Human Reproduction: Structure, Function, and Hormonal Regulation

Human Reproduction

Introduction and Learning Objectives

Human reproduction involves the interaction of specialized organs and hormones to ensure the continuation of the species. Understanding the structure and function of male and female reproductive systems, as well as the hormonal regulation of reproductive processes, is essential for comprehending human biology.

• Objective 1: Know the structure of the male and female reproductive systems and how they are adapted for their functions.

• Objective 2: Understand the roles of oestrogen and progesterone in the menstrual cycle.

• Objective 3: Understand the roles of FSH and LH in the menstrual cycle.

• Objective 4: Describe the role of the placenta in the nutrition of the embryo.

• Objective 5: Explain how the developing embryo is protected by amniotic fluid.

• Objective 6: Describe the roles of oestrogen and testosterone in the development of secondary sexual characteristics.

Structure and Function of the Male Reproductive System

Main Organs and Their Adaptations

The male reproductive system is specialized for the production, storage, and delivery of sperm. Each organ has structural adaptations that support its function.

• Testis: Produces sperm (male gametes) and the hormone testosterone. Located in the scrotum, outside the body, to maintain a temperature slightly lower than body temperature, which is optimal for sperm production.

• Scrotum: Sac of skin supporting the testes and regulating their temperature.

• Sperm Duct (Vas Deferens): Transports sperm from the testes to the urethra. Sperm mixes with fluids from glands to form semen.

• Glands: Produce seminal fluid that nourishes and protects sperm.

• Urethra: Tube running through the penis that carries urine or semen. A ring of muscle prevents mixing of urine and semen.

• Penis: Delivers semen into the vagina during sexual intercourse.

• Bladder: Stores urine (not directly involved in reproduction).

• Foreskin: Protective covering of the tip of the penis.

Male Reproductive System Table

Organ	Function
Testis	Produces sperm and testosterone
Scrotum	Maintains optimal temperature for sperm
Sperm Duct	Transports sperm
Glands	Produce seminal fluid
Urethra	Transports urine/semen
Penis	Delivers semen
Foreskin	Protects penis tip

Structure and Function of the Female Reproductive System

Main Organs and Their Adaptations

The female reproductive system is specialized for the production of ova, fertilization, and development of the embryo.

• Ovary: Contains ova (female gametes) which mature and are released in response to hormones.

• Oviduct (Fallopian Tube): Connects ovary to uterus; lined with ciliated cells to move the ovum. Fertilization typically occurs here.

• Uterus: Muscular organ with a soft lining where the fertilized egg (zygote) implants and develops into a fetus.

• Cervix: Ring of muscle at the lower end of the uterus; keeps the fetus in place during pregnancy.

• Vagina: Muscular tube leading to the outside of the body; receives sperm during intercourse.

Female Reproductive System Table

Organ	Function
Ovary	Produces ova and hormones
Oviduct	Transports ovum; site of fertilization
Uterus	Site of implantation and fetal development
Cervix	Maintains pregnancy
Vagina	Receives sperm

Gametes: Sperm and Egg Cells

Structure and Adaptations

Sperm and egg cells are specialized for their roles in fertilization. Both are haploid, containing half the genetic material of a normal body cell.

• Sperm Cell: Very small, has a head region, flagellum (tail) for locomotion, many mitochondria for energy, and enzymes in the acrosome to digest the egg's jelly coat.

• Egg Cell: Large, round, covered in a jelly-like coating, contains cytoplasm with stored energy, and is not capable of locomotion.

Comparison Table: Sperm vs. Egg

Feature	Sperm Cell	Egg Cell
Size	Very small (45 μm)	Large (0.2 mm)
Locomotion	Flagellum, motile	Not motile
Numbers Produced	Millions daily	One per month
Energy Source	Many mitochondria	Cytoplasm stores energy
Enzymes	Acrosome enzymes	Jelly coat changes after fertilization

Hormonal Regulation of the Menstrual Cycle

Oestrogen and Progesterone

Oestrogen and progesterone are ovarian hormones that regulate the menstrual cycle and prepare the uterus for pregnancy.

• Oestrogen: Stimulates thickening of the uterine lining and rise in LH levels; peaks before ovulation.

• Progesterone: Maintains the uterine lining after ovulation; inhibits FSH and LH production; drops if pregnancy does not occur, leading to menstruation.

FSH and LH

FSH (Follicle Stimulating Hormone) and LH (Luteinizing Hormone) are pituitary hormones that control ovulation and follicle development.

• FSH: Stimulates maturation of egg cells in follicles and increases oestrogen production.

• LH: Triggers ovulation and formation of the corpus luteum, which produces progesterone.

Menstrual Cycle Table

Day	Main Event	Hormone Activity
1-5	Menstruation	FSH rises
6-13	Follicle development, uterine lining thickens	Oestrogen rises
14	Ovulation	LH surge
15-28	Uterine lining maintained	Progesterone rises

Role of the Placenta in Embryonic Nutrition

Placental Function and Nutrient Exchange

The placenta is a specialized organ that connects the developing embryo/fetus to the mother's blood supply, allowing for the exchange of nutrients, gases, and waste products.

• Diffusion: Nutrients and oxygen move from maternal blood to fetal blood down a concentration gradient.

• Active Transport: Some substances are transported against a concentration gradient.

• Adaptations: Large surface area (villi), short diffusion distance, and countercurrent blood flow maximize exchange efficiency.

• Protection: The placenta acts as a barrier to some harmful substances, though certain infections and drugs can cross.

Protection of the Developing Embryo

Amniotic Fluid and Membranes

The embryo is protected by the amnion membrane and amniotic fluid, which cushion and support the growing fetus within the uterus.

• Amniotic Fluid: Surrounds the embryo, absorbing shocks and preventing dehydration.

• Uterus Wall: Provides physical protection.

• Placenta: Acts as a selective barrier and exchange organ.

Hormonal Control of Secondary Sexual Characteristics

Oestrogen and Testosterone

Secondary sexual characteristics develop at puberty under the influence of sex hormones.

• Oestrogen: Responsible for breast development, widening of hips, and onset of menstruation in females.

• Testosterone: Responsible for increased muscle mass, deepening of voice, growth of facial and body hair, and sperm production in males.

Secondary Sexual Characteristics Table

Characteristic	Male	Female
Voice	Deepens	No change
Body Hair	Chest, face, pubic area	Under arms, pubic area
Muscle Mass	Increases	No significant change
Breasts	No change	Grow, nipples enlarge
Reproductive Organs	Sperm production	Ovaries release eggs, menstruation begins

Additional info: These notes are based on biological principles and do not cover General Chemistry topics such as atomic structure, chemical bonding, or stoichiometry.