There are four types of excretory organs used by mammals. These are: lungs, skin, liver…
REPRODUCTIVE SYSTEMS IN MAMMALS
Mammals are biparental in reproduction that is sexual reproduction involves two distinct and separate individuals, sexually mature male and female.
A) Male reproductive system
The male reproductive system in man consists of testes (singular = testis), vasa efferentia, vas deferens (sperm duct), penis and glands.
1) Testes (testicles)
There are two testes. Each testis is oval in shape and is housed in a wrinkled sac called scrotum which hangs out of the body behind the penis. The scrotum functions as a thermoregulator that protects the sperm from high temperature. The body temperature is too high for the formation of sperm hence the testes are positioned out of the body that is slightly cooler (2 or 3°C lower) than the normal body temperature. When there is cold, the scrotal sacs contract and the testes are moved nearer the body.
Each testis is made of a large number (about 1000) of tiny highly coiled tubes called seminiferous tubules (sperm tubules) and interstitial cells which lie within the seminiferous tubules. The seminiferous tubules produce sperm while the interstitial cells produce testosterone (male sexual hormone).
2) Vas deferens ( Sperm duct)
Vas deferens are small number of tubes which are connected with seminiferous tubules and unite to form epididymis. Vas deferens collect sperm from the seminiferous tubules to the epididymis.
This is highly coiled tube (about 6 metre long) which receives sperm from vasa efferentia. One epididymis serves each testis. Sperm are temporarily stored to mature and develop in the epididymis.
Each vas deferens leads from the epididymis to join a common tube called the urethra. Thus the urethra serves as a passage for both urine and sperm.
It is interesting to note that both urine and semen are not passed at the same time. This is because when the penis is erect, it is difficult to urinate as a muscular valve closes the bladder and when the penis is limp, it is not possible to discharge semen.
This is an organ through which the urethra runs. The enlarged end of the penis is called glans and the skin extending downward from the glans is called foreskin (prepuce). The practice of removing the foreskin in man is called circum-cision.
Inside the penis is an erectile or spongy tissue. When the blood vessels of the erectile tissue are loaded with blood, the penis gets erect. The erection of the penis happens when there is a sexual urge. It is when the penis is erect that it can penetrate the vagina leading to the release of sperm.
Three glands open into the male reproductive system. These are seminal vesicle, prostate gland and Cowper’s gland.
The seminal vesicles is a small sac which opens into the vas deferens before it meets the urethra. The seminal vesicles secrete a thick colourless fluid that nourishes the sperm. The prostate gland is located a little below the bladder. The prostate gland secretes a white milky fluid that neutralizes any trace of acid from urine and also protects the sperm from the acidic secretion of the female body. A pair of Cowper’s glands is situated below the prostate gland. The Cowper’s glands secrete lubricant.
These three glands secrete seminal fluid in which the sperm swim. The fluid activates sperm into swimming. It provides food for the sperm and lubricates their passage. The sperm swim freely in the seminal fluid.
The seminal fluid plus the sperm make up the semen.
Functions of the male reproductive system
- Formation of sperm to fertilize the eggs of the female.
- Produces male hormones.
- Introduces sperm by the help of the penis (an intromittent organ) into the vagina of the female to fertilize the egg thus forming a zygote and an embryo.
B) Female reproductive system
The female reproductive system consists of ovaries, tubes (oviducts), uterus, vagina and vulva.
There are two oval-shaped ovaries each located in the abdominal cavity on the side of the uterus.
Each ovary is held in position by ligaments. Ovaries are small and short. Each ovary has thousands of undeveloped eggs (ova). It is only when a girl has reached the age of puberty (9-15 years) that one ripe egg is released alternatively each month by the ovaries. During the fertile period (about 30-35 years) of a woman, thirteen (13) eggs mature each year.
The ovaries stop producing eggs at about the age of 45-55 years, a condition referred to as menopause. Of about seven thousand (7000) undeveloped eggs in the ovaries, only about 400 eggs reach maturity, the rest degenerate and get absorbed.
- Ovaries produce and release mature eggs.
- They produce female sex hormones (oestrogen and progesterone).
2) Fallopian tubes (Oviducts)
Arising from each side of the uterus is a narrow Fallopian tube about 9-13cm long. The free end is funnel-shaped. The Fallopian tubes are not connected with the ovaries but lie close to them. The funnel-shaped free ends of the Fallopian tubes are lined with cells having cilia. When an egg is released the cilia beat to direct the egg into the fallopian tube.
Functions of fallopian tube
- Directs the released egg from the ovary to itself.
- Fertilization takes place inside the Fallopian tube.
- The fertilized egg (zygote) is pushed down to the uterus by the cells of the Fallopian tube.
3) Uterus (Womb)
The uterus is a wide and thick-walled muscular chamber of about 8cm long and 5cm wide. Two fallopian tubes enter into it at the top while the lower narrow part terminates as a neck or cervix.
The inner side of the uterus is lined with endometrium (a layer well supplied with blood and food to receive fertilized egg).
The uterus varies in number in different mammals. In higher mammals such as a man, there is one uterus while in rodents (rats and rabbits) there are two.
Functions of uterus
- Implantation of the fertilized egg (the zygote).
- The foetus is housed for nine months in the uterus.
- The uterus provides a place for the attachment of foetal placenta.
- Before the development of placenta, the uterus provides nutrients for the embryo.
The vagina leads from the cervix of the uterus to the outside of the body. It is an elastic and muscular tube of about 10 to 14cm long. The walls are wrinkled. The opening of the vagina is partially covered by a thin membrane called hymen. If the hymen is intact, it means that the girl is still a virgin. However, the hymen can be torn during vigorous physical exercise such as athletics or during sexual intercourse.
Functions of vagina
- It receives the male penis which deposits sperm into it.
- It serves as a birth canal during the expulsion of the foetus from the uterus.
- The woman menstruates through the vagina.
The external female organs are collectively called vulva. These include the two thick folds of skin covered with public hair (in the mature girl) called labia majora, two folds of hairless skin called labia minora, a small erectile organ called clitoris. The clitoris is a sensitive organ that is well supplied with blood vessels and nerves. It causes excitement during sexual intercourse and accelerates orgasm in females. Like the penis, it becomes erect in sexual excitement.
Cutting of the clitoris (a practice called circumcision) must not be allowed because it prevents orgasm in female and also makes childbirth difficult and harmful.
Functions of Vulva
- Vulva protects the opening of the vagina.
Functions of the female reproductive system
- It produces eggs (ova) for ovulation.
- It prepares the uterus to receive the fertilized egg (zygote).
- The woman menstruates through the vagina.
- It receives the penis during sexual intercourse so that sperm can be discharged.
- It provides food and protection for the foetus throughout the gestation period.
- It expels the mature foetus from the uterus when the foetus is ready to be born.
- It expels unfertilized egg and other unused material during menstruation.
- It provides female hormones to maintain menstrual cycle.
- The system provides milk to feed the baby after birth.
- It stops the production of new eggs at old age when the woman’s body may not be able to carry pregnancy.