside the uterus.
Childbirth is divided into three stages:
- Ranges from 1 hour to 72 hours.
- Walls of the uterus begin to contract caused by secretion of the hormone oxytocin from the pituitary.
- The amniotic sac breaks (‘breaking of the waters’).
- Parturition is the process of the baby being born (passing through the birth canal).
- The baby is usually delivered through the birth canal head first.
- The umbilical cord is clamped, cut and sealed to prevent excess blood loss.
- The afterbirth involves the passing of the placenta from the uterus.
Lactation is the production and secretion of milk by the breasts of the female.
- The production of milk by the breasts is controlled by a pituitary hormone, prolactin.
- In the days after childbirth, the breasts secrete a thick yellow substances called colostrum – a nutritious and concentrated form of milk that contains antibodies that protects the baby in the first few weeks and months of life.
Advantages of breast-feeding:
Breast-feeding is recommended for all babies up to at least 6 months of age.
- Contains all the correct nutrients in the correct proportions.
- Contains antibodies.
- Correct temperature.
- Sterile – contain no bacteria or viruses.
- Promotes a strong bond between mother and baby.
- Helps the mother’s body recover more quickly after pregnancy.
- Thought to reduce the chances of developing breast cancer later in life.
Birth control is procedures taken to limit the number of offspring produced.
Abortion is the chemical or physical removal of a embryo or foetus from the uterus.
Contraception is the intentional prevention of pregnancy by stopping fertilisation or implantation from occurring.
There are four methods of contraception:
1. Natural contraception: the couple avoid sexual intercourse during the female’s fertile period (rhythm method).
2. Mechanical contraception: a barrier is used to prevent the sperm from reaching and fertilising the egg cell (e.g. the use of a condom, diaphragms and domes)
3. Chemical contraception: using spermicides or hormones. Condoms, domes and diaphragms are often coated in a spermicide that kills the sperm as soon as they come into contact with it. Hormones are used as contraceptives in the form of the ‘contraceptive pill’ – the woman takes one pill per day – which prevents her ovulating.
4. Surgical contraception: involves the ligation (tying and cutting) of the fallopian tubes in females (tubal ligation) and of the sperm ducts in males (vasectomy).
Infertility is the inability to contribute to conception.
Male infertility can be due to:
- Low sperm count
- Low sperm mobility
- Endocrine gland failure
Female infertility may be due to:
- Fallopian tube blockage
- Endocrine gland failure
Corrective measures for infertility include:
- Hormonal treatment
- IVF (in vitro fertilisation)