0610 · 16.4Sexual reproduction in humans

Male reproductive system

0:00How can two tiny cells begin the development of a whole human being? And once those cells have joined, how does the developing baby obtain oxygen and nutrients without breathing air or eating food? The answers connect the reproductive organs, the specialised structures of sperm and egg cells, and the placenta. We will follow that story from the production of gametes to the protection and nourishment of a developing fetus.

0:21You need just a little background before we begin. A gamete is a reproductive cell, and in humans the male gamete is a sperm and the female gamete is an egg cell. Each contains a nucleus carrying genetic information. Fertilisation brings genetic information from both parents together, but producing a new individual also requires somewhere for development to happen and a continuing supply of materials. Let us start with the male reproductive system, following the route sperm take rather than learning a disconnected list of labels.

0:50The testes produce sperm and also make the hormone testosterone. The scrotum is the sac holding the testes outside the main body cavity. This keeps them slightly cooler than the inside of the body, providing suitable conditions for sperm production. Sperm ducts carry sperm from the testes to the urethra. The prostate gland adds fluid that the sperm can swim in, and the mixture of sperm and fluid is called semen.

1:13Notice the distinction between the cells and the mixture containing them. Sperm are the male gametes, whereas semen is the fluid containing those gametes. The urethra is the tube that carries semen out of the body through the penis. It also carries urine, but urine and semen do not pass through it at the same time. The penis passes semen into the vagina during sexual intercourse. So the route we need to remember is from the testes, through the sperm ducts, into the urethra and out through the penis.

1:41On a diagram, identify these structures by their positions and connections. The testes sit within the scrotum, while the sperm ducts lead upwards from them. The prostate gland lies below the bladder, close to where the sperm ducts connect with the urethra. Do not mistake the bladder for a reproductive organ simply because it appears on the same diagram.

Female reproductive system and fertilisation site

2:00Now look at the female reproductive system. The ovaries release egg cells and make the hormones oestrogen and progesterone. An ovary lies near the open end of each oviduct, which is the tube leading towards the uterus. The oviduct carries an egg cell towards the uterus, and it is also the usual location of fertilisation. The uterus is a muscular organ with a lining into which an embryo can implant.

2:21It is where the fetus develops during pregnancy. At the base of the uterus is the cervix, a ring of muscle forming its narrow opening. Below the cervix is the vagina, which receives sperm during intercourse and acts as the birth canal. A useful way to understand this diagram is to separate three different jobs. The ovaries provide the egg cells, the oviducts provide the route along which an egg travels and where fertilisation occurs, and the uterus provides the site for development.

2:48The vagina and cervix provide the route into the uterus from below. Hormonal control is covered in the video on sex hormones in humans. After semen enters the vagina, sperm can travel through the cervix and uterus towards the oviducts. If an egg cell is present in an oviduct, a sperm may reach it and fertilisation may occur. Notice that the sperm and egg approach the meeting place from different directions.

3:11Fertilisation normally happens in an oviduct, not in the vagina and not in the uterus.

Gamete adaptations

3:17How is a sperm adapted for this journey? Its flagellum is the long tail that moves to propel it through fluid. The sperm also contains many mitochondria, which release energy through respiration for movement. These features work together, because having a tail is useful only if the cell can supply the energy needed to move it. At the front of the sperm head is the acrosome, which contains enzymes. These enzymes digest a way through the egg cell's surrounding jelly coat, allowing the sperm to enter.

3:44Think of the sperm as facing two separate challenges, reaching the egg and getting through its outer covering. The flagellum and mitochondria help with the journey, while the acrosome enzymes help with entry. The egg cell has a different role, so it has different adaptations. Its cytoplasm contains energy stores that support early development after fertilisation. Its surrounding jelly coat changes at fertilisation, preventing additional sperm from entering. This matters because the new cell should receive genetic information from one sperm and one egg, rather than from several sperm.

4:16Let us compare the gametes across size, structure, movement and numbers. An egg cell is much larger than a sperm and contains much more cytoplasm, including its energy stores. A sperm is small and has a head, a mitochondria-containing section and a flagellum. Both contain a nucleus, but their overall structures reflect their different contributions to reproduction. Sperm are motile, meaning they can move themselves using their flagella. Egg cells are not motile, although they are transported along the oviduct, so not swimming does not mean remaining in one place.

4:47Sperm are produced in millions, whereas usually one egg cell is released each month. Producing many sperm increases the chance that one will successfully reach and fertilise an egg.

Fertilisation, implantation and early development

4:57Now we can state the precise definition of fertilisation. Fertilisation is the fusion of the nuclei from a male gamete, a sperm, and a female gamete, an egg cell. Fusion means that the nuclei join together, combining genetic information from both parents. A sperm merely touching an egg is therefore not the complete definition of fertilisation. The cell formed by fertilisation is called a zygote. It divides repeatedly as it travels towards the uterus, forming an embryo, initially a ball of cells.

5:26Those divisions increase the number of cells, rather than producing another fertilisation event. You will study the details of cell division in the video on inheritance. When the embryo reaches the uterus, it implants into the uterine lining. Implantation means becoming embedded in that lining, rather than simply resting loosely inside the uterus. This positions the embryo where it can establish the connection needed for its continuing development. Keep the sequence clear.

5:52Fertilisation produces the zygote, the zygote divides into an embryo, and the embryo implants.

Placenta, umbilical cord and fetal protection

5:58As development continues, the developing baby is called a fetus. In a diagram of pregnancy, find the placenta attached to the uterine wall and the umbilical cord connecting it to the fetus. Around the fetus is the amniotic sac, which contains amniotic fluid. These structures provide two essential kinds of support, exchanging materials and protecting the fetus. The amniotic sac encloses the fetus and holds the amniotic fluid around it.

6:25The fluid cushions the fetus against bumps and physical shocks. Imagine the difference between pressing directly against an object and pressing against a fluid-filled cushion surrounding it. The sac and fluid provide protection, but they are not the main route by which oxygen and nutrients reach the fetus. That exchange happens at the placenta. The placenta brings the mother's blood and the fetus's blood close together, allowing substances to pass between them.

6:50However, the two blood supplies do not normally mix. This is an exchange surface between separate circulations, not an open pipe pouring the mother's blood into the fetus. Oxygen passes from the mother's blood to the fetus's blood across the placenta. Glucose, amino acids and other dissolved nutrients also pass from mother to fetus. Oxygen and glucose are needed for respiration, which releases energy, while amino acids are needed to make proteins for growth.

7:16These are dissolved substances crossing the exchange surface, rather than whole portions of maternal blood being transferred. Exchange also happens in the opposite direction. Carbon dioxide and urea pass from the fetus's blood into the mother's blood. These are excretory products, meaning waste products of the fetus's metabolism that must be removed. The mother's body can then remove them, so the placenta supports both the delivery of useful substances and the removal of wastes.

7:42The umbilical cord carries fetal blood to and from the placenta. Blood travelling from the fetus towards the placenta brings wastes for removal, while blood returning towards the fetus brings oxygen and nutrients obtained there. The cord therefore connects the fetus to the exchange surface. It contains blood vessels, not a hollow tube carrying food directly into the fetus's stomach. The placenta is not a complete barrier against harmful substances.

8:08Some pathogens, including rubella virus and the human immunodeficiency virus, can pass across it and affect the fetus. Some toxins, including nicotine and alcohol, can also cross. Keeping the two blood supplies separate does not prevent every substance or pathogen from passing between them.

Exam answers and recap

8:24Let us work through an exam-style example using this diagram. The question asks you to identify structure A and describe its function in relation to the exchange of substances between the mother and fetus. Begin with position. Structure A is attached to the uterine wall, and the umbilical cord joins it to the fetus. That identifies it as the placenta, rather than the cord or the amniotic sac. Next, organise the exchange by direction, rather than writing a random list of substances.

8:52Ask what the growing fetus needs, and you obtain oxygen and dissolved nutrients moving from mother to fetus. Ask what its cells produce as waste, and you obtain carbon dioxide and urea moving from fetus to mother. Finally, include the distinction that exchange happens without the two blood supplies mixing. A precise answer would sound like this. Structure A is the placenta. Oxygen, glucose, amino acids and other dissolved nutrients pass from the mother's blood to the fetus's blood.

9:20Carbon dioxide and urea pass from the fetus's blood to the mother's blood. The mother's blood and the fetus's blood do not mix. Another question might ask you to explain how a sperm is adapted for fertilisation. For an explain question, connect each feature to what it achieves, rather than merely naming structures. The flagellum propels the sperm towards the egg cell. Many mitochondria release energy through respiration for movement.

9:45Enzymes in the acrosome digest a way through the egg cell's jelly coat. Watch for a few mistakes that come from mixing up locations and functions. Fertilisation occurs in the oviduct, whereas implantation occurs in the uterine lining, so these are different events in different places. The uterus is where the fetus develops, while the vagina receives sperm and forms the birth canal. The urethra carries semen out of the male body, whereas the sperm ducts carry sperm towards the urethra.

10:12Also avoid describing mitochondria as making energy, because respiration releases energy from nutrients. Do not say that the egg's jelly coat changes to let many sperm in, because the change prevents further sperm entry. And do not say that the fetus obtains oxygen from amniotic fluid. Oxygen reaches fetal blood across the placenta and is carried to the fetus through vessels in the umbilical cord. Let us bring the story together.

10:36The male reproductive system produces sperm and delivers semen, while the female reproductive system releases egg cells and provides the route and place for development. Sperm are adapted to move and enter an egg, while the egg supplies energy stores and prevents additional sperm entry after fertilisation. Their structures make sense when you connect them to these different jobs. Fertilisation is the fusion of the sperm and egg nuclei, producing a zygote that divides into an embryo and implants in the uterine lining.

11:03As the fetus develops, the placenta exchanges nutrients, gases and excretory products between separate maternal and fetal blood supplies. The umbilical cord carries fetal blood to and from that exchange surface, while the amniotic sac and fluid provide protection. Reproduction is therefore not just the joining of gametes, but the coordinated support of a developing new individual.